As nitrogen supply in intensively managed crops is usually ensured by fertilization their growth could mainly be limited by water availability. Especially on fields with heterogeneous soil properties it is helpful to know these properties site specifically to derive local yield potentials and thereby fertilizer requirements. Crop reactions to water availability and drought stress may be used to assess the plant available soil water. Plant biomass and yield are not a good measure for the water holding capacity of the soil as they integrate not only drought stress effects but also other stresses over the growing period. In contrast, the canopy temperature is a more direct measure of the actual water availability at any time. From the canopy temperature and energy balance of the crop the ratio of actual to potential evapotranspiration can be derived in the form of the crop water stress index CWSI. Therefore the canopy temperature at times of high potential evapotranspiration coupled with a model of soil water uptake and evapotranspiration on a high spatial resolution (i.e. hourly time step) gives a measure of plant available water. A soil water and crop growth model is then parameterized for soil parameters using data of plant available water at certain times derived from canopy temperature measurements. A wheat growth model partially based on algorithms from CERES Wheat was coupled with a soil water model to achieve a site specific estimation of soil water parameters for a field in northern Germany with very heterogeneous soils. This site specific calibration was used to predict drought stress and to explain a part of the spatial variation seen in plant biomass and yield.