A Comparison of Various Evapotranspiration Models for Estimating Reference Evapotranspiration in Sokoto, North Western, Nigeria

Evapotranspiration (ET) is an important component of the hydrological cycle and its accurate quantification is crucial for the design, operation and management of irrigation systems. Agricultural planning depending on evapotranspiration suffers due to inaccuracy in its estimation. The lack of meteorological data retrieved from ground stations required for accurate estimation of reference evapotranspiration (ET0) led in the development of various models for estimating ET0 . This present study compares various universally accepted ET models for estimating ET0 , the six models considered in this study for estimating ET0 for Sokoto, Nigeria (Latitude 13.02 ° N, Longitude 05.25°E and altitude 350.8 m above sea level) using measured meteorological parameters of monthly average daily global solar radiation, sunshine hour, wind speed, maximum and minimum temperatures and relative humidity covering a period of thirty one years (1980-2010) are Blaney- Morin-Nigeria, Priestly and Taylor, Makkink, Hargreaves and Samani, Abtew and the Jensen-Haise models using the FAO-56 Penman-Monteith model as a reference. Based on the FAO-56 Penman- Monteith model, the results showed that the lowest ET0 (4.6977 mmday-1) occurred during therainy season (August) while the highest ET0 (10.0600 mmday-1) occurred during the dry season (March). The statistical indicators of Root Mean Square Error (RMSE), Mean Bias Error (MBE), Mean Absolute Error (MAE) and coefficient of correlation (r) were used for the comparison of the six ET models. The results indicates that the Blaney-Morin-Nigeria is the most appropriate model for estimating for ET0 this particular study area, with lowest RMSE (1.2147mmday-1) , MBE (-1.1581 mmday-1) , MAE (1.1581 mmday-1) and highest value of r (0.9822) . Based on the overall results, the Blaney-Morin-Nigeria model is recommended as an alternative to FAO-56 Penman-Monteith model for estimating ET0 in Sokoto, North – Western, Nigeria when temperature and relative humidity data are available.