David A. Goldhamer, Irrigation and Soil Specialist
Department of Land, Air and Water Resources, Davis
UC Cooperative Extension
Division of Agriculture and Natural Resources
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Last reviewed December 19, 2002
Drought Irrigation Strategies for Deciduous Orchards
Irrigation is practiced in the vast majority of California's deciduous orchards since rainfall is insufficient to meet the plant demand for water throughout the season. Scheduling of water applications in normal years usually takes into account knowledge of the following: plant water requirements, root zone depth, soil water holding capacity, and plant response to soil water supply. When the water supply is unrestricted, plant response is considered with respect to how much water can safely be depleted between irrigations - water is applied before plant water deficits can hurt the crop yield or quality. Irrigation schedules are normally designed to meet the water needs of the trees throughout the season.
Limited availability of irrigation water requires fundamental changes in irrigation management for California orchards. Under drought conditions, reduced streamflows result in below-normal reservoir storage, and this usually limits irrigation district water deliveries to a fraction of those available in normal years. At best, growers must supplement their surface water allocations with pumped groundwater, usually increasing irrigation costs. At worst, growers without wells must get through the season with substantially less water than their trees have the potential to use. Especially under the latter scenario, water becomes a precious commodity and knowledge of how trees respond to water deficits is of paramount edge of how trees respond to water deficits is of paramount importance. It's no longer a question of irrigating to prevent plant water deficits as in nomral years; the presumption is that the trees will be deprived of water at certain times during the season. Under these conditions, irrigators must make decisions based on conserving (minimizing the waste of) water; if the tree must be deprived of water, it must only occur when the water deprivation will have the least effect on current season yield and on subsequent seasons' tree performance.
This leaflet provides information on these two most important aspects of irrigation management with restricted water supplies - limiting nonbeneficial water losses and applying water such that plant water deficits are controlled, and occur during times of the season when adverse effects on productivity are minimized.