Erin Kizer, Shrini Upadhyaya, Kelley Dreschler, Channing Ko-Madden, Julie Meyers 2017-06-28 00:25:31
Water is one of the world’s most precious resources. Yet with increasing urban demand and a world population expected to reach 9 billion by 2050, water is becoming increasingly scarce. While irrigated agriculture is a major consumer of freshwater supplies, it is also crucial for supporting the growing population. As a result, farmers are being forced to implement better irrigation management practices, and they are turning to precision agriculture to accomplish this. Precision agriculture is the use of information about the soil, plant, and environment for optimal site-specific input management. This practice can result in increased yield, decreased input, enhanced quality, and protection of the environment. When the input is water, this practice is called precision irrigation. Measurements of soil moisture content are often used to manage irrigation, but these measurements are not always representative of water availability to plants in the entire root zone. This is particularly the case in orchard and vineyard crops, which have deep and extensive root systems. The alternative is to evaluate the water available within the plant as indicated by its plant water status (PWS), or more colloquially, its blood pressure. Asking the plant directly The PWS approach is practical because it essentially means irrigating a tree when it’s thirsty. Additionally, PWS-based scheduling is uniquely suited for deficit irrigation. PWS-based irrigation management also aids in maintaining tree health, reducing the incidence of diseases like hull rot, and promoting important harvest processes like hull-splitting. The gold standard for measuring PWS is taking measurements of stem water potential. This entails bagging leaves, waiting, and dragging a pressure bomb in the orchard around solar noon in a relatively narrow time window. This process is extremely tedious and time-consuming, and it’s just not suitable for implementing precision irrigation on a commercial scale. What if there was a way to ask a tree if it was thirsty? In fact, leaf stomata open in the presence of sunlight when a tree is happy with the amount of water it’s receiving, and evaporation from the stomata cools the leaf surface. Conversely, when the plant is under stress, the stomata do not open fully and in extreme cases may not open at all. As result, the leaf surface is not cooled. By measuring leaf temperature with an infrared sensor, researchers in the Department of Biological and Agricultural Engineering at UC Davis have been able to consistently assess PWS in walnut, almond, and grape crops. The team has developed a continuous leaf monitoring system that can be installed on a leaf for the entire growing season and transmit data in real-time through a wireless network that’s accessible on the internet. This leaf monitor measures the leaf surface temperature as well as microclimatic variables that affect leaf surface temperature, including air temperature, wind speed, relative humidity, and light level. Creating management zones Irrigating an entire orchard is more complex than asking an individual tree if it’s satisfied with the amount of water it receives. For example, water applied in one part of an orchard may be received by trees at another nearby location, especially in orchards with rolling terrain. Therefore, precision irrigation is ideally applied in an orchard divided into management zones, or areas that behave similarly because of their relatively uniform characteristics. Management zones based on static soil properties are stable over the years, and the orchard’s irrigation system can be modified to align with these zones. In our research orchard, the grower’s irrigation (based on soil moisture content) and a PWS-based irrigation treatment were applied in each management zone. In general, three leaf monitors are suggested per zone so that a representative average is available. In our orchard, two additional leaf monitors were installed: the first monitor was installed on a saturated tree that received an excess of irrigation water, and the second monitor simulated dry conditions by using a broken stem. From the data provided by these monitors, the crop water stress index (CWSI) was calculated. Water savings In the last part of the 2015 growing season, the PWS-based irrigation was managed using the CWSI values. Preliminary results for this treatment indicated water savings of 30% in management zone 1 and 10% in management zone 2, compared to the grower’s irrigation. These promising results led to full PWS-based control for the 2016 growing season. Per the grower’s instructions, we controlled PWS to different stress ranges during and after hull split, using CWSI values to indicate stress, and we used measurements of stem water potential to validate the stress ranges. Irrigation was implemented whenever the stress levels were exceeded. Compared to the irrigation applied by the grower, water use was reduced by 25% and 14% in zones 1 and 2, respectively. Despite the differences in water use, both irrigation management strategies (grower and PWS-based) yielded similar results. The almond crop yield and quality considerations, such as mass per 50 kernels, kernel size, and percentage mold, were not significantly different between the two irrigation strategies. Additional data will be collected during the 2017 growing season to verify the 2016 results. California’s current freshwater consumption averages about 150% of the state’s annual precipitation. Each year, almond growers alone use over 3 million acre-feet of freshwater. Just a 10% savings from using variable-rate, PWS-based irrigation could save more than 300,000 acre-feet, or 97.7 billion gallons, annually. So why not ask the trees if they are thirsty before giving them one of the world’s most valuable resources? If we ask before we share, maybe we’ll all have enough to drink. ASABE member Erin Kizer, Research Assistant, firstname.lastname@example.org, ASABE Fellow Shrini Upadhyaya, Professor and Principal Investigator, Kelley Dreschler, Junior Specialist, Channing Ko-Madden, Research Assistant, and Julie Meyers, Undergraduate Researcher, Department of Biological and Agricultural Engineering, University of California, Davis, USA. This project was funded by the California Department of Food and Agriculture through a Specialty Crop block grant and by the Almond Board of California.
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