In semi-arid regions of the world where summer rainfall for crop production is limited, irrigation systems have become essential for sustainable crop production explains Professor Pete Jacoby of Washington State University
In the western U.S., water is also essential for many competing interests such as municipal, industrial, recreation and fisheries. During periodic droughts, water is allocated according to priorities that are often determined by public policy which may favor other users ahead of agriculture. With growing concern regarding climate change, water adjudication has impacted growers of crops that require large amounts of water to achieve commercial production goals. Even when water is supplied to agriculture, cost of water and the power to pressurize it for use in most modern irrigation systems has created a demand for improvements in crop water use efficiency.
Surface drip irrigation: Improving crop water productivity
Efforts to improve overall crop water productivity have led to the widespread use of surface drip (SD) irrigation which is the most efficient method of irrigation for many irrigated crops, especially those grown on agricultural lands that are not flat. Producers of winegrapes have some advantages over growers of other crops, in that winegrapes are relatively efficient in their use of water. This feature has allowed most winegrapes to be grown under SD irrigation. This type of irrigation uses pressure-compensated emitters that drip water from a pressurized line that is suspended from the bottom wire of the trellis support system. Research has shown that application of less water than vines lose through crop evapotranspiration (ETc) can create a moderate amount of water stress on the vine which can improve overall quality of grapes for making premium wines and can in turn yield higher prices paid by consumers. Deficit irrigation is generally applied after fruit set and prior to full maturation of the winegrape.
Use of subsurface drip irrigation (SDI) can achieve even greater crop water efficiency, but damage to buried lines from soil clogging or burrowing rodents often make its use impractical in certain regions. To apply SDI without the use of buried lines, a novel delivery system using vertical sections of plastic pipe was recently developed that permits SDI to be applied in both new and established winegrape vineyards. Research using this technique to deliver water at depths of 60 cm or more into the vine root zone has led to a type of SDI labelled “Direct Root-Zone” (DRZ) that has demonstrated water savings of up to 35% compared to the volume used for SD. Concomitantly, grape quality has been improved through combined use of DRZ and deficit irrigation.
Concurrently, new systems for scheduling irrigation events by use of soil sensors and electrically controlled pumps and water valves are currently becoming commercially available. Altogether, growers of irrigated winegrapes are being provided several new options to reduce overall water use while achieving higher quality of grapes and are becoming increasingly more sustainable into the future.
Implications of irrigation systems on winegrape producers and the global society
Development of the DRZ subsurface irrigation system illustrates some opportunities for addressing issues of global warming and impending challenges facing society such as maintaining sustainable agriculture through improved use of water resources through improved technologies and their application. In 2014 when the concept research proposal for DRZ subsurface irrigation delivery was developed, the winegrape acreage in the Pacific Northwest region and Washington (WA), in particular, was growing at a rate of over 9% per year, although WA was in a distant second place behind California (CA). A strategic planning effort forecasted the single most limiting factor for sustaining this growth was available water for new vineyards.
Most surface water had been adjudicated and either certain crops would be replaced by more water-efficient crops or improved methods of irrigation would be needed. Both options were advancing, which has allowed WA to grow to a current level of producers of winegrapes on 24,000 ha and over 1,000 wineries. Growers have also been able to reduce overall water use through adoption of SD irrigation and deficit irrigation through application of new research findings and Extension education programs.
At the outset, an advisory group of 4 experienced growers worked with the scientist leading the DRZ research project. Initial funding was awarded from Washington State University (WSU) and was soon matched by grant from the WA Wine Commission. This was followed by other grants from combined state and federal funding sources. Two commercial winegrape growers provided land for research to be conducted using established vineyard plantings. The first experiments using the DRZ method coincided with the hottest and driest year on record in 2015.
Rates of deficit irrigation set at estimated levels of 60, 30, and 15% of ETc were used for three growing seasons to compare DRZ and SD. Results were published and led to further research being funded and published in peer-reviewed international scientific journals. To date, DRZ has been installed by growers in WA, OR, CA and AZ in cooperation with the WSU scientist and continue to garner data across a wide array of soils and climatic regimes.
Root architecture, subsurface delivery and saving water
Some of the key findings include that water can be saved through SDI applied through the DRZ delivery method while maintaining commercial yields and yielding higher quality of grapes than under SD irrigation using deficit irrigation in the 70-80% of ETc. Measurements of vine activity found that vine under DRZ maintained higher rates of photosynthesis and CO2 exchange than did vines watered at the same rate by SD delivery. Research using mini-rhizotrons to observe root growth dynamics under both SD and DRZ revealed that vines under DRZ subsurface irrigation delivery had altered root architecture to become deeper rooted than vines receiving SD irrigation delivery.
This change allowed vines to remain more active during hot, dry weather conditions. Differences in water stress levels among treatments of varying irrigation rates and methods of delivery were detected using remote sensing methods on unmanned aerial vehicles (UAV) and ground-based platforms. Remote sensing methods were closely correlated with mid-day leaf water potential methods.
Future funded research for soil water
Current research involves the use of soil water content and soil water tension sensors to evaluate soil water use dynamics under both SD and DRZ delivery under varying levels of irrigation. Newly funded research is exploring the use of sensor-driven automated irrigation systems scheduling to determine the potential for maintaining more consistent levels of deficit irrigation throughout grape maturation.
A series of quarterly updates and e-books will be up-coming for those readers who wish to follow this research. Potential research collaborators and growers/cooperators are invited to contact the author to discuss partnering opportunities in the use of DRZ subsurface irrigation and sensor-driven irrigation.
