Limited irrigation capacities, high energy costs, aspiration to conserve water and need to get the best economic return for crop inputs are strong motivators for efficient water management. Approaching the end of the irrigation season, producers are hoping to optimize efficiency and minimize water losses while sustaining the crop through maturity.
Seasonal total and peak crop water requirements
Cotton seasonal water demand in the Texas High Plains can be as high as 27 to 29 inches for fully irrigated cotton. Peak water demand for cotton (occurring around peak bloom) is often between 0.3 and 0.4 inches per day. Between peak bloom and first open boll stage, the crop water demand begins to decline through boll opening and harvest. For efficient water management in cotton production, a target of 75 percent to 80 percent of the crop water demand based upon crop evapotranspiration (ET) is recommended, assuming high irrigation application efficiency.
Higher water applications may not increase yield significantly, resulting in lower water use efficiency. Drought stress should be avoided early in the season to prevent yield losses, but excess water applications late in the season can contribute to vegetative “re-growth,” provided excess nitrogen is also available. The goal is to provide adequate moisture for the last harvestable bolls to mature.
In any given field, the last irrigation will depend on seasonal conditions, soil type and the irrigation system. Ideally, the last irrigation should provide just enough plant available moisture to retain and mature all the bolls that have a reasonable chance of producing lint of acceptable quality under normal growing conditions.
General recommendations for West Texas cotton production are that furrow irrigations that bring fields to field capacity should be terminated before Sept. 1; sprinkler or subsurface drip irrigation (SDI) termination should be considered once open bolls appear. In many years the value of continued center pivot irrigation and SDI after bolls begin to open is probably questionable, unless extremely high temperatures and high water use are encountered and the field has a depleted moisture profile and a late boll load.
There is considerable management utility in using deficit irrigation with center pivots and SDI to apply small amounts of water extended into early September as needed to minimize fruit shed. Because of highly controlled irrigation amounts in center pivot or SDI, producers may need to "wean off" high yielding fields and not terminate completely at that time.
With center pivots and SDI, low amounts of irrigation can be applied if the cotton is severely stressed after initial termination. If the amount of wilting is unsuitable for the boll load, then the pivot can be passed over the field or drip applications may be made to apply additional water. These amounts could be as small as 0.75 to 1 inch per week depending on profile moisture and crop conditions. Generally, we observe about 2 percent to 5 percent boll opening per day once bolls begin to open. This implies that if the last irrigation is made at a few percent open bolls, then it should take about 10 days to reach 30 percent to 60 percent open bolls. Research projects addressing these issues relative to fiber quality are underway.
Current local crop water demand information
Local, current evapotranspiration (weather-based crop water demand) estimates can be used to refine irrigation management throughout the season. In some areas, crop water use estimates are available through local or regional Evapotranspiration (ET) networks. These networks calculate and disseminate crop-specific evapotranspiration (ET) information based upon local weather data.
Some networks, including the Texas High Plains Evapotranspiration Network (http://txhighplainset.tamu.edu ) and the Texas ET Network (http://texaset.tamu.edu ), provide crop-specific local information. Other networks, including the Oklahoma Mesonet Ag Weather (http://agweather.mesonet.org/ ), New Mexico Climate Center (http://weather.nmsu.edu/ ), Arizona Meteorological Network (http://ag.arizona.edu/azmet/ ), and California Irrigation Management Information System (CIMIS) (http://wwwcimis.water.ca.gov/ ), provide reference crop ET (grass crop or alfalfa crop) estimates, which can be used to further calculate crop-specific water use. Most of these networks also provide additional irrigation management information and scheduling tools. Stored soil moisture
The root zone of many row crops, including cotton, can be as deep as 5 to 6 feet, if soil conditions allow. Roots are generally developed early in the season, and will grow in moist (but not saturated or extremely dry) soil. Most crops will extract most of their water from the top 1 to 2 feet of soil, and almost all from the top 3 feet of soil, if water is available. Deeper soil moisture is beneficial primarily when the shallow moisture is depleted in high water demand periods. A shallow-rooted crop is more susceptible to drought and related injury.
Soil moisture storage capacity describes how much plant available water can be retained in a soil. A sandy soil generally will hold 0.6 to 1.25 inches of plant available water per foot of soil; a clay loam will hold 1.5 to 2.3 inches of water per foot of soil. Because a soil's moisture holding capacity is limited, excess water applications can be lost through deep percolation and/or runoff. Soil specific information is available from the USDA-NRCS Web Soil Survey at http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm  or from your local USDA-NRCS Service Center or county Cooperative Extension office.
Soil moisture monitoring provides very good feedback to fine-tune irrigation. Are irrigation applications adequately re-filling the root zone? Is deep moisture available, and are the roots extracting it? Soil moisture monitoring tells us where the water is stored, as well as if and where the roots are accessing it.
Management allowable depletion (MAD) is a soil moisture management concept that takes into account that as stored soil moisture is extracted from the root zone, increasing energy is required to remove the remaining moisture. Effectively, the plant has to “work harder” to get the water as the plant available water is depleted. For example, to avoid drought stress in the crop, an irrigation manager may use a MAD of 50 percent of plant available soil moisture depletion as a threshold to trigger an irrigation application. Typical MAD levels for most row crops, including cotton, are in the range of 45 percent to 60 percent; for drought sensitive crops MAD levels of 30 percent to 50 percent are more common.
Irrigation system capabilities will, of course, need to be considered in timing the last irrigation application. Center pivot and subsurface drip irrigation systems afford the capability to apply timely irrigation and flexibility to adjust irrigation depth per application. If the crop is delayed in maturity; if the weather turns unseasonably hot, dry and/or windy; or if the soil moisture has been depleted, additional irrigation applications can be delivered as needed. Cost of pumping and relative economic return (crop yield and quality) per water input must also be considered.
Water use efficiency (yield return per water applied) is generally greater under deficit irrigation management than under full irrigation management. Even so, moderate to severe drought stress, particularly early in the season, will reduce crop yield. Efficient crop water management maximizes the effective use of rainfall, stored soil moisture, local crop water use information and efficient irrigation applications to optimize the overall water use efficiency.