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Crop water use, or evapotranspiration (ET), is the movement of water through evaporation (E) from the soil and plant surfaces and transpiration (T) through the plant. Transpiration is the movement of water from the soil into plant roots, through plant stems and leaves, and back out into the atmosphere. Transpiration is an important concept because yield is related to the amount of water a plant transpires. An understanding of cotton water use and practices that help to improve water use efficiency may help improve yield potential.
Weather and climate. The ability of the atmosphere to evaporate water is the driving force for soil water evaporation and transpiration. Daily ET is influenced by solar radiation, air temperature, relative humidity, and wind. High air temperatures, low humidity, clear skies, and high winds can greatly increase evaporative demand.
Relative maturity. Longer-season products will require more water over the growing season than short-season products. While longer-season products use more water, they may also have a higher yield potential if adequate heat units and water are available.
Soil water holding capacity and soil water content. A soil’s water holding capacity indicates the maximum amount of water that will be available for plant use when the soil water profile is full (field capacity). Fine textured soils can hold more water than coarse textured soils. As the soil dries, it becomes more difficult for plants to extract water. At field capacity, plants use water at the maximum rate. Plants use less water as the water content of the soil drops.
Soil and residue management. Tillage increases the exposed surface area of the soil which increases evaporation. Tillage also destroys crop residue that can help retain soil moisture. Minimizing soil disturbance from tillage and increasing surface crop residue can reduce soil water evaporation and help the soil capture and retain more moisture by creating obstructions that limit water movement and allow more time for water to infiltrate into the soil profile.
Compaction and restrictive layers in the soil can limit root growth and increase plant susceptibility to water stress during hot, dry weather because the plant roots are unable to reach the moisture deeper in the soil.
Crops will respond differently to water stress depending on the growth stage and fruit load. Cotton is less sensitive to water stress from planting to first square and after blooming has commenced. Moisture stress during the critical water use period (first flower to peak bloom) can quickly reduce yields by reducing plant growth, reducing the number of fruiting sites, causing shedding of young bolls, and reducing boll size. Severe water deficit at this stage can also lead to shorter staple, higher micronaire, and lower fiber quality. Severe water deficits during peak bloom to open bolls can lead to square and young boll shedding, though loss of late fruit has less effect on yield potential than loss of early-season bolls.
Excess moisture and poor drainage can negatively impact crop growth and yield potential. Soils that remain saturated for long periods can result in reduced oxygen in the root zone, an increase in soil-borne diseases, and restricted root growth due to lower soil temperatures. Excessive moisture early in the season can lead to shallow root systems and excessive vegetative growth. Excessive moisture during and after cutout can delay maturity leading to delayed harvest, and can lead to boll rot and germination of cotton seeds in the boll. Too much irrigation or precipitation can also increase soil loss due to runoff and cause leaching of nitrogen.