A 'leaky bucket' soil moisture model, based on Guswa et al. (2002). Rain falls as discrete events. The mean depth and frequency of rainfall events are determined by total monthly rainfall and number of rain days. A portion of the rainfall is intercepted by vegetation and evaporates before reaching the soil. The remaining rainfall (throughflow) either infiltrates the soil or, if the soil has insufficient capacity, runs off immediately. Soil water exceeding the field capacity is lost by sub-surface leakage, at a rate determined by the degree of soil saturation. Degree of soil saturation also limits rates of soil evaporation and vegetation transpiration. The partitioning between evaporation and transpiration is influenced by fractional area covered by vegetation.

Reference:

Guswa, A.J., Celia, M.A., Rodriguez-Iturbe, I. (2002) Models of soil moisture dynamics in ecohydrology: a comparative study. Water Resources Research 38, 5-1 - 5-15.