Principles of Soil Conservation and Management PDF
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The Ohio State University, Kansas State University
2008
Humberto Blanco and Rattan Lal
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Principles of Soil Conservation and Management is a comprehensive textbook focusing on soil and water management, conservation strategies, and restoration of eroded soils. It explores various aspects of soil erosion, including water, wind, and tillage erosion, and links these to the restorative practices, soil resilience, and global food security, with a global perspective emphasizing research from developed countries like the USA to developing countries.
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Principles of Soil Conservation and Management Principles of Soil Conservation and Management by Humberto Blanco The Ohio State University, Columbus, OH, USA Kansas State University, Hays, KS, USA and Rattan Lal The Ohio State University, Columbus, OH, USA 123 Humberto Blanco...
Principles of Soil Conservation and Management Principles of Soil Conservation and Management by Humberto Blanco The Ohio State University, Columbus, OH, USA Kansas State University, Hays, KS, USA and Rattan Lal The Ohio State University, Columbus, OH, USA 123 Humberto Blanco Rattan Lal The Ohio State University The Ohio State University 2021 Coffey Road 2021 Coffey Road Columbus OH 43210 Columbus OH 43210 422B Kottman Hall 422B Kottman Hall USA USA Current address: Kansas State University Western Agricultural Research Center-Hays 1232 240th Avenue Hays, KS 67 601 USA ISBN: 978-1-4020-8708-0 e-ISBN: 978-1-4020-8709-7 Library of Congress Control Number: 2008932254 c 2008 Springer Science+Business Media B.V. No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper 9 8 7 6 5 4 3 2 1 springer.com Preface Management and conservation of soil and water resources are critical to human well-being. Their prudent use and management are more important now than ever before to meet the high demands for food production and satisfy the needs of an increasing world population. Despite the extensive research and abundant literature on soil and water conservation strategies, concerns of worldwide soil degradation and environmental pollution remain high. Several of the existing textbooks deal with principles of soil erosion, measurement, and modeling of soil erosion, and climatic (rainfall and wind) factors affecting the rate and magnitude of erosion. Yet, a state-of-the-science textbook for graduate and undergraduate students with emphasis on soil management to address the serious problems of soil erosion and the attendant environmental pollution is needed. Managing soils under intensive use and restoring eroded/degraded soils are top priorities to a sustained agronomic and forestry production while conserving soil and water resources. Management must come before conservation for the restoration and improvement of vast areas of world’s eroded and degraded soils and ecosystems. Thus, this textbook presents a comprehensive review and discussion of the: (1) severity and implications of soil erosion, (2) principles of management and conser- vation of soil and water resources, (3) impacts of water, wind and tillage erosion on soil resilience, carbon (C) sequestration and dynamics, CO2 emissions, and food se- curity, and (4) risks of soil erosion and the attendant relationships with the projected climate change and vice versa. It differs from other textbooks in that it incorporates detailed discussions about biological/agronomic management practices (e.g., no-till systems, organic farming, agroforestry, buffer strips, and crop residues), tillage ero- sion, C dynamics and sequestration, non-point source pollution (e.g. hypoxia), soil quality and resilience, and the projected global climate change. This textbook specifically links the soil and water conservation issues with the restorative practices, soil resilience, C sequestration under different land use and soil management systems, projected global climate change, and global food security. This textbook also synthesizes current information on a new paradigm of soil management which is soil quality. Being a textbook of global relevance, it links and applies the leading research done in developed countries such as in the USA to contrasting scenarios of soil erosion problems in the developing countries. v vi Preface Soil erosion history and the basic principles of water and wind erosion (e.g., fac- tors, processes) have been widely discussed in several textbooks. Thus, the present volume presents only a condensed treatise on these topics. Major attention is given to management rather than to generic factors and processes of erosion. Chapter 1 reviews the implications of soil erosion in the USA and the global hotspots and presents the state-of-knowledge of soil and water conservation research and prac- tices. Chapter 2 synthesizes the processes and factors of water erosion, whereas Chapter 3 reviews the factors and processes of wind erosion with emphasis on the management and control. Chapter 4 discusses the water and wind erosion models and presents examples of calculations of runoff and soil erosion rates. Chapter 5 introduces a relatively new topic in soil and water conservation research, which is tillage erosion. Discussions on tillage erosion have been practically ignored in soil conservation textbooks. Yet, it is an essential topic provided that erosion by tillage can be equal to or even higher than that by water or wind, especially in rolling agricultural landscapes. A larger portion of this textbook from Chapters 6 to 11 is devoted to the man- agement and control of soil erosion. These six Chapters provide comprehensive and thorough assessment of integrated management techniques and approaches to manage and conserve soil and water resources for diverse land uses. Benefits of crop residues, conservation buffers, agroforestry systems, crop rotations, and con- servation tillage (e.g., no-till) systems are discussed. Chapter 11 reviews the differ- ent types of mechanical structures used for erosion control. Erosion in forestlands, rangelands, and pasturelands is discussed in Chapters 12 and 13. Chapter 14 cov- ers the current topics addressing the implications of soil erosion and water runoff to nutrient/chemical transport causing eutrophication and hypoxia or ‘dead zones” in coastal ecosystems around the world. Water pollution caused by the excessive and indiscriminate use of agricultural chemicals on agricultural, forestry, and urban lands is discussed. Chapter 15 describes management strategies for restoring eroded, compacted, saline and sodic, acidic, and mined soils, whereas inherent potential of the inten- sively managed, degraded, and misused soils to recover from the degradation forces is discussed in Chapter 16. Chapter 17 introduces a new topic in soil management and conservation concerning sequestration of C in terrestrial ecosystems and net emissions of CO2 to the atmosphere. This chapter also discusses the transfers of soil C with sediment and runoff water and its fate. Towards the end of the textbook, relations of soil management with soil quality, food security, and global climate change are described (Chapters 18, 19, and 20). These chapters uniquely address the impacts of projected global warming on soil erosion risks and the attendant decline in food production. Finally, Chapter 21 addresses trends in soil conserva- tion and management research as well as research needs for an effective soil and water conservation and management. It identifies possible shortcomings of past and current research work in soil and water conservation and suggests measures for improvement. This textbook is suitable for undergraduate and graduate students in soil sci- ence, agronomy, agricultural engineering, hydrology, and management of natural Preface vii resources and agricultural ecosystems. It is also of interest to soil conservationists and policymakers to facilitate understanding of principles of soil erosion and imple- menting strategic measures of soil conservation and management. The contents of this textbook are easily comprehended by students with a basic knowledge of intro- ductory soils, hydrology, and climatology. Students will gain a better understanding of the basic concepts by following solved problems and doing additional problems given at the end of each chapter. The select problems are designed to further en- hance the understanding of the material discussed in each chapter. Application of basic concepts is depicted by pictures from diverse management systems, soils, and ecoregions. Hays, KS H. Blanco Columbus, OH R. Lal June 2008 Contents 1 Soil and Water Conservation.................................... 1 1.1 Why Conserve Soil?...................................... 1 1.2 Agents that Degrade Soil.................................. 2 1.3 Soil Erosion............................................. 3 1.3.1 Water Erosion................................... 3 1.3.2 Wind Erosion................................... 4 1.4 History of Soil Erosion.................................... 5 1.5 Consequences of Soil Erosion.............................. 6 1.5.1 On-site Problems................................ 6 1.5.2 Off-site Problems................................ 7 1.6 Drivers of Soil Erosion.................................... 8 1.6.1 Deforestation................................... 9 1.6.2 Overgrazing.................................... 9 1.6.3 Mismanagement of Cultivated Lands................ 10 1.7 Erosion in the USA....................................... 10 1.8 Global Distribution of Soil Erosion Risks..................... 11 1.8.1 Soil Erosion in Africa and Haiti.................... 13 1.8.2 Drylands....................................... 14 1.8.3 Magnitude of Wind Erosion....................... 15 1.9 Current Trends in Soil and Water Conservation................ 16 Summary...................................................... 17 Study Questions................................................ 17 References..................................................... 18 2 Water Erosion................................................. 21 2.1 Types................................................... 21 2.1.1 Splash Erosion.................................. 21 2.1.2 Interrill Erosion................................. 22 2.1.3 Rill Erosion..................................... 23 2.1.4 Gully Erosion................................... 24 2.1.5 Tunnel Erosion.................................. 26 2.1.6 Streambank Erosion.............................. 26 2.2 Processes............................................... 27 ix x Contents 2.3 Factors................................................. 28 2.4 Agents.................................................. 28 2.5 Rainfall Erosivity......................................... 30 2.6 Runoff Erosivity......................................... 32 2.6.1 Estimation of Runoff............................. 33 2.6.2 Time of Concentration............................ 33 2.6.3 Runoff Volume.................................. 36 2.6.4 Characteristics of the Hydrologic Groups............ 37 2.6.5 Peak Runoff Rate................................ 40 2.7 Soil Properties Affecting Erodibility......................... 41 2.7.1 Texture......................................... 41 2.7.2 Structure....................................... 42 2.7.3 Surface Sealing.................................. 42 2.7.4 Aggregate Properties............................. 43 2.7.5 Antecedent Soil Water Content..................... 44 2.7.6 Soil Organic Matter Content....................... 45 2.7.7 Water Transmission Properties..................... 46 2.8 Measuring Erosion....................................... 49 Summary...................................................... 50 Study Questions................................................ 51 References..................................................... 52 3 Wind Erosion.................................................. 55 3.1 Processes............................................... 55 3.2 Factors................................................. 58 3.3 Wind Erosivity........................................... 59 3.4 Soil Erodibility.......................................... 61 3.4.1 Texture......................................... 61 3.4.2 Crusts......................................... 62 3.4.3 Dry Aggregate Size Distribution................... 62 3.4.4 Aggregate Stability.............................. 63 3.4.5 Soil Surface Roughness........................... 63 3.4.6 Soil Water Content............................... 64 3.4.7 Wind Affected Area.............................. 64 3.4.8 Surface Cover................................... 64 3.4.9 Management-Induced Changes..................... 65 3.5 Measuring Wind Erosion.................................. 65 3.5.1 Efficiency of Sediment Samplers................... 65 3.5.2 Types of Sediment Samplers....................... 66 3.6 Management of Wind Erosion.............................. 68 3.7 Windbreaks............................................. 68 3.7.1 Reduction in Wind Velocity....................... 70 3.7.2 Density and Porosity............................. 72 3.7.3 Side-Benefits................................... 72 3.7.4 Constraints..................................... 73 Contents xi 3.8 Crop Residues........................................... 73 3.8.1 Flat and Standing Residues........................ 74 3.8.2 Availability of Residues........................... 74 3.9 Perennial Grasses........................................ 74 3.10 Conservation Tillage...................................... 75 Summary...................................................... 77 Study Questions................................................ 77 References..................................................... 78 4 Modeling Water and Wind Erosion............................... 81 4.1 Modeling Erosion........................................ 81 4.2 Empirical Models........................................ 82 4.3 Universal Soil Loss Equation (USLE)........................ 82 4.3.1 Rainfall and Runoff Erosivity Index (EI)............. 83 4.3.2 Soil Erodibility Factor (K)........................ 84 4.3.3 Topographic Factor (LS).......................... 84 4.3.4 Cover-Management Factor (C)..................... 84 4.3.5 Support Practice Factor (P)........................ 85 4.4 Modified USLE (MUSLE)................................. 88 4.5 Revised USLE (RUSLE).................................. 88 4.6 Process-Based Models.................................... 89 4.7 Water Erosion Prediction Project (WEPP).................... 89 4.8 Ephemeral Gully Erosion Model (EGEM).................... 92 4.9 Other Water Erosion Models............................... 93 4.10 Modeling Wind Erosion................................... 93 4.11 Wind Erosion Equation (WEQ)............................. 94 4.11.1 Erodiblity Index (I).............................. 95 4.11.2 Climatic Factor (C).............................. 95 4.11.3 Soil Ridge Roughness Factor (K)................... 96 4.11.4 Vegetative Cover Factor (V)....................... 96 4.12 Revised WEQ (RWEQ)................................... 98 4.12.1 Weather Factor (WF)............................. 98 4.12.2 Soil Roughness Factor (K)........................ 99 4.12.3 Erodible Fraction (EF)............................ 99 4.12.4 Surface Crust Factor (SCF)........................ 100 4.12.5 Combined Crop Factors (COG).................... 100 4.13 Process-Based Models.................................... 101 4.14 Wind Erosion Prediction System (WEPS).................... 101 4.15 Other Wind Erosion Models................................ 103 4.15.1 Wind Erosion Stochastic Simulator (WESS)......... 103 4.15.2 Texas Tech Erosion Analysis Model (TEAM)........ 103 4.15.3 Wind Erosion Assessment Model (WEAM).......... 103 4.15.4 Wind Erosion and European Light Soils (WEELS).... 103 4.15.5 Dust Production Model (DPM)..................... 104 4.16 Limitations of Water and Wind Models...................... 104 xii Contents Summary...................................................... 104 Study Questions................................................ 105 References..................................................... 105 5 Tillage Erosion................................................. 109 5.1 Definition and Magnitude of the Problem..................... 110 5.2 Tillage Erosion Research: Past and Present................... 111 5.3 Tillage Erosion versus Water and Wind Erosion............... 112 5.4 Factors Affecting Tillage Erosion........................... 113 5.5 Landform Erodibility..................................... 114 5.6 Soil Erodibility.......................................... 114 5.7 Tillage Erosivity......................................... 114 5.7.1 Tillage Depth................................... 114 5.7.2 Tillage Implement............................... 115 5.7.3 Tillage Direction................................ 116 5.7.4 Tillage Speed................................... 116 5.7.5 Frequency of Tillage Passes....................... 117 5.8 Tillage Erosion and Soil Properties.......................... 117 5.8.1 Soil Profile Characteristics........................ 117 5.8.2 Soil Properties.................................. 118 5.9 Indicators of Tillage Erosion............................... 118 5.9.1 Changes in Surface Elevation...................... 119 5.9.2 Activity of Radionuclides......................... 119 5.10 Measurement of Soil Displacement.......................... 120 5.11 Tillage Erosion and Crop Production........................ 121 5.12 Management of Tillage Erosion............................. 121 5.13 Tillage Erosion Modeling.................................. 122 5.13.1 Predictive Equations............................. 122 5.14 Computer Models........................................ 127 5.14.1 Tillage Erosion Prediction (TEP) Model............. 127 5.14.2 Water and Tillage Erosion Model (WaTEM).......... 127 5.14.3 Soil Redistribution by Tillage (SORET)............. 128 5.14.4 Soil Erosion by Tillage (SETi)..................... 129 5.14.5 Water- and Tillage-Induced Soil Redistribution (SPEROS)..................................... 129 5.15 Soil Erosion and Harvesting of Root Crops................... 130 Summary...................................................... 132 Study Questions................................................ 132 References..................................................... 133 6 Biological Measures of Erosion Control........................... 137 6.1 Functions of Canopy Cover................................ 137 6.1.1 Measurement of Canopy Cover.................... 138 6.1.2 Canopy Cover vs. Soil Erosion Relationships......... 138 6.2 Soil Amendments........................................ 139 Contents xiii 6.2.1 Classification................................... 139 6.2.2 Specificity...................................... 140 6.2.3 Soil Conditioner................................. 140 6.3 Cover Crops............................................. 140 6.3.1 Water Erosion................................... 142 6.3.2 Wind Erosion................................... 142 6.3.3 Soil Properties.................................. 143 6.3.4 Management of Cover Crops...................... 143 6.4 Crop Residues........................................... 144 6.4.1 Quantity....................................... 144 6.4.2 Soil Properties.................................. 145 6.4.3 Runoff and Soil Erosion.......................... 146 6.4.4 Crop Production................................. 147 6.5 Residue Harvesting for Biofuel Production................... 148 6.5.1 Threshold Level of Residue Removal............... 149 6.5.2 Rapid Impacts of Residue Removal................. 150 6.6 Bioenergy Plantations as an Alternative to Crop Residue Removal 150 6.7 Manuring............................................... 151 6.7.1 Manuring and Soil Erosion........................ 152 6.7.2 Manuring and Soil Properties...................... 152 6.8 Soil Conditioners: Polymers................................ 153 6.9 Polyacrylamides (PAMs).................................. 154 6.9.1 Mechanisms of Soil Erosion Reduction by Polyacrylamides................................ 155 6.9.2 Factors Affecting Performance of Polyacrylamides.... 157 6.9.3 Soil Characteristics.............................. 157 6.9.4 Polyacrylamide Characteristics..................... 157 6.9.5 Rainfall/Irrigation Patterns........................ 158 6.9.6 Soil Management................................ 159 6.9.7 Polyacrylamide vs. Soil Water Dynamics............ 159 6.9.8 Use of Polyacrylamide in Agricultural Soils.......... 160 6.9.9 Use of Polyacrylamide in Non-Agricultural Soils..... 161 6.9.10 Cost-effectiveness of PAM........................ 161 Summary...................................................... 162 Study Questions................................................ 163 References..................................................... 163 7 Cropping Systems.............................................. 167 7.1 Fallow Systems.......................................... 168 7.2 Summer Fallows......................................... 168 7.3 Monoculture............................................. 169 7.4 Crop Rotations........................................... 171 7.4.1 Soil Erosion.................................... 172 7.4.2 Soil Physical Properties........................... 173 7.4.3 Nutrient Cycling and Input........................ 174 xiv Contents 7.4.4 Pesticide Use................................... 174 7.4.5 Crop Yields..................................... 175 7.4.6 Selection of Crops for Rotations.................... 175 7.5 Cover Crops............................................. 176 7.6 Cropping Intensity........................................ 176 7.7 Row Crops.............................................. 177 7.8 Multiple Cropping........................................ 178 7.9 Double Cropping......................................... 179 7.10 Relay Cropping.......................................... 179 7.11 Intercropping............................................ 180 7.12 Contour Farming......................................... 180 7.13 Strip Cropping........................................... 181 7.14 Contour Strip Cropping................................... 182 7.15 Land Equivalent Ratio..................................... 183 7.16 Organic Farming......................................... 184 7.16.1 Definition...................................... 184 7.16.2 Background..................................... 185 7.16.3 Importance..................................... 186 7.16.4 Water Quality................................... 186 7.16.5 Soil Erosion.................................... 187 7.16.6 Soil Biological Properties......................... 188 7.16.7 Soil Physical Properties........................... 189 7.16.8 Crop Yields..................................... 189 Summary...................................................... 190 Study Questions................................................ 191 References..................................................... 191 8 No-Till Farming................................................ 195 8.1 Seedbed and Soil Tilth.................................... 195 8.2 Factors Affecting Soil Tilth................................ 195 8.3 Tilth Index.............................................. 196 8.4 Tillage.................................................. 197 8.5 Tillage Tools............................................ 198 8.6 Types of Tillage Systems.................................. 198 8.7 Conventional Tillage: Moldboard Plowing.................... 199 8.7.1 Residues....................................... 199 8.7.2 Soil Properties.................................. 200 8.7.3 Soil Compaction................................. 200 8.8 Conservation Tillage Systems.............................. 201 8.9 No-Till Farming.......................................... 201 8.9.1 Americas....................................... 202 8.9.2 Europe......................................... 204 8.9.3 Africa and Asia.................................. 205 8.9.4 Australia....................................... 205 8.10 Benefits of No-Till Farming................................ 205 Contents xv 8.10.1 Soil Structural Properties.......................... 206 8.10.2 Soil Water Content............................... 207 8.10.3 Soil Temperature................................ 208 8.10.4 Micro-Scale Soil Properties....................... 209 8.10.5 Soil Biota...................................... 211 8.10.6 Soil Erosion.................................... 211 8.11 Challenges in No-Till Management.......................... 212 8.11.1 Soil Compaction................................. 213 8.11.2 Crop Yields..................................... 214 8.11.3 Chemical Leaching.............................. 214 8.12 No-Till and Subsoiling.................................... 214 8.13 Reduced Tillage.......................................... 215 8.14 Mulch Tillage............................................ 215 8.15 Strip Tillage............................................. 216 8.16 Ridge Tillage............................................ 217 Summary...................................................... 219 Study Questions................................................ 219 References..................................................... 220 9 Buffer Strips................................................... 223 9.1 Importance.............................................. 224 9.2 Mechanisms of Pollutant Removal.......................... 225 9.3 Factors Influencing the Performance of Buffer Strips........... 226 9.4 Types and Management................................... 227 9.5 Riparian Buffer Strips..................................... 228 9.5.1 Design of Riparian Buffers........................ 229 9.5.2 Ancillary Benefits............................... 230 9.6 Filters Strips............................................. 230 9.6.1 Effectiveness of Filter Strips in Concentrated Flow Areas......................................... 231 9.6.2 Grass Species for Filter Strips...................... 232 9.7 Grass Barriers........................................... 234 9.7.1 Natural Terrace Formation by Grass Barriers......... 234 9.7.2 Runoff Ponding Above Grass Barriers............... 235 9.7.3 Use of Grass Barriers for Diverse Agroecosystems.... 235 9.7.4 Use of Grass Barriers in the USA................... 235 9.7.5 Grass Species for Barriers: Vetiver grass............. 236 9.7.6 Grass Barriers and Pollutant Transport.............. 238 9.7.7 Design of Grass Barriers.......................... 239 9.7.8 Grass Barriers and Concentrated Flow............... 240 9.7.9 Combination of Grass Barriers with Other Buffer Strips 240 9.8 Grass Waterways......................................... 241 9.8.1 Design......................................... 241 9.8.2 Management of Waterways........................ 245 9.9 Field Borders............................................ 245 xvi Contents 9.10 Modeling of Sediment Transport through Buffer Strips......... 246 9.10.1 Process-Based Models............................ 247 9.10.2 Simplified Equations............................. 248 Summary...................................................... 254 Study Questions................................................ 255 References..................................................... 256 10 Agroforestry................................................... 259 10.1 Importance.............................................. 260 10.2 Classification............................................ 260 10.3 History................................................. 260 10.4 Current Trends........................................... 261 10.5 Functions of Agroforestry................................. 261 10.5.1 Magnitude of Soil Erosion Reduction............... 263 10.5.2 Agroforestry and Non-Point Source Pollution........ 263 10.6 Agroforestry and Factors of Soil Erosion..................... 264 10.6.1 Rainfall and Runoff Erosivity...................... 264 10.6.2 Soil Erodibility.................................. 265 10.6.3 Terracing....................................... 266 10.6.4 Surface Cover................................... 267 10.7 Agroforestry and Land Reclamation......................... 267 10.8 Agroforestry Plant Species................................. 268 10.9 Alley Cropping.......................................... 269 10.9.1 Benefits of Alley Cropping........................ 270 10.9.2 Design and Management of Alley Cropping Systems.. 271 10.10 Forest Farming........................................... 273 10.11 Silvopastoral System...................................... 276 10.11.1 Silvopastoral System and Soil Erosion.............. 276 10.11.2 Establishment and Management.................... 277 10.12 Use of Computer Tools in Agroforestry...................... 277 10.12.1 Geographic Information Systems................... 277 10.12.2 Models......................................... 278 10.13 Challenges in Agroforestry Systems......................... 279 Summary...................................................... 280 Study Questions................................................ 281 References..................................................... 281 11 Mechanical Structures and Engineering Techniques................ 285 11.1 Types of Structures....................................... 286 11.1.1 Contour Bunds.................................. 286 11.1.2 Silt Fences...................................... 286 11.1.3 Surface Mats.................................... 288 11.1.4 Lining Measures................................. 289 11.2 Farm Ponds............................................. 290 11.2.1 Groundwater-fed Ponds........................... 290 Contents xvii 11.2.2 Stream or Spring-fed Ponds....................... 290 11.2.3 Off-stream Ponds................................ 291 11.2.4 Rainfed Ponds.................................. 291 11.2.5 Design and Installation of Ponds................... 292 11.3 Terraces................................................ 295 11.4 Functions of Terraces..................................... 296 11.5 Types of Terraces......................................... 296 11.6 Design of Terraces........................................ 300 11.7 Management and Maintenance of Terraces.................... 304 11.8 Gully Erosion Control Structures............................ 307 11.8.1 Types of Structures.............................. 309 11.8.2 Grassed Waterways.............................. 311 11.8.3 Gabions........................................ 311 11.8.4 Chute Spillways................................. 313 11.8.5 Pipe Spillways.................................. 313 11.8.6 Drop Structure.................................. 314 11.8.7 Culverts........................................ 316 11.8.8 Maintenance of Gully Erosion Control Practices...... 316 Summary...................................................... 317 Study Questions................................................ 317 References..................................................... 318 12 Soil Erosion Under Forests...................................... 321 12.1 Importance of Forestlands................................. 321 12.2 Classification of Forests................................... 322 12.3 Natural Forests and Soil Erosion............................ 322 12.3.1 Canopy Structure................................ 323 12.3.2 Forest Litter and Roots........................... 323 12.4 Deforestation and Soil Degradation.......................... 323 12.4.1 Soil Erosion.................................... 324 12.4.2 Soil Properties.................................. 325 12.5 Causes of Deforestation................................... 327 12.5.1 Cultivation..................................... 327 12.5.2 Grazing........................................ 327 12.5.3 Logging........................................ 328 12.5.4 Urbanization.................................... 329 12.5.5 Wildfires....................................... 329 12.6 Global Implications of Deforestation........................ 331 12.7 Methods of Land Clearing................................. 333 12.8 Water Repellency of Forest Soils............................ 333 12.9 Management of Burned Forestlands......................... 334 12.10 Reforestation............................................ 337 12.11 Afforestation............................................ 338 12.12 Management of Cleared Forestlands......................... 338 12.13 Modeling of Erosion Under Forests.......................... 340 xviii Contents 12.13.1 Empirical Models................................ 340 12.13.2 Process-Based Models............................ 341 Summary...................................................... 342 Study Questions................................................ 343 References..................................................... 343 13 Erosion on Grazing Lands...................................... 345 13.1 Rangeland Systems....................................... 346 13.2 Pastureland Systems...................................... 346 13.3 Degradation of Grazing Lands.............................. 348 13.3.1 Rangelands..................................... 348 13.3.2 Pasturelands.................................... 348 13.4 Grazing Impacts.......................................... 350 13.4.1 Soil Erosion.................................... 350 13.4.2 Soil Properties.................................. 352 13.4.3 Plant Growth.................................... 354 13.5 Grasses and Erosion Reduction: Mechanisms................. 355 13.5.1 Protection of the Soil Surface...................... 355 13.5.2 Stabilization of Soil Matrix........................ 355 13.6 Root System and Soil Erodibility........................... 356 13.7 Water Pollution in Grazing Lands........................... 359 13.8 Grazing and Conservation Buffers........................... 360 13.9 Grasslands and Biofuel Production.......................... 361 13.10 Methods of Grazing....................................... 362 13.11 Management of Grazing Lands............................. 363 13.11.1 Benefits of Grazing.............................. 364 13.11.2 Fire as a Management Tool........................ 364 13.11.3 Resilience and Recovery of Grazed Lands........... 365 13.11.4 Conversion of Pastureland to Croplands............. 366 13.11.5 Conversion of Croplands to Permanent Vegetation.... 367 13.11.6 Rotational Stocking.............................. 367 13.11.7 Restoration of Degraded Grazed Lands.............. 368 13.12 Modeling of Grazing Land Management..................... 369 Summary...................................................... 370 Study Questions................................................ 371 References..................................................... 372 14 Nutrient Erosion and Hypoxia of Aquatic Ecosystems.............. 375 14.1 Water Quality............................................ 375 14.2 Eutrophication........................................... 376 14.3 Non-point Source Pollution and Runoff...................... 377 14.4 Factors Affecting Transport of Pollutants..................... 377 14.5 Pollutant Sources......................................... 378 14.6 Common Pollutants....................................... 380 14.6.1 Sediment....................................... 380 Contents xix 14.6.2 Nitrogen....................................... 381 14.6.3 Phosphorus..................................... 382 14.6.4 Animal Manure................................. 383 14.6.5 Pesticides...................................... 384 14.7 Pathways of Pollutant Transport............................ 385 14.7.1 Water Runoff................................... 386 14.7.2 Leaching....................................... 386 14.7.3 Volatilization.................................... 387 14.8 Hypoxia of Coastal Waters................................. 387 14.9 Wetlands and Pollution.................................... 389 14.9.1 Degradation of Wetlands.......................... 390 14.9.2 Restoration of Wetland........................... 391 14.10 Mitigating Non-point Source Pollution and Hypoxia........... 391 14.10.1 Management of Chemical Inputs................... 392 14.10.2 Conservation Practices........................... 393 14.11 Models of Non-Point Source Pollution....................... 395 Summary...................................................... 395 Study Questions................................................ 396 References..................................................... 396 15 Restoration of Eroded and Degraded Soils........................ 399 15.1 Methods of Restoration of Agriculturally Marginal Soils........ 400 15.2 Compacted Soils......................................... 402 15.3 Acid Soils............................................... 403 15.4 Restoration of Acid Soils.................................. 404 15.5 Saline and Sodic Soils..................................... 406 15.5.1 Causes of Salinization and Sodification.............. 408 15.5.2 Salinization and Soil Properties.................... 409 15.6 Restoration of Saline and Sodic Soils........................ 409 15.6.1 Leaching....................................... 410 15.6.2 Increasing Soil Water Content..................... 411 15.6.3 Use of Salt-Tolerant Crop Varieties................. 411 15.6.4 Use of Salt-Tolerant Trees and Grasses.............. 412 15.6.5 Establishment of Drainage Systems................. 412 15.6.6 Tillage Practices: Subsoiling....................... 412 15.6.7 Application of Amendments....................... 413 15.6.8 Application of Gypsum........................... 413 15.6.9 Other Techniques................................ 415 15.7 Mined Soils............................................. 415 15.8 Restoration of Mined Soils................................. 417 15.8.1 Soil Restoration Practices......................... 418 15.8.2 Indicators of Soil Restoration...................... 418 15.8.3 Soil Profile Development......................... 419 15.8.4 Runoff and Soil Erosion.......................... 419 15.8.5 Soil Physical Properties........................... 420 xx Contents Summary...................................................... 421 Study Questions................................................ 421 References..................................................... 422 16 Soil Resilience and Conservation................................. 425 16.1 Concepts of Soil Resilience................................ 425 16.2 Importance.............................................. 426 16.3 Classification of Soil Resilience............................ 427 16.4 Soil Disturbance......................................... 428 16.5 What Attributes Make a Soil Resilient?: Factors............... 429 16.5.1 Parent Material.................................. 430 16.5.2 Climate........................................ 430 16.5.3 Biota.......................................... 431 16.5.4 Topography..................................... 432 16.5.5 Time........................................... 433 16.6 Soil Processes and Resilience.............................. 433 16.7 Soil Erosion and Resilience................................ 435 16.8 Soil Resilience and Erodibility.............................. 435 16.8.1 Soil Physical Properties........................... 435 16.8.2 Soil Chemical and Biological Properties............. 437 16.9 Soil Resilience and Chemical Contamination................. 437 16.10 Indicators of Soil Resilience................................ 438 16.11 Measurements of Resilience................................ 439 16.12 Modeling............................................... 439 16.12.1 Single Property Model............................ 439 16.12.2 Multiple Property Models......................... 439 16.13 Management Strategies to Promote Soil Resilience............. 442 Summary...................................................... 444 Study Questions................................................ 445 References..................................................... 446 17 Soil Conservation and Carbon Dynamics......................... 449 17.1 Importance of Soil Organic Carbon.......................... 449 17.2 Soil Organic Carbon Balance............................... 450 17.3 Soil Erosion and Organic Carbon Dynamics.................. 451 17.3.1 Aggregate Disintegration.......................... 451 17.3.2 Preferential Removal of Carbon.................... 452 17.3.3 Redistribution of Carbon Transported by Erosion..... 452 17.3.4 Mineralization of Soil Organic Matter............... 452 17.3.5 Deposition and Burial of Carbon by Transported by Erosion........................................ 453 17.4 Fate of the Carbon Transported by Erosion................... 453 17.5 Carbon Transported by Erosion: Source or Sink for Atmospheric CO 2.................................................... 454 17.6 Tillage Erosion and Soil Carbon............................ 455 Contents xxi 17.7 Conservation Practices and Soil Organic Carbon Dynamics..... 456 17.8 No-Till and Soil Carbon Sequestration....................... 456 17.8.1 Mechanisms of Soil Organic Carbon Sequestration.... 456 17.8.2 Excessive Plowing............................... 457 17.8.3 Site Specificity of Carbon Sequestration............. 457 17.8.4 Stratification of Soil Carbon....................... 457 17.8.5 Soil-Profile Carbon Sequestration.................. 458 17.9 Crop Rotations........................................... 459 17.10 Cover Crops............................................. 460 17.11 Crop Residues........................................... 460 17.12 Manure................................................. 461 17.13 Agroforestry............................................. 462 17.14 Organic Farming......................................... 463 17.14.1 Excessive Tillage................................ 463 17.14.2 Source of Soil Organic Carbon..................... 464 17.14.3 Cropping Systems............................... 464 17.15 Bioenergy Crops......................................... 464 17.16 Reclaimed Lands......................................... 465 17.17 Measurement of Soil Carbon Pool........................... 466 17.17.1 Laser Induced Breakdown Spectroscopy (LIBS)...... 466 17.17.2 Inelastic Neutron Scattering (INS).................. 467 17.17.3 Infrared Reflectance Spectroscopy (IRS)............. 467 17.17.4 Remote Sensing................................. 467 17.18 Soil Management and Carbon Emissions..................... 468 17.19 Biochar................................................. 469 17.20 Modeling Soil Carbon Dynamics............................ 470 17.21 Soil Conservation and Carbon Credits....................... 471 Summary...................................................... 472 Study Questions................................................ 473 References..................................................... 474 18 Erosion Control and Soil Quality................................. 477 18.1 Definitions of Soil Quality................................. 477 18.2 Divergences in Conceptual Definitions and Assessment Approaches............................... 478 18.3 New Perspective......................................... 479 18.4 Soil Quality Paradigm and its Importance.................... 480 18.5 Indicators of Soil Quality.................................. 481 18.5.1 Soil Physical Quality............................. 482 18.5.2 Soil Chemical and Biological Quality............... 482 18.5.3 Macro- and Micro-Scale Soil Attributes............. 482 18.5.4 Interaction Among Soil Quality Indicators........... 483 18.6 Soil Quality Index........................................ 484 18.7 Assessment Tools........................................ 484 18.7.1 Farmer-Based Soil Quality Assessment Approach..... 485 xxii Contents 18.7.2 Soil Test Kits................................... 486 18.7.3 The Soil Management Assessment Framework....... 486 18.8 Soil Quality and Erosion Relationships....................... 487 18.8.1 Soil Erosion and Profile Depth..................... 487 18.8.2 Soil Physical Properties........................... 488 18.8.3 Soil Chemical and Biological Properties............. 489 18.9 Management of Soil Quality............................... 489 Summary...................................................... 489 Study Questions................................................ 490 References..................................................... 491 19 Soil Erosion and Food Security.................................. 493 19.1 Soil Erosion and Yield Losses.............................. 494 19.2 Variability of Erosion Impacts.............................. 495 19.2.1 Soil Type....................................... 496 19.2.2 Climate........................................ 497 19.3 Soil Factors Affecting Crop Yields on Eroded Landscapes...... 497 19.3.1 Physical Hindrance.............................. 498 19.3.2 Topsoil Thickness............................... 498 19.3.3 Soil Compaction................................. 499 19.3.4 Plant Available Water Capacity.................... 499 19.3.5 Soil Organic Matter and Nutrient Reserves........... 500 19.4 Wind Erosion and Crop Production.......................... 501 19.5 Response Functions of Crop Yield to Erosion................. 502 19.6 Techniques of Evaluation of Crop Response to Erosion......... 502 19.6.1 Removal of Topsoil.............................. 503 19.6.2 Addition of Topsoil.............................. 504 19.6.3 Natural Soil Erosion.............................. 504 19.7 Modeling Erosion-Yield Relationships....................... 504 19.8 Productivity Index (PI).................................... 505 19.9 Process-Based Models.................................... 506 19.9.1 EPIC.......................................... 506 19.9.2 Cropsyst....................................... 508 19.9.3 GIS-Based Modeling Approaches.................. 508 Summary...................................................... 510 Study Questions................................................ 511 References..................................................... 511 20 Climate Change and Soil Erosion Risks........................... 513 20.1 Greenhouse Effect on Climatic Patterns...................... 514 20.1.1 Temperature.................................... 514 20.1.2 Precipitation.................................... 515 20.1.3 Droughts....................................... 515 20.1.4 Other Indicators of Climate Change................. 516 20.2 Climate Change and Soil Erosion........................... 516 Contents xxiii 20.2.1 Water Erosion................................... 516 20.2.2 Nutrient Losses in Runoff......................... 518 20.2.3 Wind Erosion................................... 519 20.3 Complexity of Climate Change Impacts...................... 519 20.4 Erosion and Crop Yields................................... 519 20.5 Impacts of Climate Change on Soil Erosion Factors............ 520 20.5.1 Precipitation.................................... 520 20.5.2 Soil Erodibility.................................. 521 20.5.3 Vegetative Cover................................ 522 20.5.4 Cropping Systems............................... 522 20.6 Soil Formation........................................... 522 20.7 Soil Processes........................................... 524 20.8 Soil Properties........................................... 524 20.8.1 Temperature.................................... 524 20.8.2 Water Content................................... 525 20.8.3 Color.......................................... 525 20.8.4 Structural Properties............................. 525 20.8.5 Soil Biota...................................... 526 20.8.6 Soil Organic Carbon Content...................... 527 20.9 Crop Production.......................................... 528 20.9.1 Positive Impacts................................. 528 20.9.2 Adverse Impacts................................. 529 20.9.3 Complex Interactions............................. 530 20.10 Soil Warming Simulation Studies........................... 530 20.10.1 Buried Electric Cables............................ 530 20.10.2 Overhead Heaters................................ 531 20.11 Modeling Impacts of Climate Change........................ 531 20.12 Adapting to Global Warming............................... 532 Summary...................................................... 533 Study Questions................................................ 534 References..................................................... 534 21 The Way Forward.............................................. 537 21.1 Strategies of Soil and Water Conservation.................... 538 21.2 Soil Conservation is a Multidisciplinary Issue................. 540 21.3 Policy Imperatives........................................ 540 21.4 Specific Strategies........................................ 541 21.5 Food Production......................................... 541 21.6 Crop Residues and Biofuel Production....................... 542 21.7 Biological Practices and Soil Conditioners.................... 543 21.8 Buffer Strips............................................. 543 21.9 Agroforestry............................................. 544 21.10 Tillage Erosion........................................... 545 21.11 Organic Farming......................................... 546 21.12 Soil Quality and Resilience................................ 547 xxiv Contents 21.13 No-Till Farming.......................................... 549 21.14 Soil Organic Carbon...................................... 549 21.15 Deforestation............................................ 551 21.16 Abrupt Climate Change................................... 552 21.17 Modeling............................................... 553 21.18 Soil Management Techniques for Small Land Holders in Resource-Poor Regions................................. 554 Summary...................................................... 556 Study Questions................................................ 556 References..................................................... 557 Appendix A........................................................ 559 Appendix B........................................................ 561 Color Plates....................................................... 565 Index............................................................. 601 Chapter 1 Soil and Water Conservation 1.1 Why Conserve Soil? Soil is the most fundamental and basic resource. Although erroneously dubbed as “dirt” or perceived as something of insignificant value, humans can not survive with- out soil because it is the basis of all terrestrial life. Soil is a vital resource that pro- vides food, feed, fuel, and fiber. It underpins food security and environmental qual- ity, both essential to human existence. Essentiality of soil to human well-being is often not realized until the production of food drops or is jeopardized when the soil is severely eroded or degraded to the level that it loses its inherent resilience (Fig. 1.1). Traditionally, the soil’s main function has been as a medium for plant growth. Now, along with the increasing concerns of food security, soil has multi-functionality including environmental quality, the global climate change, and repository for ur- Fig. 1.1 Soil erosion not only reduces soil fertility, crop production, and biodiversity but also alters water quality and increases risks of global climate change and food insecurity (Courtesy USDA-NRCS) H. Blanco, R. Lal, Principles of Soil Conservation and Management, 1 C Springer Science+Business Media B.V. 2008 2 1 Soil and Water Conservation Table 1.1 Multifunctionality of soils Food security, Water quality Projected global Production of biofuel biodiversity, and climate change feedstocks urbanization r Food r Filtration of r Sink of CO and r Bioenergy crops 2 r Fiber pollutants CH4 (e.g., warm season r Housing r Purification of r C sequestration in grasses and r Recreation water soil and biota short-rotation r Infrastructure r Retention of r Reduction of woody crops) r Waste disposal sediment and nitrification r Prairie grasses r Microbial diversity chemicals r Deposition and r Preservation of r Buffering and burial of C-enriched flora and fauna transformation of sediment chemicals ban/industrial waste. World soils are now managed to: (1) meet the ever increasing food demand, (2) filter air, (3) purify water, and (3) store carbon (C) to offset the anthropogenic emissions of CO2 (Table 1.1). Soil is a non-renewable resource over the human time scale. It is dynamic and prone to rapid degradation with land misuse. Productive lands are finite and represent only semiarid> dry subhumid areas>humid areas. Unlike water, wind has the ability to move soil particles up- and down-slope and can pollute both air and water. While arid lands are more prone to wind erosion than humid ecosystems, any cultivated soil that is seasonally disturbed can be subject to eolian processes in windy environments. 1.4 History of Soil Erosion 5 Fig. 1.2 Wind erosion reduces vegetative cover and forms large sand dunes in arid regions (Photo by H. Blanco) Wind erosion not only alters the properties and processes of the eroding soil but also adversely affects the neighboring soils and landscapes where the deposition may occur. Landscapes prone to wind erosion often exhibit an impressive network of wind ripples ( Critical shear of soil = Gully formation The widening of an ephemeral gully with successive rain storms can be expressed as per Eq. (2.14) (Foster and Lane, 1983): 26 2 Water Erosion ⌬W = 1 − exp (−t∗ ) W f − Wi (2.14) where ⌬W is change in channel width, W f is final channel width under the new storm, Wi is initial channel width, and t∗ is time. ⭸W t ⭸t t∗ = i (2.15) W f − Wi where ⭸W ⭸t i is initial rate of change in channel width with respect to the previ- ous width. A rapid approximation of the amount of soil eroded by gully erosion is done by measuring the size of the gully (length and area) and correlating it with the bulk density of the reference soil (Foster, 1986). This simple approach can be related to the whole landscape by the voided area with reference to the uneroded portions of the fields. Advanced techniques of mapping gully erosion across large areas involve aerial photographs, remote sensing, and geographic information sys- tems (GIS) tools. Conservation practices such as no-till, reduced tillage, and residue mulch are effective to control rill and interrill erosion but not gully erosion. Perma- nent grass waterways, terraces, and mechanical structures (e.g., concrete structures) are often used to control gully erosion (See Chapter 11). 2.1.5 Tunnel Erosion Tunnel erosion, also known as pipe erosion, is the underground soil erosion and is common in arid and semiarid lands. Soils with highly erodible and sodic B hori- zons but stable A horizons are prone to tunnel erosion. Runoff in channels, natural cracks, and animal burrows initiates tunnels by infiltrating into and moving thor- ough dispersible subsoil layers. The surface of tunnel erosion-affected soils is often stabilized by roots (e.g., grass) intermixed with soil while the subsoil is relatively loose and easily erodible. Presence of water seepage, lateral flow, and interflow is a sign of tunnel erosion. The tunnels or cavities expand to the point where they no longer support the surface weight and collapse forming potholes and gullies. Tunnel erosion changes the geomorphic and hydrologic characteristics of the affected areas. Reclamation procedures include deep ripping, contouring, revegetation with proper fertilization and liming, repacking and consolidation of soil surface, diversion of concentrated runoff, and reduction of runoff ponding. Revegetation must include trees and deep rooted grass species to increase water absorption. 2.1.6 Streambank Erosion It refers to the collapse of banks along streams, creeks, and rivers due to the erosive power of runoff from uplands fields (Fig. 2.3). Pedestals with fresh vertical cuts along streams are the result of streambank erosion. Intensive cultivation, grazing, 2.2 Processes 27 Fig. 2.3 Corn field severely affected by streambank erosion (Courtesy USDA-NRCS). Saturated soils along streambanks slump readily under concentrated runoff, which causes scouring and un- dercutting of streambanks and expansion of water courses and traffic along streams, and absence of riparian buffers and grass filter strips ac- celerate streambank erosion. Planting grasses (e.g., native and tall grass species) and trees, establishing engineering structures (e.g., tiles, gabions), mulching stream bor- ders with rocks and woody materials, geotextile fencing, and intercepting/diverting runoff are measures to control streambank erosion. 2.2 Processes Water erosion is a complex three-step natural phenomenon which involves detach- ment, transport, and deposition of soil particles. The process of water erosion be- gins with discrete raindrops impacting the soil surface and detaching soil particles followed by transport. Detachment of soil releases fine soil particles which form surface seals. These seals plug the open-ended and water-conducting soil pores, re- duce water infiltration, and cause runoff. At the microscale level, a single raindrop initiates the whole process of erosion by weakening and dislodging an aggregate which eventually leads to large-scale soil erosion under intense rainstorms. The three processes of erosion act in sequence (Table 2.1). The first two processes involving dispersion and removal of soil define the amount of soil that is eroded, and the last process (deposition) determines the dis- tribution of the eroded material along the landscape. If there were no erosion, there would be no deposition. Thus, detachment and entrainment of soil particles are the primary processes of soil erosion, and, like deposition, occur at any point of soil. 28 2 Water Erosion Table 2.1 Role of the three main processes of water erosion Detachment Transport Deposition r Soil detachment occurs r Detached soil particles r Transported particles after the soil adsorbs are transported in runoff. deposit in low landscape raindrops and pores are r Smaller particles (e.g., positions. filled with water. clay) are more readily r Most of the eroded soil r Raindrops loosen up and removed than larger material is deposited at break down aggregates. (e.g., sand) particles. the downslope end of the r Weak aggregates are r The systematic removal fields. broken apart first. of fine particles leaves r Placing the deposited r Detached fine particles coarser particles behind. material back to its move easily with surface r The selective removal origin can be costly. runoff. modifies the textural and r Runoff sediment r When dry, detached soil structural properties of transported off-site can particles form crusts of the original soil. reach downstream water low permeability. r Eroded soils often have bodies and cause r Detachment rate coarse-textured surface pollution. decreases with increase with exposed subsoil r Runoff sediment is in surface vegetative horizons. deposited in deltas along cover. r Amount of soil streams. transported depends on r Texture of eroded the soil roughness. material is different from r Presence of surface the original material residues and growing because of the selective vegetation slows runoff. transport process. When erosion starts from the point of raindrop impact, some of the particles in runoff are deposited at short distances while others are carried over long distances often reaching large bodies of flowing water. 2.3 Factors The major factors controlling water erosion are precipitation, vegetative cover, to- pography, and soil properties and are discussed in Table 2.2. The interactive effects of these factors determine the magnitude and rate of soil erosion. For example, the longer and steeper the slope, the more erodible the soil, and the greater the transport capacity of runoff under an intense rain. The role of vegetation on preventing soil erosion is well recognized. Surface vegetative cover improves soil’s resistance to erosion by stabilizing soil structure, increasing soil organic matter, and promoting activity of soil macro- and micro-organisms. The effectiveness of vegetative cover depends on plant species, density, age, and root and foliage patterns. 2.4 Agents Two main agents affecting soil erosion by water are: rainfall and runoff erosivity. 2.4 Agents 29 Table 2.2 Factors affecting water erosion Climate Vegetative cover Topography Soil properties r All climatic r Vegetative cover r Soil erosion r Texture, organic factors (e.g., reduces erosion increases with matter content, precipitation, by intercepting, increase in field macroporosity, humidity, adsorbing, and slope. and water temperature, reducing the r Soil topography infiltration evapotranspira- erosive energy of determines the influence soil tion, solar raindrops. velocity at which erosion. radiation, and r Plant morphology water runs off the r Antecedent water wind velocity) such as height of field. content is also an affect water plant and canopy r The runoff important factor erosion. structure transport capacity as it defines the r Precipitation is influences the increases with soil pore space the main agent of effectiveness of increase in slope available for water erosion. vegetation cover. steepness. rainwater r Amount, intensity, r Surface residue r Soils on convex absorption. and frequency of cover sponges up fields are more r Soil aggregation precipitation the falling readily eroded affects the rate of determine the raindrops and than in concave detachment and magnitude of reduces the areas due to transportability. erosion. bouncing of interaction with r Clay particles are r Intensity of rain is drops. It increases surface creeping transported more the most critical soil roughness, of soil by gravity. easily than sand factor. slows runoff r Degree, length, particles, but clay r The more intense velocity, and and size of slope particles form the rainstorm, the filters soil determine the rate stronger and more greater the runoff particles in runoff. of surface runoff. stable aggregates. and soil loss. r Soil detachment r Rill, gully, and r Organic materials r High temperature increases with stream channel stabilize soil may reduce water decrease in erosion are typical structure and erosion by vegetative cover. of sloping coagulate soil increasing r Dense and short watersheds. colloids. evapotranspiration growing (e.g., r Steeper terrain r Compaction and reducing the grass) vegetation slopes are prone reduces soil soil water content. is more effective to mudflow macroporosity r High air humidity in reducing erosion and and water is associated with erosion than landslides. infiltration and higher soil water sparse and tall increases runoff content. vegetation. rates. r Higher winds r The denser the r Large and increase soil canopy and unstable water depletion thicker the litter aggregates are and reduce water cover, the greater more detachable. erosion. is the splash r Interactive erosion control, processes among and the lower is soil properties the total soil define soil erosion. erodibility. 30 2 Water Erosion 2.5 Rainfall Erosivity It refers to the intrinsic capacity of rainfall to cause soil erosion. Water erosion would not occur if all rains were non-erosive. Since this is hardly the case, knowl- edge of rainfall erosivity is essential to understanding erosional processes, estimat- ing soil erosion rates, and designing erosion control practices. Properties affecting erosivity are: amount, intensity, terminal velocity, drop size, and drop size distri- bution of rain (Table 2.3). These parameters affect the total erosivity of a rain, but measured data are not always available in all regions for an accurate estimation of rain erosivity. Erosivity of rain and its effects differ among climatic regions. The same amount of rain has strikingly different effects on the amount of erosion de- pending on the intensity and soil surface conditions. Rains in the tropics are more Table 2.3 Factors affecting the erosivity of rainfall Amount Intensity Terminal velocity Drop size r More rain results r Intensity is the r A raindrop r Size of raindrops in more erosion amount of rain per accelerates its can range although this unit of time (mm velocity until the between 0.25 and correlation h−1 ). air resistance 8 mm in diameter, depends on r Intensity is equals the but those between rainfall intensity. normally 76 mm h (2.18) where E is in megajoule ha−1 mm−1 of rainfall, and i m is rainfall intensity (mm h−1 ). When rainfall is measured in daily totals, the E in USLE is estimated as a function of the rainfall depth (D) (mm) and intensity (i) (mm h−1 ) of rainfall as follows: D (210 + 89Log10 i) E= (2.19) 100 The i for rainfall events of different return periods required for designing erosion control practices can be represented as KTx i= (2.20) tn 32 2 Water Erosion where K , x, and n are constants specific to a location, t is the storm duration (min), and T is the return period (yr). Rainfall frequency data including rain duration from 30 min to 24 h and return periods from 1 to 100 yr are available for the USA (Hershfield, 1961). 2.6 Runoff Erosivity Runoff, also known as overland flow or surface flow, is the portion of water from rain, snowmelt, and irrigation that runs off the field and often reaches downstream water courses or bodies such as streams, rivers, and lakes. Runoff occurs only after applied water: (1) is absorbed by the soil, (2) fills up the soil pores and surface soil depressions, (3) is stored in surface detention ponds if in place, and (4) accumulates on the soil surface at a given depth. The components of water balance for runoff to occur are: Runoff = INPUT − OUTPUT = (Rain, Snowmelt, I rrigation) − (I n f iltration, Evaporation, Rain I nter ception by Canopy, W ater Absor ption, T ranspiration, Sur f ace Detention) Similar to the rainfall erosivity, runoff erosivity is the ability of runoff to cause soil erosion. Raindrops impacting soil surface loosen up, detach, and splash soil particles, while runoff carries and detaches soil particles. Interaction among rain, runoff, and soil particles results in erosion. Floating and creeping soil particles in turbulent runoff also contribute to aggregate detachment. Rain has more erosive power than runoff. The kinetic energy (E) of a rain of mass equal to m and terminal velocity (v) equal to 8 m s−1 is (Hudson, 1995) 1 E= m(8)2 = 32m (2.21) 2 Assuming that 25% of the rain becomes runoff and the runoff velocity is 1 m s−1 , the E of runoff is 1 m 1 E= (1)2 = m (2.22) 2 4 8 Thus, the E of rain is 256 times greater than that of runoff. If 50% of the rain had become runoff, the E would be greater by 128 times. Even if all the rain had become runoff, the rain would still have greater E because of the greater terminal velocity of the rain. 2.6 Runoff Erosivity 33 The capacity of runoff to scour the soil and transport particles increases with runoff amount, velocity, and turbulence. Runoff carries abrasive soil materials which further increase its scouring capacity. Early erosion models such as the USLE con- sidered only rainfall erosivity. Improved models which partition the erosive force of water in rainfall erosivity and runoff erosivity provide more accurate predictions. One such relationship which accounts for both components is the modified USLE (MUSLE) (Foster et al., 1982) represented as: Re = 0.5E I30 + α0.5Q e q p 0.33 (2.23) where Re is the rainfall-runoff erosivity, E I30 is product of rain E and its 30-min. intensity (I30 ) of the USLE (MJ. mm ha−1 h−1 ), is α is a coefficient, Q e is the runoff depth (mm), and q p is the peak runoff rate (mm h−1 ). 2.6.1 Estimation of Runoff The determination of the maximum runoff rate and total amount of runoff leaving a watershed are of great utility to: r design and construct mechanical structures of erosion control (e.g., ponds, ter- races, channels), r design and establish conservation buffers (e.g., grass barriers, vegetative filter strips, riparian buffers), r estimate the probable amount of sediment and chemicals (e.g., fertilizers, pesti- cides) transport in runoff, and r convey runoff water safely in channels or grass waterways at a reduced erosive power. Determining rate and volume of runoff involves the consideration of the various runoff factors such as topography, soil surface conditions (e.g., roughness), soil tex- ture, water infiltration, and vegetative cover. When rain falls on an impermeable surface such as a paved surface, all the rain becomes runoff. This is not the case under natural soil conditions where rainfall is partitioned into various pathways: interception by plants and surface residues, infiltration, evaporation, accumulation in surface depressions, and runoff. Any mathematical equation that attempts to esti- mate runoff from a watershed must consider all these factors. 2.6.2 Time of Concentration Time of concentration is the time required for the runoff water to travel from the far- thest point in terms of travel time to the outlet of the watershed (Schwab et al., 1993). Assume that a rain falls only at the lower end of a watershed. Such being the case, runoff water from a point near the upper end of the wetted portion would reach the 34 2 Water Erosion outlet of the watershed in a shorter time than that from the most distant point of the watershed if it rained in the whole watershed. The greatest amount of runoff results when the whole watershed is contributing to runoff under the same rainfall intensity. The time that it takes for the whole watershed to produce runoff depends on the time of concentration. The longest time may not always correspond to the most distant point from the outlet as variability in surface roughness (e.g., major depressions) even near the outlet could delay the time for the water flow to reach the outlet. The time of concentration is critical to compute the runoff hydrograph. The shape and peak of runoff rate are a function of runoff travel time in all its forms including interrill and rill flow. Development of impervious surfaces in urban areas dramat- ically decreases the time of concentration and increases the peak discharge rates. The time of concentration primarily depends on the following factors: 2.6.2.1 Surface Roughness The smoother the surface of a watershed, the smaller is the time of concentration. Growing vegetation, residue mulch, rock outcrops, ridges, depressions, and other obstacles retard the overland flow. Thus, travel time in a vegetated watershed is increased unless the flow is conveyed in constructed channels, which conduct wa- ter more rapidly. Surface roughness is expressed in terms of Manning’s roughness coefficient, which varies according to the type of obstacles (Table 2.4). Table 2.4 Manning’s coefficient of roughness for selected surface conditions (After Engman, 1986) Condition of the soil surface Manning’s coefficient (n) Bare soil 0.011 Impervious surface (paved surfaces) 0.011 Continuous fallow without residue 0.05 Cultivated soil with ≤20% of residues 0.06 Cultivated soil with ≥20% of residues 0.17 Short grass prairie 0.15 Tall and dense grass prairie including 0.24 native species (weeping lovegrass, bluegrass, buffalo grass, switchgrass, Indian grass, and big bluestem). Trees 0.40–0.80 2.6.2.2 Watershed Slope The steeper the surface of a watershed, the shorter the time that it takes for water to reach the outlet. Terracing and establishment of conservation buffers reduce the watershed slope and thereby increase the travel time of water flow. In urban areas, grading changes the slope. Channels with reduced roughness increase runoff veloc- ity and peak discharge. On the contrary, establishment of ponds and reduction of soil slope increase the time of concentration. 2.6 Runoff Erosivity 35 2.6.2.3 Size of the Watershed The larger the watershed, the greater the contributing area to runoff but longer the time for runoff to travel (Fig. 2.4). Both size and shape of the watershed influence the travel time of runoff. Runoff rate reaches its peak faster in a shorter than a longer watershed. 2.6.2.4 Length and Shape the Channel Water flow from the farthest point in flow time under field conditions is not always laminar but tends to flow in different ways including through: (1) shallow rills, (2) open channels as concentrated flow, and (3) diffuse interrill flow. After a short dis- tance, interrill or sheet flow becomes concentrated flow in channels. The longer and smoother the channel, the shorter is the travel time to reach the outlet. Sloping and straight channels accelerate the runoff velocity. Channels that are straightened out increase runoff velocity as compared to meandering and tortuous channels. The common equation to compute the time of concentration is that developed by Kirpich (1940): Tc = 0.0195L 0.77 S −0.385 (2.24) where Tc is time of concentration (min), L is maximum length of flow (m), and S is slope of the watershed (m m−1 ). Rainfall duration can be higher, lower, or equal to the time of concentration. The time of concentration for overland and channel flow is computed by sum- ming up both types of flow time as (USDA-SCS, 1986): Tc = tov + tch (2.25) where tov is time of concentration for overland flow (min) and tch is time of concen- tration for channel flow (min). C B A Fig. 2.4 A large watershed under both overland and channel flow (A) and two watersheds (B and C) of the same size but oriented differently, yielding thus different times of concentration (After Hudson, 1995). Size, shape, and orientation of the watershed influence the runoff travel time and peak runoff rates 36 2 Water Erosion L 0.6 × n 0.6 tov = (2.26) 18S 0.3 0.62 × L ch × n 0.75 tch = ch (2.27) A0.125 × Sch 0.375 where L is slope length of the watershed (m), n is Manning’s roughness coefficient for the watershed, S is average slope gradient of the watershed (m m−1 ), L ch is channel length from the farthest point in flow time (km), n ch is Manning’s roughness coefficient for the channel, A is area of the watershed (km2 ), and Sch is slope of the channel (m m−1 ). In topographically complex watersheds with a large network of channels, the concentration is estimated for each segment in the watershed as: Tc = Tc1 + Tc2 + Tc3 +............ Tcn (2.28) where Tc1 , Tc2 , Tc3 , and Tcn are time of concentration for watershed segments 1, 2, and 3, respectively, and n is the number of flow segments. Example 1. Estimate the time of concentration for a watershed of 1.5 km2 that has an overland slope length of 80 m with a slope of 5.5%. The channel length is 6 km with a slope of 0.9%. The Manning’s coefficient of roughness for the watershed is 0.15 and that for the channel is 0.014. L 0.6 × n 0.6 (80)0.6 × (0.15)0.6 4.44 tov = = = = 0.589 h 18S 0.3 18 × (0.055) 0.3 7.54 0.62 × L ch × n 0.75 0.62 × 6 km × (0.014)0.75 0.151 tch = ch = = = 0.839 h A 0.125 × Sch 0.375 (1.5 km) 0.125 × (0.009)0.375 0.180 The time of concentration for both types of flow is: Tc = tov + tch = 0.589 + 0.839 = 1.428 h. 2.6.3 Runoff Volume The total amount of runoff leaving a field can be computed using the runoff curve number (CN) method, an empirical approach widely used to compute runoff volume for different soil types and surface conditions, as follows: 2 Rday − Ia Q= (2.29) Rday − Ia + S 2.6 Runoff Erosivity 37 where Q is depth of runoff (mm), Rday is amount of rainfall (mm) for the day, Ia is initial abstraction that accounts for the surface water storage in depressions or ponding, rainfall interception by plants and litter/residues, evaporation, and in- filtration before runoff starts (mm), and S is retention parameter (mm). The Ia , a complex parameter, depends on soil surface and vegetative cover characteristics and is assumed to be equal to: Ia = 0.2S (2.30) Substituting Eq. (2.30) in Eq. (2.29) results in 2 Rday − 0.2S Q= (2.31) Rday + 0.8S Thus, S becomes the parameter which accounts for the differences in soil surface conditions, land use and management, and antecedent water content. It reflects the land use conditions through the CN, which is equal to: 25400 S= − 254 (2.32) CN Among the factors that influence CN are hydrologic soil group, land use, soil man- agement, cropping system, conservation practices, and antecedent water content. The values of CN vary from 0 to 100 depending on the soil and surface conditions (Table 2.5). Values of CN decrease with increase in surface vegetative cover. Bare soils without crop residues have the largest CN values whereas undisturbed soils covered by dense vegetation have the smallest CN values. Soils based on their infil- tration characteristics and runoff potential are classified into four main hydrologic groups: A, B, C, and D. A hydrologic soil group refers to a group of soils having the same runoff potential under similar rainstorms and surface cover conditions. Important factors which determine the runoff potential include infiltration capacity, drainage, saturated hydraulic conductivity, depth to water table, and presence of impermeable layer. 2.6.4 Characteristics of the Hydrologic Groups A: These soils are deep, highly permeable, and their textural class includes sand, loamy sand, and sandy loam. Because of the low clay content, soils in this group have very high saturated hydraulic conductivity and infiltration rates even when completely wet and thus have the lowest runoff potential. Deep loess and sandy soils are part of this group. B: This group includes silt loam and loamy soils, which are moderately deep and permeable. They transmit water at slightly lower rates than group A although the 38 2 Water Erosion Table 2.5 Runoff curve numbers for selected surface conditions for different soil hydrologic groups (After USDA-SCS, 1986) Surface condition Hydrologic condition Hydrologic soil group A B D C Urban Areas Impervious areas (roofs, streets, 98 98 98 98 parking lots, and driveways) Pervious areas (lawns, parks, golf Good 39 61 74 80 courses, etc.) Gravel streets and roads 76 85 89 91 Compacted soil surface (roads and 72 82 87 89 streets and right-of-way) Agricultural Lands Fallow: Bare soil 77 86 91 94 Fallow: Crop residue cover Poor 76 85 90 93 Good 74 83 88 90 Row crops 1. Straight rows Poor 72 81 88 91 Good 67 78 85 89 2. Straight rows + residue cover Poor 71 80 87 90 Good 64 75 82 85 3. Straight rows + contoured and Poor 65. 73 7980 81 terraced + residue cover Good 61 70 77 80 Small grains: Straight rows Poor 65 76 84 88 Good 63 75 83 87 Straight rows + residue cover Poor 64 75 83 86 Good 63 75 83 87 Straight rows + contoured and Poor 60 71 78 81 terraced + residue cover Good 58 69 77 80 Legumes or crop Poor 63 73 80 83 rotations + contoured and terraced Good 51 67 76 80 Non-Cultivated Lands Pasturelands, grasslands, and 75% cover 39 61 74 80 Woods Grazed or regularly burned 45 66 77 83 Grazed but not burned 36 60 73 79 Ungrazed 30 55 70 77 rates are still above the average values. The moderate permeability results in soils with moderately low runoff potential. C: These soils are less permeable and shallower than those in group B because of relatively high clay content or presence of slowly permeable layers below the 2.6 Runoff Erosivity 39 topsoil. Sandy clay loams are within this group. These soils have mo
