2:2.3.4 Evaporation from Ponded Water

Evapotranspiration (ET) is estimated as the sum of plant transpiration and evaporation rates. SWAT+ inherits the SWAT model simulating potential evapotranspiration (PET) rates using the Priestley-Taylor method (Priestley and Taylor, 1972), the Penman-Monteith method (Monteith, 1965), or the Hargreaves method (Hargreaves and Samani, 1985). The estimated daily PET serves as the maximum daily actual ET amount. Thus, daily ET is assumed to be the same as PET if the sum of transpiration (EP) and soil evaporation (ES) is higher than PET. In this case, the soil evaporation rate is adjusted as ES = PET – EP. In rice paddies and wetlands where standing water exists, daily ET is calculated as the sum of EP and EVP, which can exceed the calculated daily PET. However, shades of mature rice straws can limit water evaporation (Choi et al., 2017; Sakaguchi et al., 2014b).

$EVP_t=0.6*(1-\frac {LAI}{4.0})*PET, \; if LAI≤4.0$ 2:2.3.22

$EVP_t = 0, \; if LAI>4.0$ 2:2.3.23

where $EVP_t$ is the evaporation of standing water, LAI is the dimensionless leaf area index, and PET is the potential evapotranspiration.

The SWAT soil evaporation equation substitutes Equation 2:2.3.23 if standing water is entirely depleted by transmission losses.

Table 2:2-3: SWAT+ input variables used in soil evaporation calculations..

Variable Name	Definition	File Name
ESCO	$esco$: soil evaporation compensation coefficient	hydrology.hyd
LAI	Dimensionless leaf area index	plant.ini
PET	Potential evapotranspiration	codes.bsn