An analysis of the tendency of reference evapotranspiration estimates and other climate variables during the last 45 years in Southern Spain

Climate change will have important implications in the agriculture of semi-arid regions, such as Southern Spain, where the expected warmer and drier conditions might augment crop water demand. To evaluate these effects, a data set consisting of observed daily values of air temperature, relative humidity, sunshine duration and wind speed from eight weather stations in Andalusia and covering the period 1960-2005 was used for estimating reference evapotranspiration (ETo). ETo was calculated using five methods: the more complex Penman-Monteith FAO-56 (PM) equation, considered as a reference in this study, and four alternative methods with fewer data requirements, Hargreaves, Blaney-Criddle, Radiation and Priestley-Taylor. These methods were compared to PM with respect to ETo average values and trends. The non-parametric Mann-Kendall test was used to evaluate annual and seasonal trends in the main climate variables and ETo. Due to increases in air temperature and solar radiation, and decreases in relative humidity, statistically significant increases in PM-ETo were detected (up to 3.5 mm year-1). Although the Hargreaves equation provided the closest average values to PM, this method did not detect any ETo trend. On the other hand, trends found from Blaney-Criddle and Radiation ETo values were similar to those obtained from PM. In addition, after a local adjustment, these two methods gave accurate ETo average values. Therefore, Blaney-Criddle and Radiation methods have shown themselves to be the most accurate approaches for ETo determination in climate change studies, when available data provided by climate models are limited.