GRASS ET ESTIMATES USING PENMAN-TYPE EQUATIONS IN CENTRAL SUDAN

Irrigation schemes in the central clay plains of Sudan represent a major irrigated area in Africa. Annual irrigation requirements may approach 10(10)m(3). The main irrigation season extends for 9 months (mid-June to mid-March). Off-season irrigation requirements mainly for sugarcane and orchards have to compete with the water supply and hydropower for the usually scarce water resources available in the Blue Nile at this time of the year. Accurate estimates of off-season irrigation requirements become critical for such conditions. A prerequisite for this is to estimate reference evapotranspiration (ET) from meteorological and other data. In this paper four forms of the Penman combination equation were compared with measured ET data from a clipped grass lawn. The Penman-Monteith method gave the best agreement with measured data, followed by the FAO-Penman (used with the original Penman wind function) and Penman-Watts-Hancock methods, which gave almost identical results. The Penman 1963 method was the least satisfactory among the methods tested; however, its performance improved significantly when the saturation vapor pressure was calculated as the mean value at maximum and minimum air temperatures. The results of this study lend further support to the recommendations of "Evapotranspiration 1990" and "The FAO Panel of Experts-1991Both documents recommended the Penman-Monteith method for general use.