Evaporation of Intercepted Rainfall

Any free water present in the canopy is readily available for removal by evapotranspiration. The amount of actual evapotranspiration contributed by intercepted rainfall is especially significant in forests where in some instances evaporation of intercepted rainfall is greater than transpiration.

SWAT+ removes as much water as possible from canopy storage when calculating actual evaporation. If potential evapotranspiration, $E_o$, is less than the amount of free water held in the canopy, $R_{INT}$, then

$E_a=E_{can}=E_o$ 2:2.3.1

$R_{INT(f)}=R_{INT(i)}-E_{can}$ 2:2.3.2

where $E_a$ is the actual amount of evapotranspiration occurring in the watershed on a given day (mm H$_2$O), $E_{can}$ is the amount of evaporation from free water in the canopy on a given day (mm H$_2$O), $E_o$ is the potential evapotranspiration on a given day (mm H$_2$O), $R_{INT(i)}$ is the initial amount of free water held in the canopy on a given day (mm H$_2$O), and $R_{INT(f)}$ is the final amount of free water held in the canopy on a given day (mm H$_2$O). If potential evapotranspiration, $E_o$, is greater than the amount of free water held in the canopy, $R_{INT}$, then

$E_{can}=R_{INT(i)}$ 2:2.3.3

$R_{INT(f)}=0$ 2:2.3.4

Once any free water in the canopy has been evaporated, the remaining evaporative water demand$(E'_o=E_o-E_{can})$is partitioned between the vegetation and snow/soil.