Combined Term

For wind speed in m s$^{-1}$, Jensen et al. (1990) provided the following relationship to calculate $K_1*0.622\lambda\rho/P$:

$K_1 *0.622*\lambda*\rho/P=1710-6.85*\overline T_{av}$ 2:2.2.19

where $\overline T_{av}$ is the mean air temperature for the day (˚C).

To calculate potential evapotranspiration, the Penman-Monteith equation must be solved for a reference crop. SWAT+ uses alfalfa at a height of 40 cm with a minimum leaf resistance of 100 s m$^{-1}$ for the reference crop. Using this canopy height, the equation for aerodynamic resistance (2:2.2.3) simplifies to:

$r_a=\frac{114.}{u_z}$ 2:2.2.20

The equation for canopy resistance requires the leaf area index. The leaf area index for the reference crop is estimated using an equation developed by Allen et al. (1989) to calculate $LAI$ as a function of canopy height. For nonclipped grass and alfalfa greater than 3 cm in height:

$LAI=1.5*ln(h_c)-1.4$ 2:2.2.21

where $LAI$ is the leaf area index and $h_c$ is the canopy height (cm). For alfalfa with a 40 cm canopy height, the leaf area index is 4.1. Using this value, the equation for canopy resistance simplifies to:

$r_c=49/(1.4-0.4*\frac{CO_2}{330})$ 2:2.2.22

The most accurate estimates of evapotranspiration with the Penman-Monteith equation are made when evapotranspiration is calculated on an hourly basis and summed to obtain the daily values. Mean daily parameter values have been shown to provide reliable estimates of daily evapotranspiration values and this is the approach used in SWAT+. However, the user should be aware that calculating evapotranspiration with the Penman-Monteith equation using mean daily values can potentially lead to significant errors. These errors result from diurnal distributions of wind speed, humidity, and net radiation that in combination create conditions which the daily averages do not replicate.