Making informed irrigation management decisions is an essential part of good cotton management practices and often plays a vital role in optimizing productivity and profitability. Improper irrigation scheduling translates to wasting or underutilizing water resources through under – or over-irrigation and more precise irrigation practices involve applying the proper amount of water at the right time. In most years, supplementing rainfall with timely irrigation events will not only assist in supplying water for crop evapotranspiration needs, but also has the role of regulating plant growth, influencing weed and insect pest pressures and can reduce the incidence of disease. Successful growers agree that even in regions where rainfall provides most of the crop water needs during the season, developing a sound irrigation management approach is economical and increases overall resource use efficiency.
When irrigation water is readily available, there is a tendency for some to over-irrigate cotton to its detriment, thereby reducing the opportunity to maximize profit. While frequent irrigation results in low plant water stress levels and rapid canopy expansion, too much of a good thing can result. Allowing some level of water stress between irrigation or rainfall events is beneficial for cotton and allows the plant to moderate its vegetative growth. Without periodic water stress events during the boll set period, most cotton varieties have the tendency to grow rapidly, thereby shading lower branches that are important in providing photosynthates to nearby bolls. Too much shading too early can compromise lower boll retention and delay crop maturity.
Over-application of irrigation water on cotton also impacts farm resource utilization by reducing energy and nutrient efficiencies. Applying water that is not required for beneficial use increases pumping costs and creates conditions that can lead to nutrient leaching below the root zone. Figure 2.2 illustrates the relationship between pumping depth and energy requirements to apply an acre-inch of water. Note that approximately one gallon of diesel is needed for every 100 feet that water is raised. The deep percolation of water is responsible for carrying nutrients from the site, potentially contaminating adjacent ground and surface waters. Nitrate nitrogen is often the most common mobile constituent in these leachates; however, other nutrients such as phosphate as well as some pesticides can percolate into the local water table when sound irrigation management methods are ignored.
Applying too much water can result in cutting off the root's supply of air that is important for maintaining proper root function. Particularly in areas where confining layers limit downward movement of water or in high clay content soils, anoxic conditions can develop in a relatively short amount of time, limiting cotton's ability to absorb nutrients and water properly. This condition manifests by limiting the optimal expansion of roots by the plant, suppresses optimal nutrient balance and can result in chlorotic plants that have yield limitations.
Central in the practice of irrigation scheduling is the concept of plant available water. Following a significant irrigation or rainfall event, water saturates soil pores and within a day or two drains as a result of gravitational forces taking drainage water deeper into the ground, leaving the soil in a state of high water status or high water potential. Like a fully wet sponge, the soil contains a large amount of water that is readily available to the plant. This water is held by soil colloids and, no longer responding to gravitational forces, is said to be at field capacity and is unique to each soil. In this highly available state, cotton roots are in full contact with soil water and uptake occurs with great ease allowing the plant the capacity to readily take up water, fulfilling the full atmospheric demand for water through evapotranspiration (ET). Because of the ease with which water uptake occurs, the plant's water conducting tissues are also in good water status or high water potential, and plant tissues responsible for growth allow the plant to grow at a high rate.
However, as readily available water is extracted from the soil, soil water retention forces increase (tension) and the plant root, as well as above ground plant tissues, find it more difficult to transport water and match the rate at which ET proceeds. Under these conditions, both soil and plant water status (potentials) are decreased and eventually get to the point at which plant growth rates are reduced. As the continued extraction of soil water takes place, indicators of water stress increasingly begin to take form. More significant declines in plant growth rate occur, as well as rates of evapotranspiration as plant stomata begin to close and protect the plant from more serious heat stress. If there is a continuation of water loss from the soil and plant without replenishment, the plant ultimately reaches its permanent wilting point and dies. Figure 2.3 illustrates the relationship between soil moisture content and soil moisture tensions for two soil types. Studies have found cotton is nearing soil moisture stress when tension exceeds 30 to 50 centibars in the root zone depending on soil type.
In practice this condition would never be allowed; however, by estimating these quantities, total plant available water can be established as the difference between the water content held at field capacity and the water content of the soil at permanent wilting point. Once we define or approximate plant available water for a given soil or field, we can now better estimate the individual soil's ability to provide water to the crop and better plan the timing of irrigation events. There is also a relationship between plant available water and plant water stress and, once understood, will go far to assist the grower in properly establishing the scheduling of irrigation events.
Even in regions where spring and summer rainfall is considerable, the cotton plant can experience water stress to the point where productivity and quality are reduced. This is especially true during extended periods of drought and in soils that have limited plant available water. Although the soil acts as an ideal medium to store water and nutrients, the amount of plant available soil water is reduced daily following a rainfall or irrigation event. As the duration of water deficit increases, so too does the intensity of the stress and the eventual need for irrigation. Water stress can build to the point where physical and physiological changes can influence crop performance.
Indicators of heightened water stress levels in cotton include the reduced size of newly formed leaves, shortened main stem internodes, reddening of the leaf petioles and the reduced growth rate of the entire plant. And while cotton often benefits from some level of growth-limiting stress, more severe water deficits can depress the yield and quality expectations. As cotton water stress levels are elevated, more severe reductions in vegetative growth occur and the leaf stomata begin to close. Photosynthesis slows, ultimately leading to significant reductions in amount and type of carbohydrates produced by the plant. Particularly during the early and middle boll set periods, more severe water stress depresses fiber quality primarily by reducing the length and strength of cotton fiber and can simultaneously lower seed cotton yield.
Research studies have consistently shown that in humid regions, irrigation can dramatically increase cotton yields, in some cases doubling yield and improving overall quality depending on the extent and duration of the water deficit. Targeted irrigation scheduling allows the cotton producer to more precisely manage the plant by controlling the amount and duration of the water stress that, in turn, impact overall crop performance. Proper scheduling of water recognizes that climate and soil water availability primarily determine the rate at which water stress accumulates in cotton. Understanding how these two interact to affect crop water status is key to good water management practices.
Irrigation scheduling is the science of determining when to irrigate, how much to apply, where to apply, and for what purpose. The purpose may be to maximize land productivity (yield per unit land), water productivity (yield per unit water used), net profit, and/or quality and marketability. Different scheduling strategies are usually needed depending on the producer goals in mind. For instance, achieving maximum yield via maximum irrigation may not necessarily lead to the highest water productivity or net profit. In fact, research in the dry environments shows that the highest crop water productivity may coincide with irrigation amounts at levels below the full crop water requirements.
When water availability is limited, the traditional practice of maximum irrigation for maximum land productivity may no longer be wise. In such conditions, irrigation must be optimized, and may prove more profitable, by attempts to get the most crop per drop rather than the most crop per unit land. This may require practices such as deficit irrigation on all the land rather than full irrigation on part of the land.
The primary goal of irrigation is to ensure that sufficiently high crop water use rates are sustained during the season that allow non-limiting soil water conditions for optimum plant growth and development. However, establishing sound irrigation practices also provides that ample stress will build between irrigation events that keeps vegetative growth manageable and in line with the field and variety management strategy. Managed stress also promotes late season boll maturation, boll opening and improved defoliation. That strategy includes incorporating the knowledge of cultivar performance, plant growth regulator approach, and how the soil retains water and nutrients, keeping in mind that these strategies can change during the season depending on climate, pest and nutrient conditions. Given these complexities that influence irrigation management decisions, it is no wonder that many successful growers regard irrigation scheduling as both an art and a science.