Lecture 1 Watershed Management PDF

1 In the Name of Allah the Most Gracious the Most Merciful  2 CE-888 Watershed Management Lecture: 1 Introduction to Watershed Management ...

1 In the Name of Allah the Most Gracious the Most Merciful  2 CE-888 Watershed Management Lecture: 1 Introduction to Watershed Management  3 fference b/w Catchment area , watershed and Drainage basin  A river drains the water collected from a specific area, which is called its catchment area  The land area that drains runoff( rain or snow) into a lake, river , or a stream is called watershed. It is an area covered by a system of surface and subsurface water flowing into stream.  An area drained by a river and its tributaries is called a river basin or drainage basin  4 What is a watershed?  Hydrosphere & hydrological cycle gives better concept about watershed.  Hydrosphere in physical geography describes combined mass of waters found on, under and above the surface of the planet.  Hydrosphere consists waters of land (rivers and other water bodies, groundwater system etc.), oceans & atmosphere surrounding the land.  Hydrological Cycle is the change in phase of water in the hydrosphere.  Watershed topographically delineated area that is drained by a stream system. An area from which runoff resulting from precipitation flows past a single point into a stream, river, lake or an ocean.  5 What is a watershed?  Watersheds are biophysical systems that define the land surface that drains water and waterborne sediments, nutrients, and chemical constituents to a point in a stream channel or a river defined by topographic boundaries.  Watersheds are the surface landscape systems that transform precipitation into water flows to streams and rivers, most of which reach the oceans.  Watersheds are the systems used to study the hydrologic cycle and they help us understand how human activities influence components of the hydrologic cycle.  Classification of watersheds based on stability indicators:  The main goal of classifying watersheds based on stability indicator is to:-  Identify critical watershed areas and concentrate limited financial and manpower resources onto the most seriously affected lands first.  Based on their stability watersheds can be classified into:  Fragile  Instable  Moderately stable and  Stable  This classification is based on evaluation of biophysical (rainfall, slope, forest cover and watershed shape) and socio-economic (population density, proportion of arable land affected by erosion, average farm size and per capita food crop production) indicators which are easy to obtain and suitable to evaluate the stability of the watershed.  7 Classification of watershed?  Watersheds can be classified using any measurable characteristics in the area like:  Size  Shape  Location  Ground water exploitation  Land use.  However, the main classification of watershed is discussed broadly on the basis of size and land use.  Two watersheds of the same size may behave very differently if they do not have similar land and channel phases.  8 Size  The main implication of watershed size appears in terms of spatial heterogeneity of hydrological processes.  The spatial variability of watershed characteristics increases with size, therefore, large watersheds are most heterogeneous.  As the watershed size increases, storage increases. Based on size, the watersheds are divided into three classes. Small Watersheds < 250 km2 Medium Watersheds between 250 to 2500 km2 Large Watersheds > 2500 km2  9 Size  Small watersheds are those, where the overland flow and land phase are dominant. Channel phase is relatively less conspicuous. The watershed is highly sensitive to high intensity and short-duration rainfalls.  Being medium in size, the workability of medium watersheds are easy due to accessible approach. Rather than size, shape of the watershed plays a dominant role. Overland flow and land phase are prominent.  Large watersheds are less sensitive to high-intensity-rainfalls of short duration. The channel networks and channel phase are well- developed, and, thus, channel storage is dominant.  10 Land use  Land use defines the exploitation (natural and human interactions) characteristics of watersheds which affect the various hydrological processes within the watershed. The watershed classification based on the land use can be given as below:  Agricultural  Urban  Mountainous  Forest  Desert  Coastal  Mixed - a combination of two or more of the above classifications  11 Agricultural Watershed  Agricultural watershed is the watershed in which agricultural activities (crop cultivation) is dominant.  It experiences perhaps the most dynamically significant land-use change. This usually leads to increased infiltration, increased erosion, and/or decreased runoff.  Depression storage is also increased by agricultural operations. When the fields are barren, falling raindrops tend to compact the soil and infiltration is reduced.  There is lesser development of streams in agricultural watersheds. The small channels formed by erosion and runoff in the area are obliterated by tillage operations.  12 Urban Watershed  These are the watershed areas having maximum manipulation for the convenience of human being. These are dominated by buildings, roads, streets, pavements, and parking lots. These features reduce the infiltrating land area and increase imperviousness.  As drainage systems are artificially built, the natural pattern of water flow is substantially altered. For a given rainfall event, interception and depression storage can be significant but infiltration is considerably reduced. As a result, there is pronounced increase in runoff and pronounced decrease in soil erosion. Thus, an urban watershed is more vulnerable to flooding if the drainage system is inadequate.  13 Mountainous Watershed  Because of higher altitudes, such watersheds receive considerable snowfall. Due to steep gradient and relatively less porous soil, infiltration is less and surface runoff is dominantly high for a given rainfall event.  The areas downstream of the mountains are vulnerable to flooding. Due to snow melt, water yield is significant even during spring and summer.  14 Forest Watershed  These are the watersheds where natural forest cover dominates other land uses. In these watersheds, interception is significant, and evapotranspiration is a dominant component of the hydrologic cycle.  The ground is usually littered with leaves, stems, branches, wood, etc. Consequently, when it rains, the water is held by the trees and the ground cover provided greater opportunity to infiltrate.  The subsurface flow becomes dominant and there are times when there is little to no surface runoff. Because forests resist flow of overland water, the peak discharge is reduced. Complete deforestation could increase annual water yield by 20 to 40 %.  15 Desert Watershed  There is little to virtually no vegetation in desert watersheds. The soil is mostly sandy and little annual rainfall occurs. Stream development is minimal.  Whenever there is rainfall, most of it is absorbed by the porous soil, some of it evaporates, and the remaining runs off only to be soaked in during its journey. There is limited groundwater recharge due to occurrence of less rainfall in these watersheds.  16 Coastal Watershed  The watersheds in coastal areas may partly be urban and are in dynamic contact with the sea. Their hydrology is considerably influenced by backwater from wave and tidal action of the sea.  Usually, these watersheds receive high rainfall, mostly of cyclonic type, do not have channel control in flow, and are vulnerable to severe local flooding.  In these watersheds, the water table is high, and saltwater intrusion threatens the health of coastal aquifers, which usually are a source of the fresh water supply.  17 Mixed Watershed  These are the watersheds, where multiple land use/land cover exists either because of natural settings or due to a combination of natural and human interaction activities.  In these watersheds, a combination of two or more of the previous classifications occurs and none of the single characteristics dominate the area  18 Watershed characteristics  Climatic Characteristics Rainfall & its movement, Intensity, Duration, Temperature, Wind Velocity, Humidity, Transpiration, and Evaporation.  Physiographic Characteristics Geomorphology, Size, Shape, Slope, Orientation, Drainage Density, Elevation, Land use, Vegetation Cover, Soil, Geology, Hydrology, Hydrogeology, Hydrography  Socio-economics Characteristics Statistics on people and their health, wants & wishes, Cattle and Farming Practices, People Participation. Each and every watershed has distinct characteristics of its own. No Two Watersheds are Identical. Climatic Characteristics and Physiographical Characteristics will be different. All these characteristics affect the pattern of disposal of stream flow.  19 Climatic characteristics  If the climatic condition is dry before the rainfall, loss of runoff is more due to infiltration and evapotranspiration.  Climatic condition depends on Temperature, Wind Velocity, and Humidity. Transpiration, Evaporation, and Evapotranspiration depends on Climatic Condition.  The more the intensity of rainfall in the watershed, the more is the peak flow disposal from the area.  If the duration is less, time taken to dispose with lower peak flow is also less  20 Climatic characteristics  21 Climatic characteristics  For the correct design of hydraulic structures like: Dams Weirs Barrage Reservoirs Spillways Retaining walls Embankment a peak flow assessment of run-off or flood should be accurately worked out  22 Physiographic characteristics  Watershed geomorphology refers to the physical characteristics of the watershed. Basin area, basin length, basin slope, and basin shape are the physical characteristics of watersheds, significantly affecting the characteristics of runoff and other hydrologic processes. Basin Area: The area of watershed is also known as the drainage area and it is the most important watershed characteristic for hydrologic analysis. It reflects the volume of water that can be generated from a rainfall. Once the watershed has been delineated, its area can be determined by approximate map methods, planimeter or GIS.  23 Physiographic characteristics  Basin area is defined as the area contained within the vertical projection of the drainage divide on a horizontal plane. Watershed area is comprised of two sub- components; Stream areas and Inter-basin areas. The inter-basin areas are the surface elements contributing flow directly to streams of order higher than 1. Stream areas are those areas that would constitute the area draining to a predetermined point in the stream or outlet. For example, the stream area for first-order streams would be delineated by measuring the drainage area for each first-order channel.  24 Physiographic characteristics Basin Length: Conceptually the basin length is the distance traveled by the surface drainage and sometimes more appropriately labeled as hydrologic length. This length is generally used in computing a time parameter, which is a measure of the travel time of water through a watershed. The watershed length is therefore measured along the principal flow path from the watershed outlet to the basin boundary. Since the channel does not extend up to the basin boundary, it is necessary to extend a line from the end of the channel to the basin boundary. The measurement follows a path where the greatest volume of water would generally travel.  25 Physiographic characteristics Basin Length: 1.The greatest straight-line distance between any two points on the perimeter. 2.The greatest distance between the outlet and any point on the perimeter. 3.The length of the main stream from its source (projected to the perimeter) to the outlet (most commonly used).  26 Physiographic characteristics Shape: Watersheds differ in their shape based on morphometric parameters like geology and structure. The shape of watershed has a dominant effect on the characteristics of the hydrograph of the watershed such as peak flow, overland flow, and base of hydrograph (run-off time).  27 Physiographic characteristics  Different geomorphologic indices can be used for the analysis of a watershed if its shape is taken into consideration. The most frequently used index is the Gravelius's index KG, which is defined as the relation between the perimeter of the watershed and that of a circle having a surface equal to that of a watershed where: KG Gravelius's shape index A watershed area [km2] P watershed perimeter [km]  28 Physiographic characteristics Several values of the Gravelius's index for various shapes of watershed can be found in following figure:  29 Physiographic characteristics Orientation: The orientation of a watershed influences the melting speed of snow. Watersheds developed especially in North-South direction have an alternative exposure to sunrays; the melting speed of snow thus being smaller than in cases of watersheds developed towards East-West. For a precise determination of the influence of watershed orientation, it is necessary to know the direction and frequency of the dominant wind.  30 Physiographic characteristics Size: The size of watershed is governed by the size of stream or river and the development and management works taken.  For minor irrigation project, size may be few hundred square kilometers For Tank or Pond Irrigation, size is just few square kilometers  In Watershed management works in hilly or undulating topography, the size of watershed may be even much smaller. Measurement of precipitation as well as water retention, drainage from the watershed is complex for a bigger size of watershed  31 Physiographic characteristics  32 Physiographic characteristics 33  34 Physiographic characteristics Basin Slope: Watershed/basin slope affects the momentum of runoff. It reflects the rate of change of elevation with respect to distance along the principal flow path. It is usually calculated as the elevation difference between the endpoints of the main flow path divided by the length. Basin slope has a profound effect on the velocity of overland flow, watershed erosion potential, and local wind systems. Basin slope S is defined as S = h/L where h = fall in meters, and L = horizontal distance (length) over which the fall occurs  35 Physiographic characteristics  36 Physiographic characteristics Characteristic altitudes for watershed : The extreme altitudes of the watershed, such as minimum and maximum, are obtained as a starting step for topographic maps. The maximum altitude is the elevation of the highest point of the watershed, while the minimum altitude is the elevation of the lowest point, this being generally the outlet section of the watershed. These values determine the altimetry amplitude of a watershed and help to calculate the slope. The average altitude of a watershed can be deduced directly from reading the topographical map. The average altitude of a watershed is often used in the evaluation of certain hydro-meteorological parameters and can be calculated with different formulas.  37 Physiographic characteristics Drainage: The Stream Order, Drainage Pattern, and Drainage Density have a profound influence on watershed as to runoff, infiltration, land management etc. It determines the flow characteristics and erosional behavior.  If drainage density is more, peak runoff is more. 38  39 Physiographic characteristics Physiography: Type of land, its altitude and physical disposition immensely speak about a watershed as to the climate and planning the activities in greening. Hilly tract could be useful mainly for Forestry and Plains of populated areas could be utilized for Crops  40 Physiographic characteristics Hydrography: The hydrographic network is defined as the sum of all the watercourses, natural or artificial, permanent or temporary, which contribute to the runoff. The characteristics of a hydrographic network of a watershed are influenced by four main factors: geology, climate, relief and environment. The hydrographic network is one of the most important characteristics of a watershed. The classification of the watercourses was introduced by Strahler (1957). The order of the watercourses reflects the degree of ramification of the hydrographic network from upstream to downstream and it is based on the following principles: [Musy, 2001]  41 Physiographic characteristics  all watercourses without tributaries are of 1st order;  the watercourse formed by the confluence of two watercourses of different order is going to keep the highest order of the two;  the watercourse formed by the confluence of two watercourses of same order is going to have an order higher with one than the other two.  42 Physiographic characteristics Land Use: The land in watershed is used for numerous purposes such as dwelling houses cultivation  livestock  water harvesting  roads and railways Land use affects the run-off rates Vegetation Cover: Vegetation cover retard the flow and increases the infiltration and interception, there by reducing the peak flow. Vegetation cover protects the soil erosion. Detailed information on vegetation helps in choosing type, mode and manner of greening the watershed  43 Physiographic characteristics Soil: The soil such as sand absorbs a larger part of run-off water, so the rate of flow is less. Clay soil produces more flow as its absorption is less. Soil parameters such as depth, nature, moisture and fertility determines crops. Geology: : Rocks and their structure control formation of a watershed itself because their nature determines size, shape, physiography, drainage, and groundwater conditions. Geological conditions affects the run-off of watershed. If there are some cracks in rocks joining surface water and groundwater, a part of water flows quickly underground. Lakes, Storages, Swamp hold a part of excess rainfall  44 Physiographic characteristics Hydrology: The availability, quality and distribution of surface water is basic to the final goal of growing greenery in a watershed. Hydrological parameters such as intensity and duration of rainfall help in quantification of water available in watershed. If rainfall continues for longer period with low intensity, evaporation and percolation increases but run-off is less. Further it may cause landslide in sloping hillside areas. Hydrogeology: The groundwater demand for irrigation, farm plantation, drinking water , domestic use and industrial use is more as it is relatively less polluted and pure. The information about aquifer such as nature, thickness and characteristics should be known for better planning of watersheds.  45 Socio-economic characteristics A survey of people living in the watershed should be made regarding their:  needs and wants,  economic conditions,  health & hygiene, cattle,  farming practices and participation in watershed management. Demographic profile [characteristics used to categorize a group of people based on specific criteria, such as age, gender, income level, education, ethnicity (population group or subgroup made up of people who share a common cultural background or descent), marital status, and employment], sociological stratification (society’s categorization of its people into rankings based on factors like wealth, income, education, family background, and power), attitude and behavior of community will affect the watershed management to a great extent.  46 Benchmark Survey Benchmark Survey of a watershed is the survey of basic resources. It is essential for a master plan and future evaluation and monitoring of the watershed. The basic points considered in Benchmark Survey are as follows: Physiography (Name, Elevation, Boundaries, etc.) Climate (Temperature, Wind, Humidity etc.) Soil (Type, Depth, Permeability etc.) Hydrological Survey (Rainfall, Infiltration, Evaporation, Evapotranspiration) Topographical Survey (Size, Slope, Shape etc.) Land Use and Vegetation Survey Socio-economic Survey NATIONAL UNIVERSITY OF SCIENCES & TECHNOLOGY THANK YOU