On modeling reference crop evapotranspiration under lack of reliable data over Iran

This study was conducted to evaluate the performance of temperature-based models i.e. temperature-based Penman-Monteith FAO 56 fed with average wind speed (U) value (TPMU) and default U quantity (TPM2), Hargreaves-Samani (HS) and FAO Blaney-Criddle (FBC) against Penman-Monteith FAO 56 (PM) using data recorded in 1993-2015 at 146 sites over Iran. Two statistics i.e. normalized Root Mean Square (nRMSE) and relative Mean Bias Error (rMBE) were calculated to analyze the absolute error and bias magnitude of the temperature-based ET0 estimation, respectively. Except for the December-January-February (DJF), the models gave reliable seasonal estimates (i.e. nRMSE of 30 > %) for the majority of studied areas. At monthly scale, FBC gave poor estimates of ET0 in DJF for more than 60% of semi-arid and sub/humid-humid sites. ET0 in December and January was not also modeled reliably by TPM2 for 61 and 52% of the semi-arid and sub/humid-humid sites, respectively. Hence, application of FBC and TPM2 appears not to be recommendable in cold areas and months over Iran. Overall, TPMU and HS are better suited at all temporal scales under data limitation over the studied areas. In the case of data availability, calculation of TPM with local average U (instead of default quantity of 2 m s−1) is highly likely to improve the estimation accuracy. Seasonal and monthly ET0 were mostly underestimated over the hyper-arid/arid sites during the March-April-May (MAM) and June-July-August (JJA). However, TPM2 and HS overestimated ET0 for the majority of semi-arid and sub-humid/humid areas. The U anomalies were identified as the primary contributing factor to the error in temperature-based ET0 estimation for most cases. TPM2, HS and FBC provided more accurate estimates for the U range of 1.5-2.5 m s−1. These findings are of significant practical importance for agricultural, hydrological and climatic studies and applications under data sparse condition.