A Decadal Change in the Simulated Surface Runoff Response Associated with Land Use and Land Cover Change in the uMlaas Catchment, South Africa

Quantifying and knowing land use and land cover (LULC) change remains significant for tracking environmental changes and evaluating changes in a watershed’s hydrological response. This study applies the Natural Resources Conservation Service Curve Number (NRCS-CN) method to estimate surface runoff response to LULC change in the uMlaas catchment between 1995 and 2025, using four LULC scenarios (1995, 2005, 2015, and 2025), hydrological soil groups (HSGs), and fixed mean monthly and annual rainfall depths. Because the curve number (CN) values are assigned according to LULC types and HSGs, runoff estimates under this method are structurally linked to LULC composition; thus, this study quantifies the magnitude of this linkage by examining LULC change over the study period, rather than testing it as an independent hypothesis. LULC change analysis shows rapid and sustained urban expansion at the expense of grassland and cultivated land, while natural and planted forests increased modestly over the same period. The urban expansion is closely mirrored by the model-derived annual runoff, which rose from 442.55 mm to a peak of 485.44 mm in 2015 before declining slightly by 472.96 mm in 2025, remaining higher than in 1995. Simulated runoff peaks in October–April, consistent with rainfall seasons, and concentrated in the central to eastern region, coinciding with urban expansion. These results illustrate how, under the assumptions of the NRCS-CN method, the observed LULC transitions in uMlaas, particularly urban expansion and loss of vegetated land cover, translate into elevated simulated runoff. Land use planning within the catchment should prioritise runoff control, particularly in the central to eastern region (in the city of Durban).