Q_surf

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

Q_surf = (R_day - I_a)² / (R_day - I_a + S)

Q_surf = (R_day - 0.2S )² / (R_day + 0.8S)

I_a  = includes surface storage, interception, and infiltration prior to runoff =  (0.2S)

S = retention parameter = 25.4[(100/CN) - 10]

Lumps canopy interception in I_a

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)

Q_surf = (R_day - 0.2S)² / (R_day + 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  & Q_surf 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

Assumptions:

1. the soil profile is homogeneous
2. antecedent moisture is uniformly distributed in the profile
3. 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

collective term that includes all processes by which water at the earth's surface is converted to water vapor

Can occur from...

1. Plant canopy
2. transpiration
3. sublimation
4. 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

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 Q_surf

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

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

1. Penman-Monteith: requires solar radiation, air temp, rel humidity, and wind speed.
2. Priestly-Taylor: requires solar radiation, air temp, rel humidity
3. Hargreaves: requires air temp only

SWAT will also read in daily PET values 

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)

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.