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Fluid Journal : Summer 2014
9 The Fluid Journal Summer 2014 Figure 2. Corn accounts for 70% of irrigated corn acres. Much has changed on the land since the 1942 agreement. Spring water once flowed into the Republican River, keeping a steady flow throughout much of the year. Since 1942, deep wells and irrigation development in Colorado and Nebraska lowered water tables, which dried up many of the springs in the feeder streams. Soil and water conservation practices were applied to the land beginning in the late 1940s and continue through to the present time. Terraces and farm ponds kept soil and water on the farm, thus reducing runoff into the river. More recently, crop residual management, minimum tillage, and no-till farming methods have reduced runoff even further. “Reuse pits” were installed to collect runoff water from gravity irrigated fields with the water being recycled to use for irrigation again and again without leaving the farm. Inefficient gravity irrigation gave way to more efficient center pivot irrigation systems. Through the decades, the goal of soil and water conservationists who assist the growers was and is to reduce erosion by keeping rainwater and irrigation runoff to near zero, thus much less water reaches the river. On well-managed farms, water leaves the property only through evapotranspiration. Zero-sum game In the end, the Republican River controversy has resulted in a zero- sum game. Water consumed upstream cannot be used downstream and the benefit to the upstream user is to the detriment of those downstream. For many years, Nebraska argued that groundwater was not included in the Republican River Compact. Kansas filed suit in 1998 and the case wound its way to the United States Supreme Court. That case was settled in 2002 when the three states agreed to use a computer model using an algorithm including precipitation, stream flow, and assumed values for recharge from precipitation and subsurface leakage in and out of the Republican River area. The model also included water used by irrigators and others. Years of legal wrangling followed with Kansas notifying the other states in late 2007 of an alleged failure to comply with the settlement. Kansas proposed a remedy to groundwater and stream flow depletions and asked for a shutdown of all wells within 2.5 miles of the Republican River and its tributaries and a suspension of irrigation on lands added since 2000. The 2009 nonbinding arbitration meeting resulted in a no resolution conclusion among the three states. The economics of water are staggering. David Cookson, Nebraska’s chief deputy attorney general, said the shutdown of half of the irrigated acres in the Republican River basin would cost billions long-term in economic activity to the state. The number and value of irrigation-related transactions for seed fertilizer, herbicides, feed, machinery, insurance, and the sale of forage quickly mount. Not only agriculture is affected. There are also industrial, recreational, and municipal components that have a strong reliance on water. The High Plains Aquifer is a waterlogged jumble of sand, clay, and gravel that begins beneath Wyoming and South Dakota and stretches to the Texas Panhandle. The Northern portions of the aquifer hold enough water for perhaps hundreds of years of irrigated agriculture. Most of the water lies under the Nebraska Sandhills, a mass of stable sand dunes covering a third of the state, an area not suitable for farming. As one travels south, pumping water is increasingly elusive. Kansas wells that used to pump 1,600 gallons to the surface every minute now may yield only 300 gallons or be completely dry. High Plains Texas irrigators find they have to drill deeper and deeper to extract water from the declining aquifer. From 1940 to 1980, the water table was lowered by more than 100 feet in parts of Texas, New Mexico, the Oklahoma panhandle, and Kansas. Follow-up studies show that the water table has dropped an additional 40 feet or more. Replenishing the aquifer will require more than just a few seasons of rainy weather, as is often the case in surface waters. Rather, it could be hundreds or, more probable, thousands of years of rain required to restore aquifer levels even if no additional irrigation was allowed. The Great Plains area is not alone in ground water depletion. The USGS map (Figure 3) indicates known depletion areas. The villain The villain of this drama is the farmer’s friend--the center-pivot irrigation system. The pivot makes irrigating crops easy in comparison to other systems and requires much less labor. The center- pivot irrigating system is, perhaps, the most efficient way to create an oasis. Center-pivot irrigation efficiency was improved by the addition of dropped nozzles, which reduce water lost to evaporation and drift. Paradoxically, it was often found that farmers ended up applying more groundwater to fields. Rather than reducing consumption, some farmers use the efficiency “savings” to expand irrigation into poor soils or grow higher value crops such as corn, alfalfa, and soybeans, which consume more water. Texas researchers set out to determine how much water loss occurs in the air above the canopy, within the plant canopy, and from the soil surface. They compared different sprinkler devices and heights of sprinkler devices with respect to the crop canopy. Table 1 shows the water loss during irrigation and the application efficiency for 1) six-degree low angle impact sprinklers located on the sprinkler pipe, 2) spray heads located five feet above the ground, and 3) a Low