Equifinality and uncertainty in physically based soil erosion models: Application of the glue methodology to WEPP-the water erosion prediction project-for sites in the UK and USA

Despite the wealth of soil erosion models available for the prediction of b oth runoff and soil loss at a variety of scales, little quantification is m ade of uncertainty and error associated with model output. This in part ref lects the need to produce unequivocal or optimal results for the end user, which will often be an unrealistic goal. This paper presents a conceptually simple methodology, Generalized Likelihood Uncertainty Estimation (GLUE), for assessing the degree of uncertainty surrounding output from a physicall y based soil erosion model, the Water Erosion Prediction Project (WEPP). Th e ability not only to be explicit about model error but also to evaluate fu ture improvements in parameter estimation, observed data or scientific unde rstanding is demonstrated. This approach is applied to two sets of soil los s/runoff plot replicates, one in the UK and one in the USA. Although it is demonstrated that observations can be largely captured within uncertainty b ounds, results indicate that these uncertainty bounds are often wide, refle cting the need to qualify results that derive from 'optimum' parameter sets , and to accept the concept of equifinality within soil erosion models. Att ention is brought to the problem of under-prediction of large events/over-p rediction of small events, as an area where model improvements could be mad e, specifically in the case of relatively dry years. Finally it is proposed that such a technique of model evaluation be employed more widely within t he discipline so as to aid the interpretation and understanding of complex model output. Copyright (C) 2000 John Wiley & Sons, Ltd.