Validation of catchment models for predicting land-use and climate change impacts. 3. Blind validation for internal and outlet responses

The capability of the physically based, distributed SHETRAN catchment modelling system for predictive modelling of hypothetical future catchments is validated for the 0.94 km2 Slapton Wood catchment in southwest England. A ‘blind’ procedure (without sight of measured response data) is used which accounts also for uncertainty in model parameter evaluation. Internal catchment conditions as well as the outlet discharge are considered, making the test perhaps the severest to which a model can be subjected. Data collection formed an integral part of the validation procedure and was designed specifically to satisfy the needs of the modelling component. The extensive dataset which was collected included rainfall, evapotranspiration, soil property data, channel geometry, phreatic surface elevation, soil water potential and stream discharge. Following a prescribed method, blind predictions were made of ten features of the phreatic surface, soil water potential and surface runoff responses. Output uncertainty bounds were determined as a function of uncertainty in the model parameter values. Subsequent comparison of the bounds with the measured data showed that eight of the ten predictions passed the specified success criteria, constituting a successful validation. Within reasonable uncertainty bounds, and on a spatially distributed basis, SHETRAN is shown able to represent the annual catchment water balance as well as important features of the event-scale response. The results are an encouraging demonstration of the fitness of such models for predictive modelling.