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.