Groundwater-Flow Model for the High Plains Aquifer in South Dakota Active

Executive Summary:
Throughout the Upper Midwest and Great Plains areas, rapidly rising commodity prices are driving dramatic changes towards more intensive agricultural practices aimed at increased crop production. An area that could be heavily impacted by increased irrigation demand is the northern extent of the High Plains aquifer system, which in South Dakota exists between the White River and Nebraska border and extends from near Pine Ridge to near the Missouri River. In this area, the High Plains Aquifer System consists of two primary bedrock aquifers (the Ogallala and Arikaree aquifers). The High Plains aquifer system extends as far south as Texas and is the most heavily utilized aquifer system in the United States.                                                                                                              Large-scale irrigation development from the High Plains aquifer system in South Dakota could have important implications relative to water-rights issues for the Oglala Sioux Tribe (OST) and Rosebud Sioux Tribe (RST). Numerous tracts of deeded land are interspersed with tribal trust lands within the Pine Ridge and Rosebud Indian Reservations and within other adjoining non-tribal counties (Bennett,
Gregory, Mellette, and Tripp Counties) where the High Plains Aquifer System exists. Effects associated with increased irrigation demand also could be manifested in area wetlands that have important connections to the regional groundwater system and streams that derive base flow from groundwater discharge. Primary streams that could be affected include the White, Little White, and Keya Paha Rivers, as well as numerous tributaries to these rivers.
Irrigation development and associated increases in crop production could potentially be an important economic development tool for the OST and RST. However, another major consideration may be balancing irrigation development with the complicated water-rights issues and with sustainability issues associated with both the groundwater system and the interconnected base-flow components critical to the area wetlands and streams.
Development of a regional numerical groundwater flow model would be the best possible tool for assessing potential effects of increased irrigation demand across the northern extent of the High Plains Aquifer System in South Dakota. Through cooperative projects with RST, the U.S. Geological Survey (USGS) has developed two groundwater flow models for the Ogallala and Arikaree aquifers. The first model was developed for Mellette and Todd Counties (Long and others, 2003) and was subsequently
updated (Long and Putnam, 2010) to include additional data and simulations of selected potential future scenarios of demand and climate. The second model was developed for Gregory and Tripp Counties (Davis and Putnam, 2013) and simulates groundwater flow in the Ogallala aquifer. The USGS also has developed a third groundwater flow model of the Ogallala and Arikaree aquifers in cooperation with OST for applicable parts of Bennett, Jackson, and Shannon Counties (Davis and others, 2015).
For this study, the three separate groundwater flow models will be combined into a regional to evaluate potential broad-scale effects of increased irrigation demand (and possibly climate change scenarios) throughout the northern extent of the High Plains aquifer system. A regional numerical groundwater flow model would be the best possible tool for assessing potential effects of increased irrigation demand and drought stresses across the northern extent of the High Plains aquifer system in South Dakota. The model will have natural hydrologic boundaries on nearly all sides and will include all areas of this aquifer system in South Dakota. The model also will extend to the Niobrara River in Nebraska, which would provide a natural hydrologic boundary on the southern side. The model will provide an improved understanding of the hydrology and hydrogeology of the High Plains aquifer in South Dakota. Future
potential water use and climate scenarios will be simulated with the model to estimate the hydrologic effects, such as water-level declines and decreased stream base flow rates.

Objective(s):

The primary objective of this study is to assess the potential effects of increased groundwater withdrawal or prolonged drought across the northern extent of the High Plains aquifer system in South Dakota and northern Nebraska. A single, combined, and consistent groundwater flow model of the High Plains aquifer system in southern South Dakota and northern Nebraska will be
developed as a part of the assessment. Construction for the South Dakota part of the model will be based on previously published digital models of the High Plains aquifer. The combined model will be extended to the south to the Niobrara River in Nebraska, and will include natural hydrologic boundaries on all sides. Construction for the Nebraska part of the model will be based on interpretation of well logs and published hydrogeologic information. Recharge for the study period will be estimated using a soil-
water balance (SWB) program. Updated estimates of groundwater discharge as stream base flow will be estimated using digital hydrograph separation methods. Subsequent modeling efforts would commence following data collection and interpretation and would improve model input parameters.