Rosebud Sioux Tribe and U.S. Geological Survey Collaboration for Groundwater Flow Modeling of the Ogallala and Arikaree Aquifers

Executive Summary

This project consists of refining an existing numerical ground-water flow model for the Ogallala and Arikaree aquifers within the Rosebud Reservation and utilizing the refined model to simulate potential stress scenarios for the aquifers. This project builds on a previous collaborative project between the Rosebud Sioux Tribe and U.S. Geological Survey (USGS) that resulted in the development and documentation of the existing numerical model (Long and others, 2003). The first phase of the current project consists of making numerous streamflow measurements during base-flow conditions and assembling existing hydrogeologic data to be used for the revised model. The second phase consists of updating the model with new data, calibration to additional streamflow measurements, and executing the model under different climatic and hydrologic stress scenarios.

This project is part of a long-term program to obtain information critical to management of the Ogallala and Arikaree aquifers within Tribal lands, which are crucial water resources for the Tribe. These aquifers are used for municipal, irrigation, and domestic water supplies and are an important component of the water supply for the Mni Wiconi Water System, which has been under construction for about 10 years and will supply water through the Reservation. The Rosebud Sioux Tribe is concerned about potential effects of current and future large-scale withdrawals from these aquifers. Potential exists for adverse effects to water resources from three primary factors, including: (1) increased ground-water demand from development of large-scale, center-pivot irrigation systems; (2) substantially reduced recharge from continued severe drought conditions that have persisted since about 2000 (which also could further increase irrigation demand substantially); and (3) potential effects associated with long-term global climate change (combination of increased demand and reduced recharge). These factors could cause reduced ground-water levels that could result in reduced production capacity, increased pumping costs, and reduced availability of ground water in some areas. These factors also could result in decreased ground-water discharge to streams that provides substantial and important baseflow to the Little White River, Keya Paha River, and various tributaries. Adverse effects on cultural resources associated with these surface-water resources also could result.

Objectives

The objectives of the project are to:

1. Re-discretize the model to a finer modeling grid near public-supply wells.
2. Refine calibration to additional stream base-flow measurements.
3. Simulate various pumping scenarios and drought conditions to make hypothetical projections about potential effects on ground water supply.