Q: What is meant by limited or deficit irrigation?
A: Limited or deficit irrigation refers to situations where irrigation water is either managed or applied out of necessity in amounts throughout the duration of the irrigation season such that the total amount of water applied is less than that required to meet the potential evapotranspiration of the crop. Limited irrigation may be achieved by one of several techniques, including: 1) reducing acres under irrigation, 2) reducing irrigation water applied to the field per application and throughout the growing season, 3) making use of the available recycled water, and 4) modifying irrigation equipment and systems through sprinkler nozzle configurations, system pressure modifications. Limited irrigation practices are appropriate where irrigation water is not readily available to meet crop-water requirements and in areas of competing demands for ground and surface water supplies.

Factors that will determine the ultimate success of limited irrigation strategies include crop choice in relation to climate and availability of water, physical and chemical soil properties, anticipated precipitation amounts and timing throughout the year and during the growing season. Because reduced yield is often a result of limited irrigation practices, the goal is to manage crops and water use for the greatest possible return for the crop grown. For this reason, it is important to implement additional water conservation techniques in conjunction with limited or deficit irrigation strategies.

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Q: How do conservation tillage practices aid water conservation efforts?
A: Conservation or reduced tillage refers to the practice of utilizing tillage methods and equipment that strive to retain crop residue that provides a minimum of 30% soil surface coverage at all times, which includes both during crop growing periods and periods when agricultural land is idle (such as during fallow periods). Conservation tillage can enhance water conservation by: 1) reducing runoff, 2) increasing surface moisture detention by means of snow trapping and wind reduction, 3) reducing evaporation by providing shade and protection from wind. Conservation tillage and the resulting changes in soil physical properties often lead to increased infiltration rates, which can be a be a benefit to irrigated soils with low and medium intake rates. Conservation tillage, in conjunction with monitoring soil moisture particularly in the spring after sufficient winter precipitation, may allow irrigators to delay or eliminate one to two irrigation applications during the irrigation season.
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Q: Practically, or in reality, how much water can I conserve by practicing limited or deficit irrigation?
A: It is conceivable that an irrigator could reduce the amount of irrigation water applied by 10 to 20% by practicing limited or deficit irrigation. Obviously, this depends on the crop, irrigator experience, system design and efficiency, previous irrigation practices, soil and weather conditions. It also depends on the irrigator’s management and production goals.
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Q: What limitations to irrigation practices are caused by conservation tillage?
A: Concerns related to irrigation timing and uniformity of water distribution has discouraged some irrigators from adopting conservation tillage practices. Residue left on the soil surface often causes problems in furrow irrigation systems, where residue dams plug furrows and prevent uniform movement of water down furrows.
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Q: What are some other low-cost management practices that irrigators can adopt to conserve moisture when implementing deficit or limited irrigation strategies?
A: Furrow firming is a practice which can be used to reduce or even infiltration rates in soils with high intake rates. Additional changes irrigators can implement include changes in furrow stream size which determines advance time. Management of the amount of crop residue, set times, length of run, surface conditions at the trough of the furrow, and furrow size together will increase water savings.
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Q: Are some crops better suited to limited or deficit irrigation than others?
A: Some crops are drought and saline tolerant, while others may flourish in a cooler or warmer climate. Additionally, some crops are more sensitive to moisture deficit during early growth while others are more sensitive to moisture deficit during mid-season or late-season growth. Pasture is a good example of this, where warm-season grasses perform better in Texas while cool-season fescue grows better in high altitude and cool temperatures.

Cool-season, short-growth crops such as brown mustard, canola and camelina have become an essential component in on-farm agricultural water conservation strategies. They are known to use less water and allow more time to accumulate water prior to planting wheat, for example, to replace fallow in the High Plains of western United States. Choosing crops based on their plant-water requirements as well as crop rotation and fallow periods contribute to agricultural water conservation at the micro and macro level of farming systems.

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Q: What guidelines should be applied to selection of crops for limited or deficit irrigation?
A: Analyzing crop selection requires knowing the well yield and field size, the irrigation regime, the cost of irrigation, yield response to water for different crops irrigated, water requirements per crop, labor requirements, and expected climate. Before making any cropping decisions, check with the appropriate Farm Service Agency, CRC coverage, Revenue Assurance, etc. regarding any restrictions. When selecting crops, look at when the crop most needs water as well as how much water will be needed. Make a comparison between when the crop needs water and when it will be available from the irrigation district. If water is not expected to be available during a critical growth period, consider another crop. Try to make adjustments that will work in a dry year, but will not limit potential yields if the drought ends. Make sure there is an economic use for the crop being considered as well as the technical expertise and equipment necessary for growing a different crop.
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Q: Are some cropping strategies or crop rotations better-suited to limited or deficit irrigation than others?
A: When implementing limited or deficit irrigation it is important to match crop rotation with local patterns of precipitation and evaporative demand. Certain crops like soybean, edible bean, winter wheat and sunflower are the major crops with lower water requirements. Splitting fields between corn and one of these crops would reduce total water requirements for the field and distribute the water requirements across a longer portion of the growing season. For example, peak water demands for wheat are during May and June, while corn uses the most water during July and soybean water needs peak in August. Splitting the field into multiple crops allows producers with low-capacity wells or limited water supplies to more completely meet the peak requirements of all crops (Scheneekloth, Determining Crop Mixes for Limited Irrigation).
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