Seasonal variation of reference evapotranspiration and Priestley-Taylor coefficient in the eastern Free State, South Africa

Estimation of reference evapotranspiration (ETo) is crucial in crop production practices and other hydrological processes. The hourly FAO-56 Penman-Monteith (EToPM) method was used to calculate ETo at Bergville, Bethlehem and Harrismith in the eastern Free State, South Africa. Priestley-Taylor evapotranspiration coefficients (PTc) were estimated using EToPM and equilibrium evapotranspiration. The study establishes that for the mountainous semi-arid areas, aerodynamic conditions are major contributors of evapotranspiration. The aerodynamic component of EToPM is generally a dominant contributor varying between 50% and 70% of the total EToPM depending on the season and location. Dry and windy atmospheric conditions that reach their highest levels during the spring season cause water vapour pressure deficit to be a parameter with greater influence on ETo. The study further shows that the impact of solar irradiance on ETo decreases with altitude in the study area. More than 87% of EToPM from aerodynamic resistance is experienced during daytime. Agricultural water resources management activities need to consider the importance of nocturnal EToPM which contributes between 10-14% of total daily ETo depending on the station. Geometry of a surface affects total solar irradiance received at a place and the associated ETo. The commonly used average Priestley-Taylor constant of 1.26 under-estimates evaporation rates in the semi-arid environments. This study shows that the coefficients are highly variable during winter and spring, less variable in autumn, and that topoclimate has a significant impact on the PTc variability. This variability reflects a direct influence that the dry atmospheric environments of the semi-arid regions have on PTc, which is a limitation of the use of the 1.26 coefficient.