When the well’s dry, we know the worth of water. —Benjamin Franklin
Irrigation accounts for 95 percent of the water used in Nebraska. Most of this water is groundwater pumped from aquifers and used for growing corn and beans. Nebraska has approximately ninety-five thousand irrigation wells and nine million irrigated acres, the most of any state. Irrigation continues to grow, and since 2008, over four thousand new irrigation wells have been registered within Nebraska. Although there are no exact numbers available, irrigation pumping in Nebraska uses millions of acre-feet of groundwater each year. (An acre-foot is the amount of water it takes to cover an acre one foot deep).
Aquifers also provide a steady source of groundwater seepage to surface water. Known as baseflow, groundwater seepage provides a portion of flow to all of Nebraska’s river basins. If you have ever waded into a Nebraska stream during late summer or in the fall when the leaves have turned colors and the fields and corn are starting to dry, you might have noticed that the water was running particularly clear. The reason the flow is clearer when the weather turns dry is because most of the flow is being supplied by groundwater seepage into the streambed.
River basins, however, are not just the main channel of a river but also consist of numerous tributaries that feed into and supply the main channel. These tributaries are also supplied by groundwater seepage. For example, the wetlands, meadows, springs, streams, and rivers in the Sandhills are all supplied by groundwater seepage from the High Plains (Ogallala) Aquifer.
Nebraska’s aquifers, including the Ogallala, are primarily deposits of buried and saturated sand and gravel (with some areas of saturated sandstone and limestone in eastern Nebraska). Aquifers are recharged and filled over many years by percolating rain and snowmelt. Left alone and over time, the level of groundwater in an aquifer becomes balanced, and the amount of groundwater seepage into streams and rivers roughly equals the amount of recharge from precipitation. This process is not instantaneous. Water moves slowly through an aquifer and deeper older groundwater would still continue to seep into streams even if rains were to stop for many years.
Groundwater pumping changes this natural equilibrium by diverting water that would have naturally seeped to the surface and become streamflow. In areas where streams and rivers are hydraulically connected to the aquifer (which is most of Nebraska), this is a gallon-for-gallon depletion. Every gallon of groundwater pumped from an aquifer is a gallon that would have otherwise been streamflow. It is just a matter of time—the closer a well is located to surface water bodies (think rivers, creeks, streams, springs, or lakes), the sooner the surface water flow is depleted. There are a lot of surface water bodies in Nebraska, and in some areas the depletions do not take long to materialize. Small declines in aquifer levels can also result in proportionally large depletions in streamflow, and if wells are located close enough to a river or stream, pumping may actually draw the surface water back to the wells.
Overpumping occurs when aquifer levels cannot be sustained. Think of an aquifer as a storage account for groundwater. If you withdraw more than you deposit, the balance will keep falling. Those millions of gallons of irrigation water come from depletion of the aquifer’s “account” and from water captured from streams. Just like a savings account, groundwater levels in an aquifer cannot be maintained if withdrawals for irrigation are consistently greater than the amount of recharge from precipitation.
Aquifer declines and resulting surface water depletions are accumulative, and pumping of groundwater in 2014 will impact surface water in 2015 and beyond. The combination of drought and overpumping adds to the problem, and not surprisingly, recent data show large areas of aquifer declines. Temporary aquifer declines of a few feet are to be expected during times of drought and may be replenished in a few years of above-average precipitation. Steeper declines in groundwater levels may not be replenished in our lifetimes. If the aquifer declines are relatively steep compared to the amount of recharge and occur over a large enough area, groundwater seepage stops altogether and baseflow to streams in that portion of the river basin ceases.
The extent and duration of recent water shortages in the Platte, Republican, Elkhorn, and Blue river basins can be linked directly to the pumping of high-capacity irrigation wells lowering the groundwater level in the surrounding aquifers. In portions of Nebraska where groundwater levels have been dropping for several decades, aquifer declines have become semipermanent. In some areas of the state, the declines are so great that entire wetland complexes have disappeared and the overlying tributaries are so depleted, flow may not fully recover in our lifetime, if ever.
Nebraskans depend upon our aquifers for agriculture, recreation, power, and domestic use, and declining aquifer levels and resulting streamflow depletions can have significant consequences. Groundwater supplies over 85 percent of Nebraskans with drinking water, and almost all Nebraskan towns and villages have a well field that supplies drinking and domestic water. Hundreds of domestic wells are put at risk of losing their water supply if groundwater levels are significantly lowered. Municipalities, power and irrigation districts, recreation, and wildlife require steady and dependable streamflow, especially during the summer and early fall when conditions are drier and groundwater seepage supplies a large portion of Nebraska’s surface water flow.
Nebraska’s water resources, including our aquifers, are a public resource that belongs to all of us, and we share a responsibility in sustaining this exceptional resource for ourselves and for future generations.