Evapotranspiration (ET) is an important component of the hydrologic cycle and involves the exchange of water between the surrounding water bodies, soil, crop surfaces, and the atmosphere. Crop growth and yields are largely affected by the rate of ET, especially in semi-arid areas where the rate of ET is high and rainfall is not sufficient and reliable to add more water into the soil for crop use. Solar radiation, relative humidity, air temperature, rainfall, and wind velocity are some of the meteorological factors that affect ET. Therefore, this research was aimed at determining ET and its trend across Rwanda using climatic data measured at 5 sites. With the research, we accessed meteorology data measured at synaptic stations in the five provinces of Rwanda (Kigali city/central, Kawangire/Eastern, Ruberengera/Western, Ruhengeri/Northern and Gikongoro/ and used the data to calculate reference evapotranspiration (ET) for the recent 10 years (2010-2018).
Equations were used to calculate extra-terrestrial radiations, solar radiations, evapotranspiration using the FAO-56 guidelines - Turc method (1961) and Hargreaves-Samani (HS) method (1983). The obtained data was analyzed using the two methods and graph were plotted for visualization. Both methods have performed well in semi-humid and semi-arid environmental conditions. The average monthly reference evapotranspiration for both equations ranges from 3 to 5 mm/day. The Hu-Ts graphs for all the five locations shows a low value of reference ET (3 to 4 mm/day) for the year of 2018 compared to the previous years. Generally, there was consistency in variation in the reference ET throughout the 10 years for both the equations. The Turc method underestimates reference ET value compared to the HS method. The difference and variation of reference crop evapotranspiration for both the equations might have been due to the different locations since they have variations in climatic conditions that contributes a lot to the evapotranspiration value in the region
