A COMPUTER-SIMULATION DESCRIBING WATER EXTRACTION BY CROP ROOT SYSTEMS

A computer simulation of soil water extraction by crop root systems is described based on a simple, root-centered abstraction equation in which the relative rates of uptake from different soil depths depend upon the rooting densities, the differences between soil and stem (root) xylem potential and an effective resistance to water uptake per unit length of root, R(e). This effective resistance is assumed constant with depth and time and is dominated by root resistance to water inflow. Although the model needs further calibration, simulated seasonal crop water use and water extraction profiles agreed well with measured values for winter wheat in three seasons on different soil types, with R(e) equal to 10(11) MPa m s m-3. There was some indication that R(e) needs to be determined more accurately than has so far proved possible in order to fully resolve questions concerning its constancy with depth and time and the possible importance of axial resistance.