In order to make the best use of the available water this year, modification of orchard management practices to incorporate principles of water efficiency will be necessary. This involves understanding the conditions and limitations of the irrigation system and soil, how to calculate irrigation timings and amounts, and the adoption of practices that improve water penetration.
• Use of evapotranspiration data
The amount of water that is lost daily to soil moisture evaporation and tree transpiration can be calculated using climate data and then multiplied by the crop coefficient to determine how much water is lost by the respective crop. The daily evapo‐transpiration rates (ETc) are then added up to determine how much water needs to be replaced through irrigation. Watering less than this amount will lead to a deficit, forcing the tree to tap deep moisture soil reserves. Too many repeated irrigation deficits will deplete the deep soil moisture reserve and lead to drought stress and poor tree vigor.
Daily calculated ETc are available through the CIMIS Web site http://wwwcimis.water.ca.gov/cimis/welcome.jsp. Historical ETc rates can also be used to help calculate a water budget. Average ETc values for almond and walnut are on pages five and six of http://gwpa.uckac.edu/pdf/Estimating_Crop_Water_Use.pdf.
• Irrigation system efficiencies
Almond, walnut, and pistachio trees use 39, 42, and 41 inches of water annually, respectively. Inefficiencies within the differing irrigation systems affect the amount of water that is delivered into the soil profile. Sub‐surface drip irrigation is the most efficient in water usage (95 percent to 98 percent efficiency), followed by micro-sprinklers (85 percent to 95 percent), solid‐set sprinklers (75 percent to 85 percent), and then furrow and flood irrigation (60 percent to 75 percent).
Therefore, system inefficiencies must be taken in consideration when irrigating. For example, a 3-inch irrigation through a micro-sprinkler system only delivers 2.55‐2.85 inches of water to the tree. Losses occur through evaporation, leaks within the system, clogged pipes, and dysfunctional sprinkler heads. Furthermore, irrigating on windy days will lose more water to evaporation – especially in micro-sprinkler and sprinkler irrigation systems.
• Soil water holding capacity
Depending upon texture, soils may hold more or less water per foot of soil. Broadly, sandy soils hold the least amount of water per foot of soil (0.5‐0.7 inches), followed by loamy sand (0.7‐1.1 inches), sandy loams (0.8‐1.4 inches), loams (1.0‐1.8 inches), silt loams (1.2‐1.8 inches), clay loams (1.3‐2.1 inches), and clay soils (1.4‐2.4 inches). Applying too much water to coarse soils may lead to water percolation beyond the tree’s root zone. Therefore, irrigations should never exceed the amount of water that is held in 5 feet of the soil profile.
For example, an irrigation set of 4 applied acre inches on sandy soil moves water out of the rooting zone as only 2.5‐3.5 inches of water can be held in the first 5 feet of the rooting zone. Over-application of water can be used to leach salts from the rooting zone, but this practice is not advised when water is of limited supply. Orchards on coarser soils may need to irrigate less water more frequently to ensure that water is within reach of the tree’s roots.
• Timing of irrigations
The first irrigation of the growing season is the most difficult to determine. Over-watering during this period has caused tree and yield loss due to an increased incidence of Phytophthora root and crown rot and lower limb dieback. By using a pressure chamber, the tree’s “pull” of water from the soil can be measured.
The first irrigation should occur when the tree begins to “pull” water. This is determined by a pressure chamber reading more negative than the fully irrigated baseline of ‐6 bars in almond and ‐2 bars in walnut.
Subsequent irrigations are determined by using ETc calculations, soil measurements, and/or pressure chamber readings.
Irrigations should occur when the tree uses around 50 percent to 60 percent of the water within the effective rooting depth. Assuming a uniform, well‐drained soil, the effective rooting depth is estimated between 4‐5 feet for almond, 4‐6 feet for walnut, and greater than 6 feet for pistachio. Constraints such as hardpans and soil texture changes may limit the depth of the tree roots. Proper pre‐plant soil modification is needed to prevent this occurrence. Use of a soil auger or backhoe can determine if soil stratification occurs within the field.
• Improving soil water penetration
Orchard management practices should include methods of improving soil water penetration and holding capacity. Soil types vary in physical aspects that aid or prevent water penetration. Problems with water penetrations are often experienced with finer texture soils because the soil often “seals” up and prevents the flow of water into the soil.
This may be observed about a day after irrigation when the first 6 inches to 1 foot of the soil is water saturated and the deeper soil is dry. Calcium (gypsum) can be used to improve water penetration. Being a positively charged ion, it binds to the soil to keep the soil from binding to itself, providing pores for the water to move through. Amendments of calcium are not always needed to improve water penetration.
The addition of organic matter has been demonstrated to increase the water penetration rate in annual crops. Organic material increases water penetration through two ways; large objects such as plant parts and stems provide pores for the water to enter the soil, while the breakdown of organic material results in the formation of positively charged acids (humic and fulvic acids) which bind to the soil in ways similar to calcium.
Soils with high levels of organic matter have also been shown to have a higher water holding capacity. In this case, the organic matter acts similar to a sponge, holding more water within the rooting profile of the plant. Within perennial cropping systems, it is thought that organic material provides similar benefits, but research has not been completed in these areas.
Soils with poor water infiltration rates need to be watered less more frequently. It is best to match the irrigation rate with the rate that the water infiltrates into the soil. Most soils can be irrigated between 2‐4 inches. Few soils can handle irrigations larger than 4 inches.
As orchard systems continue to evolve and improve, it is important to keep in mind the above principles to increase the efficiency of water usage. Providing the right amount of water at the right time and ensuring the delivery of that water into the rooting profile is essential to maintain a healthy and productive orchard.
Advancements in irrigation technology appear to be moving into systems that improve timing of water applications. Adoption of these technologies will be reliant upon the ability to efficiently provide trees with water at the prescribed times. If any questions should arise, do not hesitate to contact your local farm advisor.