Long-term reference evapotranspiration trend and causative factors analysis in the sugarbelt area of the midlands of KwaZulu-Natal, South Africa

Global warming is widely recognised, and its effects are becoming apparent throughout the world. Evaporation and evapotranspiration, the key components of the hydrological cycle, are generally expected to increase due to the rise in air and surface temperatures. However, previous studies suggest a decrease in these phenomena despite the observed global warming. The decreasing evaporation and evapotranspiration have been attributed to various factors. In this study, reference evapotranspiration (ETo) trends estimated using the Penman-Monteith method were studied over the KwaZulu-Natal midlands area of South Africa for the period 1997–2017. This study employed the Mann–Kendall test and linear regression model to analyse annual and seasonal ETo trends. In addition, the trends of climate parameters and their contribution towards ETo variation were analysed. The results indicate a generally decreasing ETo trend for most weather stations studied over the study period. The climate variables analysed indicate an average decreasing trend in wind speed, solar irradiance, and relative humidity while average air temperature exhibited no significant change. Relative humidity and solar irradiance were found to greatly influence ETo variation in the study area. We therefore conclude that atmospheric condition studies should consider both local and global climate phenomena to understand the actual drivers of change in any atmospheric factor.