the first month of the season, when the water management between these two practices was different, indicated that K c and water use were lower in DS systems relative to WS systems when there was only one irrigation flush during this period, while two or three irrigation flushes resulted in similar values between the two systems.

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 . 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

Abstract. The impact of climate change and its relation with evapotranspiration was evaluated in the Duero River Basin (Spain). The study shows possible future situations 50 yr from now from the reference evapotranspiration (ET o ). The maximum temperature (T max ), minimum temperature (T min ), dew point (T d ), wind speed (U ) and net radiation (R n ) trends during the 1980-2009 period were obtained and extrapolated with the FAO-56 Penman-Montheith equation to estimate ET o . Changes in stomatal resistance in response to increases in CO 2 were also considered. Four scenarios were done, taking the concentration of CO 2 and the period analyzed (annual or monthly) into consideration. The scenarios studied showed the changes in ET o as a consequence of the annual and monthly trends in the variables T max , T min , T d , U and R n with current and future CO 2 concentrations (372 ppm and 550 ppm). The future ET o showed increases between 118 mm (11 %) and 55 mm (5 %) with respect to the current situation of the river basin at 1042 mm. The months most affected by climate change are May, June, July, August and September, which also coincide with the maximum water needs of the basin's crops.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 season stages (K cend ) were 0.92, 1.06, and 1.03, respectively, the length of 1 these stages being about 55, 45 and 25 days, respectively. 2

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