Percolation

Percolation is calculated for each soil layer in the profile. Water is allowed to percolate if the water content exceeds the field capacity water content for that layer and the layer below is not saturated. When the soil layer is frozen, no water flow out of the layer is calculated.

The volume of water available for percolation in the soil layer is calculated:

$SW_{ly,excess}=SW_{ly}-FC_{ly}$ if $SW_{ly}>FC_{ly}$ 2:3.2.1

$SW_{ly,excess}=0$ if $SW_{ly} \le FC_{ly}$ 2:3.2.2

where $SW_{ly,excess}$ is the drainable volume of water in the soil layer on a given day (mm H$_2$O), $SW_{ly}$ is the water content of the soil layer on a given day (mm H$_2$O) and $FC_{ly}$ is the water content of the soil layer at field capacity (mm H$_2$O).

The amount of water that moves from one layer to the underlying layer is calculated using storage routing methodology. The equation used to calculate the amount of water that percolates to the next layer is:

$w_{perc,ly}=SW_{ly,excess}*(1-exp[\frac{-\Delta t}{TT_{perc}}])$ 2:3.2.3

where $w_{perc,ly}$ is the amount of water percolating to the underlying soil layer on a given day (mm H$_2$O), $SW_{ly,excess}$ is the drainable volume of water in the soil layer on a given day (mm H$_2$O), $\Delta t$ is the length of the time step (hrs), and $TT_{perc}$ is the travel time for percolation (hrs). If the HRU has a seasonal high water table, percolation is not allowed when $SW_{ly+1} \le FC_{ly+1}+0.5*(SAT_{ly+1}-FC_{ly+1})$ where $SW_{ly+1}$ is the water content of the underlying soil layer (mm H$_2$O), $FC_{ly+1}$ is the water content of the underlying soil layer at field capacity (mm H$_2$O), and $SAT_{ly+1}$ is the amount of water in the underlying soil layer when completely saturated (mm H$_2$O). The water will instead stay ponded in the upper layer.

The travel time for percolation is unique for each layer. It is calculate

$TT_{perc}=\frac{SAT_{ly}-FC_{ly}}{K_{sat}}$ 2:3.2.4

where $TT_{perc}$ is the travel time for percolation (hrs), $SAT_{ly}$ is the amount of water in the soil layer when completely saturated (mm H$_2$O), $FC_{ly}$ is the water content of the soil layer at field capacity (mm H$_2$O), and $K_{sat}$ is the saturated hydraulic conductivity for the layer (mm∙h$^{-1}$).

Water that percolates out of the lowest soil layer enters the vadose zone. The vadose zone is the unsaturated zone between the bottom of the soil profile and the top of the aquifer. Movement of water through the vadose zone and into the aquifers is reviewed in Chapter 2:4.

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