High Plains Groundwater Availability Study
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Water availability is a function of many factors, including the quantity and quality of water and the laws, regulations, economics, and environmental factors that control its use. The focus of the High Plains Groundwater Availability Study is on improving fundamental knowledge of the water balance of the basin, including the flows, storage, and water use by humans and the environment. An improved quantitative understanding of the basin's water balance not only provides key information about water quantity but also is a fundamental basis for many analyses of water quality and ecosystem health.
A new groundwater-flow model for the northern High Plains aquifer has been developed and is being used as a tool to understand how the aquifer responses to the continuing and in some cases growing demands on the groundwater resources in the northern High Plains aquifer. Additionally, through the collection of existing information a water budget will be developed for the entire aquifer.
The High Plains aquifer is a nationally important resource that serves as the primary source of drinking water for residents of this eight-state region and sustains more than one fourth of the Nation's agricultural production. The highly publicized aquifer depletions that have been documented in the High Plains have lead to local and national concern for its sustainability. The proposed assessment of water availability in the High Plains aquifer will provide local water managers and planners with current information on the status of the aquifer and a set of tools with which to make informed decisions about the effects of future water-use and water-management practices on the aquifer. This work will provide increased understanding of the hydrogeology of the aquifer, water-use and consumption, aquifer recharge, and the forecasted effects of continued groundwater withdrawal. These data will enable informed decisions that will be critical to ensuring water availability for the nation's "breadbasket", notwithstanding hypothesized future climatic changes.
We are developing information and tools that can be used to assess the groundwater availability of the northern High Plains aquifer (and how it has changed over time) by evaluating regional and temporal trends in the mass balance of groundwater resources for the entire aquifer and to quantify the natural and anthropogenic processes that affect these water budgets. This will provide temporal information about the primary inflows, outflows, and changes in water storage in specific sub-regions of the aquifer, and result where possible to forecast system responses to future anthropogenic and environmental stresses. Within this broad goal, specific tasks include:
- Refine both pre-development and contemporary water budgets for the entire High Plains aquifer based on compilation, refinement, and integration of existing water budgets, recent and newly developed groundwater flow models, and newly available data. Quantify the current status of the aquifer’s groundwater resources and emphasize regional gradients in land-cover, climate, and water use.
- Develop a methodology that uses remotely sensed data and land-cover modeling to determine the spatial distribution and temporal changes in evapotranspiration and irrigation application of water for the northern High Plains aquifer.
- Construct a MODFLOW groundwater flow model for the northern High Plains aquifer that utilizes the most current conceptual and geohydrologic models of the aquifer. Incorporate the remotely sensed evapotranspiration, land cover, and irrigation application data developed as part of this study.
- Assess groundwater availability for the northern High Plains aquifer. Use the MODFLOW groundwater flow model for the northern High Plains aquifer to extrapolate the effects of predicted natural (climatic) and anthropogenic stresses on the revised water budgets. Evaluate the implications of these stressors and forecasted effects on the long-term water supply for the northern High Plains aquifer.
The High Plains aquifer occupies the higher elevations of the Great Plains physiographic province which lies east of the Rocky Mountains. Elevations range from about 8,000 feet along the northwestern boundary of the aquifer in Wyoming to about 1,000 feet along the eastern boundary. The topography is characterized by flat to gently rolling terrain which is a remnant of a vast plain of Tertiary and Quaternary sediments deposited by streams originating in the ancestral Rocky Mountains.
Land use over the High Plains aquifer is characterized by a sparse rural population and a few moderate sized cities and towns dominated by an agricultural economy. Population in 2000 was about 2.3 million people with 77 percent residing in rural areas and smaller towns and cities (McMahon and others, 2007). Rangeland and agricultural land were the dominant land uses/land covers in the High Plains in 2001 with 56 percent of the land being classified as rangeland and 38 percent as agricultural land (McMahon and others, 2007).
These conditions of concentrated agricultural land use, semi-arid and variable climatic conditions (which demands agricultural irrigation), and the hydrogeologic setting combine to produce the documented aquifer water-level declines.
Because of its size and importance to the Nation, the High Plains aquifer is one of the most-studied aquifers in the world, and describing all previous studies of the aquifer is beyond the scope of this web site. The High Plains Groundwater Availability Study will use groundwater-flow models as a tool to assess the availability of water in the aquifer, and thus the previous studies described herein relate to groundwater-flow models.
The last aquifer-wide groundwater-flow model for the High Plains aquifer was completed in 1986 under the USGS Regional Aquifer-Systems Analysis (RASA) Program. This groundwater-flow model was completed prior to the availability of the USGS MODFLOW modeling code and used a modified Trescott-Pinder-Larson code. The aquifer was divided into three individual sub-regions (the northern, central, and southern High Plains areas) and groundwater flow in each area was simulated in two dimensions for a heterogeneous, isotropic, unconfined aquifer. Grid spacing for the RASA model was a uniform network of nodes spaced 10 miles apart. It was assumed that the regional scale of the model resulted in vertical components of flow that were small enough to be neglected without causing significant errors in the results; hence the 2-D model. Depending on the sub region being modeled, the models were calibrated for both predevelopment (before 1940-60) and development periods (1940-60 through 1979). Model projections were made for 1980-2020. During this time period, and assuming continuation of the 1980 economic trends and groundwater-management policies, the RASA study concluded that water level declines would be greater than 200 ft in some areas of the High Plains and average more than 25-36 ft over much of the High Plains (Luckey and others, 1988).
Additional modeling efforts for selected portions of the High Plains aquifer have been completed since the RASA effort. These models were developed independently by various agencies for diverse applications and so have varying assumptions and design features. Each of these more recent models were developed using the MODFLOW modeling code (various versions) and generally have a finer discretization than the RASA effort of the 1980s.
One unique data set that will be of great benefit to an assessment of water availability in the High Plains aquifer is the groundwater-level monitoring network maintained by the USGS for the entire aquifer.
In addition to the more recent models of groundwater flow and aquifer depletions there have been a few other local studies on the hydrogeologic framework of the aquifer and the land-use and irrigation impacts associated with the agricultural activities of the area. These studies, mainly completed by other federal agencies (for example, the U.S. Department of Agriculture) and local universities, are another important source of information that can be incorporated into a regional assessment of groundwater availability.
Steve Peterson's Nebraska Water Center Spring Seminar (video): The High Plains Groundwater Availability Study: Abundant Groundwater Doesn't Necessarily Mean Abundant Surface Water