Cotton, primarily the Gossypium species, is a major global fiber crop that is often characterized by its high water requirements. Growers must manage water carefully because the amount needed for a successful harvest is not a fixed number. The total water required varies drastically based on geography, climate, and the specific stage of plant development. Understanding the dynamic nature of cotton’s water demand is fundamental to maximizing yield and fiber quality while conserving resources.
Total Seasonal Water Demand
The total water required by a cotton plant over its entire growing season, measured as evapotranspiration (ET), typically falls within a broad range. This seasonal total is often between 500 and 900 millimeters (20 to 35 inches) in temperate climates, but can rise to over 1,200 mm (nearly 50 inches) in hotter, arid environments with a long growing period. ET includes all water lost from the field through evaporation from the soil surface and transpiration from the plant itself.
A more telling measure of cotton’s water use is its Water Use Efficiency (WUE), which quantifies the amount of cotton lint produced per unit of water consumed. Globally, the amount of water needed to produce one kilogram of cotton lint averages around 1,931 liters of irrigation water and 6,003 liters of rainwater. However, this metric shows wide regional variation, demonstrating that local practices and climate dramatically influence the efficiency of water use.
The total water needed (ET) must be distinguished from the irrigation water supplied by the farmer. In regions with substantial rainfall, the need for supplemental irrigation is much lower, or even zero, especially in humid regions where up to 65% of cotton is grown without irrigation. Conversely, in arid climates, nearly all of the 500 to 900 mm requirement must be delivered through irrigation systems. The goal of a water budget is to determine the gap between effective rainfall and the crop’s total water demand.
Critical Water Periods During Plant Development
Cotton’s water requirements change dramatically as the plant progresses through its life cycle, making the timing of water application more important than the total volume. In the initial stage, from planting to the first square, the plant has a low water demand, often less than 2.5 mm (0.1 inches) per day, as the plant canopy is small and root development is still occurring. Allowing some mild, controlled water stress during this early period can actually encourage a deeper root system, which helps the plant access water later in the season.
The demand increases rapidly during the squaring and flowering stages, becoming the most water-sensitive period for the plant. During this phase, daily water use can peak at 7 to 9 mm (0.28 to 0.35 inches) per day and even exceed 14 mm (0.55 inches) on hot, windy days. A water deficit during the 9 to 10 days of peak bloom can cause significant yield loss by triggering the plant to shed its squares and young bolls.
Water demand remains high during the boll formation and fill period, as the plant directs resources toward developing the fiber inside the fruit. Water stress at this time severely impacts fiber quality, resulting in shorter staple length and high micronaire. As the crop enters maturation and boll opening, the water requirement naturally tapers off. Excessive water supplied too late can delay the final maturity and negatively affect the quality of the harvest.
External Factors Modifying Water Needs
Environmental and agronomic variables modify the actual water required by the cotton crop. Climate factors, such as high air temperature, low humidity, and high wind speed, directly increase the rate of evapotranspiration, thereby raising the crop’s daily water demand. A hot, windy day places a high evaporative demand on the field, which must be met to prevent plant stress.
Soil Type
The soil type on which the cotton is grown plays a major role in how water is retained and made available to the roots. Sandy soils have a low water-holding capacity, meaning they require more frequent, but smaller, applications of water. Conversely, clay soils hold water much better, allowing for less frequent irrigation, but they can be susceptible to waterlogging if too much water is applied.
Variety and Rainfall
The specific variety of cotton planted also influences the overall water need and stress tolerance. Some modern varieties are bred for shorter growing seasons or for enhanced drought tolerance, which can reduce the total seasonal water requirement. Moreover, the amount and distribution of in-season rainfall directly reduce the need for irrigation input, as effective rainfall contributes to meeting the crop’s total water demand.
Water Delivery and Efficiency Methods
Farmers utilize various technologies to deliver water precisely and improve production efficiency. Drip irrigation, particularly subsurface drip irrigation (SDI), is the most water-efficient method, delivering water directly to the root zone with up to 95% efficiency by minimizing surface evaporation. Although the initial cost is high, SDI can lead to significant water savings and improved yields.
Center pivot and other sprinkler systems are also highly efficient, typically operating at 85% to 95% efficiency, especially when equipped with low-pressure application nozzles. These systems are well-suited for large, circular fields and allow for uniform water distribution.
Traditional furrow or flood irrigation, where water flows down furrows or covers the field surface, is generally the least efficient method, often resulting in significant water loss through runoff and deep percolation.
Precision irrigation relies on monitoring tools to inform farmers exactly when and how much to irrigate. Devices like soil probes, tensiometers, and capacitance sensors measure soil moisture levels in real-time, providing actionable data to prevent underwatering and overwatering. Remote sensing and satellite imaging can map moisture variability across large fields, allowing for zone-specific scheduling that optimizes water application and enhances water use efficiency.