Modelling the sources and transport of ammonium nitrogen with the SPARROW model: A case study in a karst basin

The assessment of nutrients delivered to aquatic ecosystems is important for water quality controls. Mainly due to the complex terrains and subsurface hydrological systems in karst regions, it is challenging to understand nutrient delivery pathways and quantify contributions of various sources to surface waterbodies in karst basins, e.g., via the use of mechanistic models. To resolve this issue, a statistical water quality model (i.e., the SPAtially Referenced Regression On Watershed attributes-SPARROW model) was tested to estimate the mean annual transport, concentration, and yield of ammonium nitrogen (NH4+-N) contributed by natural and human sources in the Wujiang River Basin (WRB) in southwest China, where the nutrient inputs and pathways have been significantly altered by anthropogenic activities. Overall, the modelling results explained about 86% of the variability in the observed mean annual NH4+-N fluxes, attesting the applicability of the SPARROW model for estimating NH4+-N transport at mean annual time scales in karst basins. Moreover, the results indicated that the anthropogenic sources (i.e., fertilizer, livestock manure, and waste water) were the main origins of NH4+-N, accounting for a total load of 66.8% in waterbodies. In addition, the leakage of NH4+-N into groundwater from the karst area was evaluated, leading to a reduction of NH4+-N delivery to surface waterbodies for about 36.9% with a range from 31.3% to 52.2% in the seven main subbasins in the WRB. The damming effect of the constructed reservoirs on the NH4+-N delivery differed noticeably with the lowest reduction rate (2.4%) in the Suofengying reservoir and the highest rate (79.1%) in the Hongfenghu reservoir. It was found that the interception efficiency of lake-type reservoirs was generally higher than that of river-type reservoirs in the WRB. The results of this study demonstrate the usefulness of the SPARROW model for evaluating nutrient transport and pathways in karst regions, which can provide critical information for better management to control nutrients in those regions. •A statistical model (SPARROW) was used to quantify transport of NH4+-N in a karst basin.•River discharge and reservoir surface area affected NH4+-N transport.•The areal distribution of karstification influenced NH4+-N transport to streams.