Considerations for Limited Irrigation
Producers can make some adjustments to compensate for reduced allocations or well capacity.
Limited irrigation results from restricted water supplies that cannot meet the full evapotranspiration demands of a crop. Reasons that producers may be limited on the amount of available water include limited capacity of the irrigation well or reduced surface water supplies.
When producers cannot apply water to meet crop ET, they must realize that with typical management practices, yields and returns will be reduced as compared to a fully irrigated crop. To properly manage the water for the greatest return, producers must have an understanding of how crops respond to water, how cropping mixes can be adjusted to better match water availability, and how changes in agronomic practices can influence water needs.
Yield vs ET and Irrigation
Crop
yields increase linearly with the water that is used by the crop (Figure
1). Crops such as corn, respond with more yield for every inch of
water that the crop consumes as compared to winter wheat or sunflower.
However, high water use crops such as corn also require more water for
plant development and maintenance before grain yield is produced. Corn
requires approximately 10 inches of ET as compared to 4.5 and 7.5 inches
of ET for wheat and sunflower before any yield is produced. These crops
also require less ET for maximum production compared to corn.
Irrigation results in increased ET and thus grain yields by supplementing rainfall in periods when ET is greater than precipitation. However, not all of the irrigation water applied will result in ET because of losses resulting from irrigation system inefficiencies. As yield is maximized, more losses occur since the soil is close to field capacity and more prone to losses such as deep percolation and runoff (Figure 2). As shown in figure 2, a reduction in water applied from point A to point B can save water with little or no yield reduction.
Limited
Water Management – Reduced Allocations
When producers are faced with reduced surface water supplies, they have
three management options that can be utilized: 1) reduce irrigated acreage
2) reduce the amount of irrigation applied to the entire field or 3) include
different crops that require less irrigation. Option one idles potentially
productive ground while option two reduces yields for the entire irrigated
acres unless precipitation is above normal. The third option involves
using crops that require less water for maximum production and then using
any “saved water” to fully irrigate reduced acreage of traditionally
irrigated crops.
The following is an example of these three options under reduced allocations using ET from Longmont. A grower could produce all irrigated corn or irrigate some corn and a lower water use crop such as dry beans. Corn grown around Longmont (see Table 1 in previous article) requires 17.3 inches of net irrigation (assuming 85% efficiency) and dry bean requires 10.4 inches. If the allocation from the ditch limits a producer to 14 inches of water the producer could raise 80% of their acres to irrigated corn and the remainder in dry-land production or idle. He could also raise 100% of his acres to corn and apply only 80% of the irrigation required for maximum production. The final option would be that he could raise 50% of his acres to dry bean and 50% of his acres to corn and maintain maximum production on all of his acres.
Limited Water Management – Low Capacity Systems
When managing for maximum production, irrigation systems must have minimum
capacities to meet crop water requirements during peak water use periods
(see
fact sheet 4.704). If irrigation system capacities are below what
is normally required, reduced yields are expected with normal precipitation.
Management strategies to compensate for low capacity include pre-irrigation
and beginning irrigation events at higher soil moisture contents. These
strategies may maintain yields in above normal precipitation years but,
are less effective in below normal precipitation years. One management
strategy to alleviate this problem is splitting fields under one irrigation
systems into 2 or more crops that have different peak water needs.
Fields split into different crops help to spread the irrigation season over a greater time period, but on fewer acres irrigated at any one time, as compared to a single crop. Crops such as corn, soybean and wheat have different timings for peak water use (Figure 3). With low capacity wells, planting multiple crops on smaller acreages allows for water to be applied at amounts and times when the crop needs the water. The advatage of irrigating fewer acres at any one point in time is that peak ET demand of that crop can be better met with the lower capacity. A more definited description of this concept is available in the Neb Guide, “Irrigating for Maximum Economic Return with Limited Water”.
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| Figure 3. Example of daily ET during the growing season. |
Another option is to plant the entire pivot or field to a single crop. Irrigation management with low capacity systems requires that a producer maintain soil moisture at or near field capacity early in the growing season when the system capacity exceeds ET. When the ET for the crop is greater than the capacity of the system, plants will use stored soil moisture to maintain ET. This strategy intends to maintain soil moisture for the crop when they reach the reproductive growth stage, which is also the peak water demand which the system cannot keep up. However, if precipitation is less than anticipated, soil moisture, during peak water demand, may fall below critical levels and yields will probably be reduced.
