Estimation of reference evapotranspiration in a mountainous mediterranean site using the Penman-Monteith equation with limited meteorological data

In recent decades many authors have adopted the Penman- Monteith formula as the standard way to estimate reference evapotranspiration from climate data. The main drawback associated with the Penman-Monteith method is the relatively high data demand: temperature, solar radiation, relative humidity and wind speed are the minimum inputs required by the formula. In the Spanish Pyrenees historical databases usually consist of temperature and rainfall only, although an improvement in data acquisition is expected in terms of parameters monitored. Under situations of data scarcity, some authors recommend the use of less data intensive methods, such as the empirical Hargreaves equation. Other authors suggest that it is better to estimate the missing parameters and to apply the Penman-Monteith equation. This paper presents a study on the accuracy of the Penman-Monteith method for a situation where some parameters have to be estimated from available temperature, and wind speed data must be replaced by a constant value. The results have been compared with the information available for a location in the central Spanish Pyrenees (period 1999-2003) where parameters required by the Penman-Monteith method have been monitored. A comparison is then made with the Hargreaves model in order to assess the most appropriate method for calculation of the reference evapotranspiration. The results indicate that the use of the Penman-Monteith formula results in errors in reference evapotranspiration ETo estimation of different magnitude and sign throughout the year. However, in general, it offers a more accurate estimation of reference evapotranspiration than the Hargreaves formula.