POTENTIAL VINEYARD EVAPOTRANSPIRATION (ET) DUE TO GLOBAL WARMING: COMPARISON OF VINEYARD ET AT THREE LOCATIONS IN CALIFORNIA DIFFERING IN MEAN SEASONAL TEMPERATURES

An increase in global temperature has been predicted to increase evaporative demand as it is controlled by temperature, net radiation, wind and relative humidity. This would suggest that vineyard evapotranspiration (ET) and/or irrigation requirements will increase under future climate scenarios. Crop evapotranspiration (ETc) can be calculated using the following equation: ETc=ETo*Kc, where ETo is reference ET and Kc is the crop coefficient. Vineyard ET was estimated during the 2009-growing season at three locations in California: Carneros (southern Napa Valley), Lodi (northern San Joaquin Valley) and Parlier (southern San Joaquin Valley). It was assumed that the vineyard sites used the same cultivar, training system and row spacing. The seasonal Kc values used were a function of degree-days. Mean seasonal (April 1 to October 31) temperatures for 2009 at Carneros, Lodi and Parlier were 15.2, 18.2 and 20.6°C and accumulated degree days (base of 10°C) were 1575, 2147 and 2670, respectively. Seasonal ETo in 2009 was 1136 mm at the Parlier site while ETo was 95 and 84% of that value at Lodi and Carneros, respectively. The increase in the Kc to the seasonal maximum value (0.82) was delayed at the Carneros site, compared to the Lodi and Parlier sites. Estimated ETc was 778, 732 and 599 mm at the Parlier, Lodi and Carneros locations, respectively. A mean seasonal temperature increase of 5.4°C (Carneros/Parlier comparison) resulted in a 70% increase in accumulated degree-days. Using the same two locations comparison, ETo increased ~19% while that for estimated ETc increased ~30%. The increase in ETc from Carneros to Lodi to Parlier was due to both increases in evaporative demand and temperature, the latter accelerating canopy development (affecting the Kc).