Scenarios are accepted as useful tools for assessing economic and environmental impacts of land use changes, for involving stakeholders in environmental issues and thus for decision-making. In most studies involving comparison and evaluation of scenarios, a reference situation from which different scenarios will be developed is defined. When dealing with rural areas, scenarios, and thus reference situation, should not only consider land use but also cropping systems. The usual lack of data concerning cropping system distribution in a given region can be overcome by the use of expert knowledge. In this paper, we present a procedure for the spatial allocation of cropping systems that integrates expert knowledge and quantitative data. The aim of this procedure is to develop a reference situation that makes explicit the link between cropping systems and the factors responsible for their geographical location (location factors). The procedure consists of five steps: (1) simplifying the cropping systems according to the main aspects that are significant regarding the study area and the studied issue, (2) identifying relevant location factors, (3) dividing the study area into support units, (4) collecting and analysing qualitative expert knowledge and (5) allocating cropping systems by deterministic algorithms and minimisation of cross-entropy. We present an example of the method’s application. It concerns the Neste system, a catchment in south-western France where irrigation is a key issue because of scarcity of water. The reference situation is a distribution of the crops and their associated irrigation management. We have identified farm and soil types as location factors. They can serve to elaborate distributions of cropping systems on territories which could be used as discussion support in local debates on irrigation.