Impact of land use changes on water quality at lower Oranger River catchment, South Africa.

Increased nutrient loads from agricultural land use have been associated with water quality deterioration. The Orange River catchment in South Africa faces water quality issues linked to nitrogen (N) leaching and Phosphorus (P) pollution. It is well established that land use changes from natural to anthropogenic purposes have an impact on water quality. An increase in the application of inorganic fertilizer due to the expansion of intensive agriculture and especially irrigation may contribute to water pollution. This research, therefore, presents the effect of land use change on water quality in the Lower Orange River Basin using the Riet_Modder River in the Free State, South Africa watershed. Historical GEMStat water quality data and multi-year LULC mapped from Landsat imagery were used for the analysis. Nutrient (NH4N, NOxN and DRP) concentrations and loads from 1990 to 2020 at the Riet-Modder River watershed were analysed for the trends and their correlation to various land cover classes. Main LULC changes included an eightfold increase in commercial irrigated land area, bare lands by 96%, subsistence lands by 81% and wetlands by -83%. Water quality data at the outlet point showed an increase in NH4N and DRP over the period of study, while NOxN decreased over the period.  NH4N and NOxN loads were reduced, while DRP indicated an increase in all seasons. From the findings, the Riet-Modder River watershed is threatened by water quality deterioration associated with increased commercial irrigated lands, reduced wetlands, subsistence croplands and waste from urban centres. Nitrogen elements, especially (ammonia, ammonium) concentration and phosphorus concentrations, are on the rise, and the water sources in the area are classified as eutrophic. Phosphorus loads transported out of the watershed are also increasing, which will further deteriorate ambient water quality.