Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth. Under non-limiting soil water conditions, the increase in root mass at any depth is governed by a logistic equation so that the root length density (R(V)) cannot exceed the maximum value. The maximum R(V), is assumed to be the same for all depths. Additional dry matter partitioned to roots is initially distributed according to the current root mass at each depth. As the root mass approaches the maximum value, less dry matter is partitioned to that depth. When soil water is limiting, a water deficit factor is introduced to further modify the distribution of root dry matter. It is assumed that the plant is an ''energy minimiser'' so that more root mass is partitioned to the wetter regions of the soil where least energy will be expended for root growth. Hence, the model allows for enhanced root growth in areas where soil water is more easily available. Simulation results show that a variety of root distribution patterns can be reproduced due to varying soil water conditions. It has been demonstrated that broad patterns of root distribution reported in the literature can also be simulated by the model.