ORIGINAL PAPER Hussein M. Al-Ghobari Estimation of reference evapotranspiration for southern region of Saudi Arabia Received: 16 January 1998 Abstract The reference crop evapotranspiration (ET r ) for four areas in Saudi Arabia was estimated using ®ve dierent methods: FAO-Penman, Jensen-Haise, Blaney & Criddle, pan evaporation, and calibrated FAO-Pen- man under local conditions (Penman -SA ). Comparison was also made between the estimated ET r and the measured ET r of alfalfa grown in lysimeters in the Ri- yadh area. Regression analysis revealed that estimated ET r values were highly correlated with measured ET r values. In addition, linear regression relationships be- tween ET r values estimated by the Penman -SA method and other methods were determined. The results of this study indicated that the calibrated Penman -SA method can be transferred successfully to other locations, and this method could be used for the estimation of ET r values in all areas in the southern region of Saudi Ara- bia. Introduction With growing population, urbanization and irrigated agriculture in arid regions in general and in Saudi Ara- bia in particular, water shortages are increasing. As a result of increasing demand for water resources, com- petition for existing water supplies is becoming more critical each year, calling for wiser use of the limited available water. In Saudi Arabia, the agriculture sector accounts for more than 80% of the total annual water consumption. As demand intensi®es the eective con- servation of water is of primary importance to agricul- tural development. Finding methods that increase water use eciency and reduce the excessive application of water are of importance for conserving water. The knowledge of crop evapotranspiration (ET) is one of the most important factors in irrigation scheduling, proper water management and water conservation. The estimation of ET involves calculating the po- tential ET or the reference crop ET (ET r ), and then applying a suitable crop coecient (K c ). Potential ET is de®ned as the rate at which water would be removed from wet soil and plant surfaces expressed as the rate of latent heat transfer per unit area, or as a depth of water per unit time. ET r is de®ned as the rate at which water would be removed from the soil and plant surfaces ex- pressed as the rate of latent heat transfer per unit area, or as a depth of water per unit time evaporated and transpired from a reference crop. The use of ET r for a speci®ed crop surface has largely replaced the use of the more general potential crop ET. This is because of the ambiguities involved in the interpretation of potential ET. Also, the use of a reference crop ET permits a physically realistic characterization of the eect of the microclimate of a ®eld on the evaporative transfer of water from the soil-plant system to the atmospheric air layers overlying the ®eld (Wright 1996). Alfalfa and grass are commonly used as reference ET surfaces, and the alfalfa ET r has been used more for arid climates (Wright and Jensen 1972, 1978; Allen et al. 1989; Jensen et al. 1990; Abo-Ghobar and Mohammad 1995). Alfalfa has higher ET r rates in arid areas where there is con- siderable advective sensible heat input from the air. ET r obtained with such an alfalfa surface will usually be greater than that for a clipped grass surface, particularly in windy arid areas (Burman et al. 1981). Therefore, it can be advantageously used as a reference crop in arid areas (Wright and Jensen 1972). Numerous scientists and specialists worldwide have developed many methods for estimating ET r over the last 50 years. These methods were subject to rigorous local calibration and proved to have limited global va- lidity (Smith et al. 1996). Doorenbos and Pruitt (1977) adopted the concept ET r and adjusted several existing methods to yield identical ET r estimates varying from Irrig Sci (2000) 19: 81 ± 86 Ó Springer-Verlag 2000 H.M. Al-Ghobari Agricultural Engineering Department, College of Agriculture, P. O. Box 2460, King Saud University, Riyadh 11451, Saudi Arabia e-mail: hghobari@ksu.edu.sa, Fax:+966-1-4678502