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Irrigation Management

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Fruit trees require adequate moisture to maintain growth, take up nutrients and produce high quality fruit. North Central Washington’s semi-arid climate receives only 7-9 inches of precipitation annually, making orchard irrigation imperative. Good irrigation planning and management are essential for orchard health and grower profitability. Scientists at WSU’s Irrigated Agricultural Research and Extension Center (IAREC) are working with growers to develop sustainable irrigation practices, as well as new technologies for better irrigation management decisions. One such tool is the mobile irrigation scheduler. This is a basic soil water budget model, that uses AgWeatherNet. Researchers at the WSU Center for Precision and Automated Agricultural Systems (CPAAS), in collaboration with scientists at U.C. Davis, are currently developing precision canopy and water management of specialty crops through sensor-based decision-making. To learn more visit the project websites for CPAAS or University of California.

The amount of water required by fruit trees depends on the type and age of trees, their spacing in the tree row, soil type, site location, climate, and other environmental variables. Irrigation management influences tree health, the rate of tree growth, utilization of nutrients, fruit size and crop yield.

Newly planted trees need to be watered as soon as possible after planting to avoid water stress, and this is especially important with larger trees and when temperatures are high during or immediately after planting. Growers try to maximize tree growth for the first few years to ensure adequate canopy growth before production starts, so precise irrigation is important. Young trees need a consistent water supply to achieve the greatest growth potential.

There are certain times during the growing season when lack of water can be detrimental to fruit production. The month after full bloom is the time when the most rapid cell division in fruit is occurring. Receiving the right amount of water at this time is critical for good final fruit size and quality. For stone fruits, especially, the final fruit swell phase that occurs approximately two-three weeks before harvest is also an important time for trees to receive enough water for fruit growth. Care needs to be taken during this phase, in sweet cherries, for example, to avoid excess water that can lead to fruit cracking or splitting making them unmarketable.

Choosing the correct irrigation system to deliver the appropriate amounts of water at the needed times is essential. Inadequate water can lead to tree stress, inadequate nutrient uptake and shoot growth, reduced fruit size and quality.
Excess water can lead to erosion, loss of nutrients due to leaching, higher pumping costs, increased root disease and potential adverse off-target environmental impacts when chemicals may be leached into streams and other bodies of water.

The goal of irrigation is to recharge the available water in the top foot of soil. Soil water is held in the pores and empty spaces around soil particles. When all of the pore space is completely filled, the soil is saturated. The available water holding capacity (WHC) is the amount of water a particular soil type will hold for plants to use. Different soil types have different water holding capacities. For example, sand has a limited water holding capacity because water will drain quickly through large permeable pores. Clay on the other hand, has a large water holding capacity due to small pores that are not as permeable. Water holding capacity is also affected by the number of trees utilizing the water in the soil, as there is not as much water holding capacity in high density plantings as in low density plantings (reference). Preferred orchard soil is a stable, sandy loam that is granular and porous, enabling good water and air movement, and adequate field capacity (soil water content).  It is important to determine the soil type(s) at your orchard site, when considering the type of irrigation system to install. Soils are quite variable in North Central Washington, and usable water storage can range from 0.8 to 3.5 inches. For more information about soils in orchards please see our Soils and Nutrition webpage.

Resources:

Terminology

Irrigation scheduling is the process of determining when and how much to irrigate. Growers monitor the soil, and the trees, and calculate the water used so that the timing and amounts of water applied are in sync with the requirements of the fruit trees and soil water holding capacity. The WSU irrigation scheduler mobile app is a useful tool to help plan timing.

Evapotranspiration (ET) is the loss of water through evaporation and transpiration. Transpiration is the movement of water through a plant from the soil to roots, up the stem, and out through the leaves. AgWeatherNet’s weather stations located around Washington State measure the weather parameters (solar radiation, air temperature, humidity and wind speed) needed to calculate ET. You can calculate ET using the Penman plant water use calculator here.

Field capacity Water will drain from soil over time to a point where the soil will hold a certain amount of water indefinitely against the downward pull of gravity. This is call field capacity.

Permanent wilting point occurs when the soil dries out to a point where the suction or pull of the soil on the water exceeds the plant’s ability to absorb water. Without subsequent irrigation, plants under this soil moisture condition will wilt and die.

Soil water tension is how hard a dry soil is pulling on soil water, measured using vacuum or pressure units (psi, cbars). For more information on soil water tension, see Irrigation monitoring using soil water tension, C. C. Shock, et. al., Oregon State University Extension publication EM8900, 2013.

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