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Fluid Journal : Summer 2014
11 The Fluid Journal Summer 2014 Dr. Lohry is President and Dennis Zabel is Senior Agronomist at Nutra-Flo Company in North Sioux City, SD. goals of up to 100 percent, water savings of up to 40 to 80 percent, and associated fertilizer, pesticide, and labor savings over conventional irrigation systems. It is potentially the most efficient irrigation system available, but that efficiency depends on the irrigation system itself, its proper design, installation, and management. Only if designed, installed, and managed correctly can SDI be more efficient than any other irrigation system. Drip lines are buried 13 to 18 inches below the soil surface so the soil surface stays dry and practically no irrigation water is lost due to evaporation. Because of the potential for high irrigation efficiency, it may be a good alternative for areas where irrigation water is limited. Researchers in Kansas have reported that net irrigation needs could be reduced by 25 percent with SDI, while maintaining high corn yields. Increased water use efficiency reduces pumping cost. Since no excess irrigation water is applied, nutrient leaching, with it potential to enter into surface and subsurface waters, is minimized. SDI can be automated to apply fertilizers and other chemicals such as acids, chlorine, and even pesticides with irrigation water. SDI systems are often managed to apply small amounts of water and other inputs daily or even several times a day. Spoon feeding water and nutrients could, theoretically, result in increased yields and decreased nutrient and water losses. One of the main disadvantages of SDI is its high initial cost. The University of Nebraska estimates an average gross cost of between $500 and $800 per acre. Center pivot systems cost about half as much per acre. Kansas estimations suggest that as the fields become smaller SDI becomes more cost-effective. However, even with smaller fields it may be more cost- effective to just dryland farm and not irrigate at all. Much depends on the value of the crop grown and the availability of water. SDI lends itself well to specialty and tree crops under limited water situations. SDI systems are being installed in field corners where center pivots cannot reach. A typical pivot irrigates about 134 acres out of a 160-acre quarter section. After taking out country road right–of-ways there might be 24 or 25 acres of excellent soil in the corners that aren’t irrigated. Farming the corners as dryland creates management challenges. An SDI system can complement the center pivot by bringing the field corners under irrigation. This simplifies management when all acres in the field are irrigated. The entire quarter section will have similar seeding and fertilizer rates. It also simplifies record keeping for crop insurance and farm program benefits. Deficit irrigation Deficit irrigation is the practice of applying less water than a crop needs for a full yield potential. Studies have shown that a reduction in irrigation is usually less than the reduction in yield. The marginal productivity of irrigation water is lower when water application reaches full irrigation. Applying 75 percent of full irrigation may result in 90 percent of the fully irrigated yield. One study, based on 28 years of corn production data, showed that applications of 50 percent of the non- yield limiting irrigation rate reduced yield only 13 percent. Yield variability at lower irrigation levels is usually higher and in the previous study mentioned year-to- year yield variance increased fourfold. Deficit irrigation at lower levels increases economic and weather uncertainties. Dryland yields, as a fraction of fully irrigated yields (relative yield), are more variable than deficit irrigated yields. So, deficit irrigation mitigates some of the economic and weather uncertainties, but not to the extent of fully irrigated conditions. Under deficit irrigation conditions, corn grown in rotation with another crop is often found to yield better than corn following corn. In one study in Western Nebraska, under semiarid conditions, corn in a wheat-corn-soybean rotation was able to use more stored soil water than the continuous corn crop. Increased use of stored soil water led to less dependence on irrigation. Water quality It is not just the quantity of water, too little or too much, but the quality that is getting much attention. As the water table is drawn down, total dissolved solids have been increasing. In areas vulnerable to leaching, nitrates and hazardous chemicals have been increasing in concentration to levels adverse to human health. Increasing nitrate levels have been especially serious in areas of Nebraska and Kansas. Nitrogen fertilizers and animal manure applied to farmland susceptible to leaching are the main contributors. Herbicides such as atrazine and metolachor are commonly found in both surface and ground waters. Carbon tetrachloride and ethylene dibromide, used to fumigate grain, are found under or near grain elevators. Chemicals associated with military bases and associated industries such as RDX and TNT and the commonly used degreaser trichloroethylene are found in some areas. Large trichloroethylene plumes in Hastings, Nebraska, and Wichita, Kansas, are the result of industrial solvent disposal. Modern farming practices are demonstrating their ability to improve water quality. Groundwater nitrate concentrations are an example. Certain areas in Nebraska, like the Central Platte Valley, have been using more environmentally friendly farming techniques such as not applying nitrogen in the fall, using nitrification inhibitors, and applying scientifically justified nitrogen rates. Once elevated, levels of groundwater nitrate are markedly lower due to implementation of sound research. The evidence is clear in the Central Platte Valley, because the aquifer is close to the surface and changes can be observed in a geologically short period of time. Other areas where the vadose zone is much larger will take much longer to see change. Summing up Balancing the social, ecological, economic, and agricultural interests in water is clearly complex. Agreements become unworkable as time goes on. Efforts to improve irrigation efficacy result in higher consumption of groundwater and aquifer depletion. Compliance with watershed agreements results in the expense of transferring water from one source to another and the draining of recreational impoundments. Reductions and moratoriums on wellhead development result in fewer calories for food and feed. It seems that simply allowing market forces to operate does not adequately balance the social, economic, and environmental needs for the sustainable use of water. Even our legal system seems inadequate to provide enduring solutions. Complex problems cannot be solved simplistically. Protecting our surface and underground water sources, our environment, and mankind will require a holistic approach. Solutions will be regionally based but implemented and applied appropriately on a farm scale. Components to the solution include increasing irrigation efficiency, deficit irrigation, crop rotations, using surface water sources that may be more abundant, planting less water- hungry crops and crop combinations, withdrawing some land from irrigation, and improving crop water use efficiency through advanced plant breeding techniques.