# Shared Flashcard Set

## Details

SWAT 2
Final Review
9
Engineering
04/29/2014

Term
 Hydrology : Surface Runoff
Definition
 Qsurf   Surface runoff occurs whenever the rate of water application to the ground surface exceeds the rate of infiltration   SWAT provides 2 methods for estimating surface runoff Modified SCS CN    Green & Ampt infiltration method
Term
 Surface Runoff : SCS Curve Number Method
Definition
 Qsurf = (Rday - Ia)2 / (Rday - Ia + S) Qsurf = (Rday - 0.2S )2 / (Rday + 0.8S) Ia  = includes surface storage, interception, and infiltration prior to runoff =  (0.2S) S = retention parameter = 25.4[(100/CN) - 10] Lumps canopy interception in Ia SCS CN is a function of the soil's permeability, land use, and antecedent soil water conditions (approp. for ~5% slope)   CN is modified based on antecedent soil moisture condition   wilting point : minimal point of soil moisture the plant requires not to wilt   field capacity: amount of soil moisture or water content held in soil after excess water has drained away around 2-3 days after a rain CN modification always modifies CN for moisture condition II : pg. 5 Table 2:1-2 for CN tables Retention parameter (S) -- 2 methods 1.) Based on soil profile water content S=25.4[(100/CN) - 10] CN = 25400/(S+254) Qsurf = (Rday - 0.2S)2 / (Rday + 0.8S) **this method predicts too much runoff in shallow soils. most commonly used 2. Based on Plant ET S = 0.9Smax [see pg 6 for full formula] CN  & Qsurf are the same Slope adjustments   see pg 7 for equation. will need pg 5 or CNs CN for moisture cond II are approp. for 5% slopes. Equation adjusts CN to a different slope.   SWAT DOES NOT ADJUST CN FOR SLOPE
Term
 Surface Runoff : Green Ampt method
Definition
 Assumptions: the soil profile is homogeneous antecedent moisture is uniformly distributed in the profile as water infiltrates into soil, the soil above the wetting front is completely saturated and there is a sharp break in moisture content at wetting front This method requires sub-daily precipitation data supplied by user see pg 8 for equations Wetting front matric potential is a function or porosity, % sand, and % clay
Term
 Hydrology : Evapotranspiration
Definition
 collective term that includes all processes by which water at the earth's surface is converted to water vapor Can occur from... Plant canopy transpiration sublimation evaporation from soil Primary mechanism by which water is removed from a watershed. Roughly, 62% of precipitation that falls on the continents in evapotranspired Potential VS Actual ET Potential Et is rate at which Et WOULD occur from uniformly covered, large area, with unlimited supply of soil water, not exposed to advection or ht storage effects Actual ET is rate which Et DOES occur from imperfect area
Term
 Evaporation : Canopy Storage
Definition
 Water intercepted by vegetative surfaces (canopy) where it is held and made available for evaporation   Interception of rainfall by the canopy must be calculated separately when using Green & Ampt to calc Qsurf   SWAT allows max amt of water that can be held in canopy storage to vary from day to day as a function of leaf area index (pg 10)   Canopy storage is filled before any water is allowed to reach the ground
Term
 Evaporation : Potential ET
Definition
 Penman: The rate at which evapotranspiration would occur from a large area uniformly covered with growing vegetation that has access to an unlimited supply of soil water and that was not exposed to advection or heat storage effects.   Penman: "The amount of water transpired by a short green crop, completely shading the ground, of uniform height and never short of water." (Grass = ref. crop or alfalfa 30-50cm)   3 methods incorporated in SWAT Penman-Monteith: requires solar radiation, air temp, rel humidity, and wind speed. Priestly-Taylor: requires solar radiation, air temp, rel humidity Hargreaves: requires air temp only SWAT will also read in daily PET values
Term
 Potential ET : Penman Monteith
Definition
 combines components that acct for energy needed to sustain evaporation, the strength of the mechanism required to remove water vapor, and aerodynamic and surface resistance terms. (pg 12)
Term
 Priestley-Taylor Method
Definition
 Simple version of the combination equation for use when surface areas are wet. the aerodynamic component was removed and the energy component was multiplied by a coefficient (1.28) when the general surroundings are wet or under humid conditions.   Provides potential evapotraspiration estimates for low advective conditions. In semiarid or arid areas where the advection component of energy balance is significant, the Priestley-Taylor equations will underestimate potential evapotranspiration.
Term
 Potential ET : Hargreaves Method
Definition
 simplified (not as accurate?)
Supporting users have an ad free experience!