The water budget is calculated applying the continuity equation in terms of liquid water as to the portion running on the surface, and in terms of soil moisture for water present in the soil.
Precipitation can be snow or rain according to the air temperature. Liquid precipitation is reduced to account for vegetation interception and the effective precipitation is used as input for the slope-channel routing scheme. Infiltration is tackled by solving Richards'equation [Richards,1931] and subsurface flow are derived accordingly.
Rainfall-run off routing is described in GEOtop according to two schemes: a kinematic approach describes the water drainage along vegetated slopes towards channels, whereas a simplified solution of de Saint-Venant equation is implemented to describe the flow through the network to the outlet of the watershed.
Parameters
Important parameters that need to set to describe the rainfall-run off mechanism are:
- nDtwater, in the _parameters file, block 2. It is a reduction factor that can be applied to reduce the simulation time step to improve the integration of water balance equation. E.g. let's assume the simulation time step is 3600 s, it nDtwater is set to 10 then the time step for water balance is decreased to 360 s.
- D [m^2/s] is the diffusivity and describes the speed with which the wave is changing its shape, becoming wider and reducing its peak height. In theory it can range from 0 (kinematic case) to very high values. For modeling purposes it can be taken between 1 and 10000. The sensitivity to this parameter is shown in Figure
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