An exponential root-water-uptake model with water stress compensation

Modelling soil water flow under cropped conditions requires a description of water uptake by plant roots. Most macroscopic root-water-uptake models distribute potential transpiration across the root zone based on the root distribution pattern, without accounting for the distribution of water stress in the soil profile. An exponential root-water-uptake model was modified by incorporating a weighted stress index which accounts for both root distribution and soil water stress. This new model, called water stress compensating exponential root-water-uptake model, is represented as a function of potential transpiration, soil water availability and root-length density. The root length fraction in the various soil layers can be estimated from its value in the surface (0–10 cm) layer. Both the exponential- and the water stress compensating- root water uptake models were incorporated in the Soil-Water-Atmosphere-Plant (SWAP) simulation model, and tested against soil water content data from a long-term crop rotation experiment in the semi-arid region of western Canada. The results showed that the new water stress compensating model simulated soil water contents significantly better than the exponential model, especially during the second half of the growing season at the lower depths (60–120 cm). The model is user-friendly and application oriented, and can be used for various cereal crops.