HydroResearch (Jan 2023)
Parametric calibration of Hargreaves–Samani (HS) reference evapotranspiration equation with different coefficient combinations under the humid environment
Abstract
Evapotranspiration (ET) is a major hydrological component in irrigation system design, irrigation scheduling, and hydrologic and drainage design. Estimating crop ET requires accurate reference evapotranspiration (ET0) measurement. In this work, numerous modified Hargreaves models for ET0 were tested and calibrated against the standard FAO-56 Penman-Monteith eq. (F-56 PM) for further improvement in performance in the humid subtropical environment of Imphal West District, Manipur.The Martinez-Cob and Tejero-Juste (2004) equation resulted in best region-wide ET0 values of 3.536 mm d−1, mean absolute error (MAE) = 0.367 mm d−1, standard error of estimate (SEE) = 0.348 mm d−1, correlation coefficient (r) = 0.855, coefficient of determination (R2) =0.731, and index of agreement (d) = 0.891. Trajkovic (2007) equation has the second-best ET0 values of 3.383, MAE = 0.413 mm d−1, SEE = 0.3.77 mm d−1, r = 0.827, R2 = 0.685, and d = 0.852. The third-best equation, Ravazzani et al. (2012), with average ET0 values of 4.015 mm d−1, MAE = 0.506 mm d−1, SEE = 0.395 mm d−1, r = 0.855, R2 = 0.731, and d = 0.79.The observed wind speed data were sorted into five groups based on the average wind speed in different months in the region (1 m/s,2.5 m/s, 3.5 m/s, 4.5 m/s, and 5.5 m/s), and the estimated evapotranspiration was used to calibrate 14 selected combinations of the Hargreaves models. The calibration process significantly improved the performance of the different identified Hargreaves models.The analysis indicated that the improvement in estimation decreases with increasing wind speed. The revised coefficient predicted evapotranspiration is closer to F-56PM, with MAE (mm d−1) ranging from 0.125 at 1 m/s to 0.248 at 4.5 m/s and SEE(mm d−1) from 0.16 at 1 m/s to 0.245 at 5.5 m/s, as compared to the original Hargreaves equation, whose MAE is 0.543, and SEE is 0.400.More research must be conducted to extend the applicability of this equation to other regions of the State.
