Elkhorn-Loup Model

Science Center Objects

In the Elkhorn River and Loup River Basins, Natural Resources Districts (Upper Elkhorn, Lower Elkhorn, Upper Loup, Lower Loup, Middle Niobrara, Lower Niobrara, Lewis and Clark, and Lower Platte North) are collecting data and developing tools to assist with water resource planning. Critical planning issues in the Elkhorn River and Loup River basins are focused on the availability of the groundwater resource, the effect of anthropogenic stresses on the groundwater resource, and the interaction of groundwater and surface water. Specifically, the Natural Resource Districts and the Nebraska Department of Natural Resources are concerned with the effect of groundwater withdrawal on the availability of surface water and the long-term effects of groundwater withdrawal on the groundwater resource.

 

graphic map of the ELM study area

Elkhorn-Loup Model area showing the Natural Resources Districts that participated in the study.

The Natural Resources Districts (Upper Elkhorn, Lower Elkhorn, Upper Loup, Lower Loup, Middle Niobrara, Lower Niobrara, Lewis and Clark, and Lower Platte North; referred to as NRDs) and the Nebraska Department of Natural Resources (NDNR) are concerned with the effect of groundwater withdrawal on the availability of surface water and the long-term effects of groundwater withdrawal on the groundwater resource in the Elkhorn River and Loup RIver Basins. The Elkhorn-Loup Model, is a USGS Nebraska Water Science Center project, done in cooperation with the NRDs and NDNR,  and is designed to assist the NRDs and NDNR by characterizing the groundwater system within the Elkhorn River and Loup RIver Basins and by providing a regional groundwater-flow model.

The Elkhorn-Loup Model, a multi-phase project, is a study of surface-water and groundwater resources in the Elkhorn River basin upstream from Norfolk, Nebraska, and the Loup River basin upstream from Columbus, Nebraska. The study area covers approximately 30,800 square miles, and extends north to the Niobrara River and south to the Platte River. The eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska.

Phase One

The first phase of the study began with construction of a groundwater-flow model using previously collected data. The model was constructed with a single layer vertically to represent the aquifers of the Tertiary-age Ogallala Group and Quaternary-age alluvial deposits, with a uniform node spacing of 2 miles. The model was calibrated to measured groundwater levels and estimated groundwater discharge to streams for the pre-groundwater-development period (approximately 1940) and the simulation of the 1940-2005 period was calibrated to measured groundwater-level changes. The calibrated groundwater-flow model was used to assess current and future impacts of groundwater pumping on surface water, and could be used to provide information to the NRDs for groundwater-management planning.

Phase Two

Continuation of the study, phase two, was part of a larger, ongoing effort to enhance the current knowledge of hydrogeology,  improve the understanding of stream-aquifer interactions, and compile reliable data describing hydrogeologic properties such groundwater recharge, groundwater pumpage for irrigation, and groundwater discharge to evapotranspiration in the study area.

Phase two included updates to the groundwater-flow simulation using

Photo of Steve Peterson making a stream discharge measurement.

USGS Nebraska Water Science Center Elkhorn-Loup Model project chief, Steve Peterson, making a stream discharge measurement. The project collected hundreds of discharge measurements in 2006 to determine base flow in the study area.(Credit: Robert B Swanson, USGS Nebraska Water Science Center. Public domain.)

  • newly collected data,
  • supporting analyses done in 2007 and 2008,
  • improved simulation calibration methods, and
  • additional approaches for analyzing the effects of agricultural irrigation using the simulation.

Newly collected data include

In addition to enhancing the data to the simulations, parameter-estimation techniques were used for phase-two simulation calibration, providing a more robust calibration. Other enhancements to the simulations included refining the grid discretization, using time-variable recharge from precipitation, time-variable base-flow estimates, improved estimates of groundwater withdrawals for irrigation, and refined delineation of active evapotranspiration grid cells.

Phase Three

The third phase of the study continues to use new methods and data to refine the groundwater-flow model developed in phases one and two. Implementation of these new methods and data will increase the understanding of the availability of groundwater and the effect of anthropogenic stresses on the groundwater and surface-water resources in the Elkhorn and Loup River basins. Phase-three objectives include

Checking water levels at a test well in the Elkhorn and Loup River Basins
  • An investigation of the phase-two model sensitivity to recharge using the Soil-Water-Balance (SWB) model, newly developed by USGS. SWB will help assimilate readily available public data (climate, soils, land use, etc.) to estimate the spatial and temporal patterns of recharge.
  • Continued collection of geologic data to better characterize the aquifer. The combination of test-hole drilling, bore geophysical logging, and surface geophysical surveys will provide a more detailed picture of the aquifer units and base of the aquifer.
  • Installation of monitoring wells in test-hole drilling locations to give NRDs better information on water levels.
  • Spatial and temporal refinement of the groundwater-flow model including re-discretization of the model grid to ½-mile-cell spacing and the refinement of stress periods from an annual time step to at least seasonal so that the model can represent input changes that occur within one model year.

Finally, the results of the phase-three model will undergo calibration via parameter estimation similar to the calibration done for phase two, as well as the completion of additional analysis runs.

 

Middle Loup River, Nebr.