MEASURING WATER USE BY JUICE AND WINE GRAPES IN EASTERN WASHINGTON

Authors: Tarara, Julie; Ferguson, John

Submitted to: Proceedings of the Washington State Grape Society
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2002
Publication Date: 7/31/2002
Citation: TARARA, J.M., FERGUSON, J.C. MEASURING WATER USE BY JUICE AND WINE GRAPES IN EASTERN WASHINGTON. PROCEEDINGS OF THE WASHINGTON STATE GRAPE SOCIETY. 2002.

Interpretive Summary: Water resources in eastern Washington are finite and are shared by sometimes competing interests: food production, industry, growing towns and cities, and environmental protection. Vineyard managers recognize the need to improve irrigation efficiency for several reasons. First, grape quality is directly related to irrigation--too little water stresses the vines, reducing yield and quality, but too much water delays ripening and can reduce quality. Furthermore, the cost of pumping irrigation water into vineyards continues to increase. Finally, to meet new standards for water quality, many growers have shifted from rill and furrow irrigation to overhead sprinkler and drip irrigation. These systems improve the efficiency of delivering water to the vineyard and require different management approaches. Grape growers increasingly are seeking science-based "decision aids" to assist them in determining when and how much to irrigate their vineyards. Several science-based methods for managing irrigation currently exist, some of which can be implemented directly on-farm, and others of which growers can access via public agricultural weather networks and web-based services, both private and public. This paper summarized a range of scientific methods available for improving irrigation scheduling and water management. Data were presented on direct measurements of water use by both juice and wine grapes in eastern Washington vineyards. Daily and seasonal patterns of water use by the vine were discussed.

Technical Abstract: Several science-based methods for improving irrigation management in vineyards are summarized. Currently, the most widely-used method for improving the scientific basis of irrigation scheduling is the use of soil water measurements, representing the "supply" portion of the water management equation. Some soil moisture equipment has been cost-prohibitive for small growers, but new technologies and market competition are making soil water measurements more affordable. Irrigation by leaf water potential partly addresses the "demand" end of the water management equation. Leaf water potentials are particularly useful under regulated deficit irrigation, as growers manage water specifically for a pre-determined level of water stress in the vine. The technique is less useful for juice grapes, where water stress is undesirable. Only limited numbers of discrete measurements are possible because of time and technique constraints. Measurements or estimates of ET combine soil- and plant-driven evaporation, and are dependent on accurate crop coefficients. The currently available set of coefficients for grapevines in Washington are for well-watered V. vinifera, so do not reflect the current practice of regulated deficit irrigation, nor any biological differences between V. vinifera and V. labrusca, which is managed under well-watered conditions. One of the first steps in reconstructing valid Kc values is the separation of E from T. Direct measurements of sap flow, which provide T, were collected in both a well-watered V. labrusca vineyard and a deficit-irrigated V. Vinifera vineyard. Diurnal and seasonal courses of water use were discussed.