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

A two-dimensional physically-based model for the daily simulation of root c ompetition for water in an alley cropping system associating Gliricidia sep ium with Digitaria decumbens is developed. This paper deals with the impact of root distribution on soil water partitioning. By adapting existing prin ciples of root water uptake modelling for pure crops, the model accounts si multaneously for the sink terms of each species in a defined soil domain. S oil-root water transport functions are solved at the level of discrete volu mes of soil; each of them are characterized by the inherent soil physical p roperties, root length density, soil-root distances, and the calculated sin k terms of each species. The above ground boundary conditions, such as tran spiration 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 ove r a 10-day period following a rainfall event. With both root maps, the simu lated soil water potential profiles at the row, at 0.75 m and 1.50 m from t he row did not differ significantly, and were in good agreement with the me asurements. However, although water was not limiting during this period, th e simulated cumulative water absorption profiles of G. sepium and D. decumb ens contrasted markedly, and matched their observed root distribution. This model, although still under further development, forms the basis for devel opment of an above and below ground coupled model to simulate plant interac tions for water in intercrops or agroforestry.