Evaluating coupled influences of slope class and land use change on water quality using single and composite indices in an agricultural basin

Understanding the critical source areas (CSAs) of pollution and their key relationships with land use is essential for efficient water quality improvement. Despite much efforts to examine their influences on water quality, the role of topographic characteristics in land use processes acting on water quality in lowland regions remains unclear. Therefore, taking a typical rural lowland catchment-the Stör catchment (in northern Germany) as the study area, the influences of different land use types with varying slopes on sediment, total phosphorus (TP), and total nitrogen (TN) loads, as well as on the relevant CSAs of a single or combined index (I_j) were determined using an integrated approach that applies hydrologic model SWAT_3s (Soil and Water Assessment Tool, adapted with three groundwater aquifers) and multivariate statistical techniques. The results indicated: (1) sediment and nutrients have markedly declined from 1990 to 1991 over 2009–2011 to 2018–2019, particularly in summer and in comparatively steeper areas with slopes above 5%; (2) the subbasins in the southwest or northeast of the catchment dominated by larger arable fields with slope > 5% and urban areas were identified as CSAs; (3) the spatial heterogeneity of overall water quality was more explained in summer (75% on average) and particularly by croplands (50% on average); (4) water quality indicators were mostly and positively affected by cropland and pasture with relatively steeper slope (>5%). In particular, cropland with slopes > 5% was among those land use indicators producing the highest values of sediment, TP, TN, and I_j. This study demonstrates that the identification of seasonal CSAs could provide insights into the seasonal targeting of conservation practices. Furthermore, communicating model outputs to cause-effect analysis can help formulate slope-adaptability measures for water quality protection.