A PET reduction factor, PETFACT, is employed to account for lower evaporation during days with rain. The default value is 0.1.
The potential evapotranspiration will first be fulfilled by the evaporation from depression storage and then the active soil zone. The available potential evapotranspiration for active soil zone, is calculated as:
The ET rate from the active soil zone is calculated using the approach employed in Stanford Watershed Model for ET from lower soil zone (Warren Viessman, 1977), which is:
where r is the evapotranspiration opportunity representing the maximal amount of water available for evapotranspiration at a particular location. It’s calculated as
LZS = the current soil moisture storage (mm),
LZSN = a nominal storage level (mm)
K3 = an input parameter that is a function of watershed cover as shown in Table 10.
In continuous OTTHYMO, LZS is set equal to SSTOR and LZSN is set to / 2.
TABLE 10 TYPICAL LOWER ZONE EVAPOTRANSPIRATION PARAMETERS
(WARREN VIESSMAN, 1977)