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

• plant canopy storage
• transpiration 
• sublimation
• evaporation from the soil

Actual ET = canopy storage + transpiration + soil evaporation

Potential ET dicatates the max ET that can happen in a single day

3 methods incorporated in SWAT for estimating Potential ET

• Penman-Monteith method: requires solar radiation, air temp, rel. humidity and wind speed
• Priestley-Taylor method: requires solar radiation, air temp, and rel humidity
• Hargreaves method: requires air temp only

Once total PET is determine, Actual ET must be calculated

1. SWAT evaporates any rainfall intercepted by the plant canopy
2. SWAT calculates the max amount of transpiration, sublimation, & soil evaporation
3. The actual amount of sublimation and evaporation from soil is calculated

• Any free water present in canopy is readily available for removal by ET
• Amt of actual ET contributed by intercepted rainfall is esp significant in forests
• SWAT removes as much water as possible from canopy storage in calc. actual ET

If potential ET (E₀) < amt free water in canopy (R_int) 

E_a=E_can=E₀

R_int(f)=R_int(i)-E_can

If PET (E₀) is greater than the amt of free water held in canopy (R_int(i)) then:

E_can = R_int(i)

R_int(f) = 0

Remaining evaporative water demand is partitioned b/t the vegetation and snow/soil

E₀' = E₀ - E_can

The coefficients in the equations were selected so that 50% of the evap demand is extracted from the top 10mm of soil and 95% of the evap demand is extracted from the top 100 mm of soil.

Diff b/t evap demands calc at upper and lower boundaries of soil layer = amt of evap demand for soil layer

SWAT DOES NOT ALLOW A DIFFERENT LAYER TO COMPENSATE FOR THE INABILITY OF ANOTHER LAYER TO MEET ITS EVAPORATIVE DEMAND.

evap demand not met by soil layer = reduction in actual evap for the HRU (it goes with the 50%-10mm rule)

esco is a coeff incorporated to allow user to modify the depth distribution used to meet soil evap demand.

E_soil,ly = Esoil,zl-E_soil,zu*esco

esco: soil evap compensation coeff

As value for esco is reduced, the model is able to extract more of the evap demand from lower levels

When water content of soil layer is below FC-evap demand for layer is reduced

SWAT:

• limits the amount of water removed by evap in dry conditions
• defines max value of water that can be rmoved at any time. (this value is 80% of plant avail water on given day
• plant avail water is total water content of soil layer minus water content of soil layer at wilting pt (-1.5MPa))

Other Hydrology

Peak runoff rate

Surface runoff lag

Transmission losses

Sediment generate, flow and sediment routing, etc. are affected by

• Peak runoff rate
• Surface runoff lag
• transmission losses

max runoff flow rate that occurs w/ a given rainfall event.

An indicator of erosive power of a storm: used to predict sediment loss

SWAT calculates peak runof rate with modified rational method based on assumption that if a rainfall of intensity i begins and time t=0 and continues indefinitely, the rate of runoff will increase until the time of concentration t=t_conc when the entire subbasin area is contribution the flow at the outlet

time of concentration = amt of time from beginning of a rainfall event until the entire subbasin area is contributing to flow at the outlet

TOC is calculated by summing overland flow time (remotest point in subbasin to reach the channel) & channel flow time (flow in upstream channels to reach the outlet)

In large subbasins with time of concentration > 1 day, only a portion of surface runoff will reach channel in day of generation.

SWAT--surface runoff storage feature to lag a portion of the surface runoff release to channel

surlag: as surlag decreases in value, more water is held in storage. Delay in release of surface runoff will smooth the streamflow hydrograph simulated in the reach.

abstractions of streamflow caused by ephemeral channels in semiarid and arid watersheds

Reduce runoff volume as the flood waves travels downstream

Transmission losses from surface runoff are assumed to percolate into the shallow aquifer