Management of irrigation water in Al-Baha, Saudi Arabia, using a simple alternative to the Penman–Monteith equation

Estimating reference crop evapotranspiration (ET_o) is very important in the planning and scheduling of irrigation water. A simple method for the estimation of ET_o is needed, with fewer meteorological factors that are required for estimation using the Penmen–Monteith equation (PME). The aim of this study was to manage irrigation water in the Al-Baha region of Saudi Arabia using a simple equation to estimate ET_o based on air temperature as an effective alternative to the PME. Four simple temperature-dependent equations proposed by Thornthwaite, Blaney-Criddle, Hamon and Linacre were selected for analysis. Of these four equations, the Linacre equation (LE) was found to be best for estimating ET_o. Although the ET_o rate was overestimated when using the LE compared with the PME, it had the lowest error percentage (9.55%) and the highest R² (0.97). After deducing a new and accurate equation based on the LE with the ability to estimate the ET_o rate, the modified LE produced values closest to the results obtained using the PME for the Al-Baha region. It can thus be relied upon for estimating irrigation needs in the Al-Baha region with the irrigation systems used in the region.