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  1. Agricultural IPM
  2. Field Crops
  3. Small Grains
  4. Irrigation
Small Grains 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
  • May 2015
    Irrigation of Small Grains in Arizona
    Publication (PDF, 532KB)
    Authors: Steve Husman and Michael J. Ottman
    Wheat and barley use about 2 ft of water in Arizona, but 3 to 3.5 ft of applied water is often required with surface flood irrigation due inefficiencies in the irrigation system. Some suggestion on how to irrigate your small grains are made, including when and how much.
  • May 2015
    Small Grain Growth and Development
    Publication (PDF, 443KB)
    Authors: Michael J. Ottman
    Small grain growth and development can be divided into several major and minor stages. This article discusses those stages and what to look as the crop develops. A chart also details the timing of management operations during crop development.
  • May 2015
    Planting Methods For Small Grains in Arizona
    Publication (PDF, 320KB)
    Authors: Michael J. Ottman
    Small grains are planted for a variety of reasons, but their rotational benefit makes them a popular crop all over the world and influences the way they are planted. One of the major benefits of small grains as rotational crops is that they cover the soil and suppress weeds
  • Jun 2014
    Sensor-based management of Nitrogen of irrigated durum wheat in Arizona
    Publication
    Authors: Pedro Andrade-Sanchez and Michael J. Ottman
    It can be difficult to accurately apply the proper amount of nutrients to wheat due to their varying sizes and densities. Current field equipment can already vary the rate of fertilizer dispersal, but it needs to be controlled by an algorithm. An algorithm is being developed for that purpose.
  • Jan 2014
    Recommendations for Growing Standard-Height Wheat Varieties in Arizona
    Publication (PDF, 537KB)
    Authors: Mike Ottman and Joy M. Hought
    Until the introduction of semi-dwarf wheat in the late 1960s, wheat varieties were typically one and a half to two times their current height. Most "standard height" wheat varieties are adapted to lower-input conditions, and cannot tolerate high-fertility environments without lodging. Planting date, seeding rate, nitrogen rate, phosphorus rate, and irrigation are discussed in this article.
  • 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
    Small Grain Irrigation
    Publication (PDF, 413KB)
    Authors: Shawna Loper and Mike Ottman
    This document has a quick breakdown of some of the most important factors to consider when irrigating small grains. Topics such as seasonal water use, the first irrigation, last irrigation, and soil water balance methods.
  • 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.
  • Jun 1989
    Winter Grass Pasture For Low Desert Valleys In Arizona
    Publication (PDF, 863KB)
    Authors: Michael Ottman and David Kopec
    Crops that can be used for winter grass pasture are barley, oats, wheat and common or tetraploid ryegrasss. Suggestions on planting time, seeding rate, fertilizing, irrigating, harvest and management are discussed.

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Irrigation Presentations

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