Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Summer 2014
13 The Fluid Journal Summer 2014 in the 1970s and 1980s to about 1,000 lbs lint/A beginning in the mid 2000s. Dryland yields range from 250 to 300 lbs lint/A to 500 to 600 lbs lint/A, depending on the year’s rainfall and timeliness of the rain. Cotton defined Botanically, cotton is defined as a woody perennial with an indeterminate growth habit, which means that it does not have to produce seed to survive as a plant species. When the cotton plant is subjected to environmental stress, such as limiting water supply, high temperatures, or nutritional deficiencies, it becomes a survivor rather than a producer. This requires the grower to fully understand the growth and developmental processes of the cotton plant and intensely manage the resources he can control. An adequate water supply throughout the life cycle is the single most limiting resource to attaining maximum genetic yield potential of a cotton crop. Proper management of the total water resource (rain and irrigation) is critical to maximizing yield and product quality. PET Water use by every growing plant is a function of the amount of leaf surface area relative to ground area (leaf area index = leaf area/ground area) and the potential evaporating of the atmosphere (PET). PET is determined using net solar radiation, air temperature, water vapor content of the air relative to water holding capacity of the air (relative humidity), and wind speed close to the crop (6-foot height). The standardized and widely accepted PET equation is the modified Penman. Weather Station Networks have been established in most states that provide hourly and daily weather information that is widely used to calculate PET on a daily basis to aid in water management, especially in irrigation water applications. Daily PET varies considerably from day to day within a location (see Figure 1, diagram of typical daily PET in West Texas). In West Texas, seasonal PET averages about 40 inches from early May through September. When plants are small and cover very little of the ground area the majority of the water loss from the field is from bare soil evaporation. However, when the plant begins to develop floral structures at the 6th to 7th main stem node until it begins to flower, leaf area index increases rapidly and daily water use increases proportionately. Water use The ratio of crop water use to total daily evaporative demand is defined as the crop coefficient (see Figure 2, crop coefficients for cotton grown in West Texas). The total crop water use for a season averages about 24 inches in West Texas. The volume of water can produce around 1,500 lbs of lint and 2,100 lbs of cotton seed. Cotton, similar to all other seed producing plants, is most sensitive to limited water supplies during the reproductive development phase. Due to its indeterminate growth habit, cotton never loses the ability to produce main stem nodes and leaves. Fruiting sites are produced from axillary branches beginning from the 6th main stem node through the 18th. Each fruiting branch is capable of producing 3 to 4 fruiting sites. This requires 36 to 40 days to complete the production of fruiting sites. Water stress during the production of fruiting sites stage of development results in fewer main stem nodes and thus fewer reproductive branches with each branch producing fewer floral structures, resulting in a severe reduction in potential boll number and thus yield. Each fruiting site requires about 500 heat units to reach the flowering stage. The first flower at the 6th to 7th node occurs at 1,000 total heat units or about 60 days of age in an average year. Flowering progresses up the plant in 3-day intervals and out of a fruiting branch in 5- to 7-day intervals. The first five days following Figure 2. Crop coefficients—Cotton 0 2013COTTONWATER USE Figure 3. 2013 Cotton water use