Using a Groundwater Model To Analyze Depletion Mapping For The Mississippi Alluvial Plain Groundwater Project

Monday, December 4, 2017
Steven M, Peterson , Nebraska Water Science Center, U.S. Geological Survey, Lincoln, NE

The Mississippi Alluvial Plain (MAP) is among the most productive agricultural regions in the United States. This agriculture is enhanced through substantial groundwater irrigation, resulting in groundwater level declines in parts of the area. Efforts are currently (2017) underway to update the information and science available to support water-resources decisions and sustain future water availability. Groundwater models representing the major regional groundwater flows in a larger area encompassing the MAP have been published from 2009-2017. Even prior to integration of new data and approaches, these groundwater models also can be used in their current state to map depletions, which are like responses to increased stresses.

For a depletion mapping analysis, a model of an area for a specified time period is first run under a baseline condition. Simulated groundwater flows to various simulated boundary conditions are recorded, such as for streams, groundwater levels, and wells. Next, a new well is added to one model cell, the model is rerun, groundwater flows to various boundaries are again recorded, and the results are compared with the baseline condition. Results are commonly expressed as the change in the boundary flow as a percentage of the new well’s flow. Subsequently, the new well is moved from one cell to the next, and each cell is mapped. Depletion maps have been used to establish water resources management boundaries in Nebraska for more than a decade. In addition, these maps reveal characteristics of the groundwater system of an area and of the simulation representing that system. Depletion maps generated for the MAP area demonstrate where the new wells caused increased groundwater level declines (loss of storage) or decreased groundwater discharge to streams. The depletion maps also show some characteristics and details of the groundwater model inputs, such as variations in the hydraulic conductivity assigned to simulated streams.

Steven M, Peterson, Nebraska Water Science Center, U.S. Geological Survey, Lincoln, NE
Steve has worked in groundwater hydrology and flow modeling for over 20 years, mainly on the High Plains of Nebraska. At the USGS Nebraska Water Science Center, Steve incorporates the latest groundwater and integrated modeling approaches to provide comprehensive information for science-based water resources management at local, state, and federal levels. Steve has recently completed the High Plains Groundwater Availability Study, assessing the processes and current and future status of the water resources of the High Plains aquifer, a region sustaining approximately one-quarter of the Nation’s agricultural production through groundwater irrigation. Steve’s other local or regional modeling studies include depletion mapping, optimization modeling, soil-water balance modeling, landscape hydrology modeling, parallel computing for parameter estimation and collaboration on integration of geophysical and geologic data in groundwater models.

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