A comparative evaluation of four evapotranspiration models based on Eddy Covariance measurement over a grass covered surface in Ile-Ife, Southwestern Nigeria

Abstract

An Eddy Covariance (EC) system was set up to measure vertical transport of water vapour fluxes over a grass covered surface at a site located within Obafemi Awolowo university campus (7°33′N, 4°35′E) southwestern Nigeria between 31st of May and 14th of June, 2013. The EC measurement was used as a benchmark to evaluate the performances of four evapotranspiration models (the standardized FAO-56 Penman–Monteith (PM), Priestly-Taylor (PT), Makkink (MK) and Turc) which were employed to estimate evapotranspiration in the study area. The ET estimates from the models showed similar diurnal variation with the direct measurement from EC technique with daytime mean (mm/day) ranging between 0.79 and 2.37 for EC, 1.02–3.75 for PM, 1.58–5.46 for PT, 1.13–4.02 for MK and 1.21–4.27 for Turc. Based on regression analysis and standard error of estimates (SEE), the performances of the models ranked from PM (R = 0.96, slope, b = 0.687, SEE = 0.049), MK (R = 0.97, b = 0.569, SEE = 0.395), Turc (R = 0.97, b = 0.539, SEE = 0.553) to PT (R = 0.97, b = 0.386, SEE = 1.32). Recalibration of models coefficients using least square method showed significant improvement in their estimates and thus, the models were found very reliable for predicting ET which is a relevant parameter for irrigation scheduling.