Considerations towards the start of irrigation for deciduous trees

Cameron Zuber, Orchard Crops Farm Advisor, Merced and Madera Counties

While we cannot control the amount of precipitation nor when it occurs during the fall and winter, this can dictate when to start irrigating deciduous trees at the beginning of the season as the amount and timing of precipitation affects the amount of water in the soil that is available to trees. Irrigation is needed when the soil moisture is depleted to a point where trees begin to be too water stressed. Water is depleted in the soil from two processes, soil evaporation and plant transpiration which are combined into the term evapotranspiration.

Deciduous trees enter a dormancy period in the fall where minimal evapotranspiration occurs. Eventually, leaves emerge, evapotranspiration increases, and water is depleted in the soil. Understanding when the soil becomes too dry, and thus when to start irrigating, understanding both rate of water loss through evapotranspiration and how much water is in the soil is important.

Photos of almond tree during dormancy period (left) and after leaf emergence (right).

While the rate of evapotranspiration can be calculated using variables like the observed evapotranspiration from the California Irrigation Management Information System (CIMIS), this only determines how much water may be lost from the soil. It does not determine the amount of water in the soil nor when it becomes too dry. This would be like knowing the miles per gallon of gas a vehicle uses but covering the gas gauge when driving. You need to know how much is being used and how much is left to accurately determine how far the vehicle can go.

The same is true when irrigating. The water in the soil is the gas in the tank and the evapotranspiration is the amount of water or gas being used. Both are important as only determining one may result in lost water from overirrigating or water stressed trees from irrigating too late.

The irrigation schedule from an almond research site in Parlier, CA, provides an example on why understanding the amount of water in soil heading into the growing season is important. At the end of March in 2020 and 2021 the soil moisture at a shallow depth was 2.0 and 1.2 inches of water per foot of soil depth, respectively. Basically, at a similar point in time the gas tank in 2020 had nearly twice as much gas as it did in 2021.

The amount of evapotranspiration was similar for this period. Using CIMIS measurements in April, almond evapotranspiration was 3.2 inches in 2020 and 3.9 inches in 2021. However, for this site in 2020 the irrigation was started in early May, while in 2021 it began at the end of March. This difference in start times was primarily based on the difference in soil moisture for those years.

Images of approximate periods of dormancy and irrigation for almond site in Parlier, CA for the 2019-2020 (top) and 2020-2021 (bottom).

If irrigation were started at the same time for both years three to four inches of water may have been wasted in 2020 because there was enough water in the soil to supply the water demand of the trees. Three to four inches is approximately 81,000 to 109,000 gallons of water for each acre of land.

When heading into the growing season it is important to understand how much water may be in your soil, how this may differ year-to-year, and how this affects when to start irrigation. Monitoring soil moisture depletion or plant water stress can provide more site-specific information to inform management choices on when to begin irrigating this season. This then circles back to the original comment at the start of the article, at which point should you start irrigation?

While many trees can survive extremely dry soil conditions, productivity declines. Therefore, determining the allowable depletion is recommended, but no single allowable depletion amount can be recommended for all situations as many factors, like root depth, soil texture, and weather, play a big role on how trees respond to a specific soil moisture. However, when speaking generally, a recommendation of 50% allowable depletion is considered reasonable. Each soil texture can hold a different amount of water that is available to trees. A safe level to start irrigating is when your soil’s available soil moisture is depleted by 50% from its maximum available soil moisture amount.

Table of different soil types’ range, average, and 50% allowable depletion of available soil moisture. Table adapted from UC ANR Almond Production Manual, 1996.

As soil moisture is typically given as an amount of water per foot of soil depth, taking into account the depth of the tree’s roots is important. For woody tree crops, there is a misperception that the rootzone is similar to the height and size of the above ground portion of the tree. This is incorrect, as the majority of a tree’s roots tends to spread out and only go to about 1.5- to two-foot deep for older trees.

Conceptual images of rootzone of woody trees. Left image more correct with majority of roots in first two feet of soil (adjusted from source, University of Minnesota Extension). Right image less correct with roots similar as above ground portions of trees (adjusted from source, Janice Austin).

To begin understanding when to start irrigation at the beginning of the year, first determine the soil moisture at that time. Then the allowable depletion based on your soil type and the suspected depth of tree roots. Once that point is reached begin irrigating. Determining that point can be done my monitoring soil moisture, calculating loss of soil moisture (i.e., evapotranspiration), or monitoring plant water stress.

If you are interested in learning more about irrigating trees please check the links for how to irrigate young almonds, how to use a pressure chamber to monitor plant water stress and inform irrigation management, or other resources regarding irrigation and water management.

Original article written for Merced County Farm Bureau Newsletter. Thank you to C. Scott Stoddard (Merced County Director and Farm Advisor for vegetable crops and soils) for reviewing this article and providing edits.