A comprehensive sensitivity and uncertainty analysis for discharge and nitrate-nitrogen loads involving multiple discrete model inputs under future changing conditions
Authors: Schürz Christoph , Hollosi Brigitta , Matulla Christoph , Pressl Alexander , Ertl Thomas , Schulz Karsten , Mehdi Bano 
Published: March 2019

Abstract. Environmental modeling studies aim to infer the impacts on environmental
variables that are caused by natural and human-induced changes in
environmental systems. Changes in environmental systems are typically
implemented as discrete scenarios in environmental models to simulate
environmental variables under changing conditions. The scenario development
of a model input usually involves several data sources and perhaps other
models, which are potential sources of uncertainty. The setup and the
parametrization of the implemented environmental model are additional sources
of uncertainty for the simulation of environmental variables. Yet to draw
well-informed conclusions from the model simulations it is essential to
identify the dominant sources of uncertainty. In impact studies in two Austrian catchments the eco-hydrological model Soil
and Water Assessment Tool (SWAT) was applied to simulate discharge and
nitrate-nitrogen (NO3--N) loads under future changing
conditions. For both catchments the SWAT model was set up with different
spatial aggregations. Non-unique model parameter sets were identified that
adequately reproduced observations of discharge and NO3--N
loads. We developed scenarios of future changes for land use, point source
emissions, and climate and implemented the scenario realizations in the
different SWAT model setups with different model parametrizations, which
resulted in 7000 combinations of scenarios and model setups for both
catchments. With all model combinations we simulated daily discharge and
NO3--N loads at the catchment outlets. The analysis of the 7000 generated model combinations of both case studies
had two main goals: (i) to identify the dominant controls on the simulation
of discharge and NO3--N loads in the two case studies and
(ii) to assess how the considered inputs control the simulation of discharge
and NO3--N loads. To assess the impact of the input scenarios,
the model setup, and the parametrization on the simulation of discharge and
NO3--N loads, we employed methods of global sensitivity
analysis (GSA). The uncertainties in the simulation of discharge and
NO3--N loads that resulted from the 7000 SWAT model combinations
were evaluated visually. We present approaches for the visualization of the
simulation uncertainties that support the diagnosis of how the analyzed
inputs affected the simulation of discharge and NO3--N loads. Based on the GSA we identified climate change and the model parametrization
as being the most influential model inputs for the simulation of discharge and
NO3--N loads in both case studies. In contrast, the impact of
the model setup on the simulation of discharge and NO3--N loads
was low, and the changes in land use and point source emissions were found to
have the lowest impact on the simulated discharge and NO3--N
loads. The visual analysis of the uncertainty bands illustrated that the
deviations in precipitation of the different climate scenarios to historic
records dominated the changes in simulation outputs, while the differences in
air temperature showed no considerable impact.