In the humid Mid-South U.S., cotton irrigation is a challenge because of the variations in rainfall, temperature, and cloudiness during the growing season. Cotton crop characteristics, as well as the prevailing environmental conditions, are critical in determining cotton water use. Its use increases gradually from the initial stage (dominated by water loss from evaporative surfaces) to developmental stage, and finally peaking at the mid-season stage. This peak water-use stage coincides with a stage of full canopy and maximum boll load of the cotton plant which is normally in August (a month typically characterized by high air temperature and solar radiation) in the Mid-South U.S.
Cotton uses water throughout its lifecycle through the combined processes of evaporation and transpiration, often referred to as evapotranspiration (ET). Water use, or ET, includes the amount of water transpired by the growing plant and evaporated from the soil in which it grows. ET is therefore a function of weather variables (mainly solar radiation, wind, air temperature, and humidity), as well as soil characteristics, crop characteristics, and cultural practices.
Reference evapotranspiration (ETo) is the combined processes of evaporation and transpiration measured over a reference surface (typically a grass surface). Crop evapotranspiration (ETc) represents the amount of water lost through the process of evaporation (from soil surface) and transpiration (from plant tissues) from a crop, grown in a large field, under a given climatic condition. The amount of water used to balance this loss is often referred to as the crop water use. ETc is estimated by first calculating reference ETo, which quantifies the evaporative demand of the environment. ETo is then adjusted by a crop-specific crop coefficient function, Kc, which accounts for specific crop and growth-stage conditions. As the crop changes throughout the growing season, the crop coefficient adjusts to account for differences in plant growth and water use.
The best way to measure ETc when determining a crop coefficient is with a lysimeter that measures the weight of water loss or gain during the day (Figure 4.1). The variability among environmental and cultural factors across regions requires the determination of local ETo, ETc, and crop coefficients, Kc for a given crop for irrigation scheduling.
Typical cotton water use at important stages (initial, developmental and mid-season) of a cotton plant is presented in Table 4.1. Data were collected from a cotton planted with Stoneville 5458 B2RF grown on a Sharkey clay soil in Northeast Louisiana. At the initial stage of the crop (approximately 0-25 days past planting), daily crop water use (ETc) ranged from 0.03 – 0.20 inch/day with an average of 0.09 inch/day or 0.63 inch/week. Average water use was 0.22 inch/day (approximately 1.5 inch/week) at the crop developmental stage and 0.28 inch/day (approximately 2.0 inches/week) at midseason. The corresponding average Kc values are 0.48, 1.02 and 1.44 for initial, developmental and midseason stages, respectively (Table 4.1).
Crop water use in Stoneville, Mississippi was similar but the Kc values there were lower. Water use ranged from about 0.05 inch/day early in the season to a peak of 0.28 inch/day, and then decreased after boll opening to 0.12 inch/day. Crop coefficient values ranged from 0.4 during the initial period to 1.2 during midseason, and then decreased to 0.6 at the end of the season. Average daily water use and crop coefficient functions are shown in Figure 4.2.
A typical 24-hour variation in the mass of a weighing lysimeter planted with cotton crop in Louisiana is illustrated in Figure 4.3. Figure 4.4 represents a water use (crop evapotranspiration) curve showing the seasonal water use characteristics of cotton at essential growth stages. Daily water use is expressed as a function of days past planting. Water use was observed to increase steadily from planting to first open boll and tended to decline slightly afterward. This suggests the need of maintaining well-watered field conditions until the first open boll. At about 60% boll opening, water use tends to substantially decline (Figure 4.3).