At the beginning of each time step, sprinkler irrigation and snowmelt is added to the available precipitation. This available precipitation first fills the interception storage. If Sint max is exceeded, the excess water is added to the current amount of ponded water on the ground surface.
Then direct bypass infiltration to SZ is calculated and the depth of ponded water is updated.
After the calculation of the bypass infiltration, the depth of ponded water is updated for drainage to paved areas (see ).
After the removal of bypass flow and drainage to paved areas, the infiltration to the upper UZ layer is calculated, limited by
· the amount of available water for infiltration - the depth of ponded water, dOL,
· the rate of infiltration, and
· the estimated amount of water than would raise the water table to the ground surface in the current time step.
The rate of infiltration is adjusted for two special conditions. In the first case, the infiltration can be higher when the soil is very dry (see Saturated Zone (V1 p. 753)). In the second case, the infiltration rate will be reduced if you have specified an Surface-Subsurface Leakage Coefficient (V1 p. 269) that is lower than the specified leakage coefficient in the 2-Layer UZ soil properties (V1 p. 294) dialogue.
The amount of infiltration can also be limited when the amount of ponded water is below a specified threshold, since the 2-Layer water balance method does not include evaporation from the soil surface. Thus, even a small amount of water on the ground surface will infiltrate. If you define a depth of overland water that must be exceeded before infiltration will occur, small amounts of precipitation will not infiltrate, but rather evaporate instead. The calculated infiltration is simply reduced if the remaining overland water depth will be smaller than the specified threshold value. See Threshold depth for infiltration (2-Layer UZ) (V1 p. 752).
The actual infiltration to the upper UZ layer is then calculated - subject to the limits described above. Any surplus water above qmax is added to the lower UZ layer. In the lower UZ layer, any surplus water above qFC is added to the upper SZ layer. Finally, ponding depth and the water contents in the upper and lower UZ layers are recalculated and updated.