Impact of rainwater harvesting on water resources of the modder river basin, central region of 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 collecting and use of surface runoff for different purpose in the upstream catchment. Increased water consumption at upstream level is an issue of concern for downstream water availability to sustain ecosystem services. The upper Modder River basin, located in a semi arid region in the central South Africa, is experiencing intermittent droughts causing water shortages for agriculture, livestock and domestic uses. To address this problem a technique was developed for small scale farmers with the objective of collecting and concentrating of rainwater for crop production. However, the hydrological impact of a wider adoption of this technique by farmers has not been well quantified. In this regard, the SWAT hydrological model was used to simulate potential hydrological impact of such practices. The scenarios studied were: (1) baseline scenario, based on the actual land use of 2000, which is dominated by pasture (combination of natural and some improved grass lands) (PAST); (2) partial conversion of actual land use 2000 (PAST) to conventional agriculture (Agri-CON); and (3) partial conversion of actual land use 2000 (PAST) to in-field rainwater harvesting which was aimed at improving the precipitation use efficiency (Agri-IRWH). SWAT was calibrated using both observed daily as well as monthly streamflow data of a sub-catchment (419 km2) in the study area. SWAT performed well in simulating the streamflow giving Nash and Sutcliffe efficiency of 0.57 for the monthly streamflow calibration. The simulated water balance results showed that the highest peak mean monthly direct flow was obtained under the Agri-CON land use (18 mm), followed by PAST (12 mm) and Agri-IRWH land use (9 mm). These were 19%, 13% and 11% of the mean annual rainfall, respectively. The Agri-IRWH scenario reduced the annual direct flow by 32% compared to Agri-CON which is significant at p < 0.02 level. On the other hand it was found that the Agri-IRWH contributed to more groundwater recharge (40 mm/year) compared to PAST (32 mm/year) and Agri-CON (19 mm/year) scenarios. Although there was observable impact of the rainwater harvesting technique on the water yield when considered on a monthly time frame, the overall result suggests that the annual water yield of one of the upper Modder River Basin quaternary catchment will not be adversely affected by the Agri-IRWH land use scenario despite its surface runoff capture design.