Developing a model to assess the impact of farm dams and irrigation for data-scarce catchments

Productive agricultural supply chains require the support of functional ecosystems, but intense agricultural practices change local hydrological systems (e.g., river diversion). In this study, the impact of farm dams was assessed for the Verlorenvlei catchment, a sensitive ecosystem currently under a state of hydrological change in South Africa. We developed a new module to the JAMS/J2000 rainfall-runoff model to assess the streamflow impact from the points of abstraction, losses during storage and irrigation. The model achieved a satisfactory streamflow calibration with efficiencies (NSE, logNSE) of 0.52 and 0.51. The irrigated area reduced simulated streamflow by 12-19%. While the results from the study agree with remotely sensed evapotranspiration, measured lake surface water levels and streamflow, uncertainty remains in the total simulated dam evaporation. While many catchments lack the data required for a detailed irrigation impact assessment, this approach considers total water use, dam storage to area relationships and general farming practices.