| Abstract |
The Upper Cubango-Kavango River Basin (UCKRB) has undergone substantial land use and land cover (LULC) changes and climate change (CC). The combined effects of LULC change as well as CC on discharge and water quality remain underexplored. A systematic review examined the applications and challenges of the LULC change and CC studies in the UCKRB. The search was conducted in English and Portuguese across four digital libraries: the Science Direct, Scholar Google, Web of Science, and Academia. A total of 19 articles were identified from a pool of 103 papers published between 2014 and 2024. In addition, the research applied a Water Quality Index (WQI) to evaluate the suitability of surface water for human consumption in the UCKRB with limited in situ data. The study also quantified the LULC and CC influence on discharge. Furthermore, it assessed the influence of the Q and Water Temperature (WT) on Nitrate (NO₃-) and Orthophosphate (PO₄³-) loads. About 176 water samples were collected throughout the river in four phases: the dry season (August 2023 and 2024) and the wet season (November 2023 and March 2024). Water quality was analysed both in the field and in the NamWater laboratory using a HANNA HI 9829 multiparameter instrument and an ion spectrophotometer Metrohm. The WQI was calculated based on the weighted arithmetic method using the following water parameters: TN, NO₃-, Nitrite (NO₂-), Ammonia (NH₃), Organic Nitrogen (ON), TP, PO₄³-, Electrical Conductivity (EC), pH, Dissolved Oxygen (DO), Total Dissolved Solids (TDS), and Chlorophyll-a (Chl-a). The simulation of streamflow and NO3- and PO4-3 loads was conducted using the QSWAT+ version 3.0.8. Furthermore, the SWAT+ Toolbox 3.0.6 was utilised for calibration from 1994 to 2011 and for validation from 2012 to 2024. The preliminary order sensitivity analysis identified CN2, ESCO, CANMX, PERCO, and ALPHA as the most sensitive parameters. The LULC maps, with a resolution of 30 m, were created using Google Earth Engine (GEE), which utilised Landsat 5 TM (1994 and 2009) and Landsat 9 OLI (2024) data. Daily temperature and precipitation data were downloaded from NASA's Power platform and the Climate Hazards Information Requirements Programme (CHIRPS). The WQI values ranged as excellent water (22.7) at Nkurenkuru to good water (29.8) in Divundu during the dry season. In the wet season, the WQI value ranged as good water as well (27.9) at Calai and 34.7 at Divundu. The study showed that the WQI values indicated that the overall water quality was suitable for drinking. However, continuing monitoring needs to be done. The LULC change and CC were responsible for a 74 mm decrease in discharge of the UCKRB between 1994 and 2024. SWAT+ has proven to be an effective model for simulating streamflow and nutrient loads. The NSE, PBIAS, R2, P factor, and R factor, with values of 0.68, -2.52, 0.66, 0.87, and 2.25% during the calibration period, and 0.67, 2.24, 0.68, 0.81, and 2.6%, respectively, during the validation period, showed good performance. However, further
calibration of the model is necessary to enhance the accuracy of nutrient concentration. The study indicated that discharge had a greater impact on nutrient loads than WT. The predominant influences of the LULC and CC were identified as a decline in discharge, accompanied by an increase in EC, TDS, salinity, Chl-a, and DO decline. The insight of this study will enhance the development of sustainable measures to preserve the ecology and maintenance of the socioeconomics services. |
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