Researchers explore deficit irrigation on thirsty SJV crops

Deficit irrigation is becoming a common practice in permanent crops, such as wine grapes and tree nuts, as growers strategically withhold water to help manage pests or impact fruit quality.

In field and row crops, including water thirsty crops like cotton and alfalfa, the practice is still relatively untested. But as water supplies tighten, growers are looking at alternative strategies to produce profitable crops under less applied water.

Fresno County University of California Cooperative Extension Farm Advisor Dan Munk said that depending on the crop, water supply and cost of water, deficit irrigation can be a good option for keeping land in production under dwindling water allocations.

“In some fields growers may be forced to deficit irrigate whether they want to or not,” Munk said. “The question is do we cut land from production or change our irrigation management through things like deficit irrigation?”

Regulated deficit irrigation in cotton requires a good understanding of crop phenology and physiology and a strategic plan to withhold water based on that knowledge, Munk said.

The latest recommendations from the UC Cooperative Extension call for minimum water stress during prebloom and only modest water stress during actual bloom. There is potential, however, to strategically inflict water stress by withholding water later in the season, after late flowering.

Munk said that post-cutout, once vegetative expansion is done and plants start to set fruit, is the best opportunity for regulated deficit irrigation in cotton.

“After cutout, we think we can delay irrigations to about minus-20 bars without risking yield,” he said.

He noted there are still many unanswered questions, including the impact on fiber length and quality, nutrient transport, earliness, and potential problems with accumulated salt buildup.

“Water stress can lead to more widespread problems in the field so there is a limitation on how far we want to move in this deficit irrigation arena,” Munk said. “We do see a fairly regular reduction in staple length so the impact on quality needs to be part of the economic angle as well.”

Munk said the best strategy to reduce risks from using reduced deficit irrigation as a water management strategy in cotton is to start early, withhold water conservatively during the early vegetative stage by delaying irrigations a few days, and build stress from there with each subsequent irrigation.

“Start conservatively and plan ahead,” he said.

In addition to work on deficit irrigation in cotton, Blaine Hanson with the Department of Land, Air and Water Resources at UC Davis, is exploring mid-summer deficit irrigation as a strategy in alfalfa. With more than 1 million acres of alfalfa grown in California, water use on the crop accounts for about 5.3 million acre-feet annually, making it the state's single largest agricultural water user.

Hanson is working with UC farm advisors Steve Orloff in Siskiyou, Khaled Bali in Imperial County and Blake Sanden in Kern County, and Dan Putnam, UC Davis forage specialist, to compare yields and returns on alfalfa fields irrigated to full evapotranspiration with fields in which irrigations are terminated in July and August.

Hanson said when water is readily available, deficit irrigation as a practice would not likely make sense. But today, with diminishing surface water supplies — particularly on the West Side of the San Joaquin Valley — growers are looking at ways to maintain profits under reduced water deliveries.

Federal water agencies have announced drastic cutbacks in surface water allocations for West Side farmers and many water agencies are already looking at rationing. Under that scenario, Hanson said alfalfa growers essentially will have two choices. They can fully irrigate alfalfa fields on a reduced amount of acreage or deficit irrigate by reducing the amount of water they apply to their full acreage.

Hanson recently analyzed the effect of three different scenarios: Farming fewer acres of fully irrigated alfalfa; a deficit irrigation strategy of reducing the amount of applied water to the entire field throughout the season; or deficit irrigating the full acreage by withholding water during peak use periods in mid-summer, when yields generally decline.

He found that eliminating irrigation sets during July, August and into September offers a better return than simply reducing the amount of applied water throughout the year or farming fewer acres under full irrigation.

Hanson assumed surface water allocations of 50 percent of normal on a 100-acre field. Not accounting for other input costs, Hanson found that the best strategy was to apply irrigations at full ET for the first three harvests and no irrigation for the last three harvests.

Under that strategy, Hanson found the first three harvests from a commercial field near Davis, Calif. yielded from 1.56 to 1.66 tons per acre, while the fourth cutting yielded .66 tons per acre and the two remaining cuttings were not harvested. The field's annual harvest totaled 5.46 tons per acre. On a 100-acre field at $110 per acre, the field grossed $60,060. By comparison, only irrigating half a similar 100-acre field for maximum yield at full ET grossed $44,550 and irrigating the full 100 acres using 50 percent of the applied water throughout the season grossed $46,200.

Mid-summer deficit irrigation did in fact reduce both yield and evapotranspiration rates, although the extent varied from site to site.

“Yields generally recovered the following year except at one site,” Hanson said.

In the study, Hanson and the others also sought to explore whether water transfer policies based on evapotranspiration rates of deficit-irrigated alfalfa make sense. The Department of Water Resources bases its amount of farm water transferred to urban or environmental uses on evapotranspiration rates of a crop typically produced on the field. In fallow fields, the amount of water transferred is based on the full ET of the crop not planted. But in the case of alfalfa, DWR proposes the amount of transferable water be the difference in the evapotranspiration rate of a fully irrigated crop and the evapotranspiration of a deficit-irrigated alfalfa field.

Hansen found that basing water transfer levels on ET differences between full and deficit irrigated alfalfa was not practical due to variability in ET and yield differences between sites and because alfalfa cuttings under deficit irrigation would likely not be harvested. Water transfer amounts instead should be based on the ET of full irrigation, he said.

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