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  1. Agricultural IPM
  2. Field Crops
  3. Cotton
  4. Irrigation
Cotton Irrigation
Section Shortcuts
Consumption Chart
Plant Available Water
Consumption Chart

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Plant Available Water
Irrigation

When the soil profile is full of water, reaching what is called field capacity (FC), the profile is said to be at 100% moisture content or at about 0.1 bars of tension. Tension is a measurement of how tightly the soil particles hold onto water molecules in the soil: the tighter the hold, the higher the tension. At FC, with a tension of only 0.1 bars, the water is not being held tightly and it is easy for plants to extract water from the soil. As the water is depleted by the plants, the tension in the soil increases. Figure 1 shows three typical curves for sand, clay and loam soils. As Fig. 1 shows, the plants will use the water in the soil until the moisture level goes to the permanent wilting point (PWP). Once the soil dries down to the PWP, plants can no longer extract water from the soil and the plants die. Although there is still some moisture in the soil below the PWP, this water is held so tightly by the soil particles that it cannot be extracted by the plant roots. The PWP occurs at different moisture levels depending on the plant and soil type. Some plants, which are adapted to arid conditions, can survive with very little moisture in the soil. With most agronomic crops, PWP occurs when the tension in the soil is at 15 bars. This means that the soil is holding on very tightly to the water in its pores. In order for plants to use this water, they must create a suction greater than 15 bars. For most commercial crops, this is not possible. At 15 bars, most plants begin to die. The difference between field capacity and PWP is called the plant available water (PAW).

 

Irrigation targets are usually set as a percent depletion of the PAW. This depletion level is referred to as Management Allowable Depletion (MAD). The bulk of irrigation research recommends irrigating row crops such as grain or cotton when the MAD approaches 50%. For vegetable crops, the MAD is usually set at 40% or less, because they are more sensitive to water stress. These defined amounts insure that water stress will not be so severe as to cause any appreciable yield losses. Careful monitoring of the PAW needs to be done throughout the season so that the appropriate point of irrigation can be anticipated.

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az1220-2014_0 (pdf)
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Irrigation Publications
  • Jun 2015
    Soil Management and Soil Testing for Irrigated Cotton Production
    Publication (PDF, 436KB)
    Authors: J. C. Silvertooth
    Whenever studying cotton, it is a natural tendency to focus on the above ground portions of the plant. However, an equally important part of the plant is the root system, The soil is a focal point of any farming operation.
  • Jun 2015
    Water Management for Defoliation
    Publication (PDF, 316KB)
    Authors: Silvertooth, Jeffrey C.
    There are several factors which are important to consider in managing defoliation. Factors such as plant-water relations, Nitrogen (N) fertility status, the extent of honeydew deposits on the leaves from insects such as the sweet potato whitefly or aphids, and weather conditions following the defoliant application are all important in terms of the final defoliation results.
  • Aug 2014
    Methods of Measuring for Irrigation Scheduling - When
    Publication (PDF, 1.3MB)
    Authors: Martin, Edward C.
    Proper irrigation management requires that growers assess their irrigation needs by taking measurements of various physical parameters. Some use sophisticated equipment while others use tried and true common sense approaches. Whichever method used, each has merits and limitations. In developing any irrigation management strategy, two questions are common: “When do I irrigate?” and “How much do I apply?” This bulletin deals with the WHEN.
  • Jul 2013
    Measuring Water Applications
    Publication (PDF, 144KB)
    Authors: Ayman Mostafa
    One of the challenges in using surface irrigation systems is determining the effective amount of water applied to a field. To get a good estimate, you must first calculate the gross amount of water applied and then, taking into system efficiencies, determine the amount of effective water applied (what the plants actually receive).
  • Dec 2011
    Measuring Water Flow in Surface Irrigation Ditches and Gated Pipe
    Publication (PDF, 1.28MB)
    Authors: Edward C. Martin
    Measuring water in surface irrigation systems is critical for peak efficiency management. Irrigation management decisions should be made based on the amount of water applied and how this relates to the consumptive use demands of the plants and the soil water holding capacity.
  • Dec 2011
    Determining the Amount of Irrigation Water Applied to a Field
    Publication (PDF, 535KB)
    Authors: Edward C. Martin
    Critical to any irrigation management approach is an accurate estimate of the amount of water applied to a field. Estimating the amount of water applied to a field or to a set is fairly easy for surface systems.

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Irrigation Presentations
  • Feb 2013
    Pre-season and Early Season Cotton Management
    Presentation (PDF, 1.56MB)
    Authors: Guangyao (Sam) Wang
    Location Presented: Avondale, AZ

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