Hydrologic conditions and budgets for the Black Hills of South Dakota, through water year 1998

The Black Hills are an important recharge area for aquifers in the northern Great Plains. The surface-water hydrology of the area is highly influenced by interactions with the Madison and Minnelusa aquifers, including large springs and streamflow loss zones. Defining responses of ground water and streamflow to a variety of hydrogeologic influences is critical to development of hydrologic budgets for ground- and surface-water systems.

Hydrographs for 52 observation wells and 1 cave site are used to show ground-water response to cumulative precipitation departures. Aquifers considered include the Precambrian, Deadwood, Madison, Minnelusa, Minnekahta, and Inyan Kara aquifers, with wells completed in the Inyan Kara aquifer generally showing small response to precipitation patterns. Many wells completed in the other aquifers have large short- and long-term fluctuations in water levels. Madison and Minnelusa wells in the southern Black Hills show a general tendency for smaller water-level fluctuations than in other areas.

Streamflow characteristics and relations with precipitation are examined for 33 gaging stations representative of five different hydrogeologic settings that are identified. The ?limestone headwater? setting occurs within outcrops of the Madison Limestone and Minnelusa Formation along the ?Limestone Plateau,? where direct runoff is uncommon and streamflow consists almost entirely of base flow originating as ground-water discharge from headwater springs. Thus, variability in daily, monthly, and annual flow is small. Annual streamflow correlates poorly with precipitation; however, consideration of ?moving averages? (involving up to 11 years of annual precipitation data for some stations) improves relations substantially.

The ?crystalline core? area is encircled by the outcrop band of the Madison and Minnelusa Formations and is dominated by igneous and metamorphic rocks. Base flow ranges from about 41 to 73 percent for representative streams; however, monthly flow records demonstrate shortterm response to precipitation, which probably indicates a relatively large component of interflow. Streamflow generally correlates well with annual precipitation, with r2 values ranging from 0.52 to 0.87.

Downgradient from the crystalline core area is the ?loss zone? setting, where streamflow losses occur to outcrops of the Madison and Minnelusa Formations. Relations between streamflow and annual precipitation are defined by a power equation for the only two representative gages in this setting. The loss zone and ?artesian spring? areas are combined because many artesian springs are located along stream channels that are influenced by streamflow losses and several artesian springs are within outcrops of the Minnelusa Formation. Streamflow characteristics for artesian springs generally have small variability and poor correlations with annual precipitation because of large influence from relatively stable ground-water discharge. The ?exterior? setting is located downgradient from the outcrop of the Inyan Kara Group, which coincides with the outer extent of the loss zone/artesian spring setting. Large flow variability is characteristic for this setting, and base flow generally is smaller than for other settings.

Basin yields are highly variable, with the largest yields occurring in high-altitude areas of the northern Black Hills that receive large annual precipitation. Relations between annual yield efficiency and precipitation were applied by previous investigators in developing a method for estimating annual precipitation recharge, based on annual precipitation. The resulting ?yield-efficiency algorithm? compares spatial distributions for annual precipitation, average annual precipitation, and efficiency of basin yield. This algorithm is applied in estimating precipitation recharge on aquifer outcrops and in estimating streamflow yield from various outcrop areas, for purposes of developing average hydrologic budgets