The original use of crop models was to calculate crop growth and development for a single field with supposedly homogeneous soil, climate, initial conditions and management practices. This is indeed still a basic use of crop models. However, there is also more and more interest in studies that concern multiple fields (see Leenhardt et al., 2003, Hansen and Jones, 2000; Russell and Van Gardingen, 2000; Hartkamp et al, 1999). In some cases each field can be treated independently, but it is the combined result from all fields that is of interest. Examples include the calculation of crop yields or forage yields on a regional or national basis (e.g. Rijks et al., 1998; Thornton et al., 1997; Rosenthal et al., 1998; Chipanshi et al., 1999; Lal et al, 1993; Donet et al., 1999; Faivre; Yun, 2003) the calculation of the water requirements for agriculture within the area served by a water provider (e.g. Sousa and Pereira, 1999; Heineman et al., 2002; Leenhardt et al., 2003) or total emission of nitrogen oxides from agricultural land in a region. In other cases, it is necessary to model not just individual fields but also interactions between fields or between a field and non-crop surroundings. For example, the problem addressed might involve nitrogen, herbicide or pesticide pollution of streams or ground water due to runoff or leaching from agricultural land (e.g. Beaujouan et al., 2001; Gomez and Ledoux, 2001). Another example would involve transfer of genetically modified pollen from one field to surrounding fields. In all of these problems there is a need to use the crop model for multiple fields, perhaps hundreds or thousands of fields, with different soils, climate and management.