SPATIAL AND TEMPORAL CHARACTERISTICS OF REFERENCEEVAPOTRANSPIRATION IN AMHARA REGION, NORTHERN ETHIOPIA

The FAO Penman-Monteith equation is the standard method which is commonly used for estimating reference evapotranspiration (ETo) in most parts of the world. This method requires use of full weather data measured at the principal stations. Here, we used 25 principal stations in the Amhara region to calculate ETo. In addition to this, we tested Hargreaves, Thornthwaite and Blaney-Criddle techniques to calculate ETo so as to choose best ETo estimating model for the region at 19 temperature stations using Penman-Monteith as a standard. The statistical analysis of the results of these models showed that Hargreaves model can be safely used among the tested techniques. As a result, this model is then calibrated and empirical coefficients at each of the temperature stations were determined for estimating ETo. Finally, ISO-ETo map for the Amhara region is developed using thin-plate smoothing splines interpolation method in Geo-statistical analysis based on a total of 44 stations. This work can be used as an input for agricultural research and development in water resource planning, irrigation scheduling, design of irrigation schemes and hydrological studies in the Amhara region.