in Increasingly Dry Times
By Jim Steadman, Editor, Cotton Grower Magazine
Depending on your perspective, cotton is one of the most water-efficient crops on the planet or one of the most water-dependent crops on the planet. Truth is, cotton can survive and thrive on less water than many other crops, and newer cotton varieties are being bred specifically for use in more arid conditions. Yet, timeliness of that water, matched with key growth stages of the plant, is often the difference between a successful crop and a sub-par result.
Across the U.S. Cotton Belt in 2021, cotton growers enjoyed their best crop in years as timely moisture helped get plants off to a quick, healthy start and provided the boosts needed to finish the crop out impressively. Unfortunately, in 2022, severe drought conditions hampered much of the cotton crop in California, Arizona, and the High Plains of Texas — also known as “the world’s largest cotton patch.” In Texas alone, the abandonment rate for the state’s cotton crop reached nearly 60%.
Water shortages in California caused the state’s acreage of cotton and other crops to drop as reservoirs dried down to record low levels. Across West Texas, concerns over the diminishing level of the Ogallala Aquifer, combined with the worst drought conditions since 2011, left many growers searching for the best methods and options for dryland cotton farming.
And severe drought over the past two years has left Arizona struggling with significant water issues. Lake Mead reached a critical level last year, setting in motion contingency plans to help conserve and deal with shortages along the Colorado River.
“This is the first season in many decades that growers will have zero Colorado River water delivered to them,” states Kelly Thorp, a USDA-ARS researcher based in Maricopa, AZ. “It’s a dire situation, and growers are reacting in several ways including reducing acres and fallowing land. They’re again facing a total reliance on groundwater, and there’s a need for improved irrigation management.”
Finding new ways to conserve and reduce water use — whether through improved irrigation systems, conservation plans, or a combination of both — are driving several new Cotton Belt research efforts.
Presenting at the 2023 Beltwide Cotton Conferences, Thorp outlined a two-year study using several different crop models along with data from a water content measurement system to schedule irrigation in Arizona.
“The Maricopa Stanfield Irrigation District in central Arizona is primarily surface irrigated because it’s set up to receive Colorado River water through the irrigation system,” Thorp says. “But use of sprinkler irrigation is becoming more popular as is drip irrigation, especially on tribal lands that have water rights and more access to water. They want to use the most modern irrigation systems to conserve as much water as they can. Other growers are going to have to adapt to survive in this new reality that we face in Arizona.”
The research focused on developing irrigation scheduling tools using rate studies that evaluated various aspects of timing and rate of irrigation on cotton, the fiber quality, and yield. Drone applications were used for mapping and irrigation management, using a variable rate irrigation system based on various aspects that were sensed in the field.
Conducted during 2021 and 2022, the study consisted of 48 plots and six irrigation management systems. Three systems used standalone crop models while the other three used the same crop models plus soil water content data provided by neutron-based probes throughout the season. Two NexGen cotton varieties — NG 3195 B3XF and NG 4936 B3XF — were used in both years of the study.
Weather data was compiled using an Arizona Meteorological Network station at the field site, a seven-day forecast from the National Digital Forecast Database, and the average of long-term weather from their weather station. Models were used to predict water needs for the week, and irrigation was scheduled for two days each week. Simulated root depth from each model was used to define the control volume, and the root zone water depletion from the probe data and from the model simulation was analyzed to determine the difference in depletion between the measured and simulated data.
“There’s a large variability in the rate recommendations due to the real field variability that exists,” Thorp points out. “One of our issues is that we have some limitations of the irrigation system. Things must be bounded, and the pressures maintained at a certain level to have the irrigation system work correctly. We’re also managing flowing canals, so we don’t want to overtop the canals or run them dry. So, we do need to put limits on the rates recommended by these models.”
The results to date show:
The three stand-alone models overestimated water requirements, possibly due to the aridity at the Maricopa station. Marginal to no yield improvement was seen with the extra water.
Neutron probe-assisted models reduced irrigation amounts by 8% to 19%. Yield was not significantly impacted in most cases.
Combining soil water content data with irrigation scheduling models appears practical and promising to help improve irrigation recommendations.
Farther east in the Mid-South sits a swath of intensively irrigated and intensively farmed land. Lying underneath is the Mississippi River Valley Alluvial Aquifer, which like most other aquifers, is showing signs of decline because of the amount of withdrawals used in agricultural production throughout the region.
“There are ways to conserve water,” says Carson Roberts of Mississippi State University, who also reviewed findings from his research study during the 2023 Beltwide Cotton Conferences. “A lot of those are through an irrigation system approach, looking at different irrigation systems. My work is more focused on what we can do with conservation systems.”
One of the key components of Roberts’ study — conducted in conjunction with USDA-ARS and Cotton Incorporated — is exploring the benefits of cover crops in cotton production. Roberts notes that earlier studies have indicated that use of cover crops can help improve water infiltration and help increase lint yields in abnormally dry years in dryland situations.
“Irrigation can often mask the benefits of good soil quality,” he says. “We may have poor soil reports, poor soil quality, and other soil issues, but you may not see them because we irrigate.”
Roberts is working to find a conservation system that meshes with the irrigated Mid-South. “There are a lot of benefits to conservation systems outside of the water, including erosion management,” he explains. “We also want to see if the use of cover crops and tillage reductions can help reduce the need for irrigation water.”
The study, conducted in 2021 and 2022, evaluates seven different cropping systems incorporating a mix of reduced tillage, subsoil, winter fallow, strip till, no till, and a rye vetch cover crop. Plots were irrigated using surface water irrigation — a common practice in the Mid-South — and were set up so that each plot could be irrigated independently of the others. The entire plot area is surrounded by levees to help contain and measure runoff water.
“We can measure every single drop of water that goes on to each plot and every single drop that leaves the plot,” says Roberts.
Soil moisture sensors were used to help with irrigation scheduling.
Because of the amount of rainfall received in 2021, no irrigation was needed. Only yields were evaluated, and, to little surprise, the standard reduced tillage/subsoil/winter fallow production system topped the plots. But 2022 followed a more normal and drier pattern, providing a better environment for the research.
“The 2022 results are a lot more interesting,” Roberts explains. “When we look at the soil moisture and remember the soil moisture sensors, the higher the number, the drier the soil. So, our conventional system is a lot drier than any of the cover crop systems. When you separate these out between the winter fallow and the cover crop systems, we found that there was a definite difference between using cover crops and not using cover crops as far as soil moisture goes.”
When yields were evaluated, Roberts anticipated that the more conventional system would perform better since it used so much more water. In reality, the conventional system ended up yielding less than some of the other conservation-based systems.
The use of cover crops in cotton production systems continues to grow steadily throughout much of the Cotton Belt as growers discover the soil health and weed management advantages provided. Now, add water management to the list of benefits — something that Roberts plans to continue exploring.
