Contrary to popular belief, cotton is a very drought-tolerant plant. And, in fact, the majority of U.S. cotton is produced without applied irrigation—just the water of natural rainfall. When irrigation is used, it simply supplements rainfall during dry periods.
New irrigation systems and strategies used today, particularly in the U.S., are substantially more water efficient than in previous decades. These improved systems aid in land use efficiency by ensuring that the soil is as productive as possible, regardless of changing weather, and contribute to higher and more consistent yields, which helps to manage supply chain expectations, reduce farmers’ risks, and provide essential products.
Today, agriculture accounts for 70% of global water use. Global cotton production makes up 3% of total agricultural water.
Existing natural water resources can sustain cotton production in many areas of the world with minimal environmental impact. Cotton has been wrongly cited as a water intensive crop, when in reality it is very drought tolerant. In fact, in many regions of the world, cotton gets all of its water from rainfall—water that would be used by whatever vegetation is present. For example, about 64% of the U.S. cotton crop is produced without irrigation, and irrigation is used in most of the remaining 35% of U.S. cotton crop only to supplement crop needs.
The cotton plant is drought-adapted and responds favorably to periods of water stress sufficient to slow vegetative growth.
Water quality is also preserved in modern cotton productions systems. The increase in conservation tillage practices has resulted in a reduction of runoff from agricultural lands, decreasing non-point source pollution of fertilizer and pesticides. Intensive local monitoring of surface water and sub-soils has demonstrated the benefits of no-till cotton in protecting both ground and surface water resource.
Better nutrient management and precision technologies are insuring inputs are used by the crop and are not entering ground or surface waters.
Like all crops, a cotton plant's water requirements vary depending upon the environment in which it grows. The drier and hotter the environment is, the more water the plant requires. In the U.S., this amount varies significantly. Moving from the West Coast to the East Coast, for example, the desert Southwest requires a maximum of about 40 inches of water per year, while the humid Southeast can go as low as about 18 inches. While water requirements are higher in the West, so are yields, and modern cotton varieties tend to provide at least 60 pounds of lint and 90 pounds of seed for every inch of water used. (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.)
As cotton has been bred to be a drought-tolerant crop in many parts for the world, it is often grown without any supplemental irrigation and relies solely on rainfall. For example, only 35% of the U.S. cotton crop is produced on irrigated land, which compares to the average annual rainfall levels in the U.S. A cotton plant's water requirements are less than annual rainfall from central Texas to the East.
For other cotton areas that are irrigated, a small amount of irrigation at key times in the growing season can greatly improve yields. Viewed in this way, irrigated agriculture is consistent with the goals of responsible production because it maximizes efficiency in land use.
For the 36% of U.S. cotton acres receiving supplemental irrigation, there are several methods of applying the water. Some of the first irrigation systems were siphon tubes that were placed in a ditch and channeled water between the rows of the crop, or diverted water to "flood" a basin all at one time. This approach is referred to as "surface" irrigation (also known as flood or furrow irrigation) systems because the water travels along the surface of the field. If the field is well designed with the right slope, and properly operated, surface irrigation can be an efficient method of delivering water to crop. However, those conditions are not always met and surface systems are often very labor-intensive.
Sprinkler Irrigation Systems
Another method used to irrigate crops is with sprinklers, similar to the ones many people use to water their lawns. In most cases, the sprinkler irrigation systems used for cotton are mounted on a "center pivot." The earliest versions of the center pivot had sprinklers mounted on top of the pipe carrying the water but, in arid areas, this can lead to a great deal of water loss due to evaporation. Therefore, most cotton producers in arid areas use sprinklers placed just above the plant or are implementing "Low Energy Precision Application" (LEPA) systems on their pivots. These sprinklers on top of the center pivot are replaced with drop lines that "lay" the water down between crop rows.
The transition from surface to sprinkler systems is evidenced by data that show the percentages of irrigated cotton acres by surface and sprinkler systems equalizing over time.
Irrigation delivery methods continue to be refined to make sure producers get the "most crop per drop". 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; if there is a portion of the field that does not need irrigation (for example a low spot where rainfall collects), the pivot is programmed to turn off the sprinklers over that area.
The latest trend in cotton irrigation systems, particularly in West Texas where water resources are becoming limited, is subsurface drip irrigation systems. 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 is a very efficient way to deliver water directly to the root zone of the plant. Tubes are spaced either under each row, or between every other row depending on the soil type and environment.
Since water is a limited resource, and due to economic constraints related to costs of water, pumping and labor needed to apply irrigation, producers are prudent in managing this resource. A number of approaches are used to decide when to irrigate, including:
- Computer models that predict water use based on the growth stage of the plant and weather data
- Soil moisture probes that determine if there is sufficient water present to meet crop needs
- Thermal infrared thermometers (IRTs) that measure the temperature of the cotton leaves—as the plant begins to run out of water, its leaf temperature will increase. Some companies are now offering thermal images so producers can see leaf temperatures across the entire farm.
Research continues to develop more accurate and easier ways to determine crop water needs and, in fact, new instruments help researchers predict when cotton plants need water. Indeed, Cotton Incorporated collaborated recently on a publication titled “Cotton Irrigation Management for Humid Regions,” detailing the benefits of irrigation and why water management is important; cotton water requirements in humid areas; growth stages that are sensitive to water stress; and a review of tools for irrigation scheduling, which can be downloaded here.
Improving water use necessitates creative and innovative technology in agricultural production systems; as a result, numerous emerging technologies and management schemes have been developed.
Research at the NESPAL Environmental Center at the University of Georgia has shown increased efficiency of precision water placement using a combination of center pivot irrigation, sensors to detect water needs, preset geographic maps and variable rate nozzles to vary water application in accordance with soil type and water holding capacity. Adaptation was influenced by the declining ground water resources and the cost of pumping from increasingly deep wells. Additionally, in the past 25 years, low energy precision application (LEPA) using drop tubes in Texas has decreased water losses dramatically.
Installation of subsurface drip irrigation (SDI) has also increased, and this has turned out to be a desirable delivery method for supplemental crop irrigation because its installation below ground eliminates evaporation from the soil surface. In fact, 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.
The potential for introduction of reusable water into these new age irrigation systems could increase the sustainability of SDI systems even further. One method would be through the introduction of pulsed-flow waters from aquaculture. Wastewater effluent has also been utilized in SDI systems using both tapes with emitters, and gravel trenches. However, caution must be taken to monitor sodium and phosphorus levels, as well as salinity, sodicity, nutrients, trace elements, and microbial contamination.
Salinization affects about 20-30 million hectares of the world's current 260 million hectares of irrigated land and limits world food production. No data is available for cotton land, specifically. However, cotton may have an advantage in this arena because it is more tolerant to high salt levels than other crops. For agriculture, salinity can be managed through drainage, leaching during the cool season and changes to more salt-tolerant crops, such as cotton. Cultural practices such as more frequent irrigation, water source blending, land grading and timing of fertilization make salinity management easier.
An example of a measurement system that can directly reduce the use of water is the Biologically Identified Optimal Temperature Interactive Console (BIOTIC) developed by the scientists at the USDA-ARS. The system provides irrigation scheduling based upon measurements of canopy temperatures and the temperature optimum of a given crop species.
The threshold values to schedule an irrigation event are calculated from the thermal optimum for the plant and the amount of time that a given species can exceed a temperature threshold and adequately recover. In a three-year study of the BIOTIC for scheduling irrigation in cotton, it was determined that lint yields declined 343 kg/ha for each 1 hour that the temperature exceeded 28 C9. Information like this can help optimize productivity in relation to water use.
Agriculture biotech companies are currently developing drought tolerant crops that should be launched toward the end of this decade or early next decade. Crops with improved water use efficiency, whether through traditional breeding or biotechnology, will be extremely important because they will increase the stability of production in drought conditions.