At each irrigation, enough water is applied to refill 50% of plant available water (PAW) in our targeted root zone volume around each plant: a 1 m-wide, 0.5 m-deep cylinder. Irrigating at this stage prevents undue stress on plants, and is the practice used most often by “smart” controllers. The information on plant available water for each site was accessed from the online SoilWeb app that provides USGS Soil Survey information. Based on this data, the wetted soil volume in Davis, for example, contains 19.41 gallons or 3.68 in. of plant available water. Therefore we irrigate when 1.84” has been removed, applying 9.8 gal, which is the closest to 50% of 19.41 gal that is practical with our system.
|Site||Davis||South Coast REC|
|Soil type||Yolo silty clay loam||San Emigdio fine sandy loam|
|Plant Available Water||Volume of Wetted Area (L)||392.7||392.7|
|Available Water Storage (%)||19%||15%|
|Plant Available Water (gal)||19.41||15.56|
|Irrigation Amounts||% depletion of PAW before irrigation||50%||50%|
|Volume of water to be applied (in)||1.84||1.48|
|Volume of water to be applied (gal)||9.705||9.337|
Researchers irrigate plants regularly during their first 12-18 months in the ground to prevent stress and develop a healthy, well established plant with a deep root system. Near the end of the first summer, irrigations are space gradually further apart to slowly "wean" plants from the regular water and encourage them to send roots deep. Irrigation is turned off during the winter, as most plants are able to survive our mild, wet winters without irrigation. However, ET is monitored continuously and irrigation may occur during unusual periods of winter drought.
Researchers vary the frequency of irrigations for each treatment, applying water more frequently to the High treatment and less frequently to the Low treatment. This is achieved by modifying the daily reference evapotranspiration (ETo) by the percentage associated with each treatment the way a crop or landscape coefficient is used.
If CIMIS reported ETo for the day at 0.3" and the High treatment is 80% of ETo, these two values are multiplied by each other and their product is recorded as the adjusted ET for this treatment this day (80%, or 0.8 × 0.3 = 0.24”). Adjusted ET values for each treatment are accumulated until 50% of PAW (1.48") has been removed. When this is estimated to occur, the treatment is irrigated. An example table is below.
|% of ETo||Adjusted ET
Cumulative ET (in.)
Three treatments are currently imposed: High, Moderate, and Low. These treatments and the percentage of ETo used correspond to three of the categories of water needs listed in WUCOLS:
|Treatment||Percentage of ETo||WUCOLS Category of Water Needs|
The focus of the trials changed over time, resulting in adjustments to the irrigation protocols over the past 15 years. The original iteration of the trials was to see "how low can they go," irrigating when 90% of PAW was removed. In response to the development of the Model Water Efficient Landscape Ordinance (MWELO), researchers have adjusted irrigation practices to align with WUCOLS and common landscape irrigation practices, irrigating when 50% of PAW has been removed and evaluating plants at levels commensurate with WUCOLS' High, Medium and Low categories of water needs.
As a result of transitioning from "how low can you go" to our current methods, changes have been made to the percentage of PAW removed before irrigation, the number of treatments, the treatment percentages, and the number of plants on each treatment.
|Year||PAW Removal||Treatments (% of ETo)|
|2005-2013||90% (3.17")||80%, 60%, 40%, 20%|
|2014-2016||74% (2.72")||80%, 60%, 40%, 20%|
|2017||74% (2.72")||80%, 50%, 20%|
|2018-Current||50% (1.84")||80%, 50%, 20%|