SIMPLIFIED EQUATIONS FOR DAILY AND SUBHORARY REFERENCE EVAPOTRANSPIRATION IN SOUTHERN BRAZIL
Keywords:
Daily evapotraspiration, ETo, Penman-Monteith.Abstract
Evapotranspiration is an important component of the hydrological cycle. It consists of the transfer of water vapor in a given area conditioned by its climatological and physical characteristics. Hydrological processes in small basins occur in reduced temporal scales, requiring the use of data in hourly or sub-hourly intervals. Monitoring equipment is sensitive. With absence or deficiency in maintenance, they can cause failures in the time series or invalidate results. Thus, simplifications in the complete Penman-Monteith equation (PM, equation 1) in estimating evapotranspiration becomes necessary. The simplifications adopted were: 1) dispensing with moisture data, excluding the aerodynamic term from the PM equation (equation 2); 2) replacement of net radiation by global radiation, so that net longwave radiation is not calculated (equation 3); 3) omission of wind speed (equation 4). Equation 4 was modified, considering the term s / (s + γ) (s: slope of the saturated vapor pressure curve and γ: psychrometric constant) calculated by the daily average temperature. The use of any simplified equation usually yields individual ETo values less reliable. However, if you want to use a simplified equation, Equation 4 or 4 modified is the best choice, depending on whether you need hourly (or shorter term) values (4) or just daily data (4 modified). The ETo values calculated with these equations are the closest to the true values and require the least amount of input parameters.
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