Comparison of pesticide leaching models: results using the Weiherbach data set

The leaching of the herbicide isoproturon and the water tracer bromide in the profile of a silty loam soil at the Weiherbach Catchment area in Germany was simulated by eight different persons using the models MACRO, SIMULAT, LEACHP, WAVE, GLEAMS and PELMO (only bromide). Experimental results from different laboratory studies and from a field study were available to parameterise the models. Model results concerning water contents, tracer and pesticide profiles were provided for two different test periods in the field (winter and spring). The results for the behaviour of the herbicide and the tracer in the field study in spring were not known beforehand to the model users. Therefore this is considered to be a blind test. After the model users had estimated the parameters and presented the modelling results a consensus parameter set was provided by the data set co-ordinator to reduce the subjective influence of individual users and to identify the effect of different model concepts. The most striking points of the modelling exercise were the diversity of parameters estimated by different modellers from the same basic data set and the problems of simulating the field behaviour using laboratory derived pesticide parameters. An overwhelming influence of the individual users on the model results was remarkable for the Weiherbach data set. It was not possible to estimate, from the laboratory data, alone a parameter combination suitable for describing the field behaviour of isoproturon satisfactorily. The use of literature data or calibration to the field results of the winter period did improve the simulation results significantly.