Responses of evapotranspiration to changing climate for the past six decades in the upper catchment of the Olifants River basin

In river basins such as the Olifants, the evapotranspiration has been documented as the biggest water user and therefore one of the most important hydrological processes in the basin. Thus, using the Soil and Water Assessment Tool (SWAT) model, this study attempted to assess the hydrological responses of evapotranspiration (ET) to climate change in the Olifants River basin over a period of six decades, focusing mainly on the upper catchment (UC) of the basin. The study showed, on average, a general decline of seasonal ET with a decrease of 10.5% in spring, 29.8% in summer and 17.3% in winter. However, autumn ET showed an average increase of 12.5%. With (1960 - 1970) decade considered as the baseline, the study also found that annual ET in the UC, has been on the decline consistently since the 1960s with the most recent decade (2010 - 2018) taking the biggest share of the drop (12.8%). Here, the average annual decline of ET of 1.75 mm/yr was observed with most of the decline happening in the last decade. This general decadal trend of annual ET mortality in the area coincides with the temperature rise and precipitation decrease of the area, signifying the impact climate change may have had on the evapotranspiration for the past six decades.