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Agricultural Irrigation Management: Reduce the need for irrigation: maintain crop residue, reduce tillage
Leaving higher levels of crop residue and doing less tillage can increase the soil water balance by increasing the amount of water that infiltrates the soil from irrigation or precipitation, and decreasing the amount of water that runs off the soil surface. More residue and less tillage also reduce the rate of evaporation of water from the soil. Maintaining residue on the soil surface and doing less tillage can significantly reduce the amount of irrigation water needed to grow a crop.
Infiltration and runoff
Crop residue protects the soil surface from erosion by absorbing the impact energy of water droplets, thus reducing soil particle detachment. Soil particle detachment can cause crusting and sealing of the soil surface which results in decreased infiltration and increased runoff. Residue also slows the velocity of runoff water, allowing more time for infiltration. Long term no-till leads to better soil structure, less soil crusting, greater infiltration of water, and less surface runoff.
University of Nebraska-Lincoln research with a rainfall simulator at Sidney, Neb., demonstrated these differences in infiltration and runoff in a wheat-fallow rotation. In the experiment, more than 3.75 inches of water was applied, in 90 minutes, on continuous no-till before runoff started compared to only 1.0 inch of water applied, in 20 minutes, on tilled (plowed) soil.
Evaporation of water from the soil is reduced when reducing tillage because with more residue, less solar energy reaches the soil surface and wind speed (air movement) is reduced at the soil surface.
When the soil surface is wet, evaporation from an uncovered soil (no residue or crop canopy) will occur at a rate that equals the atmospheric demand. The evaporation rate will decrease drastically, because of a rapidly drying soil surface (Figure 1). Water that is deeper in the soil cannot be transported quick enough through this dry surface soil to satisfy atmospheric demand.
If the soil is covered, e.g. with residue, the residue insulates the soil from solar radiation and reduces air movement at the soil surface. This reduces the evaporation rate from a residue covered surface, compared to an uncovered soil. If there is no rain or irrigation for a long period, the surface moisture under the residue will continue to slowly evaporate. A few days after the wetting event, evaporation from the covered surface can exceed that of the uncovered surface (Figure 1).
Figure 1. Evaporation rates, relative to atmospheric demand, from covered and uncovered soil after a single wetting event (irrigation or rainfall).
Residue reduces, but does not eliminate evaporation. It still takes place from the soil, the residue itself, and from the crop canopy every time they get wet. This loss has been estimated to be around 0.08 to 0.1 inch for each wetting event. This is why light, frequent rains or irrigations are less effective than longer, soaking ones. Some center pivot irrigators have problems with runoff on tilled soils so they apply small amounts frequently, typically only 0.5 inch at a time. One tenth of an inch of evaporation out of 0.5 inch is a 20 percent loss. When adopting continuous no-till, the pivot can apply a greater amount of water before runoff occurs. With more water applied per event, but less often, the evaporation losses are reduced.
Often soils dry to the depth of tillage. An average silt loam soil holds about 2 inches of plant available soil water per foot of soil. Tilling the soil can result in a loss of 0.5 to 0.75 inch of soil water with each tillage trip. With multiple trips, there may not be adequate soil water in the seed zone for uniform germination and emergence, resulting in lower yields, even though there may be sufficient soil water the rest of the year.
Finally, more residue, especially standing residue, means more snow trapping during winter (Figure 2), thus storing more water in the soil once the snow melts, which can be used for crop production later on.
Figure 2. Standing residue traps snow during winter. This provides water for the following crop.
Experiments on crop residue and evaporation in Kansas and Nebraska
Research at Garden City, Kan., showed that up to 30 percent of evapotranspiration (ET) can be evaporation (the E in ET) during the irrigation season for corn and soybean on silt loam soils. The study suggests that a 2.5 to 3.0 inch water savings is possible, when wheat straw or no-till corn stover is present from early June to the end of the growing season. Dryland research indicates that stubble can save an additional 2 inches of water in the non-growing season if the soil profile has not been filled to capacity. In water-short areas or areas where water allocations are below full irrigation, 5 inches of water translates into possibly 20 bu/ac of soybean and 60 bu/ac of corn. For more information, go to this PDF report from Kansas State University.
Earlier UNL research results at North Platte, Neb., largely agree with the findings from Kansas. They showed that 6000 lb/ac of wheat stubble laying flat could reduce bare-soil evaporation by half under a fully irrigated corn crop. This crop received nine irrigation events during the growing season each year. The projected full season evaporation (120 days) is shown in Table 1.
Table 1. Growing season evaporation (inch) under a fully irrigated corn crop.
Water conservation with no-till
UNL Extension Engineer Paul Jasa compared dryland soybean yields in reduced- and no-till to those in a clean-tilled moldboard plow system. His data came from the Rogers Memorial Farm (near Lincoln, Neb.) in low rainfall years (2000 and 2006). In 2000, the no-till system yielded 24 bu/ac more than the moldboard plow system. In 2006, the difference was 19 bu/ac (Table 2). It is believed that these differences were largely caused by more water being available to the crop in the no-till system.
Table 2. Dryland soybean yields from a long-term tillage study and the yield increase over the clean-tilled moldboard plow system for the dry years of 2000 and 2006.
Jasa estimates that about 5 to 12 extra inches of water are available over the entire season for continuous no-till compared to tilled, depending on rainfall events and frequency. The more often rainfall occurs or the more intense a rainfall event, the greater the water savings with no-till. Likewise, the more often a crop is irrigated, the greater the water savings.
More of the story from the CropWatch archives
Crop yield, residue mass and cover
There are rules of thumb to estimate the amount of residue a crop is expected to produce, based on crop yield. For wheat, multiply the yield (bu/ac) by 100 to get residue mass in lb/ac. For example, a 60 bu/ac wheat crop is expected to produce approximately 6000 lb/ac of residue, which is the amount of residue that was used in the North Platte research project discussed above. For corn, multiply the yield by 50 and for soybean by 40. So, a 180 bu/ac corn crop is expected to produce approximately 9000 lb/ac of residue.
Sometimes we want to know the amount of residue in terms of the percentage of the soil surface it covers, so it is useful to be able to convert residue mass to residue cover. Figure 3 provides the means to do this for a number of crops. For example, for wheat, 6000 lb/ac corresponds to a residue cover of almost 100% and 1000 lb/ac of corn residue corresponds to a cover of 30%.
Figure 3. Relationship of residue mass to percent residue cover for various crops.
Current and future research
Questions remain on how much water can be conserved under various tillage conditions and residue levels. In 2007, a study was initiated on the effect of crop residue on soil water content and crop yield at the UNL West Central Research and Extension Center in North Platte. The experiment was conducted on a Cozad, Neb., silt loam soil on a set of plots planted to field corn. There were two treatments: residue-covered soil and bare soil. Bare-soil plots were created by using a dethatcher (Figure 4) and subsequent hand-raking (Figure 5), removing most of the residue (Figure 6).
Figure 4. Removing residue to create bare-soil plots using a dethatcher.
Figure 5. Removing more residue after the dethatcher.
Figure 6. Research plots after dethatching and hand-raking.
The residue plots were left untreated. The residue mass on these untreated plots was about 3000 lb/ac, mostly from previous no-till soybean crops. The experiment consisted of 8 plots (2 treatments times 4 replications). Each plot was 40 by 40 ft. Winter and spring 2007 were very wet at North Platte and the corn was only irrigated 3 times with a total of 4.5 inches of water. The crop was purposely water-stressed, so that any water conservation in the residue-covered plots might translate into higher yields.
Differences in soil water content between the residue-covered and the bare-soil plots were small. Average corn yield was 197 bu/ac in the residue-covered plots and 172 bu/ac in the bare-soil plots (Figure 7). This yield difference may be interpreted as an additional amount of water of 2-4 inches available to the crop in the residue-covered plots. Or, in other words, we estimate that it would take an additional 2-4 inches of water on the bare-soil plots to reach the same yield as we obtained in the residue-covered plots.
Water conservation of such a magnitude will help irrigators to significantly reduce pumping cost and more water would be available for competing needs including those of wildlife, endangered species, municipalities, hydroelectricity plants, and compacts with other states.
Figure 7. Corn yield on bare soil (avg. 172 bu/ac) and residue-covered soil (avg. 197 bu/ac) at North Platte on small plots.
UNL is extending this type of experiment to larger fields, working with producer cooperators and using fields at the UNL West Central Water Resources Field Lab near Brule, Neb. We recently started research there on the effect of corn stalk grazing and baling on residue levels, soil water balance, and crop yield. In 2008, UNL started a study on ET and soil water balance for fields under long-term no-till vs. disk-till in Fillmore County and near Holdrege, Neb., for a corn-soybean rotation under center pivot irrigation.
Switching To No-till Can Save Irrigation Water, from EC196-Chapter 3, 10 Easy Ways to Boost Profit $20/Acre (PDF, 978 KB; 5 pages)
Harvesting Crop Residues, G1846. (PDF version 659 KB; 4 pages)
Issues of crop residue harvest, including nutrient removal and effects on erosion, soil quality, water loss, and yield are discussed in this NebGuide.
We developed a simple calculator to calculate the economic benefit of the kind of water savings discussed here:
Less irrigation water is pumped when you save water with more residue/less tillage. This will also give you a savings in pumping cost.