Modelling daily root interactions for water in a tropical shrub and grass alley cropping system.

Abstract

A two-dimensional physically-based model for the daily simulation of root competition for water in an alley cropping system associating Gliricidia sepium with Digitaria decumbens is developed. This paper deals with the impact of root distribution on soil water partitioning. By adapting existing principles of root water uptake modelling for pure crops, the model accounts simultaneously for the sink terms of each species in a defined soil domain. Soil-root water transport functions are solved at the level of discrete volumes of soil; each of them are characterized by the inherent soil physical properties, root length density, soil-root distances, and the calculated sink terms of each species. The above ground boundary conditions, such as transpiration and soil evaporation, were managed by simple equations found from the literature or provided by experimental measurements. Running the model with two contrasting observed root maps, an evaluation was carried out over a 10-day period following a rainfall event. With both root maps, the simulated soil water potential profiles at the row, at 0.75 m and 1.50 m from the row did not differ significantly, and were in good agreement with the measurements. However, although water was not limiting during this period, the simulated cumulative water absorption profiles of G. sepium and D. decumbens contrasted markedly, and matched their observed root distribution. This model, although still under further development, forms the basis for development of an above and below ground coupled model to simulate plant interactions for water in intercrops or agroforestry.