CALGOS is a cotton simulation model, modified from GOSSYM for semi-arid irrigated conditions, emphasizing the correct simulation of plant response to irrigation and fertilizer application. A new setup of the soil slab grid, allowing asymmetrical processes, is suggested, and new routines for water transport in the soil slab, for drip irrigation and for root growth are presented. The simulation of water transport is based on hydraulic conductivity and matric potential difference between soil cells, and the numerical solution involves a combination of implicit and explicit methods. The simulation of the root system is designed to be applicable also to other crop models, requires input of user-defined parameters, and simulates taproot growth. Some examples are given, illustrating the simulation of the effects of two drip irrigation treatments on the distribution of water an active roots in the soil.