As part of the Tropenbos-Guyana programme, a forest water balance model was developed. The Soil Atmosphere Plant model (SOAP) is a research tool designed to simulate detailed hydrological measurements and to estimate those parts of the water balance that are difficult to measure directly (e.g. evapotranspiration). The model simulates the vertical water movement from the top of the canopy to the groundwater layer. The publication consists of two parts: the first part is a soil water balance model based on a finite element solution using basic hydraulic properties; the second part consists of a series of modules, which simulate the above-ground micro-climate, using basic meteorological data and vegetation structure. The programme has the capacity to calculate the forest water balance of up to one year, with a vertical division of the above ground forest structure in up to 20 layers. The possibility exists to specify and combine six different soil properties. The programme can be run in Windows or directly as a file, but requires at least a computer with an 80386 processor and a co-processor. Required input data are: rainfall intensities, meteorological variables, wind speed, pressure, temperature, humidity and radiation, measured at least several times a day, and soil hydraulic properties -pF curve and conductivity-.