The winemaker watches closely the amount of water given to the grape vines during the growing season.
Too little irrigation and the vines will suffer excessively from water stress. Without adequate soil moisture normal transpiration of water from the soil, through the roots, through the vine, and out the stomata on the underside of the leaves is shut down. Transpiration is necessary for photosynthesis. Photosynthesis is necessary for the production of sugar, and sugar based products that give grapes their color and flavor. Excessive stress means grapes do not ripen properly. Often the last part of ripening, where sugars increase to the levels you want for winemaking, water stress may cause sugar increase to come from dehydration, not photosynthesis. The wines that result from such grapes are often unbalanced and coarse.
The opposite can also be problem. Give the vines too much water and the vines react by growing vegetative. The vine makes shoots, and leaves, and grows larger in size. The vine focuses its energies on growth, not on reproduction. With the vines energies focused on vegetative growth grapes do not ripen properly. Resultant wines can have herbaceous, vegetable like aromas and weak or thin flavors.
Given sufficient, but restricted soil moisture the vine exhibits controlled growth and during ripening produces attractive aromas and flavors. Proper control of this factor comes with careful observation and experience. Each year the way the vines grow and the wines they make tell the winemaker how well they have done. Each year the winemaker will use this new information to more finely tune the vines performance the following year.
Fortunately, the well drained, hillside soils on Spring Mountain are just as forgiving as those in Bordeaux if they get too much water.The advantage a Spring Mountain winemaker has over a Bordeaux winemaker is that on Spring Mountain they can control when, where, and how long it "rains." The moisture from drip irrigation doesn't harm the fruit, as rainfall can.