Modifying Hargreaves-Samani Equation for Estimating Reference Evapotranspiration in Dryland Regions of Amudarya River Basin

Abstract

Reference evapotranspiration (ETo) is a key factor in determining the amount of water needed for crops, which is crucial to correct irrigation planning. FAO Penman-Monteith (EToPM) is among the most popular method to estimate ETo. Apparently sometimes it is difficult to compute ETo using Penman-Monteith due to challenges on data availability. FAO Penman-Monteith method requires many parameters (solar radiation, air temperature, wind speed and humidity), while Hargreaves-Samani method calculates ETo based on air temperature. Because Central Asia is a data limited region with weather stations unable to provide all required parameters for the PM method, this study aimed to estimate ETo using the Hargreaves and Samani (HS) method in Karshi Steppe, in Kashkadarya province, in southern Uzbekistan, based on data from 2011 to 2017. Reference evapotranspiration calculated by non-modified HS method is underestimated during the summer months. The reason for this underestimation might be higher air temperature and wind speed during these months. Therefore, the HS method in its original form cannot be used in our study area to estimate ETo. Modification of the EToHS, through application of a bias correction factor, had better performance and allowed improving the accuracy of the ETo calculation for this region. The calculated ETo values can inform decision making and management practices regarding water allocation, irrigation scheduling and crop selection in dry land regions of Amudarya river basin and the greater Central Asia area.

Prediction of monthly evapotranspiration by artificial neural network model development with Levenberg–Marquardt method in Elazig, Turkey

“…Regarding this issue, Ali Ghorbani et al ( 2018 ) stated that the main disadvantage of empirical models is the lack of consistency, as their accuracy is highly dependent on local calibrations and climatic conditions. Based on the studies of Cobaner et al ( 2017 ) and Gafurov et al ( 2018 ), the simple empirical models lack the universal approach required for ET forecasting. Citakoglu et al ( 2014 ) compared the performance of ANN and ANFIS in estimating the ET values in Turkey.…”

Calibration of empirical equations for estimating reference evapotranspiration in different climates of Iran

“…While the PM-FAO56 equation shows relatively stable results, the equations for empirical models based on temperature, solar radiation and mass transfer use fewer climatic parameters in the calculation of ETref. Therefore, the development and calibration of empirical model equations in different climates can be more effective for agricultural and hydrological projects where only a few climatic variables are available (Heydari and Heydari, 2014;Gafurov et al 2018).…”