STEM-SM combines a simple ecosystem model (modified version of VSEM; Hartig et al. 2019) with a soil moisture model (Guswa et al. (2002) leaky bucket model). Outputs from the soil moisture model influence ecosystem dynamics in three ways. 

(1) The ratio of actual transpiration to maximum evapotranspiration (T/ETmax) modifies gross primary productivity (GPP).

(2) Degree of saturation of the soil (Sd) modifies the rate of soil heterotrophic respiration.

(3) Water limitation of GPP (by T/ETmax) and of soil nutrient availability (approximated by Sd) combine with leaf area limitation (approximated by fraction of incident photosynthetically-active radiation that is absorbed) to modify the allocation of net primary productivity to aboveground and belowground parts of the vegetation.

Ecosystem dynamics in turn influence flows of water in to and out of the soil moisture stock. The size of the aboveground biomass stock determines fractional vegetation cover, which modifies interception, soil evaporation and transpiration by plants.

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.

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.