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Environmental modeling studies aim to infer the impacts on environmentalvariables that are caused by natural and human-induced changes inenvironmental systems. Changes in environmental systems are typicallyimplemented as discrete scenarios in environmental models to simulateenvironmental variables under changing conditions. The scenario developmentof a model input usually involves several data sources and perhaps othermodels, which are potential sources of uncertainty. The setup and theparametrization of the implemented environmental model are additional sourcesof uncertainty for the simulation of environmental variables. Yet to drawwell-informed conclusions from the model simulations it is essential toidentify the dominant sources of uncertainty. In impact studies in two Austrian catchments the eco-hydrological model Soiland Water Assessment Tool (SWAT) was applied to simulate discharge andnitrate-nitrogen (NO3--N) loads under future changingconditions. For both catchments the SWAT model was set up with differentspatial aggregations. Non-unique model parameter sets were identified thatadequately reproduced observations of discharge and NO3--Nloads. We developed scenarios of future changes for land use, point sourceemissions, and climate and implemented the scenario realizations in thedifferent SWAT model setups with different model parametrizations, whichresulted in 7000 combinations of scenarios and model setups for bothcatchments. With all model combinations we simulated daily discharge andNO3--N loads at the catchment outlets. The analysis of the 7000 generated model combinations of both case studieshad two main goals: (i) to identify the dominant controls on the simulationof discharge and NO3--N loads in the two case studies and(ii) to assess how the considered inputs control the simulation of dischargeand NO3--N loads. To assess the impact of the input scenarios,the model setup, and the parametrization on the simulation of discharge andNO3--N loads, we employed methods of global sensitivityanalysis (GSA). The uncertainties in the simulation of discharge andNO3--N loads that resulted from the 7000 SWAT model combinationswere evaluated visually. We present approaches for the visualization of thesimulation uncertainties that support the diagnosis of how the analyzedinputs affected the simulation of discharge and NO3--N loads. Based on the GSA we identified climate change and the model parametrizationas being the most influential model inputs for the simulation of discharge andNO3--N loads in both case studies. In contrast, the impact ofthe model setup on the simulation of discharge and NO3--N loadswas low, and the changes in land use and point source emissions were found tohave the lowest impact on the simulated discharge and NO3--Nloads. The visual analysis of the uncertainty bands illustrated that thedeviations in precipitation of the different climate scenarios to historicrecords dominated the changes in simulation outputs, while the differences inair temperature showed no considerable impact.

hydrology and earth system sciences

A comprehensive sensitivity and uncertainty analysis for discharge and nitrate-nitrogen loads involving multiple discrete model inputs under future changing conditions