SWAT Literature Database for Peer-Reviewed Journal Articles

Title:Evaluating the impacts of climate change and crop land use change on streamflow, nitrates and phosphorus: A modeling study in Bavaria 
Authors:Mehdi, B., R. Ludwig and B. Lehner 
Journal:Journal of Hydrology: Regional Studies 
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Broad Application Category:hydrologic and pollutant 
Primary Application Category:climate change and land use change 
Secondary Application Category:pollutant cycling/loss and transport 
Watershed Description:980 km^2 Altmühl River is located in the northwest part of the state of Bavaria in southeast Germany. The agriculturally dominant part of the Altmühl basin was examined, which comprises 130 km of river, from its source to the outlet gauge in Treuchtlingen. The soils in the upper part of the basin are mainly loamy clay and loamy gravelly sand, with pockets of gravelly sand. Located along the floodplain, below the Altmühl Lake are mainly clayey silt soils. Near the outlet, clay loam soils are predominant, with a few areas of Karst in the southwestern tip of the basin. The land use in the watershed is primarily agricultural (56%) and forest (39%). The 2008 crop land use map provided by the Bavarian State Office for Agriculture shows 23% (22,331 ha) of the watershed is planted to cereals (mainly winter- andsummer-wheat), 20% (20,900 ha) is pasture, 8% (8236 ha) is maize, 3% (2788 ha) is oilseeds (mainly rapeseed), 3% is (2847 ha) urban, 1.4% (1136 ha) is natural grassland, 1.2% (1269 ha) is legumes, and0.6% (342 ha) is tuber crops (mostly potatoes). 
Calibration Summary:The Sequential Uncertainty Fitting algorithm (SUFI-2, Abbaspour et al., 2004) is a semi-automated inverse modeling tool used for calibrating the SWAT simulated outputs to the available timeseries data of streamflow, NO3−-N and TP loads. SUFI-2 is a stochastic procedure drawing independent parameter sets using Latin Hypercube sampling. SWAT was calibrated sequentially for streamflow, NO3−-N, and TP as per Arnold et al. (2012). Based on a sensitivity analysis, 19 parameters were chosen for calibration in SUFI-2.The Nash–Sutcliffe Efficiency (NSE, Nash and Sutcliffe, 1970) statistical criterion was chosen as the primary objective function for calibration. The NSE determines the relative magnitude of the variance of the residuals compared to the variance of the observed data. Several other objective functions were performed post-validation (PBIAS, R2, bR2) so that a variety of best-fit criteria were used to show theq uality of model performance. 
Validation Summary:the model was able to reproduce the timing of daily dry spells and peak flows. The winter peak runoff and the transport of TP were overestimated, while too little NO3−-N was transported below the soil by infiltration. The peak flow overestimation in winter did not dominate the annual streamflow errors since overall SWAT underestimated the monthly flow (PBIAS of 13%). The modeled monthly variance was fairly well captured (R2= 0.78) but the magnitude was weaker (bR2 of 0.68–0.73). The NO3−-N and TP simulations represented the timing of the events well, although the simulated TP was overall lower and NO3−-N was overestimated from 1982 to 2005. Figs. 5b and 6b show this to be especially true for the low to medium loads. The statistical criteria listed in Moriasi et al. (2007), suggested that the overall performance of the SWAT model after the sequential calibration of the three variables “satisfactorily” captured the most important hydrological processes in the watershed. The model could therefore be used with reasonable confidence to reproduce the nutrient transport observed in the basin. 
General Comments: 
Keywords:SWAT, nitrate, total phosphorus, Crop land use change, Agriculture, Climate change