Responses of reference evapotranspiration to changes in atmospheric humidity and air temperature in Spain

We studied the sensitivity of reference evapotranspiration (ET o ) to global warming in Spain at the end of the 21st century. The FAO-56 Penman-Monteith equation was used to estimate ET o , and we examined the sensitivity of the latter to changes in temperature and relative humidity. Changes in stomatal resistance in response to increased CO 2 concentration were not evaluated, nor were the changes in wind velocity and solar radiation. Different scenarios were used for estimation of future ET o in different river basins as a consequence of trends in the maximum and minimum temperatures and maximum and minimum humidities during the period 1973-2002, as observed from 38 meteorological stations. The temperature increases ranged between 0.3 and 0.7C decade -1 , and the relative humidities fluctuated between 0.1 and -3.7% decade -1 . Four scenarios were simulated that considered the variations in linear tendency of the maximum and minimum temperatures and maximum and minimum relative humidities. The trends of the 4 scenarios were incorporated with the data from 338 agrometeorological stations to estimate future ET o . In all cases, there was an annual increase in ET o of 11, 21, 36 and 7% above the annual ET o (1196 mm) for Scenarios 0, 1, 2 and 3, respectively. The river basin most affected by these changes was the Ebro River valley. The most affected months were May, June, July and August, while the least affected months were November, December and January.