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

An alternative procedure for calculating root water extraction from differe nt depths of a soil profile is proposed, The procedure is based on the conc ept that the root water extraction entails energy expenditure by the plant and that the plant seeks to minimize the total rate of energy expenditure d uring water uptake. The model, therefore, considers root water extraction t o be a minimization problem whose solution can be sought using a dynamic pr ogramming framework. We tested the model by simulating the variations of th e 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, (P lant and Soil 204:183-201, 1998), Simulated patterns follow the observed wa ter content distribution quite well. Furthermore, results from numerical ex periments show that the model is capable of simulating a range of water ext raction patterns in a realistic manner. Patterns of water extraction from u niformly wet soil profiles follow those of the root distribution with depth . The extraction rate is highest in the section where the root length densi ty is also highest. Once the soil profile dries out, water extraction patte rns cease to bear any similarity to root distribution. Model simulations al so 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 as sumptions about the pattern of the root water extraction.