Soil and Water Assessment Tool Theoretical Documentation Version 2009

movement, sediment movement, crop growth, nutrient cycling, etc. are directly modeled by SWAT using this input data.Benefits of this approach are  watersheds with no monitoring data (e.g.stream gage data) can be modeled  the relative impact of alternative input data (e.g.changes in management practices, climate, vegetation, etc.) on water quality or other variables of interest can be quantified  uses readily available inputs.While SWAT can be used to study more specialized processes such as bacteria transport, the minimum data required to make a run are commonly available from government agencies. is computationally efficient.Simulation of very large basins or a variety of management strategies can be performed without excessive investment of time or money. enables users to study long-term impacts.Many of the problems currently addressed by users involve the gradual buildup of pollutants and the impact on downstream water bodies.To study these types of problems, results are needed from runs with output spanning several decades.SWAT is a continuous time model, i.e. a long-term yield model.The model is not designed to simulate detailed, single-event flood routing.The climate of a watershed provides the moisture and energy inputs that control the water balance and determine the relative importance of the different components of the hydrologic cycle.The climatic variables required by SWAT consist of daily precipitation, maximum/minimum air temperature, solar radiation, wind speed and relative humidity.The model allows values for daily precipitation, maximum/minimum air temperatures, solar radiation, wind speed and relative humidity to be input from records of observed data or generated during the simulation. WEATHER GENERATOR.Daily values for weather are generated from average monthly values.The model generates a set of weather data for each subbasin.The values for any one subbasin will be generated independently and there will be no spatial correlation of generated values between the different subbasins.GENERATED PRECIPITATION.SWAT uses a model developed by Nicks (1974) to generate daily precipitation for simulations which do not read in measured data.This precipitation model is also used to fill in missing data in the measured records.The precipitation generator uses a first-order Markov chain model to define a day as wet or dry by comparing a random number (0.0-1.0) generated by the model to monthly wet-dry probabilities input by the user.If the day is classified as wet, the amount of precipitation is generated from a skewed distribution or a modified exponential distribution.SUB-DAILY RAINFALL PATTERNS.If sub-daily precipitation values are needed, a double exponential function is used to represent the intensity patterns within a storm.With the double exponential distribution, rainfall intensity exponentially increases with time to a maximum, or peak, intensity.Once the peak intensity is reached, the rainfall intensity exponentially decreases with time until the end of the storm GENERATED AIR TEMPERATURE AND SOLAR RADIATION.Maximum and minimum air temperatures and solar radiation are generated from a normal distribution.A continuity equation is incorporated into the generator to HYDROLOGYAs precipitation descends, it may be intercepted and held in the vegetation canopy or fall to the soil surface.Water on the soil surface will infiltrate into the soil profile or flow overland as runoff.Runoff moves relatively quickly toward a stream channel and contributes to short-term stream response.Infiltrated water may be held in the soil and later evapotranspired or it may slowly make its way to the surface-water system via underground paths.The potential pathways of water movement simulated by SWAT in the HRU are illustrated in Figure 0.5. CANOPY STORAGE.Canopy storage is the water intercepted by vegetative surfaces (the canopy) where it is held and made available for evaporation.When using the curve number method to compute surface runoff, canopy storage is taken into account in the surface runoff calculations.However, if methods such as Green & Ampt are used to model infiltration and runoff, canopy storage must be modeled separately.SWAT allows the user to input the maximum amount of water that can be stored in the canopy at the maximum leaf area index for the land cover.This value and the leaf area index are used by the model to compute the maximum storage at any time in the growth cycle of the land cover/crop.When evaporation is computed, water is first removed from canopy storage. INFILTRATION.Infiltration refers to the entry of water into a soil profile from the soil surface.As infiltration continues, the soil becomes increasingly wet, causing the rate of infiltration to decrease with time until it reaches a steady value.The initial rate of infiltration depends on the moisture content of the soil prior to the introduction of water at the soil surface.The final rate of infiltration is equivalent to the saturated hydraulic conductivity of the soil.Because the curve number method used to calculate surface runoff operates on a daily time-step, it is unable to directly model infiltration.The amount of water entering the soil profile is calculated as the difference between the amount of rainfall and the amount of surface runoff.The Green & Ampt infiltration method does directly model infiltration, but it requires precipitation data in smaller time increments. REDISTRIBUTION.Redistribution refers to the continued movement of water through a soil profile after input of water (via precipitation or irrigation) has ceased at the soil surface.Redistribution is caused by differences in water content in the profile.Once the water content throughout the entire profile is uniform, g redistribution will cease.The redistribution component of SWAT uses a storage routing technique to predict flow through each soil layer in the root zone.Downward flow, or percolation, occurs when field capacity of a soil layer is exceeded and the layer below is not saturated.The flow rate is governed by the saturated conductivity of the soil layer.Redistribution is affected by soil temperature.If the temperature in a particular layer is 0C or below, no redistribution is allowed from that layer. EVAPOTRANSPIRATION.Evapotranspiration is a collective term for all processes by which water in the liquid or solid phase at or near the earth’s surface becomes atmospheric water vapor.Evapotranspiration includes evaporation from rivers and lakes, bare soil, and vegetative surfaces; evaporation from within the leaves of plants (transpiration); and sublimation from ice and snow surfaces.The model computes evaporation from soils and plants separately as described by Ritchie (1972).Potential soil water evaporation is estimated as a function of potential evapotranspiration and leaf area index (area of plant leaves relative to the area of the HRU).Actual soil water evaporation is estimated by using exponential functions of soil depth and water content.Plant transpiration is simulated as a linear function of potential evapotranspiration and leaf area index.POTENTIAL EVAPOTRANSPIRATION.Potential evapotranspiration is the rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation which has access to an unlimited supply of soil water.This rate is assumed to be unaffected by micro-climatic processes such as advection or heat-storage effects.The model offers three options for estimating potential evapotranspiration: Hargreaves (Hargreaves et al., 1985), Priestley-Taylor (Priestley and Taylor, 1972), and Penman-Monteith (Monteith, 1965). LATERAL SUBSURFACE FLOW.Lateral subsurface flow, or interflow, is streamflow contribution which originates below the surface but above the zone where rocks are saturated with water.Lateral subsurface flow in the soil profile (0-2m) is calculated simultaneously with redistribution.A kinematic storage model is used to predict lateral flow in each soil layer.The model accounts for variation in conductivity, slope and soil water content.SURFACE RUNOFF.Surface runoff, or overland flow, is flow that occurs along a sloping surface.Using daily or subdaily rainfall amounts, SWAT simulates surface runoff volumes and peak runoff rates for each HRU.SURFACE RUNOFF VOLUME is computed using a modification of the SCS curve number method (USDA Soil Conservation Service, 1972) or the Green & Ampt infiltration method (Green and Ampt, 1911).In the curve number method, the curve number varies non-linearly with the moisture content of the soil.The curve number drops as the soil approaches the wilting point and increases to near 100 as the soil approaches saturation.The Green & Ampt method requires sub-daily precipitation data and calculates infiltration as a function of the wetting front matric potential and effective hydraulic conductivity.Water that does not infiltrate POTENTIAL AND ACTUAL TRANSPIRATION.The process used to calculate potential plant transpiration is described in the section on evapotranspiration.Actual transpiration is a function of potential transpiration and soil water availability. NUTRIENT UPTAKE.Plant use of nitrogen and phosphorus are estimated with a supply and demand approach where the daily plant nitrogen and phosphorus demands are calculated as the difference between the actual concentration of the element in the plant and the optimal concentration.The optimal concentration of the elements varies with growth stage as described by Jones (1983). GROWTH CONTRAINTS.Potential plant growth and yield are usually not achieved due to constraints imposed by the environment.The model estimates stresses caused by water, nutrients and temperature. ROUTING IN THE MAIN CHANNEL OR REACHRouting in the main channel can be divided into four components: water, sediment, nutrients and organic chemicals. FLOOD ROUTING.As water flows downstream, a portion may be lost due to evaporation and