In 2006, a cooperative study was established to compile reliable data describing groundwater and surface-water interactions in the Elkhorn and Loup River Basins. The purpose of the study was to address state legislation that requires a sustainable balance between long term water supplies and uses of surface water and groundwater. A groundwater-flow model [hereinafter referred to as the Elkhorn-Loup Model (ELM)] was constructed as part of the first two phases of that study as a tool for understanding the effect of groundwater pumpage on stream base flow and the effects of management strategies on hydrologically connected groundwater and surface-water supplies. The third phase of the study was implemented to gain additional geologic knowledge and update the ELM with enhanced water-budget information and refined discretization of the model grid and stress periods. As part of that effort, the ELM is being reconstructed to include two vertical model layers, whereas phase-one and phase-two simulations (Peterson and others, 2008; Stanton and others, 2010) represented the aquifer system using one vertical model layer. The goal for defining the base of the upper model layer was to divide the model vertically so that the upper layer could have different water transmitting and storage characteristics than the lower layer. Texture descriptions were used in most cases to identify the depth in a test-hole, water-well, or surface-geophysical log at which dividing the aquifer produced contrasting texture characteristics for the upper and lower model layers. The study area covers approximately 30,000 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary roughly coincides with the western boundary of the Upper Loup NRD, and the eastern boundary roughly coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska (University of Nebraska, 2005). This data release consists of a point shapefile attributed with values representing the elevation of the base of the upper layer of the two-layer phase-three Elkhorn-Loup Model (ELM) above the vertical datum (National Geodetic Vertical Datum of 1929).

In 2006, a cooperative study was established to compile reliable data describing groundwater and surface-water interactions in the Elkhorn and Loup River Basins. The purpose of the study was to address state legislation that requires a sustainable balance between long term water supplies and uses of surface water and groundwater. A groundwater-flow model [hereinafter referred to as the Elkhorn-Loup Model (ELM)] was constructed as part of the first two phases of that study as a tool for understanding the effect of groundwater pumpage on stream base flow and the effects of management strategies on hydrologically connected groundwater and surface-water supplies. The third phase of the study was implemented to gain additional geologic knowledge and update the ELM with enhanced water-budget information and refined discretization of the model grid and stress periods. As part of that effort, the ELM is being reconstructed to include two vertical model layers, whereas phase-one and phase-two simulations (Peterson and others, 2008; Stanton and others, 2010) represented the aquifer system using one vertical model layer. The goal for defining the base of the upper model layer was to divide the model vertically so that the upper layer had different water transmitting and storage characteristics than the lower layer. Texture descriptions were used in most cases to identify the depth in a test-hole, water well, or surface-geophysical log at which dividing the aquifer produced contrasting texture characteristics for the upper and lower model layers. The study area covers approximately 30,000 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary roughly coincides with the western boundary of the Upper Loup NRD, and the eastern boundary roughly coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska (University of Nebraska, 2005). This data release consists of a line shapefile of contours attributed with values representing the elevation of the base of the upper layer of the two-layer phase-three ELM above the vertical datum (National Geodetic Vertical Datum of 1929).

In 2006, a cooperative study was established to compile reliable data describing groundwater and surface-water interactions in the Elkhorn and Loup River Basins. The purpose of the study was to address state legislation that requires a sustainable balance between long term water supplies and uses of surface water and groundwater. A groundwater-flow model [hereinafter referred to as the Elkhorn-Loup Model (ELM)] was constructed as part of the first two phases of that study as a tool for understanding the effect of groundwater pumpage on stream base flow and the effects of management strategies on hydrologically connected groundwater and surface-water supplies. The third phase of the study was implemented to gain additional geologic knowledge and update the ELM with enhanced water-budget information and refined discretization of the model grid and stress periods. As part of that effort, the ELM is being reconstructed to include two vertical model layers, whereas phase-one and phase-two simulations (Peterson and others, 2008; Stanton and others, 2010) represented the aquifer system using one vertical model layer. The goal for defining the base of the upper model layer was to divide the model vertically so that the upper layer had different water transmitting and storage characteristics than the lower layer. Texture descriptions were used in most cases to identify the depth in a test-hole, water well, or surface-geophysical log at which dividing the aquifer produced contrasting texture characteristics for the upper and lower model layers. The study area covers approximately 30,000 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary roughly coincides with the western boundary of the Upper Loup NRD, and the eastern boundary roughly coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska (University of Nebraska, 2005). This data release consists of a line shapefile of contours attributed with values representing the elevation of the base of the upper layer of the two-layer phase-three ELM above the vertical datum (National Geodetic Vertical Datum of 1929).

The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District (NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model (ELM) area using a ground-water-flow model. The ELM area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow 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 will be calibrated to measured ground-water levels and estimated ground-water discharge to streams for the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 period will be calibrated to measure ground-water level changes. The study results will assist Nebraska Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for managing hydrologically connected waters. This dataset is one of three geospatial datasets that together revise previously published maps of the configuration of the base of the principal aquifer and of the geologic units that form the base of the principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is an unnamed alluvial aquifer. This dataset consists of the location of test holes and registered wells with information about the altitude of the base of the principal aquifer and the identity of the geologic unit that underlies the principal aquifer in the Elkhorn-Loup Model area, north-central Nebraska.

The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District (NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model (ELM) area using a ground-water-flow model. The ELM area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow 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 will be calibrated to measured ground-water levels and estimated ground-water discharge to streams for the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 period will be calibrated to measure ground-water level changes. The study results will assist Nebraska Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for managing hydrologically connected waters. This dataset is one of three geospatial datasets that together revise previously published maps of the configuration of the base of the principal aquifer and of the geologic units that form the base of the principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is an unnamed alluvial aquifer. This dataset consists of the location of test holes and registered wells with information about the altitude of the base of the principal aquifer and the identity of the geologic unit that underlies the principal aquifer in the Elkhorn-Loup Model area, north-central Nebraska.

The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District (NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model (ELM) area using a ground-water-flow model. The ELM area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow 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 will be calibrated to measured ground-water levels and estimated ground-water discharge to streams for the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 period will be calibrated to measure ground-water level changes. The study results will assist Nebraska Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for managing hydrologically connected waters. This dataset is one of three geospatial datasets that together revise previously published maps of the configuration of the base of the principal aquifer and of the geologic units that form the base of the principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is an unnamed alluvial aquifer. This dataset consists of contour lines of the base-of-aquifer altitude above the vertical datum (National Geodetic Vertical Datum of 1929). The purpose of this dataset is to serve as the lower aquifer boundary in the ground-water-flow model of the Elkhorn-Loup Model area, north-central Nebraska. This dataset is not intended to be used at scales larger than 1:350,000. The density of registered wells and test holes with data about the depth to the base of aquifer varies greatly across the map area. The accuracy of the base-of-aquifer contours in a given area is related directly to the density distribution and availability of registered well and test-hole data with information about the depth to the base of aquifer in that area.

The U.S. Geological Survey (USGS), in cooperation with the Lewis and Clark Natural Resources District (NRD), Lower Elkhorn NRD, Lower Loup NRD, Lower Platte North NRD, Lower Niobrara NRD, Middle Niobrara NRD, Upper Elkhorn NRD, and Upper Loup NRD, have agreed to cooperatively study water resources from prior to the beginning of irrigation development to 2005 in the Elkhorn-Loup Model (ELM) area using a ground-water-flow model. The ELM area covers approximately 30,800 square miles, and extends from the Niobrara River in the north to the Platte River in the south. The western boundary of the ELM area coincides with the western boundary of the Middle Niobrara, Twin Platte, and Upper Loup NRDs; the eastern boundary coincides with the approximate location of the westernmost extent of glacial till in eastern Nebraska. The initial ground-water-flow 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 will be calibrated to measured ground-water levels and estimated ground-water discharge to streams for the pre-ground-water development period (approximately 1940) and the simulation of the 1940-2005 period will be calibrated to measure ground-water level changes. The study results will assist Nebraska Department of Natural Resources and the NRDs in the ELM area to develop long-term strategies for managing hydrologically connected waters. This dataset is one of three geospatial datasets that together revise previously published maps of the configuration of the base of the principal aquifer and of the geologic units that form the base of the principal aquifer in the study area; the revisions to the base-of-aquifer altitude are based on currently available or reinterpreted geologic logs of test holes and selected registered wells. The principal aquifer is the High Plains aquifer except in the northeast part of the model area, where the principal aquifer is an unnamed alluvial aquifer. This dataset consists of contour lines of the base-of-aquifer altitude above the vertical datum (National Geodetic Vertical Datum of 1929). The purpose of this dataset is to serve as the lower aquifer boundary in the ground-water-flow model of the Elkhorn-Loup Model area, north-central Nebraska. This dataset is not intended to be used at scales larger than 1:350,000. The density of registered wells and test holes with data about the depth to the base of aquifer varies greatly across the map area. The accuracy of the base-of-aquifer contours in a given area is related directly to the density distribution and availability of registered well and test-hole data with information about the depth to the base of aquifer in that area.

The U.S. Geological Survey and its partners have collaborated to complete airborne geophysical surveys for areas of the North and South Platte River valleys and Lodgepole Creek in western Nebraska. The objective of the surveys was to map the aquifers and bedrock topography of selected areas to help improve the understanding of groundwater-surface-water relationships to be used in water management decisions. Frequency-domain (2008 and 2009) and time-domain (2010) helicopter electromagnetic surveys were completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic information for refinement of groundwater model inputs. To make the geophysical data useful for multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inversion model, in conjunction with sensitivity analysis, geological ground truth (boreholes), and geological interpretation, is used to characterize hydrogeologic features. The two- and three- dimensional interpretation provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of the actual flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models represent the hydrogeology at a level of accuracy not achievable using previous data sets.

The U.S. Geological Survey and its partners have collaborated to complete airborne geophysical surveys for areas of the North and South Platte River valleys and Lodgepole Creek in western Nebraska. The objective of the surveys was to map the aquifers and bedrock topography of selected areas to help improve the understanding of groundwater-surface-water relationships to be used in water management decisions. Frequency-domain (2008 and 2009) and time-domain (2010) helicopter electromagnetic surveys were completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic information for refinement of groundwater model inputs. To make the geophysical data useful for multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inversion model, in conjunction with sensitivity analysis, geological ground truth (boreholes), and geological interpretation, is used to characterize hydrogeologic features. The two- and three- dimensional interpretation provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of the actual flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models represent the hydrogeology at a level of accuracy not achievable using previous data sets.

Several pre-existing datasets that characterize portions of the Northern High Plains aquifer base were merged together in order to define the entire base of the Northern High Plains aquifer. This digital dataset consists of the aquifer base elevation contours (100-foot contour interval) for the northern portion of the High Plains aquifer in the central United States. The boundaries were chosen to simplify boundary conditions for a computer simulation model being used for a hydrologic study of the Northern High Plains.