Theory

The infiltration rate, q_in, is a function of the infiltration capacity at the soil surface, i_cap, calculated from the saturated conductivity of the topsoil and the actual gradient in pressure head from the soil surface (ψ=0) to the middle of the uppermost layer according to Darcy’s law:

                                                     (2.16)

where q_th is the throughfall of precipitation to the soil surface. In case of above surface irrigation, q_th also includes the irrigation water; however, for all types of drip irrigation the water is directly transferred from the storage tank to the soil layers. If soil evaporation is greater than infiltration and the surface pool divided by the simulation time-step is greater than soil evaporation, an extra infiltration of water from the surface pool takes place. The amount of extra infiltration is equal to soil evaporation.

If throughfall exceeds the infiltration capacity a surface pool of water is formed on the soil surface. Water in the surface pool can either infiltrate with a delay into the soil or be lost as surface runoff. The surface runoff, q_surf, is calculated as a first order rate process:

                                             (2.17)

where a_surf is an empirical coefficient, W_pool is the total amount of water in the surface pool and w_pmax is the maximal amount, which can be stored on the soil surface without causing any surface runoff. See viewing function Surface Runoff Function. If W_pool is smaller than w_pmax then there is no surface runoff, q_surf,.

The fraction of the total soil surface that is covered with water, f_cspool, is given by:

                                              (2.18)

when the total amount of water is less than f_wcovtot, which is a parameter value. See viewing function Ponded soil cover function.

During conditions with frost in the soil the saturated conductivity can be reduced because of the ice content in the soil (see “Influence of ice on water”).

A physical barrier for infiltration such as a roof can also be simulated by setting a value larger than zero for the i_scov parameter.

Another special feature is the simulation of a furrow similar pattern on the soil surface (see switch Furrow). In this case a fraction, f_infbypass, of the infiltration is going directly to the second compartment of the soil. This means that the top layer receives only 1-f_infbypass of the total infiltration rate originating either from the surface pool or from precipitation.