Impacts of land cover change on water balance components in Upper Mzingwane sub catchment of Zimbabwe /

Abstract

The Upper Mzingwane sub-catchment is critical for water supply to various users, including domestic (especially the city of Bulawayo), mining, agriculture and industry. However, monitoring and management of water resources in the subcatchment is fraught with significant problems due to the lack of a robust network of monitoring stations. The objective of this study was to assess the suitability and effectiveness of the Soil and Water Assessment Tool hydrological model (SWAT) in simulating water balance components in the Upper Mzingwane sub-catchment, considering different land cover change scenarios. The land use map, soil map, digital elevation model (DEM), soil and weather data were used to build the hydrological model SWAT. Observed discharge data from Zimbabwe National Water Authority (ZINWA) gauging stations within the catchment were used for model calibration (1993 - 2002) and validation (2003 - 2016) using the Sequential Uncertainty Fitting (SUFI -2) method in SWAT-CUP. In the sensitivity analysis, CN2 (curve number for moisture condition II), GWQMN (threshold water level in shallow aquifer for baseflow) and REVAPMN.gw (groundwater "revap" coefficient) were found to be the most sensitive parameters. The statistical performance measures of the model show that the calibration results agree well with the observed and simulated monthly discharge data (R2 = 0.81, NSE = 0.68 and PBIAS = 4.1). The study investigated how water balance components in the Upper Mzingwane sub-catchment responded to changes in land use/land cover (LULC). Results showed that surface runoff (SURQ) increased by 8.76 mm (10.2%) from 1990 to 2000, followed by a decrease of 8.06 mm (-8.5%) from 2000 to 2011 and 7.37 mm (-8.52%) from 2011 to 2020. Similar decreasing trends were observed for groundwater flow (GW_Q), lateral flow (LATQ) and evapotranspiration (ET) between 2000 and 2020.These changes were linked to decrease vegetation cover, an increase in cropland, settlements increased degradation within the sub-catchment. Evapotranspiration (ET), soil moisture (SM), groundwater contribution to streamflow (GW_Q), surface runoff (SURQ), and water yield exhibited similar spatial and temporal variability, with higher values in the southern and northern regions and lower values in the middle of the sub-catchment. Overall, the study concludes that the SWAT hydrological model effectively captures the impact of land use patterns on runoff data in this sub-basin, offering valuable insights to enhance water resource monitoring and management in the region.