In the arid landscapes of South Africa, where every drop of water counts, a breakthrough in agricultural water management could soon be making waves. Researchers from the University of the Witwatersrand have published findings that could transform how farmers and irrigation schemes measure crop water use—potentially saving millions of liters of water annually in water-scarce regions.
The study, led by P. E. Ratshiedana from the School of Geography, Archaeology and Environmental Studies, evaluated four remote sensing algorithms to estimate actual evapotranspiration (ETa)—the combined process of water evaporation from soil and transpiration from plants. Accurate ETa data is critical for irrigation scheduling, yet direct measurements are scarce in South Africa. Using 22 Landsat 8 satellite images spanning 2019 to 2021, Ratshiedana and the team assessed ETa across over 36,000 hectares, comparing algorithm performance against ground-truth data from a smart lysimeter and weather stations.
The results were striking. The Vegetation Index (VI)-based ETa algorithm emerged as the most reliable, showing a strong correlation with lysimeter measurements (r = 0.92, R² = 0.85) and the lowest error margins (RMSE = 0.58 mm/day; MAE = 0.44 mm/day). Close behind was the Surface Energy Balance Algorithm for Land (SEBAL), which also delivered robust performance. Meanwhile, the Surface Energy Balance System (SEBS) showed moderate accuracy, and the Crop Water Stress Index (CWSI)-based approach underperformed.
Speaking to the implications of these findings, Ratshiedana emphasized the potential for remote sensing to bridge data gaps in regions lacking ground-based monitoring. “The VI-based algorithm not only matched lysimeter data closely but did so with computational efficiency—making it a practical tool for large-scale irrigation management,” he noted. This efficiency could be a game-changer for water-scarce agricultural regions, where over-irrigation is a common yet costly issue.
The study also highlighted the broader commercial impact for the energy sector. Accurate ETa estimation enables more precise irrigation scheduling, which can reduce energy consumption associated with pumping water—a major operational cost for irrigation schemes. By minimizing water waste, farmers could lower energy demand, contributing to both cost savings and environmental sustainability.
Published in *Hydrology and Earth System Sciences* (translated from the German *Hydrologie und Erdsystemwissenschaften*), the research underscores the growing role of geospatial technologies in sustainable agriculture. As climate change intensifies pressure on water resources, tools that enhance precision in water management will be invaluable. For industries reliant on agricultural supply chains, from food producers to energy providers, these findings could pave the way for smarter, more resilient water-use strategies.