The water balance in active soil zone (pervious area) is simulated as

= the amount of water stored in soil (mm),
= the initial amount of water in soil (mm),
= the runoff from depression storage (mm),
ICRO = the effective runoff from indirectly connected impervious land surfaces (mm),
RUNOFF = surface runoff (mm),
ET = the evapotranspiration (mm)
GWI = groundwater infiltration (mm).