Actual infiltration and surface runoff

The actual infiltration $INF_{act} \ [mm]$ is now calculated as:

\[INF_{act} = min (INF_{pot}, W_{av} - D_{pref,gw})\]

where $INF_{pot}$ is the potential infiltration capacity of the layers 1a and 1b; $W_{av}$ is the available water for infiltration; $D_{pref,gw}$ is the preferential bypass flow. All these quantities are expresses in $[mm]$.

$INF_{act}$ is added to the superficial and upper soil layers (1 and 2, respectively). First, the actual infiltration is added to the superficial soil layer 1, until reaching its maximum storage capacity ($w_{s,1}$). The remainder amount of water is added to the upper soil layer 2. This distribution of $INF_{act}$ within the layers 1 and 2 is necessary because the infiltration potential is computed considering both the layers. The amount of moisture in the superficial and upper soil layers is updated as follows:
$w_{1} = \min [ w_{s,1} , (w_{1} + INF_{act}) ]$
$w_{2} = w_{2} + \max [ (w_{1} + INF_{act}) - w_{s,1} , 0 ]$
$w_1 = w_{1} + w_{2}$

Finally, the surface runoff $R_s \ [mm]$ is calculated as:

\[R_s = R_d + (1 - f_{dr}) \cdot (W_{av} - D_{pref,gw} - INF_{act})\]

where $R_d$ is the direct runoff (generated in the pixel’s ‘direct runoff fraction’). If the soil is frozen (F > critical threshold) no infiltration takes place.