Seasonal and Annual Evaluation of Temperature and Radiation Based Reference Evapotranspiration Methods Over Cross River Basin Nigeria

The accurate estimation of reference evapotranspiration (ETo) on seasonal and annual scale is vital for determination of water requirement of crops and impact of climate change in irrigated agriculture. This study investigates the relationship between the climatic variables and FAO-Penman-Monteith method (PM-ETo), and comparison of the performance evaluation of temperature-based and radiation-based ETo models over Cross River basin in the rainy season (March-October), dry season (November-February) and on annual basis. Estimation of ETo was done using temperature-based models; Schendel, Samani, Trajkovic, Droogers & Allen-2, Dorji, Hadria, Hargreaves-samani, Blaney-Morin-Nigeria, and radiation-based models; Jensen Haise, Stephens and Stewart, Oudin, Abtew, Irmak-1, Copais, Tabari & Talaee-4, and Hargreaves under humid-tropic condition using the PM-ETo as a reference. Remotely sensed meteorological variables (minimum air temperature (Tmin); maximum air temperature (Tmax); mean relative humidity (RH); solar radiation (Rs) means; wind speed (Ws)) across 31 years (1987–2017) were sourced from the Climatic Research Unit (CRU) database. These meteorological parameters were used for the computation of ETo. The performance evaluation of the empirical models was determined on seasonal and annual basis with the use of coefficient of determination (R ² ), a root mean square error (RMSE), and Willmott’s index of agreement (D). Sensitivity analysis (± 5%) revealed that PM-ETo was most sensitive to Rs (3.263) and Tmean (2.430), followed by less sensitivity to RH, while Ws was found to be less significant. Further analysis revealed that radiation driven models out-performed the temperature-based models. Copais (RMSE of 0.244mm/d, R ² of 0.678 and D of 0.871) is the best radiation-based model (+ 0.1mm/d), while Blaney-Morin-Nigeria (RMSE of 0.578mm/d, D of 0.680, R ² of 0.604) is the best temperature-based model (-0.2mm/d) in the dry season. For rainy season, Abtew model (RMSE = 0.186mm/d, D = 0.868, and R ²  = 0.955) is the best radiation-based model (+ 0.4mm/d), while Hargreaves–Samani (RMSE = 0.405mm/d, D = 0.744, and R ²  = 0.773) is the best temperature-based model (-0.2mm/d). On annual basis, Tabari & Talaee-4 out-performed the Hargreaves–Samani model. Rs driven models are more suitable for ETo estimation in the region, Tmean-based models are viable alternatives. An increasing trend of PM-ETo over Cross River basin further suggests global warming scenario, which is a threat to irrigation management and crop yield.