Use of caesium-137 data for validation of spatially distributed erosion models: the implications of tillage erosion

Validation of spatially distributed models using spatially distributed data represents a vital element in the development process; however, it is rare ly undertaken. To a large extent, this reflects the problems associated wit h assembling erosion rate data, at appropriate temporal and spatial scales and with a suitable spatial resolution, for comparison with model results. The caesium-137 (Cs-137) technique would appear to offer considerable poten tial for meeting this need for data, at least for longer timescales. Nevert heless, initial attempts to use Cs-137 for model validation did not prove s uccessful. This lack of success may be explained by the important role of t illage erosion in redistributing soil within agricultural fields and, there fore, contributing to the Cs-137-derived soil redistribution rates. This pa per examines the implications of tillage erosion for the use of Cs-137 in e rosion model validation and presents an outline methodology for the use of Cs-137 in model validation. This methodology acknowledges and addresses the constraints imposed by the need to: (1) separate water and tillage erosion contributions to total soil redistribution as represented in Cs-137 derive d rates; (2) account for lateral mixing of Cs-137 within fields as a result of tillage translocation; (3) simulate long-term water erosion rates using the model under evaluation if Cs-137-derived water erosion rates are to be used in model validation. The methodology is dependent on accurate simulat ion of tillage erosion and tillage translocation. Therefore, as greater und erstanding of tillage erosion is obtained, the potential for the use of Cs- 137 in water erosion model validation will increase. Caesium-137 measuremen ts remain one of the few sources of spatially distributed erosion informati on and, therefore, their potential value should be exploited to the full. ( C) 1999 Elsevier Science B.V. All rights reserved.