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102 WINES&VINES January 2016 GRAPEGROWING PRACTICAL WINERY & VINEYARD C alifornia is a thirsty state with an erratic sup- ply of water. The most populous state in the nation, California serves the water needs of approximately 38 million individuals—nearly 12% of the entire U.S. population. California added roughly 3 million people in the past 10 years, and esti- mates of growth for the next 10 years look similar. Taking into account that California's irrigation-fed agricultural sector is vital to the food needs of the state and nation, and considering that climate change could further desta- bilize an already unpredictable water supply, it is impor- tant to find ways to conserve water wherever possible. The expense of irrigating a vineyard impacts the grower's bottom line, and in dry years the concern can intensify from a ques- tion of cost to a question of supply. One example is a directive by the State Water Resources Control Board in 2009 for a 25% reduc- tion in water use in Sonoma County and a 50% reduction in Mendocino County. 11 Although these reductions were not solely directed at viticulture, an effort to reduce water use by grapegrowers was initiated using a public dem- onstration of the water savings possible through improved irriga- tion regimes and soil-moisture monitoring. 6 The use of cultural practices to limit water use has been shown to have dramatic effects on the amount of crop produced per unit of water supplied. One study in the arid, sandy soils of the Okanagan Valley of British Columbia converted sprinkler- irrigated vineyards to drip and reduced applied water by ap- proximately 64%. 2 Although yields dropped approximately 40% in the first year, in the three subsequent years yields were equivalent. Presumably, in the first year of cultural change, the vines were adapting to the new conditions by increasing the root density in the small volume of soil wetted below the emitters. But irrigation reductions usu- ally come with a penalty. The strong relationship between water availability and crop productivity is, by now, an agricultural princi- ple. A plant with more available water will transpire more during a growing season, and the rela- tionship between transpired water and dry matter produced by plants per unit area can be de- scribed with simple formulas. This formula is simplest when condi- t i o n s a r e r e l a t i v e l y h u m i d , wherein the relationship is: plant dry matter = total transpiration X n, where "n" is a constant value. 1,4 This relationship is apparent in irrigation studies with grapes. 7 When own-rooted Thompson Seedless was irrigated such that total evaporation plus transpira- tion was reduced approximately 25% from a maximum value, yield declined approximately 15%. 7 When all irrigation was withheld, total evaporation plus transpira- tion was reduced by approxi- mately 60%, and yield declined approximately 40%. 7 In a Sonoma County field demonstration using soil-moisture probes and plant water-stress measures to better gauge water application, irrigation was reduced 60% to 100%, but yield loss occurred in all treat- ments, ranging from 13% to 42%. 6 Complexities in water use If pursued no further, the excel- lent studies that document the Breeding for Drought-Tolerant Vines Understanding and exploiting differences in root architecture By Kevin Fort and Andrew Walker A Riparia Gloire vine displays lateral root growth following four weeks of green- house culture in a rhizotron. The green stem at top-center is a vine trunk.