Australian Journal of Basic and Applied Sciences, 5(6): 1058-1069, 2011 ISSN 1991-8178 Modified Hargreaves’ Method as an Alternative to the Penman-monteith Method in the Kingdom of Saudi Arabia 1 ElNesr, M.N., 2 Alazba, A.A., 3 Amin, M.T. 1,3 Assistant Prof. Shaikh Mohammad Alamoudi, Chair for Water Researches, King Saud University, Kingdom of Saudi Arabia. 2 Professor, Shaikh Mohammad Alamoudi, Chair for Water Researches, King Saud University, Kingdom of Saudi Arabia. Abstract: Due to the lack of standard Agro-climatic Weather Stations (AWS) in Saudi Arabia, simple conversion formulas were derived to find the average daily ET o value per month for 29 weather stations. The Hargreaves (HRG) formula was used instead of the Penman-Monteith formula due to the ease of calculation of HRG formula, as it almost require only temperature. For each station, two formulas were derived considering both standard and non-standard AWS. Results show very good statistical representation of the formulas, with average root mean squared deviations of 0.73 and 0.57 and the mean percent errors of 1.22%, and 1.12% while considering the standard and non-standard AWS, respectively. Results also confirm the importance of temperature correction when using data from non-standard AWS, especially for the stations in the Middle to Midwest region (e.g. Arriadh, AlQassim, and Almadina); where the errors of using non-standard AWS data may be up to +24% of the correct ET o value, which results in a massive waste of water. Key words: Evapotranspiration; Weather stations; Hargreaves; Penman Monteith; Temperature correction. INTRODUCTION Estimation of reference evapotranspiration (ET o ) is important for several hydrological and agricultural sciences, like water resources management, crop-water requirements, irrigation scheduling, and land use planning.( Mardikis et al., 2005; and Trajkovic and Gocic, 2010,). Though, a good amount of research is done by the scientists to estimate the ET o accurately, especially for the sensitive fields like irrigation scheduling (Trajkovic, 2007). Due to its comprehensive theoretical base, The Penman–Monteith method (PM) method is recommended by the United Nations Food and Agricultural Organisation (FAO) as the sole method to calculate (ETo) and for evaluating other ET o calculation methods as well (Allen et al., 1998; and Khoob, 2008). The FAO approach to calculate ETo using the PM method was published in the FAO irrigation and drainage paper number 56, hence abbreviated as FAO-56 method. The FAO-56 PM requires measurements of air temperature, relative humidity, solar radiation, and wind speed. The FAO-56 PM equation is expressed as follows (Allen et al., 1998): (1)       2 2 900 0.408 273 1 0.34 n s a a o R G U e e T ET U           Where ET o : reference evapotranspiration [mm day -1 ]; R n : net radiation at the crop surface [MJ m -2 day -1 ]; G: soil heat flux density [MJ m -2 day -1 ]; T a : mean daily air temperature [°C]; U 2 : wind speed at 2 m height [m s -1 ]; (e s - e a ): vapor pressure deficit [kPa]; e s : saturation vapor pressure [kPa]; e a : actual vapor pressure [kPa].; e s : slope vapor pressure curve [kPa °C -1 ]; γ: psychrometric constant [kPa °C -1 ]. The formulas of the equation’s parameters are detailed at Allen et al., (1998), chapters 2 and 3. Nonetheless, the FAO-56 method requires climatic data from standard weather stations. Standard FAO56 weather stations are stations having “a uniform surface of dense, actively growing vegetation having specified height and surface resistance, not short of soil water, and representing an expanse of at least 100 m of the same or similar vegetation” (ASCE-EWRI, Corresponding Author: ElNesr, M.N., Kingdom of Saudi Arabia, Riyadh, King Saud University, POBox: 2460/11451 Riyadh. Phone: +96614673737, Cell: +966544909445, Fax:+96614673739. E-mail: melnesr@ksu.edu.sa or drnesr@gmail.com 1058