What´s Reference Evapotranspiration: A Case Stydt in North Khorasan province, Iran

Reference crop evapotranspiration (ETo) is a major component of the hydrologic cycle, and its estimate is widely used in irrigation engineering applications and consequently agricultural water management. In this study, in order to estimate reference crop evapotranspiration ETo and because of few synoptic stations (four stations) and also recorded meteorological data quantity (three stations with less than six years of data) in North Khorasan province, Iran, seven stations from neighbor provinces have been used. Reference evapotranspiration was calculated by six different methods including: Class A pan, Hargreaves-Samani, Priestly-Taylor, Turc, Makkink and Allen et al (1998) proposed method to estimate ETo with missing data (hereafter as EPM). Due to no lysimetric data available, the standard FAO-Penman-Monteith method was used as a best method for evaluation of other models. Since there was no agreement for appropriate method to calculate ETo in selected stations, using significance test of regression lines, a linear regression equation has been suggested for each month and the best calculating method converted into FAO-Penman-Monteith model. Best calculating methods were Hargreaves-Samani in 9 months of year and EPM in the other months. Assessment of proposed equations, showed their appropriate accuracy especially for cold months (mean absolute error values ranged from 0.27 to 2.47 mm day-1). Moreover, high R2 (0.87 to 0.98) and acceptable root mean square error values (0.507-1.44 mm day-1) indicate sufficient accuracy of presented equations in comparison with literatures. Proposed equations can provide ETo estimates with acceptable accuracy using limited data, which will result in improving management of agricultural waters.

Keywords: Semi-Arid Climate, FAO-Penman-Monteith, Hargreaves-Samani, Reference Crop Evapotranspiration, Regression.

1. Introduction

Reference crop evapotranspiration (ETo) is a major component of the hydrologic cycle, and its estimate is widely used in hydrological and irrigation engineering applications (Exner-Kittridge and Rains, 2010). A quantitative evaluation of evapotranspiration is necessary for purposes such as management of agricultural water resources, irrigation scheduling, and environmental assessments in the irrigated areas throughout the world. Increasing competition for water use among consumers has stressed the need for more accurate estimates of reference evapotranspiration.

Lysimeter is the most accurate and reliable method for measuring actual evapotranspiration. Several researchers have compared the accuracy of other methods including FAO-Penman-Monteith (PM) with lysimeter (Bakhtiari et al., 2011 for semi-arid climate of Kerman Province, southeastern parts of Iran; Lopez-Urrea et al., 2006 for semi-arid climate of province of Albacete, Spain; Yoder et al., 2005 for Cumberland Plateau located in the humid Southeast United States; Dehghanisanij et al.