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) Soil moisture limitation affects gross primary productivity (GPP).

(2) Soil moisture limitation affects the rate of soil heterotrophic respiration.

(3) The extent to which GPP is limited by leaf area vs soil resources (moisture, nutrients) determines the fractional allocation of NPP to aboveground vs belowground plant biomass.

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 the fraction of the soil surface covered by vegetation, which modifies rates of water loss by interception, evaporation from bare soil and transpiration by vegetation.

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) Soil moisture limitation affects gross primary productivity (GPP).

(2) Soil moisture limitation affects the rate of soil heterotrophic respiration.

(3) The extent to which GPP is limited by leaf area vs soil resources (moisture, nutrients) determines the fractional allocation of NPP to aboveground vs belowground plant biomass.

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 the fraction of the soil surface covered by vegetation, which modifies rates of water loss by interception, evaporation from bare soil and transpiration by vegetation.

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) Soil moisture limitation affects gross primary productivity (GPP).

(2) Soil moisture limitation affects the rate of soil heterotrophic respiration.

(3) The extent to which GPP is limited by leaf area vs soil resources (moisture, nutrients) determines the fractional allocation of NPP to aboveground vs belowground plant biomass.

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 the fraction of the soil surface covered by vegetation, which modifies rates of water loss by interception, evaporation from bare soil and transpiration by vegetation.

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) Soil moisture limitation affects gross primary productivity (GPP).

(2) Soil moisture limitation affects the rate of soil heterotrophic respiration.

(3) The extent to which GPP is limited by leaf area vs soil resources (moisture, nutrients) determines the fractional allocation of NPP to aboveground vs belowground plant biomass.

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 the fraction of the soil surface covered by vegetation, which modifies rates of water loss by interception, evaporation from bare soil and transpiration by vegetation.

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) Soil moisture limitation affects gross primary productivity (GPP).

(2) Soil moisture limitation affects the rate of soil heterotrophic respiration.

(3) The extent to which GPP is limited by leaf area vs soil resources (moisture, nutrients) determines the fractional allocation of NPP to aboveground vs belowground plant biomass.

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 the fraction of the soil surface covered by vegetation, which modifies rates of water loss by interception, evaporation from bare soil and transpiration by vegetation.

STEM is a modified implementation of Hartig et al.'s (2019) Very Simple Ecosystem Model (VSEM). The vegetation part of the model has two stocks of biomass carbon (C): aboveground C and belowground C.  The soil part of the model has a single stock of soil organic C. Carbon flows into the biomass C stocks via net primary productivity (NPP). Carbon flows out of these stocks and into the soil organic C stock via the loss of aboveground/belowground C through senescence (i.e., abscission of dead leaves and roots). SOC loss is due to heterotrophic respiration of the soil organic matter.

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.