Impact of land use on water resources of the Modder River basin in central South Africa

Abstract

Along the path of water flowing in a river basin are many water-related human interventions that modify the natural systems. Rainwater harvesting is one such intervention that involves abstraction of water in the upstream catchment. Increased water withdrawal at upstream level is an issue of concern for downstream water availability to sustain ecosystem services. The Modder River basin, located in the central South Africa, is experiencing intermittent meteorological droughts causing water shortages for agriculture, livestock and domestic purpose. To address this problem a technique was developed for small scale farmers with objective of harnessing rainwater for crop production. However, the impact of a wider adoption of this technique by farmers on the water resources has not been quantified. In this regard, the SWAT hydrological model was used to simulate the impact of such practice on the water resources of the river basin. The scenarios studied were: pasture (PAST), conventional agriculture (Agri-CON) and agriculture using rainwater harvesting (Agri-IRWH). The result showed that the highest mean monthly direct flow was obtained on Agri-CON land use (18 mm), followed by PAST (12 mm) and Agri-IRWH land use (10 mm). The Agri-IRWH scenario reduced runoff by 38% compared to Agri-CON, which justifies its intended purpose. On the other hand, it was found that the Agri-IRWH contributed to more groundwater recharge (40 mm) compared to PAST (32 mm) and Agri-CON (19 mm) scenarios. Although, there was a visible impact of the rainwater harvesting technique on the water yield when considered on a monthly time frame, the overall result showed that there was a substantial benefit of using the rainwater harvesting technique for agricultural production (Agr-IRWH) without impacting significantly on the mean annual water yield.