CROP YIELD STUDIES
Using daily evapotranspiration estimates to increase irrigation efficiency with subsurface drip systems
By Randy Norton, PhD
In the arid regions of the desert Southwest, water is by far the most limiting resource for crop production systems. As such, it is an extremely valuable resource to producers, and incentives to conserve and utilize it with utmost efficiency are of paramount importance. Irrigation and crop production systems in the desert Southwest have evolved to include techniques that conserve water resources and improve irrigation efficiencies. Some of these adaptations include higher efficiency delivery systems such as subsurface drip irrigation [SDI].
Daily ET
These systems have increased efficiencies with respect to delivery of irrigation water to the field and have allowed growers to more precisely manage crop water status by supplying more closely the amount of water the crop requires and when it is required. Crop water use can be estimated on a daily basis by utilizing reference evapotranspiration [ETo] data that can be obtained from weather monitoring systems maintained by universities or government agencies. An example of daily reference ETo for the low deserts of Arizona is shown in figure 1. Arizona desert ETo ranges from a low of 0.075 inches daily to just under 0.40 inches.
Knowing the reference ET is important, but it does not tell how the crop will use water based upon growth stage. The other component to estimating crop water use is a well-validated crop coefficient curve. A crop coefficient [Kc] is a factor used to adjust the ETo value to account for crop water use at different stages of growth. Crop coefficients depend on stage of growth and are typically presented as a function of time. The Kc developed for Arizona cotton
16 Irrigation TODAY | October 2017
is presented as a function of heat units [HU, thermal time], which is correlated to various stages of crop development (see fig. 2). Crop coefficients have been developed for many crops, but for the purposes of this article the crop coefficient developed in Arizona for cotton will be used.
Utilizing both ETo and Kc for a given day or other period of time, one can effectively determine the crop water use with the following equation, where ETa is the actual crop water use on that particular day:
ETa = KC ETo
Figure 3 shows the progression of crop water use [ETa] as a function of HU accumulated after planting [HUAP]. Also shown on this figure is the cumulative ETa as a function of HUAP. As an example, at peak bloom the crop coefficient for cotton is 1.2 (see fig. 2). The average date that the crop will reach peak bloom given an
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
1-Jan 1-Feb 1-Mar 1-Apr 1-May 1-Jun 1-Jul 1-Aug 1-Sep 1-Oct 1-Nov 1-Dec 1-Jan Figu e 1. Av F gurre verag ETo ates in inches ge ET rates inches of wa e as water a a functiunc ion of of day ay of year or a for the e low deserts ts of Ar zona Arizon
average April 10 planting would be June 22. The average ETo for that date is 0.25 inches. Take the ETo (0.25) and multiply it by the Kc (1.2), which results in 0.30 inches of crop water use that day. This data can then be used to estimate irrigation needs and timing based upon estimates of water- holding capacity of the soil.
System efficiencies
Once the amount of crop water use is determined, it is important to consider the other factors that influence the amount of water needed to achieve that replacement. Irrigation system efficiencies need to be considered when determining total amount of water to be applied to meet crop demand. If a high-efficiency SDI system is being used, irrigation efficiency may be close to 90 percent, meaning an additional 10 percent of applied water is needed to meet crop demand. Irrigation with SDI allows a producer to replace daily
Average daily evapotranspiration {in.}
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