Identification of critical source areas for diffuse water pollution

Input into streams due to erosion and runoff of pesticides and nutrients from agricultural fields pose a threat to our water bodies. Field studies indicate that these losses originate from limited parts of a given catchment. This holds especially for fine sediments, pesticides and phosphorus, which are mainly transported by fast flow processes that are generated only on certain locations. These critical source areas (CSAs) seem to cover in many cases about 20 % of the total area. The best empirical evidence for CSAs exists for erosion, where losses can be observed after an erosive event. For P losses, the concept is also used fairly wide-spread in many countries outside Switzerland. However, the empirical data base supporting the concept is rather limited. Even less data exist for pesticides. For nitrogen, the CSA concept is not appropriate. For identifying CSA in space, several tools are available. We have tested some of them on four different test farms. The risk areas for erosion agreed well with the field experience of the local farmers. The risk areas for runoff and erosion did not overlap in many situations. Identifying risk areas in Switzerland is in many situations severely hampered by the coarse soil maps that are available.