SPECIAL REPORT
How To Manage Desert Southwest Water Efficiently BY RANDY NORTON UNIVERSITY OF ARIZONA
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n 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 an incentive to conserve and use with utmost efficiency. Irrigation and crop production systems in the desert Southwest have evolved to include techniques that conserve water resources and improve irrigation efficiencies. Some of the adaptations include higher efficiency delivery systems, such as notched ditches, sprinkler, micro-sprinkler, and surface and subsurface drip irrigation systems. They have increased efficiencies with respect to irrigation water delivery to the field. Additionally, production practices, such as minimum tillage and no-till systems and cover crops, have changed to increase soil water-holding capacity and retention. Genetic advances have created more waterefficient crops and varieties. The introduction of higher efficiency irrigation systems has allowed growers to more precisely manage crop water status by more closely supplying the amount of water the crop requires and when it is needed. Reference Evapotranspiration Data Crop water use can be estimated on a daily basis by using reference evapotranspiration (ET o) data that can be obtained from weather monitoring systems maintained by universities or government agencies. Arizona desert ETo ranges from a low of 0.075 inches daily to just under 0.40 inches. Knowing 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 Twitter: @CottonFarming
Notched ditches let trash pass through and support better water control. coefficient (K c) is a factor used to adjust the ETo value to account for crop water use at different growth stages. Crop coefficients depend on growth stage and are typically presented as a function of time. The Kc developed for Arizona cotton was developed as a function of heat units (HU – thermal time), which is well correlated to various stages of crop development. Using both ETo and K c for a given day or other time period, one can effectively determine the crop water use. This calculation can be used to monitor crop water status and schedule subsequent irrigation. Once the amount of crop water use is determined, it is important to consider other factors that influence the amount of water needed to achieve that replacement. Take a look at irrigation system efficiencies when determining total amount of water to be applied to meet crop demand. If a high-efficiency subsurface drip irrigation system is used, irrigation efficiency may be close to 90% or better, meaning an additional 10% of applied water is needed to meet crop demand. Less efficient systems may be
around 75% where an additional 25% is needed to meet crop demand. Leaching Requirement In the arid Southwest, salinity management is another critical factor due to the potential of salt buildup in the soil that will negatively impact crop growth and development. A leaching requirement should always be employed. This is simply a percentage increase in applied irrigation water to effectively manage salts in the soil profile. The leaching requirement is based on the salinity of the irrigation water being applied and the salinity of the soil at which less than 100% yield potential may be realized for a given crop. Managing the incredibly precious water resource in the desert is critical to sustaining farming operations in the western United States. Advances in irrigation technologies and strategies in crop production go a long way in helping to effectively oversee this resource. As producers implement these technology advances, they are becoming more effective in producing environmentally and economically sustainable crops.  JULY 2020 | COTTON FARMING
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