Estimation of monthly reference evapotranspiration (ETO) for areas with in adequate meteorological data in Ethiopia
Often long-term measured meteorological data are not available or in adequate in water resources development areas such as dam and irrigation sites. Under this condition, design estimates of monthly evaporation from resenJoirs and monthly evapotrampiration from crops are unreliable. Estimation of mean monthly evaporation, in this case, can he obtained using regional regression methods that relate evaporation with readi(v available data such as altitude. In this paper monthly reference evapotranspiration (ETo)- altitude regression equations are developed for the country using 125 stations meteorological data. ETo are calculated based on radiation (sunshine hours), temperature, relative humidity, and wind speed data. FA O-Penman Monteith formula is applied to calculate monthly ETo for 125 stations. The developed relationships are valid over the altitude range from 100 to 3000 meters above mean see level. The developed equations can he used in areas (irrigation and dam sites) where meteorological data such as radiation (sunshine hours), temperature, relative humidity, and wind speed are not available. A mean monthly evaporation from reservoirs, ponds, and lakes are also estimated by multiplying the monthly ETo by 1.2. Estimate of monthly crop evapotranspiration is made by multiplying monthly ETo by corresponding monthly crop coefficients.