Date of this Version
Uwase, E. 2021. Evaluating evapotranspiration values in Rwanda using the FAO-56 PM, Turc, and Hargreaves-Samani equations. Working paper. University of Nebraska-Lincoln, Department of Biological Systems Engineering: Lincoln, NE.
Evapotranspiration (ET) is an important component of the hydrologic cycle and involves exchange of water between the surrounding water bodies, soil, crop surfaces, and the atmosphere. Therefore, this research was aimed at determining ET and its trend across Rwanda using climatic data measured at five sites. Equations were used to calculate ET using extra-terrestrial radiations, solar radiations, evapotranspiration using the FAO-56 guidelines - Turc method (1961) and Hargreaves-Samani (HS) method (1983). The obtained data from the weather stations were analyzed using the two methods and graphs were plotted for visualization. The average monthly reference evapotranspiration for both equations ranges from 3 to 5 mm/day. The Hargreaves-Samani and Turc graphs for all the five locations showed low values of reference ET (3 to 4 mm/day) for the year of 2018 compared to the previous years. The Turc method underestimates reference ET values compared to the HS method. The difference and variation of reference crop ET for both the equations may be attributed to the different locations since they have variations in climatic conditions that significantly affects ET in the region. The HS and Turc ET estimates did not capture the day-to-day variability in ET when compared with the standard FAO Penman-Monteith method. Hence, the Turc and HS equations may not be implemented for estimating daily ET but may be more acceptable for estimation of ET on a weekly or monthly basis.