Irrigation

Fruit and nut trees will grow well if irrigated regularly. Drought stress will reduce fruit size and stunt growth especially in young trees. If the water status of the plant is severely deficient the leaves will wilt, curl, and sunburn. The fruit can be dramatically affected, too, through reduction in size, water loss and shrivel, and sunburn.

Good irrigation practices in California include the application of water at sufficient intervals in order to never induce significant plant stress. This will ensure the maximum plant growth, fruit size, and yield. In some circumstances, however, a slight water stress induced at specific growth stages can improve fruit flavor, enhance sugar or oil content, and limit vegetative growth.

Water quality may be an issue in parts of California with salt, or mineral excess problems. Irrigation water should be tested for its mineral content to avoid toxicity to plants if there is a problem expected.

More trees are lost to over-irrigating than probably to any other cause. Over-irrigation combined with poor drainage especially leads to tree death. For the period after leaf drop in the fall and until shoot and leaf growth get underway in the spring, trees normally will not need irrigation.

Irrigation recommendations are often stated in the following ways: “irrigate when needed,” “irrigate thoroughly, but not frequently.” The meaning of these statements are unclear for many, especially the novice and those with the tendency to irrigate more when plants appear unhealthy. The key to irrigating any plant is “how much” and “how often.”

Watering Optimally

To water optimally you must know:

1. Daily water use
2. Soil type
3. Amount of water applied
4. The area a plant covers
5. Rooting depth
6. Statistics
7. Efficiency adjustments

1. Daily water use

   Daily water use is called evapotranspiration (ET), which is the amount of water evaporating off the soil surface plus the water used (transpired) by the tree. ET is expressed in inches per day. A typical summer reference ET is 0.3 inches per day. Reference ET is measured or based on a grass crop, which is typically more than the actual ET for trees and other plants. Using a percentage of the reference ET, e.g. 70 or 80% (or 100% with mature trees with cover crop), will closely estimate the actual ET for your own trees or orchard. For most deciduous fruits (not nuts which are higher), mid-summer percentage of reference ET is 86% (110% with cover crop).

   Since the summer days in California are predictably sunny and warm, the [PDF] Water Management Guide for Temperate Fruit Trees, based on evapotranspiration rates and tree size, can be used to determine the water use requirements for fruit trees. In the spring and fall when the days are shorter and cooler the trees need less water.

   The CIMIS map of California ETo zones [265K] characterizes elevational effects that occur in California. Note the differences across those variables. ETo is published in some newspapers and other publications during the season. More information about ETo is also available at the California Department of Water Resources–CIMIS web site.

2. Soil type

   Soil type will help you estimate water holding capacity (inches of water available to plants). Soil texture influences the water holding capacity of soils. The proportion and absolute amount of water available to the plant in coarse-textured, sandy soils is less than in fine-textured, clay soils, therefore clay holds the most water.

   Each foot of soil depth holds:

   Sands	1.0–1.5 inches of water
   Loam	1.5–2.0 inches of water
   Clays	2.0–2.5 inches of water

3. Amount of water applied

   • Drip irrigation: measure emitter output in gal/hr.
   • Sprinkler irrigation: use the “catch can test” to determine in/ft². If sprinkler irrigating, a simple way to determine flow rate in inches is the catch can test: Set several straight-walled cans at different locations along the radius beneath sprinklers. Run the sprinklers for one hour or other known period of time. Measure with a ruler the water collected. The average from the multiple cans will be the flow rate against time.

4. The area a plant covers

   Square feet (ft²) to the drip line. (% canopy)

5. Rooting depth

   Soil depth down to an impermeable layer, usually. Rooting depth varies among plants and is often overlooked in discussing water needs. Roots extract water differently along the vertical profile of soil. The concentration or mass of roots is typically greater near the soil surface. Because of that, water extraction or absorption from soils is greater near the surface. In general, about 40% of the total water extracted occurs in the upper quarter of the root zone, 30% from the second, 20% from the third, and 10% from the lowest quarter.

   Effective rooting depth (ERD) is a term used to note the depth where water absorption is most active. ERD for trees would be a minimum of two feet – more in deeper soils. A number of variables affect ERD including soil texture and depth, tree species, age, rootstock differences, and more.

6. Statistics

   • 1 acre-inch = 27,154 gal
   • 1 acre = 43,650 ft²

7. Efficiency adjustments

   Adjustment must be made for young trees under drip irrigation. Two times more water should be applied to small trees less than 20% full size, gradually reducing the adjustment until trees reach 70% full cover.

Putting It Together

The amount of water a fruit tree uses depends primarily on how big it is and how hot the day is. Several other factors influence water use such as relative humidity and wind, but they are less important. The water use by fruit trees is amazingly similar between species. The goal is maximum growth in the early years to fill the allotted space and maximum production of large fruit. This requires a lot of water in a state where the days are hot and dry. All fruit trees grown for high production have green succulent growth. If the amount of leaves covering an area is the same then the species or variety of tree does not make much difference.

The greatest difference in water use is due to tree size. In the Guide, look at the difference between a tree that occupies 36 ft² (6 ft. × 6 ft.) and one roughly three times that size 100 ft² (10 ft. × 10 ft.). The water use is three times (5.6 gallons per day compared to 15.6 gallons per day).

Water use for a medium sized semi-dwarf fruit tree is about 16 gallons of water per day on a hot summer day on the coast of California without any fog influence (0.25"/day). That same tree in the Sacramento or San Joaquin Valley would be about 19 gallons per day (0.3"/day). Therefore, a tree with two, one-gallon drip emitters on each side would have to be irrigated about 8–9 hours every day.

Drip Irrigation

The theory and practice of drip irrigation is to provide just what the tree needs every day. Not enough water is applied to leave any in storage in the soil for the next day, so it needs to be watered again the next day. Drip irrigation is a good delivery system because it only wets a small area so that weed growth is limited and the system is easily adapted to many landscape situations. Fortunately only a small fraction (10–20%) of the root area needs to be watered in order to achieve good results.

Soil type or depth has very little influence on drip irrigated trees since the water use rate is determined by weather and trees size. Soil water holding capacity is unimportant due to daily irrigations. Based on tree response from irrigation studies, it has been determined that for young trees it is beneficial to irrigate them by a factor of 2 (double) until the trees reach 70% full cover. It seems that “over irrigated” young trees grow even better than if they receive their daily water use allotment based on evapotranspiration. See the example below:

For drip irrigation, start irrigating in early spring before much soil moisture has been used because this stored water may be needed later in case the system is accidentally shut down. Soil type or depth is almost inconsequential, and only 10–20% of the rooting area need be wetted for good tree performance.

Mini/Micro-Sprinklers

Mini-sprinklers are small sprinklers with the water delivered through drip irrigation tubing. Each individual mini-sprinkler usually delivers about 10 gallons per minute or 10 times the average drip emitter. The mini-sprinkler system is typically run twice to three times per week with some water held in the soil in storage. Run times can be calculated (from the Guide), multiplied by number of days between irrigation intervals. Care must be taken to investigate the depth that the irrigation water is reaching for mini-sprinklers since some of them shoot the water so far that they would have to run continuously for days in order to water down 24 inches.

Most fruit tree roots are located between 6 inches and 24 inches of the top of the soil. This is also the area with all the nutrients (topsoil) and the oxygen. Keep this area moist at all times and really focus on maintaining adequate moisture there. The old adage of forcing the tree roots down deep is just that it is forcing the tree and causing stress. Home orchard trees that are on deep soils can get by with less intensive irrigation management because the tree roots are deeper and there is a buffering capacity for drought stress. Shallow soils need to be managed much more intensely with frequent lighter irrigations.

Sprinkler Irrigation

Sprinkler irrigated trees use the same amount of water as drip irrigated trees (which is based on how hot it is) plus an additional 20% for loss to evaporation and non-uniformity of application. The real difference is that with sprinkler irrigated trees, more water is applied at once, it is stored in the soil for 2-3 weeks before the next irrigation, and the entire area is watered. When the whole area under the trees is irrigated, water can not be saved based on tree size. Weed growth also covers a much greater area.

Another important difference for sprinkler irrigated trees is that soil rooting depth (volume of soil) and soil water holding capacity (soil type, sand or clay) becomes important since water is stored in the soil. If trees are over irrigated water is lost beyond the root zone. Under irrigation is usually caused by not running the sprinklers long enough to wet the entire depth of the root zone or miscalculating the amount of water stored in the particular soil type and going too long between irrigation intervals.

For sprinkler irrigation, water is not applied daily, but on a periodic basis to fill the soil, which acts as a storage reservoir for water available to the plant. Soil type and rooting characteristics are very important. Recent research shows beneficial results from irrigating at or before 50-75% depletion of the (soil-stored) available water, then applying what has been used + 20% for efficiency loss.

For a more complete, detailed discussion of this subject, see Micro-Irrigation of Trees and Vines: A Handbook for Water Managers, by Schwankl, Hanson and Prichard, published for University of California Irrigation Program, by the University of California, Davis, 1995.