The Soil-Plant-Atmosphere module (SPAM) module deals with competition for light and water among the soil, plants, and atmosphere. It models processes of soil evaporation and plant transpiration, thus providing estimates of crop water use as actual evapotranspiration (ET). Soil temperatures at layer depths are also estimated in this routine.
Actual evapotranspiration (ET) depends on total ETo demand, which is estimated using one of two options: Priestley-Taylor (1972), based on standard weather data input, or FAO-56 (Allen et al., 1998), which requires daily wind speed and relative humidity as additional inputs. After ETo is calculated, it is partitioned to the potential transpiration of the crop canopy (Ep) or potential evaporation of the soil (Es) as a function of the LAI and an energy extinction coefficient (Kep). Kep differs for each crop in CROPGRO. For the CERES crops, a “mixed” function of extinction of photosynthetically active radiation (PAR) is used. The actual soil evaporation depends on the potential Es and the soil water content, using either the older Stage 1 (square root of time method) or the Suleiman-Ritchie method (Ritchie et al., 2009).
The actual transpiration of the crop is the minimum of the potential Ep or the water uptake. Potential root water uptake from successive layers follows the approach described by Ritchie (1998), and it is dependent on root length density and the fraction of available soil water content in each layer. Total root water uptake is integrated over all layers, and transpiration is reduced if potential root water uptake is less than potential Ep. Under water deficits, the daily photo-assimilation is reduced as a function of actual transpiration (root uptake) over potential Ep, using a drought stress factor called SWFAC. Expansive processes are reduced somewhat sooner by a similar factor called TURFAC. See Boote et al. (2009) for a review of water balance, evapotranspiration, and simulation of water stress effects in the CROPGRO model.