FROM THE GROUND UP
Agronomy News

Limited Irrigation Management – Getting the Most Crop per Drop
Principles and Practices

by Joel Schneekloth, Extension Northern Region Water Resource Specialist

Colorado producers irrigate approximately three million acres of pasture, hay, and row crops yielding receipts worth more than five billion dollars per year. However, irrigation water availability for these enterprises is declining. Dwindling agricultural water supplies due to drought, compact compliance requirements, urban transfers, alluvial well pumping restrictions, and declining ground water from non-renewable aquifers has reduced the water available to irrigated agriculture. These water shortages have been occurring in almost every irrigated watershed and ground water basin in some degree for the past several years.

The purpose of this newsletter is to build upon concepts and suggestions for limited irrigation management, provide updates on limited irrigation research projects around the state, and suggest further resources for managing under tight water supplies. This is the second issue of Agronomy News that has focused on limited water. Readers are encouraged to review the Drought issue of April 2003 available at the URL address provided on page 2 for more information on this topic.
Full irrigation results when applied irrigation water is sufficient to meet the crop water demand or evapotranspiration (ET) not supplied by natural precipitation and soil water storage. Limited irrigation occurs when water supplies are restricted, either in timing and/or amount so that full ET demands cannot be met for the entire growing season. Limited irrigation situations include:
1. Reduced surface water supplies or storage due to drought or low snow pack.
2. Restricted pumping allocations in alluvial aquifers. In some instances, the allocations are considerably less than what is required to fully irrigate the crops typically grown.
3. Low capacity irrigation wells due to limited saturated depth of the aquifer. Well yields are then insufficient to meet the peak ET demands of the crop.
Under limited irrigation using typical management practices, yields and returns from the irrigated crop will generally be reduced as compared to a fully irrigated crop. Management strategies that can help minimize yield loss and preserve net return when coupled with careful input management include: understanding the relationships between grain yield and water use (evapotranspiration); irrigation timing to manage water stress to crops during critical growth stages; crop residue management for water conservation; plant population management; crop rotations to balance water use; and improved irrigation efficiency.

Yield and Evapotranspiration and Water Timing
Evapotranspiration (ET) is the driving force behind crop yields (Figure 1). Crops such as corn, respond with more yield for every inch of water of ET as compared to winter wheat or sunflowers. However, corn requires more water for development or maintenance and requires approximately 10 inches of ET to produce the first increment of yield as compared to 4.5 and 7.5 inches of ET for wheat and sunflowers, respectively. These crops also require less ET for maximum production. Forage crops such as alfalfa produce harvestable yield with the first increment of ET and thus are reasonable crop choices for many producers under limited water. See the article Estimated Yield of Some Alternative Crops under Varying Irrigation in Northeast Colorado on page 8 for a more detailed explanation of crop response to water.

Crops respond to water stress differently at several growth stages. Most grain crops are not as impacted by water stress during the vegetative growth stage and during late reproductive or grain fill growth stages (Figure 3). However, crops are sensitive to water stress during the reproductive growth stages and water stress during these stages will significantly impact yields. When producers have control over when they can irrigate, limiting water during the growth stages that are least sensitive to water stress while saving water for the critical growth stages is important to maximizing the return to water. When restricted upon the total amount of water that can be applied, saving that water for the reproductive growth stages is the most advantageous to grain yield.

Crop rotations that include lower water use crops such as sunflowers or winter wheat can reduce irrigation needs. Schneekloth et al. (1991) found that when limited to 6 inches of irrigation, corn following wheat yielded 13 bu/acre (8 percent) more than continuous corn. The increased grain yield following wheat was due to increased stored soil moisture during the non-growing season that was available for ET during the growing season. With low capacity wells, planting multiple crops on smaller acreages within a field allows for water to be applied at amounts and times when the crop needs the water. Crop rotations also spread the irrigation season over a greater time period as compared to a single crop. When planting multiple crops such as corn and winter wheat under irrigation, the irrigation season is extended from May to early October as compared to continuous corn, which is predominantly irrigated from June to early September. Crops such as corn and wheat have different timings for peak water use (Figure 2). The net effect of irrigating fewer acres at any one point in time is that ET demand of that crop can be better met. Irrigation management can be as needed rather than in anticipation of crop ET.

Other resources for understanding crop rotations under limited irrigation include: Cropping options for limited water supplies in Northeast Colorado, 2003 available at: http://www.extsoilcrop.colostate.edu/Newsletters/2003/croppingoptions_2003.htm and Seasonal Water Needs for Colorado Crops at http://www.extsoilcrop.colostate.edu/Newsletters/2003/Drought/seasonal.html.
Residue management in limited irrigated systems for capturing and storing rain and snow becomes as critical as dryland systems. Crop residues and reduced tillage can significantly increase the capture and storage of water due to reduced evaporation from tillage operations and runoff and increased snow catch. Standing residue is more effective than flat residue for snow catch. Studies in Akron found that standing sunflower residue increased the amount of snow captured nearly 2 inches in increased soil moisture over flat residue (see http://www.extsoilcrop.colostate.edu/Newsletters/2003/Drought/stubble.html). During the growing season, residue can also have important impact upon water conservation. Researchers in Kansas found that wheat residue reduced the amount of evaporation from the soil during the growing season for irrigated corn as compared to bare soil. The reduction in evaporation amounted to nearly 2.5 inches for the growing season. Most of these saving occurred before the corn crop reached full canopy (see http://www.extsoilcrop.colostate.edu/Newsletters/2003/Drought/residue.html). Residue also reducers runoff from precipitation allowing for better infiltration and decreases rainfall and irrigation impact upon surface sealing which increases infiltration rates.
Efficient application is critical when irrigation water is limited. Thus runoff, deep percolation, and evaporative losses must be minimized by tweaking the existing or upgrading to a different irrigation system. Center pivot irrigation efficiency can vary depending upon management and system design, so seek the advice of an irrigation professional to make sure your nozzle package correctly fits your pumping capacity and soil characteristics. Surface irrigation systems are inherently inefficient, but can be improved by shortening row length, increasing stream size and cutback, using optimum set times, packing furrows and/or using surge valves or manually surging rows. A longer discussion of these adjustments can be found at: http://www.extsoilcrop.colostate.edu/Newsletters/2003/Drought/tips.html.
Pre-irrigation is a strategy that is often recommended under limited irrigation, particularly under low capacity systems to ensure that the soil profile is filled to field capacity before the growing season. However, producers should remember that the storage efficiency of pre-irrigations is low. Lamm and Rogers (1985) found that the storage efficiency of non-growing season precipitation was reduced as the fall available soil water content was closer to field capacity. Although pre-irrigation may be needed in years with low precipitation, decisions on whether to pre-irrigate are more reliable in the spring to take advantage of non-growing season precipitation.
Crop yields and gross returns from limited irrigation will generally be lower than a fully irrigated crop production system. However, changes in agronomic and irrigation management practices can help maintain respectable yields and net returns. A combination of management strategies such as rotations with lower water use crops, reduced tillage, water timing, and improved irrigation efficiency can help stretch limited water supplies in many situations.
Joel Schneekloth: jschneek@ext.colostate.edu
CSU Extension – Northern Region

References
Lamm, F.R. and D.H. Rogers. 1985. Soil water recharge function as a decision tool for preseason irrigation. Trans. of the ASAE. 28(5):1521-1525.
Schneekloth, J.P., N.L. Klocke, G.W. Hergert, D.L. Martin and R.T. Clark. 1991. Crop rotations with full and limited irrigation and dryland management. Trans. of the ASAE. 34(6):2372-2380.