Digitial map of geologic units forming the base of the principal aquifer and areas where the base of aquifer is not well defined, 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 identifies and delimits approximately the extent of the geologic units forming the base of the principal aquifer and the areas where the base of principal aquifer surface is not well defined. Ambiguity in the calculated base of aquifer existed wherever this calculated base-of-aquifer altitude was above the mapped land surface. The calculated base of aquifer was above the mapped land surface either because of inaccuracy in the mapped land surface or base of aquifer calculation or because the principal aquifer was thin or not present.美国地质调查局（U.S. Geological Survey, USGS）与刘易斯与克拉克自然资源区（Lewis and Clark Natural Resources District, NRD）、下埃尔克霍恩自然资源区（Lower Elkhorn NRD）、下卢普自然资源区（Lower Loup NRD）、下普拉特北自然资源区（Lower Platte North NRD）、下尼奥布拉拉自然资源区（Lower Niobrara NRD）、中尼奥布拉拉自然资源区（Middle Niobrara NRD）、上埃尔克霍恩自然资源区（Upper Elkhorn NRD）以及上卢普自然资源区（Upper Loup NRD）合作，达成协议将利用地下水流模型（ground-water-flow model），对埃尔克霍恩-卢普模型区（Elkhorn-Loup Model Area, ELM）内灌溉开发启动前至2005年的水资源开展联合研究。ELM区域总面积约30800平方英里，北起尼奥布拉拉河，南至普拉特河。其西界与中尼奥布拉拉、双子普拉特及上卢普NRDs的西界重合；东界大致与内布拉斯加州东部冰碛物的最西端分布位置一致。初始地下水流模型采用单层垂向结构，用以表征第三纪（Tertiary-age）奥加拉拉组（Ogallala Group）地层与第四纪（Quaternary-age）冲积沉积物（alluvial deposits）所构成的含水层，节点间距统一为2英里。该模型将以地下水开发前（约1940年）的实测地下水位与估算的地下水河道排泄量进行校准；1940年至2005年的模拟结果将以实测地下水位变化量开展校准。本研究成果将助力内布拉斯加州自然资源部及ELM区域内的各NRD制定水文连通水体管理的长期战略。 本数据集为三套地理空间数据集之一，三者共同修订了此前发布的研究区内主要含水层底部构型及构成主要含水层底部的地质单元分布图；含水层底部海拔的修订依据现有或重新解译的试钻孔地质录井资料与选定的注册井数据。除模型区东北部以外，研究区的主要含水层为高平原含水层（High Plains aquifer），而东北部的主要含水层为未命名的冲积含水层。 本数据集明确并大致划定了构成主要含水层底部的地质单元分布范围，以及主要含水层底部界面界定不清晰的区域。当计算得到的含水层底部海拔高于已绘制的地表高程时，便存在含水层底部计算结果的不确定性。此类情况的成因包括已绘制的地表高程数据存在误差、含水层底部计算存在偏差，或是主要含水层厚度较薄甚至不存在。