Forest Hydrology PDF

SFI 106 Lecture 7 Forest Influences on Forest Hydrology  Basic Definition Water is the fundamental agent that links all components (living and non-living) in watersheds (Chang 2012). Watershed management generally revolves around water as a central theme...

SFI 106 Lecture 7 Forest Influences on Forest Hydrology  Basic Definition Water is the fundamental agent that links all components (living and non-living) in watersheds (Chang 2012). Watershed management generally revolves around water as a central theme. This presentation will be devoted to examining the pathways and mechanisms by which water moves from the atmosphere, to the watershed surface and subsurface, into and out of biological communities, and ultimately downstream to the ocean or subsequent rivers.  HYDROLOGY is the scientific discipline concerned with the waters of the Earth, including their occurrence, distribution and circulation via the hydrologic cycle and interactions with living things (https://www.britannica.com/science/hydrology).  is the science of water that is concerned with the origin, circulation, distribution and properties of water of the earth ( U.S. Geological Survey. Retrieved 7 October 2015). F O R E S T H Y D R O L O G Y , R A N G E H Y D R O L O G Y , WILDLAND HYDROLOGY is the branch of hydrology which deals with the effects of land management and vegetation on the quantity, quality and timing of water yields, including floods, erosion and sedimentation In recent years, scientists and progressive thinking resource managers have called for management programs to be held at the watershed level  Forest hydrology studies the distribution, storage, movement and quality of water and the hydrological processes in the forest dominated ecosystems. (USDA, Forest Service)  Forest hydrology is the study of water in forests that includes the distribution, storage, movement and quality of water; hydrologic processes within forested areas; and the delivery of water from forested areas (NRC, 2008).  Forest hydrology is viewed as one of the foundational sciences in integrated watershed management (Brooks et al., 2012)  Forest hydrology is the study of water in forests: the movement, distribution, and quality of water as regulated by forests.  Forest hydrology addresses the hydrologic processes within forested areas and the output of water resources from forested areas.  Disturbances—both planned and unplanned— and management activities in forests can significantly alter hydrologic processes. A. Precipitation  In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, graupel or hail.  Precipitation is moisture that falls from the atmosphere as drizzle, rain, sleet, snow, graupel and hail. 3 types of precipitation 1. Convectional 2. Orographic 3. Cyclonic Convectional precipitation Orographic Precipitation Cyclonic (Frontal)Precipitation Instruments 1. Rain/precipitation Rain gauge Rainfall Sensor 2. Air Temperature Thermometer Thermograph Hyterograph 3. Relative humidity Hygrograph Hygrometer Psychrometer 4. Wind Anemometer Wind Vane 2. Evaporation Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. 3. Transpiration 4. Evapotranspiration Studies have revealed that transpiration accounts for about 10 percent of the the moisture in the atmosphere, providing nearly 90 percent, and a tiny amount coming from sublimation… Instruments Evaporation Pan Atmometer Lysimet er Lysimeter Atmometer Instruments 5. Interception Interception Factors Affecting Interception: 1. Wind speed 2. Rainfall intensity and duration and the type 3. Extent and condition of vegetation (canopy storage, deposition, leaf area index.) Interception measurement  Measurement  To be able to measure interception loss and net precipitation, throughfall, drip, and stemflow should be measured.  To measure throughfall and drip, interceptometers or ordinary raingages are randomly placed under plant canopy. The average value is then computed for a particular area. I = P – Net Rainfall Net Rainfall = amount of rain reaching the soil = Sf + Th + Dr Sf = stemflow, Th = throughfall, Dr = drip 6. Infiltration: the entry of water into the soil surface. Importance of Infiltration: 1. It determines how much water is recharged into the groundwater. 2. It affects the amount of water that is available for plant use. 3. Infiltration also influence the amount of water that is a available for surface runoff formation. Factors Affecting Infiltration 1. Vegetation 2. Soil Characteristics 3. Antecedent Soil Moisture 4. Rain intensity and duration 7. Percolation A downward movement of water into the soil. It is possible that there is no percolation even there is infiltration. This when the bedrock is oriented diagonally. The seepage also possible if parent material is so shallow and the soil layer is very thin. Factors Affecting Percolation 1. Rate of infiltration high infiltration; high percolation 2. Hydraulic gradient/head determined by the depth of water being applied or supplied. greater amount of water; greater gravitational force; greater percolation 3. Permeability of the soil high permeability; high percolation if there are tubular spaces, the water moves faster downward 4 Cross sectional area of flow water moves freely when gravitational force is greater water moves faster when capillary force is smaller 5. Other biological factors microorganism, soil borers, earthworms) high humus; soil borers - facilitates percolation Measurements of Percolation: 1. Lysimeter 2. Mathematical equation 3. Use of observation wells 4. Water budget approach Pe = P - ET - SRO - I Where Pe = percolation P = precipitation ET = Evapotranspiration SRO =Surface-runoff I = Interception Water table distribution Vertical distribution of Water 6. Surface Runoff  Surface runoff is precipitation runoff over the landscape Surface Runoff: 1. It is m ajor force c a u s i n g s o i l e ro s i o n (especially when the soil is unprotected by vegetation) 2. It contributes to the immediate streamflow (SRO has instantaneous effect on the volume of water on stream) Meteorological factors affecting runoff Type of precipitation (rain, snow, sleet, etc.) Rainfall intensity, amount, and duration Distribution of rainfall over the drainage basin Direction of storm movement Precipitation that occurred earlier and resulting soil moisture Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season Effects of Forest Vegetation on Surface Run-off The relative amounts of the two classes of run-off, surface and seepage, cannot be determined separately. The total run off, however, can be measured with a dair degree of accuracy. Surface run-off if it results to flood water is often of destructive significance, whereas seepage run-off gives streams their sustained flow and is of great constructive significance. It is therefore, very easy to see that any factor which decreases surface run-off, consequently increases seepage, and is of utmost economic importance in the utilization and conservation of water in the soil. The relative proportion of run-off to seepage is determined by a large number of variable factors which are: 1. amount and character of precipitation 2. topographical characteristics of the area 3. physical characteristics of the soil, kind of parent materials, and nature of geologic substratum. 4. structural characteristics of forest vegetation.  The forest vegetation is of great importance as a factor, but it is the only one factor which is entirely under the control of man. The above four factors determine what proportion of the rainfall will escape as floodwater and what will be absorbed by the soil. Some Observations A slowly falling, prolonged rain, even on bare soil on steep slopes is practically all taken up by the soil, but a heavy rain of equal amount but of short duration may largely be lost as surface run-off. The effect of a dense forest canopy is to prolong materially the time over which rain reaches the soil thus lessening surface run-off. It slows down the terminal velocity of falling rain, thus much of throughfall becomes absorbed by the soil as infiltration water.  Forest soil, on the whole, absorbs water more rapidly that most other soils. Forest soils have higher rain water infiltration rates. There is practically no superficial run-off, except on steep slopes, in every forest where leaves and other organic debris (litter) form a thick covering over the mineral soil, but when these organic materials have been removed by fire and other causes, surface run-off becomes excessive as shown by rapid and destructive deep gully erosion.  The humus formed from the decomposition of organic debris from the forest floor when incorporated with the mineral soil, increases the water holding capacity and water percolation rate or it improves the structure and power of water absorption and retention of forest soils. Surface run-off from forested slopes may be about ½ as much as from deforested slopes, whereas on hilly land covered with dense stands of timber and with abundant leaf litter, the surfaces run-off is little or nothing.  In order that a forest may have its maximum effect in reducing surface run-off, the soil should have an unbroken leaf litter as contact cover. It should not be burned over or grazed. The reduction of surface run-off not only increases seepage but it also prevents soil erosion. Both of these are factors of importance in the conservation of water for power and for human consumption. Effects of Forest Vegetation on Seepage and Retention of Soil Water Forest vegetation, by reducing surface run-off, increases the amount of water that percolates into a soil. How much water percolates into the soil depends largely upon the: 1) volume of soil or the depth of the mineral soil and 2) the physical characteristics of the soil. Some Observations When soil becomes saturated with water, and rain continues, the effect of forest in regulating water movement ceases. This is because the water is no longer held back, it finds its ways into water courses as rapidly as rain falls. If the soil is shallow, it can hold back a comparatively small amount of precipitation. Forest cover, contributes to the increased volume of soil in mountainous regions over the solid rock foundations, thus it increase seepage. The living and non-living vegetation peculiar to forest, absorbs vast qualities of water in proportion to its volume or weight.  Forest soil with its overlaying organic layers is in a real sense a vast sponge capable of absorbing much more water per unit area than the soil in the open. Sphagnum moss absorbs water 10 times or more than its weight and humus layers absorb water 2 to 4 times their weight.  Forest litter not only reduces surface run-off, particularly from finer-textured soils, but this influence continues long after the litter is completely saturated. Forest litter filters suspended soil particles in run off water which otherwise would seal the pores and seepage openings in the soil thus reducing the absorption or infiltration rate of the soil. This explains the marked differences in rate of absorption in bare soil than in soils covered with litter, thus destruction of forest litter, and the consequent exposure of the forest soil greatly decreases the absorption or infiltrationn rate of the soil. Effects of Forest Vegetation on Springs Part of the water that seeps into the soil at higher elevation reappears at the surface, usually in lower elevations in the form of a spring. A forest through its influence in increasing seepage and decreasing surface run-off provides a larger supply of ground water, particularly in mountainous and hilly regions for the feeding of streams  The volume of water flow from springs in mountainous forests is not only greater but also is much more uniform than in denuded regions which are otherwise. This is due to the permeability of the forest soil and the depth to which water percolates before it reappears as springs.  In the mountainous regions, springs usually diminish their flow and exhibit greater fluctuations in flow with the removal of the forest trees as soil cover. In level areas, the general effect of the forest upon them is of minor importance.  Disappearance of springs often occur after forest destruction in mountainous regions, particularly in regions where rainfall is scarcely adequate for vigorous forest vegetation. Springs may dry up or noticeably decrease their flow after deforestation and some springs reappear after reforestation. Effect of Forest Vegetation on Stream Flow Stream flow includes that part of the rainwater which remains after the loss through interception, transpiration and the small amount which percolates to such depths that it does not reappear on the surface of a drainage area. Stream flow includes surface run off, or floodwater, and seepage run off, or that which reaches streams after percolating through the soil. Streamflow Groundwater runoff (Baseflow) Interflow (subsurface runoff) Direct channel precipitation Overland flow (surface runoff) Importance of Streamflow 1. major source of water supply especially for domestic and irrigation purposes 2. a means for transport 3. parameter which can be used to measure the behavior and response of watershed to management activities 4. a channel for transporting soil nutrients and materials from the watershed to the lowland parts, seas, farms 5. for recreational purposes 6. serves as habitat to many aquatic species Factors affecting streamflow Factors Affecting Streamflow 1. climate/rainfall high intensity rainfall produces low quality water high intensity; high streamflow 2. topography of the watershed 3. high surface-runoff high streamflow; high sediment 4. vegetation good; less SRO; low streamflow; high infiltration; high recharged water 5. land-use 6. characteristic of channel/drainage pattern high drainage intensity; higher streamflow high density; faster velocity geomorphology Streamflow Measurement Volume of water passing through a given cross section per unit time. Q=AXV where : Q = streamflow discharge (cu m/sec) V = streamflow velocity (m/sec) A = cross sectional area ( sq m)  Peak flow vs Base flow Peak Flow: The maximum flow of a stream in response to a rainstorm event. Base Flow: The portion of streamflow contributed by groundwater. Some observations The amount of water available as stream flow in flat country covered with dense forest vegetation, is less than in flat country denuded of vegetation directly because of the large consumption in supplying the transpiration demand. In level regions where there is no surface run off, ground water is more noticeable in regions of deficient rainfall.  On well-timbered catchment or watershed areas the maximum streamflow does not occur for some time, often several hours after precipitation ceases, and its On non-forested catchment areas, on the other hand particularly small ones, maximum stream flow may occur within an hour or two after maximum rainfall, and after precipitation ceases, the decrease in streamflow is very rapid.  Although a forest may use more water during the growing season than it receives from precipitation, it stores much more water in the soil during the so called dormant season. It forms a reservoir which is filled with the excess water when flood danger is greatest and is slowly given out to vegetation and to sustain stream flow at time of drought. Effect of Forest Vegetation on Floods Floods result when the river overflows its bank and water drains the flood plains. This can directly be traced to very heavy and prolonged continuous rains. Forests, in reducing surface run off and increasing seepage, extend the time over which precipitation reaches streams. As a result, a flood is slower in rising and the crest or peak flow is not so high. Some Observations During downpour or heavy rains, forest soil, owing to its larger volume and greater absorptive capacity, is capable of taking up and holding much more water than denuded soils that are otherwise having similar soil physical characteristics. After the ground is saturated, a forest has little influence upon the floodwater until it dries out again.  Climate, soil, topography, and meteorological conditions play the most important role in causing floods, however, deforestation must also be considered as important factor and one which is under the control of man. Forests cannot entirely be depended upon to prevent the occurrence of floods, however, they aid materially by rendering the stream flow throughout the year more uniform, thus making the low stages of stream flow higher and high stages lower.  Floods can be controlled effectively only under a coordinated program of national land management. Soil conservation, reforestation land use management, and upstream engineering on the headwaters, together with down streams, engineering works on the larger rivers.  Basic Definition  WATERSHED, or CATCHMENT, is a topographic area that is drained by a stream, that is, the total land area above some point on a stream or river that drains past that point.  The watershed is often used as a planning or management unit. Natural environment unit.  WATERSHED MANAGEMENT is the process of guiding and organizing land and other resource use on a watershed to provide desired goods and services without affecting adversely soil and water resources.  Forests affect the hydrology of watersheds in various and complex ways: by increasing evapotranspiration increasing soil infiltration, intercepting cloud moisture reducing the nutrient load of runoff  The forest plays an important role in the maintenance of the dynamic balance of the physio- ecosystem, as well as in the exchange of material flux and energy flux.  Effects of Forests on Hydrologic Functioning Canopy Forest Floor 1. depth and surface roughness greatly increase infiltration of precipitation 2. highly variable ability to hold water 3. in riparian zones, litter slows overland flow, traps sediments, and sequesters nutrients 1. intercepts precipitation, especially during low rainfall Roots events 2. changes drop size and reduces 1. roots stabilize soil aggregates velocity and stream banks 3. “throughfall” alters rainfall 2. increase macropore space and chemistry preferential flow/infiltration 4. ET generally maximum in forest 3. take up water and nutrients from canopy deep in the soil profile 5. Leaf structure and water use greatly influence canopy effects More lectures next meeting