Performance Evaluation of Class A Pan Coefficient Models to Estimate Reference Evapotranspiration in Mymensingh Region of Bangladesh

Khalid  Mahmud; Md. Sifat  Siddik; Mst. Khadija  Khatun; Md. Touhidul  Islam

Authors

Khalid Mahmud Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
Md. Sifat Siddik Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
Mst. Khadija Khatun Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
Md. Touhidul Islam Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh

Keywords:

Evapotranspiration, Evaporation, Penman-Monteith method, Snyder method

Abstract

Reliable estimation of reference crop evapotranspiration (ET₀) is of great importance for irrigation planning and operation as well as for climatologic and hydrologic studies. This study evaluated the performances of pan evaporation-based ET₀ estimation methods. First, we detected the correlation between observed pan evaporation (E_p) and ET₀ estimated by Penman-Monteith FAO-56 (PMF-56). Second, we estimated ET₀ from E_p using six pan evaporation models (Cuenca, Allen and Pruitt, Snyder, modified Snyder, Pereira and Orang methods) and compared them with ET₀ by the PMF-56 method. The accuracy of the models was assessed based on three performance statistics such as R², mean absolute error, and root mean square error. We used daily meteorological data recorded at the Mymensingh weather station for the period of 2007–2016 to estimate class A pan coefficients (K_p) using the empirical equations proposed by the selected models. Daily ET₀ was then estimated by multiplying the K_p values with the corresponding daily E_p values. Daily E_P and ET₀ values showed moderate correlation whereas monthly values showed high correlation only for February, August, and September. The moderate correlation between daily values is mainly due to the dissimilar response of E_p and ET₀ to their influencing meteorological factors. In estimating daily and monthly ET₀, overall all methods showed poor performances with underestimated PMF-56 ET₀. However, in the case of August ET₀ estimation, we noticed better performances from pan evaporation models in terms of lower errors and high R² (> 0.70). Particularly, the Snyder model ranked first among the selected pan evaporation models as it closely predicted PMF-56 ET₀. So, after necessary calibration, this method can be considered for the estimation of ET₀ under the climatic condition of Bangladesh. To conclude, the findings of this study will be a useful reference for adopting a comparatively easier ET₀ estimation method in the country.