Groundwater

f_I

Infiltration from the surface 

f_EU

Evapotranspiration from the upper zone which is a fixed fraction of the un-used surface evaporation

f_U

Percolation from the upper to lower zone which depends on the upper zone moisture content & depth d_U

f_EL

Evapotranspiration from the lower zone, which is a function of the depth of the upper zoned_U

f_L

Percolation from the lower zone to deep groundwater which depends on the lower zone depthd_L

f_G

Lateral groundwater interflow to the drainage system, which depends on the lower zone depth d_L as well as the depth in the receiving channel or node