Abstract
Abstract. Model predictions of biogeochemical fluxes at the landscape scale are highly uncertain, both with respect to stochastic (parameter) and structural uncertainty. In this study 5 different models (LASCAM, LASCAM-S, a self-developed tool, SWAT and HBV-N-D) designed to simulate hydrological fluxes as well as mobilisation and transport of one or several nitrogen species were applied to the mesoscale River Fyris catchment in mid-eastern Sweden. Hydrological calibration against 5 years of recorded daily discharge at two stations gave highly variable results with Nash-Sutcliffe Efficiency (NSE) ranging between 0.48 and 0.83. Using the calibrated hydrological parameter sets, the parameter uncertainty linked to the nitrogen parameters was explored in order to cover the range of possible predictions of exported loads for 3 nitrogen species: nitrate (NO3), ammonium (NH4) and total nitrogen (Tot-N). For each model and each nitrogen species, predictions were ranked in two different ways according to the performance indicated by two different goodness-of-fit measures: the coefficient of determination R2 and the root mean square error RMSE. A total of 2160 deterministic Single Model Ensembles (SME) was generated using an increasing number of members (from the 2 best to the 10 best single predictions). Finally the best SME for each model, nitrogen species and discharge station were selected and merged into 330 different Multi-Model Ensembles (MME). The evolution of changes in R2 and RMSE was used as a performance descriptor of the ensemble procedure. In each studied case, numerous ensemble merging schemes were identified which outperformed any of their members. Improvement rates were generally higher when worse members were introduced. The highest improvements were achieved for the nitrogen SMEs compiled with multiple linear regression models with R2 selected members, which resulted in the RMSE decreasing by up to 90%.
Highlights
IntroductionAnthropogenic influence on environmental systems has been demonstrated
1.1 Catchment modellingIn recent decades, anthropogenic influence on environmental systems has been demonstrated
In hydrological sciences the catchment is considered as the basic unit and numerous different models were created from the 1960s onwards
Summary
Anthropogenic influence on environmental systems has been demonstrated. Balanced biogeochemical cycles such as the nitrogen cycles have been deeply altered (Galloway et al, 2004; Vitousek et al, 1997) since the middle of the 18th century. For about 50 years the increasing speed of computers allowed scientists from different fields to simulate such systems (e.g. atmosphere, hydrosphere) behaviour through different sets of mathematical equations. Boughton (2005) reviewed 13 different rainfall-runoff models developed in Australia alone in the second half of the 20th century. Numerical models are nowadays used as management tools from local to global scale and are able to give an approximation of the effects of different changes on a natural system (e.g. land use change, global warming)
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