alley cropping is a crop production system in which food crops are grown in the alleys created by hedgerows of selected tree species, preferably legumes. the objective of this study was to model a cassia /maize alley cropping system in the semi-arid district of machakos in kenya. the modelling was done with respect to the system's ground water dynamics as it affects its biomass production. first the dynamics of water in a bare soil was modelled, and then cassia and maize were introduced separately and their biomass production modelled with respect to water. finally, a combined cass/a/maize/ground water model was build and analysed. the model suggests that water is a critical factor in the biomass production of cassia and maize. the main achievement of this project has been to provide a starting point for a framework in which empirical research results can be integrated and interpreted in a more holistic manner.