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Maximizing Value of Limited Water

Friday, February 5, 2016
filed under: Irrigation/Water Use

When referencing the impact of irrigation on crop yield, the measuring stick has, historically, focused on the amount of yield gain per unit of water applied. In this era of droughts and often-inadequate irrigation water supplies, however, a team of researchers in Colorado has been taking a different tact. Their approach is based on the best use of deficit irrigation — i.e., working to answer the question: what is the optimal management, under water-deficit conditions, for irrigation that will in the end save water while producing acceptable yields?

These scientists are part of the USDA-ARS Water Management and Systems Research Unit (WMSRU), based at Fort Collins. They are conducting their deficit irrigation experiments about 30 miles away on a 50-acre limited irrigation research farm at Greeley. The farm, located adjacent to the city’s airport, is owned by Colorado State University and operated by ARS under a long-term lease.  

A previous WMSRU study at the Greeley site, conducted from 2008 through 2011, encompassed four crops common to the High Plains: corn, wheat, sunflower and pinto beans. The current project, though, is focusing on just sunflower and corn. A big part of the reason for sunflower’s inclusion is its attractiveness as an alternative crop for producers with limited irrigation capacity, points out Louise Comas, plant physiologist with the WMSRU team*. “We’re looking at the response between yield and ET (evapotranspiration),” Comas explains. If the response line is too steep (as is the case with corn), “you’re proportionately losing the same amount in yield as you are saving water, so the benefit of saving that water is muted — unless we can minimize losses through strategic seasonal deployment of deficits.  

“With sunflower, though, it’s a very shallow response line,” she adds. “So there’s already a lot more ‘room’ to save water and with minimal yield loss.”

The Greeley site — half in corn and half in sunflower — contains a total of 96 plots, each with 12 rows about 140 feet long and planted in 30-inch row spacing. The plots are grown under drip irrigation to have as “tight” control of the water applications as possible and to also maximize efficient water use. Comas defines evapotranspiration (ET) as “how much water is lost from the crop system from both plant transpiration and evaporation.” With the drip system, “once you get full ground cover, we’re losing virtually none from evaporation.”  

Measurements are taken from the center four rows of each plot so that the “edge effect” of neighboring plots is not a factor in the results. There are 12 different treatments (i.e., differing water amounts and irrigation timing regimens), each replicated four times.

Data are collected on several fronts. The WMSRU scientists measure evapotranspiration (ET) by water balance, volumetric soil moisture by neutron probe, crop canopy temperature, vegetative canopy cover and growth stage/biomass. They also conduct multispectral and thermal imaging, examine root phenology and density, measure sap flow (movement of water through the plant), gas exchange, stomatal conductance — and, of course, crop yield.

An on-site CoAgMet (Colorado Agricultural Meteorological Network) weather station provides the baseline ET data that the team uses in calculating and interpreting their plot readings.

A single hybrid is planted in all of the sunflower plots (likewise in the corn plots). “We’re not looking at differences in hybrids and their responses; we’re focusing on water management,” Comas points out. “So we picked a variety that’s commonly used this area.” Planting date, plant population, fertility and other production practices — with the exception of water management — are similar across all plots.

At the core of the WMSRU experiments is the understanding that crop yield — including that of sunflower — will be reduced if the crop experiences water shortages during certain growth periods, such as pollination. But, there are other times when water shortages do not necessarily impact final yield to a significant degree. So that’s been the objective of the Greeley experiments: to produce sunflower and corn under several different seasonally varied deficit irrigation regimens (avoiding deficit during pollination) in order to see which one(s) best (1) save water while (2) maintaining yield.

The 12 different water treatments — all applied through the farm’s drip irrigation system — range from 100/100 all the way down to 40/40. (The first number equates to the percent ET applied during late vegetative growth stages, with the second number being percent ET applied during the crop maturation stage.) The “100/100” is considered “full irrigation,” while the “40/40” basically equates to nearly dryland conditions with supplemental irrigation during pollination. All of the treatments are controlled from the drip system’s control manifold, with the desired amount of water then pumped to the appropriate plot. They typically irrigate every four to five days.  

Should the Greeley research site receive rainfall during the experiment, “we adjust the next irrigation to compensate,” Comas says. “Before and after every single irrigation and precip event, we have the soil moisture readings of what’s in the soil profile. From that, we can do water balance and determine how much ET the plants have actually extracted from the soil and used.”  

The Greeley WMSRU project has one more season to go before its conclusion, so results are not yet finalized. However, some findings are taking shape regarding the sunflower portion of the study.

“Early results show that reducing crop water use by 35% during the late-vegetative stage, with full or nearly full irrigation during the rest of the season, maintained high yields similar to full-irrigation treatments,” Comas reports. “This seasonally applied irrigation deficit saved approximately 15% of crop water used over the season.”

Water shortfalls during the late-vegetative stage produced short plants with reduced leaf area. “However, sunflower appears able to achieve high seed yield if water is applied during the reproductive and seed fill stages,” she says. By contrast, though, water deficits during seed fill resulted in considerable yield losses.

“Seasonally varied deficit irrigation, with deficits in appropriate times of the season, shows promise for increasing crop water productivity (yield per amount of water used by the crop) when water is limited,” Comas states.

Work in 2016 will include repeating the experiment to test the reliability of the responses found thus far. The WMSRU group also will evaluate more-extreme treatments to test just how much water can be saved during the late-vegetative stage before yields are affected. Additionally, WMSRU is working on remote sensing techniques to provide farmers with spatially variable water stress information that can be useful in making irrigation management decisions.

The ultimate goal, Comas reiterates, is to develop better answers about the relationship between deficit irrigation and final crop yield. Given the increasing demand for water — by urban entities as well as agricultural — and the full likelihood of future drought events, it’s more important than ever to use available water supplies as efficiently as possible. — Don Lilleboe 
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