Abstract
The Penman-Monteith equation is often recommended as the standard for both computing the reference evapotranspiration (ET0) and evaluating other ET0 equations. The main shortcoming of this equation is it requires weather data, e.g. solar radiation or sunshine hours (n), that are not always available. Since neither solar radiation nor n data are available in Libya, the main objective of the present study was to develop a reliable equation for estimating the sunshine hours (n(e)) to estimate monthly ET0 in arid regions. In this study, three equations were developed for predicting n(e) based on mean monthly temperature, relative humidity and wind speed. The derived equations were used for calculating the monthly ET0 based on the Penman-Monteith equation. The ET0 estimates were compared with ET0 calculated using observed n. The results indicate that the ET0 calculated using n(e) matched well with the ET0 calculated by n. The ET0 calculated by the three equations were tested against ET0 calculated by both the California Irrigation Management Information System (CIMIS) and Hargreaves-Samani equation. The ET0 based on estimated sunshine was closest to ET0 obtained from CIMIS compared to the Hargreaves-Samani equation. In conclusion, the equation to estimate sunshine hours should provide good estimates of the ET0 for similar desert locations that lack sunshine hours or solar radiation data. Copyright (c) 2015 John Wiley & Sons, Ltd.