Water Stewardship – United States
Improving quantity and quality
Despite its reputation, cotton is not a water-intensive crop. It has been bred to be drought tolerant, and in many parts of the world, it relies solely on rainfall. The same is true across most of the United States – about 60% of U.S. cotton is produced without irrigation. Most irrigation used for the remaining 40% simply supplements crop needs. Only 4% of the U.S. cotton harvested acres in 2008 required irrigation to grow (USDA, 2010).
A small amount of irrigation at key times in the growing season can greatly improve cotton yields – helping growers maximize land use efficiency. Cotton is also highly tolerant of soil and water salinity levels, so it can be grown with water and soil resources unsuitable for most other crops (Hanson et al., 1999; and Ayers and Westcot, 1985). It can even make use of drainage or reclaimed water that otherwise would require environmentally challenging waste disposal (Goyal et al., 1999).
The changing climate is increasing the frequency of volatile weather systems – causing both prolonged droughts and intense rains that quickly run off the land before soaking in. In the face of these extreme conditions, U.S. growers know they must continuously improve their water use efficiency. In the last decade or so, cotton water stewardship in the U.S. has seen tremendous progress thanks to improved irrigation and tillage practices, plus the advent of precision agriculture. Taking a longer view, the efficiency of irrigated water used has increased by almost 82% since the early 1980s (Field to Market, 2016).
Not only does modern cotton production strive to conserve water – it also preserves water quality by reducing fertilizer and pesticide runoff. No-till cotton protects both ground and surface water resources (Smith and Johnson, 2003), while precision ag and nutrient management technologies help growers apply inputs much more efficiently, preventing any excess from escaping into water or air.
"The efficiency of irrigated water used has increased by almost 82% since the early 1980s."
– Field to Market, 2016
Cotton water requirements by region
Over the vast geography of the United States, cotton’s water requirements vary significantly.
For instance, water requirements are higher in the West, but so are yields, and modern cotton varieties tend to provide at least 50 pounds of lint and 70 pounds of seed per acre for every inch of water used (Zwart and Bastiaanssen, 2004). (An “inch” is a common way to describe crop water requirements, and is the same unit used to measure rainfall. The “inch” represents the depth of water per unit area.)
For this reason, water use efficiency for one region is not compared with that of another, but taken as a national average.
Evolving irrigation systems
From 1988 to 2008, the percentage of irrigation delivered by efficient water delivery systems more than tripled in the U.S. (USDA, 2010). Irrigation methods continue to be refined – and adoption of more efficient systems and new precision ag technologies has only increased. Below is an overview of the progress.
Some of the first irrigation systems were referred to as “surface” irrigation (also known as flood or furrow irrigation) systems because the water traveled along the surface of the field. Most U.S. producers who still use surface irrigation systems use computer model simulations to determine the exact flow rate of water needed for each furrow to maximize water use efficiency. They also use tail water recovery systems, which reduce nutrient runoff, lower sedimentation in streams, and decrease water usage requirements.
Sprinklers systems and drop lines
Many U.S. growers, however, have transitioned from surface to sprinkler systems. In most cases the sprinkler irrigation systems used in cotton are mounted on a “center pivot.” To reduce water loss due to evaporation, cotton producers in arid regions use sprinklers placed below the pipe and just above the plant, or they use “Low Energy Precision Application” (LEPA) systems, in which sprinklers are replaced with drop lines that “lay” the water down between crop rows.
Within the last few years, new technology has also become available that allows individual sections of a center pivot to be turned on or off. This leads to more water savings, as the pivot can be programmed to turn off the sprinklers when pointed toward portions of the field that do not need irrigation (for example, a low spot where rainfall collects).
Most center pivots are operated at low pressure to save energy and reduce evaporative water loss. They can be turned on with the touch of a switch, making it easy for producers to irrigate exactly when water is needed.
The latest trend in cotton irrigation systems, particularly in water-limited West Texas, is subsurface drip irrigation (SDI). This type of system is expensive to install and maintain, as it involves running a series of tubes about 6 to 18 inches below the surface of the entire field. However, it maximizes efficiency by delivering water directly to the root zone of the plant. Studies have shown that cotton grown under SDI decreased daily crop evapo-transpiration by 75% and had the highest water use efficiency for lint production (Bhattarai et al., 2006).
Advanced precision, advanced scheduling
Whatever the system used, growers also have an increasing number of tools to help them decide when to irrigate, including:
- Computer models that predict water use based on the growth stage of the plant and weather data
- Thermal infrared thermometers (IRTs) that measure the temperature of the cotton leaves – as the plant begins to run out of water, its leaf temperature increases
- Soil moisture probes that determine if there is sufficient water present to meet crop needs
Agricultural experts predict that growers will increasingly adopt irrigation scheduling technologies. A number of studies have demonstrated the importance of soil moisture sensors, which allow growers to increase their water use efficiency and their yield levels compared to systems that do not utilize sensors for scheduling (Leib et al., 2015). More specifically, data from Daystar et al. (2017) show that producers using sensor-based irrigation were achieving 100 pounds higher yield without increased water use compared to producers not using sensors.
Cotton water management research
Improving cotton water use necessitates innovative technology and management schemes – and researchers have risen to the challenge. Research continues to develop easier and more accurate ways to determine and respond to cotton’s water needs. The diversity of conditions within the U.S. Cotton Belt make it ideal for water research.
Studies in both the humid southeastern U.S. and the more arid conditions of west Texas have shown cotton is even more drought tolerant early in the season before flower formation begins (Meeks et al., 2017; Bordovsky et al., 2015). The practical application for growers is significant: they can use higher irrigation thresholds early in the season and, if located in a water-limited region, they can postpone irrigation until flowering begins.
Soil type is also an important consideration in irrigation decisions, as recent studies have highlighted. For instance, studies in Tennessee showed that while silt loam soils did not require any irrigation under certain weather conditions, coarse-textured soils in the same conditions did benefit from supplemental water (Grant et al., 2017). These results will be used to design site-specific irrigation schedules for fields with multiple soil types where precision pivot control is available.
On-farm technology is rapidly advancing to help growers use water and other inputs more efficiently, from easy-to-use smartphone applications to advanced computer simulation models. The simulation models, in particular, are being used to develop water management and allocation strategies in arid regions like west Texas (Modala et al., 2015). The same models are also being tested for possible use by growers for real-time irrigation management decisions (Thorp et al., 2017).
Agriculture seed companies are also continually working toward more drought-tolerant crops. Crops with improved water use efficiency, whether through traditional breeding or biotechnology, will be extremely important in increasing the stability of production in drought conditions.
U.S. regulation of agriculture water
Water quality and quantity are regulated to minimize risk to the environment and people, and producers are subject to state and federal laws that regulate water use and quality. Examples of such laws include …
- Clean Water Act: Regulation by the Environmental Protection Agency (EPA) through section 319 of the Clean Water Act (CWA) establishes a Non-Point Source Management Program that includes oversight of agricultural operations (EPA, 2002).
- The Food, Conservation, and Energy Act of 2008: Operates several programs that assist farmers to continue adopting new technologies to improve water management and protect water quality. A specific example is the Agricultural Water Enhancement Program.
- Western states (including the cotton states of California, Arizona and New Mexico): There is a long history of carefully allocating and monitoring water resources in the West. A summary of the efforts to sustain the water resources in the Western United States was compiled by Bell and Taylor (2008).
- Texas: Surface water withdrawals must be permitted by the Texas Natural Resource Conservation Commission.
- Mississippi: Water quality and water withdrawals are regulated by the Mississippi Department of Environmental Quality.
- Tennessee: Under the authority of the Water Resources Information Act of 2002, TCA, Section 69-8-103, water withdrawals of 10,000 gallons or more on any day in Tennessee must be registered.
- Georgia: The Georgia Soil and Water Conservation Commission oversees efforts to ensure sustainable use of agricultural water resources in the state. Water withdrawals require state permits and all agricultural withdrawals are metered.
Continue to learn
These resources can provide more in-depth information related to cotton and water usage.
- NRCS Water Quality and Quantity Resource Page contains information on all aspects of water-related issues in agriculture.
- 2008 Farm and Ranch Irrigation Survey provides a very detailed look at irrigation trends in the U.S. for all crops, including cotton. The information is provided by the USDA, National Agriculture Statistics Service.
- Examples of water management research funded by Cotton Incorporated can be found on the Cotton Cultivated website.