Issue link: http://winesandvines.uberflip.com/i/66144
GRAPE GRO WING Determine the coefficient R esearch has shown that vine water use is correlated to the amount of shade that the vine canopy casts on the vineyard floor in the middle of the day. When you know this, you can use this equation to determine the crop coefficient (equation at right). As an example, let's imagine the average shade on a vineyard floor. If the vines shade 50% of the area, then we can calculate crop coefficient (example at right). Water is applied daily by the California Irrigation Manage- ment System (CIMIS) and can be accessed online and in some local newspapers. (It is also possible to purchase weather stations to measure this in your own vineyard, but it is expensive.) The larger the canopy of the vines, the more water the vine will use. In general, winegrape vines do not use the amount of water determined by ETo (they use less in most cases.) To schedule irrigation, you must determine a crop coefficient (Kc), which is measured as a percentage of ETo. You need to know how long it takes to apply a given amount of water to your vineyard. Since evapotranspiration is de- scribed in inches or millimeters, you will taking this into account: ETo (in inches or millimeters) x Kc (in percent) = the amount of water that will be applied when you irrigate relative to measured evapotranspiration. Applying more water than that amount will stimulate extra vine growth and reduce fruit quality. Crop coefficient Kc = 0.002 + (the percent shaded area x 0.017) Example: vines shade 50% of area Kc = 0.002 + (50 x 0.017) = 0.852 need to know the precipitation rate at which you are applying water. With this information, you can determine how much water is stored in your soil, how much water is being depleted by the vines due to evapotranspiration and how much water you need to apply to replace the wa- ter removed by the vines. There are work- sheets available that can be downloaded (see "Additional Resources" on page 65) to help you with your scheduling. The analogy to a bank account is a good one: You start with a balance (stored moisture), you write a check (water use by the vines), and then you need to replace the money that you re- moved if you are going to continue to write new checks (irrigating the vineyard). How and when to start irrigation Growers use a variety of techniques to de- termine when to start irrigation including visual inspections (elongation of shoots, shoot tip condition), but by far the most accurate technique is the pressure cham- ber (pressure bomb.) PMS Instrument Co. of Albany, Ore., is the best-known wine industry supplier of the chambers, which determine leaf water potential expressed in bars of tension. Basically, this is a mea- surement of how tightly the vine is holding onto water—the more stressed the vine, the more tightly water is held in the leaf. For most red varieties, irrigation is initi- ated when leaf water potential is measured between -11 and -13 bars (this varies be- Read it, Search it, Share it, Download it! (866) 453-9701 winesandvines-digital.com 64 Wines & Vines AUGUsT 201 1