Article review: Crop coefficients and water use of young almond orchards.

Article reviewed: Drechsler, K., A. Fulton, and I. Kisekka. 2022. Crop coefficients and water use of young almond orchards. Irrigation science.Young almond tree water use has always been of interest. Within my time at the UC, a lot of time was spent evaluating different fertility and irrigation treatments on newly planted orchards. This was often done in combination with fumigation studies. Now, with the management of several thousand acres of young trees, it is always interesting to learn how to improve irrigation management.This topic was revisited recently by a team of researchers from UC Davis. Using the energy balance method, Drechsler and colleagues (2022) determined water use of several developing almond orchards. Through this work, they were able to refine estimates of young tree water use.Their methods were relatively straight forward. Three orchards in the Sacramento valley of California were followed for multiple years and ranged in age from one to five years. All the orchards were part of the same farming operation, spaced 6.7mx4.3m (22’x14’) and were irrigated using micro-sprinklers at 36.7 liters/hr (9.7 gph).  Plant stress was measured using mid-day stem water potential and Tule.Total water use within the orchards was monitored using energy flux towers. These instruments measure the small changes in temperature that occur within the orchard. When accounting for the temperature of the soil, plant, amount of solar radiation, and climate conditions, they can determine the amount of water lost through evaporation and transpiration. This is due to the cooling effect that water has as it evaporates. Measurements are taken very frequently, then eventually averaged over a specific period to determine total water use. To determine the crop coefficient value, the baseline evapotranspiration (ETo) is calculated from the weather station data and back calculated out of the equation. An additional method, soil water balance, was also used to confirm water use. This method utilized neutron probe readings and water flow meters to measure the depletion of the stored soil moisture and the amount of water applied to the field.To account for canopy expansion, light interception of the tree canopy taken at mid-day (PAR) was measured. These measurements were taken in early to mid-June. This variable is critical in determining water needs for orchards outside of the study area. Water use can be divided by PAR to determine how much water a tree needs for the expanding canopy.The developing orchards grew relatively quickly. The two-year-old orchard had 9% canopy coverage, while the 3^rd, 4^th, and 5^th year measurements averaged 24%, 35%, and 55%, respectively. The team found that water use increased as the canopy size increased. The mid-season crop coefficients for the first five years of the developing orchards were found to be equivalent to 33%, 52%, 84%, 98% and 93% of mature orchard crop coefficient values, respectively. These values show a rapid increase in water demand for young trees to maximize growth. Figure 1 shows the linear correlation, in which 50% light interception indicates that the orchard requires 100% of mature orchard water use to prevent water stress.This study of young orchard water use within California is the first in 40 years. The last study evaluating young orchard water use occurred in Colusa and Fresno Counties between 1978-1981 and was conducted by E. Fereres and colleagues (1982). This study utilized the soil water balance method to determine water use on trees aged between 1 and 6 years over the three-year study period. This study found that young orchard water use changed based on the developing canopy, and they concluded that young orchard ETc reached mature orchard ETc at 50% canopy coverage (figure 2). Additionally, they determined that the proportion of mature orchard water use is about 2.5 times the shaded area of the developing orchard. This means that 10%, 20%, and 30% canopy coverage would require 25%, 50%, and 75% of mature water use.The work by Dreschler et al, 2022 supports the results of Fereres, et al. Additionally, Dreschler et al’s research provides refinement of the proportional water needs of smaller canopied trees, indicating that more water is needed for younger orchards than previously thought. This difference may be due to the increased tree vigor and rate of canopy development within modern orchards. Tree density, rootstocks, irrigation systems, and fertility programs were different between these two studies, influencing tree vigor and, most likely, the associated water use. This modern work provides a critical update to our understanding of young orchard water use.The similarity of the results between the two studies indicate that young almond trees use water at a disproportional amount when accounting canopy coverage in comparison to mature almond orchards. As such, it should be assumed that young orchards, especially if the canopy size is greater than 40%, will be impacted as much by water stress as mature orchards. Therefore, cutting water applications on young orchards in time of drought most likely will have the same outcome as stressing mature orchards, impacting both current and future yield potential (Fereres, et al, 1981).Citations and works referenced:

• Drechsler, K., A. Fulton, and I. Kisekka. 2022. Crop coefficients and water use of young almond orchards. Irrigation science. https:\\doi.org/10.1007/s00271-022-00786-y
• Fereres, E., T. Aldrich, H. Schulbach, D. martinich. 1981. Responses of young almond trees to late season drought. California Agriculture: July-August.
• Fereres, E., D. Martinich, T. Aldrich, J. Castel, E. Holzapfel, and H. Schulbach. 1982. Drip irrigation saves money in young almond orchards. California Agriculture, September-October 1982.