On the simulation of root water extraction: Examination of a minimum energy hypothesis

An alternative procedure for calculating root water extraction from different depths of a soil profile is proposed, The procedure is based on the concept that the root water extraction entails energy expenditure by the plant and that the plant seeks to minimize the total rate of energy expenditure during water uptake. The model, therefore, considers root water extraction to be a minimization problem whose solution can be sought using a dynamic programming framework. We tested the model by simulating the variations of the soil water content using the time and depth of a maize-sorghum intercrop experiment reported for an 8-day drying cycle by Ozier-Lafontaine et al, (Plant and Soil 204:183-201, 1998), Simulated patterns follow the observed water content distribution quite well. Furthermore, results from numerical experiments show that the model is capable of simulating a range of water extraction patterns in a realistic manner. Patterns of water extraction from uniformly wet soil profiles follow those of the root distribution with depth. The extraction rate is highest in the section where the root length density is also highest. Once the soil profile dries out, water extraction patterns cease to bear any similarity to root distribution. Model simulations also show increased root activity at greater depths when the top sections of the soil dry out. Generally, the model avoids the need to make any prior assumptions about the pattern of the root water extraction.