Douglas G. Emerson

Surface-water hydrology of Hay Creek watershed, Montana, and West Branch Antelope Creek watershed, North Dakota

Hydrologric data were used to determine the premining surface-water conditions in two small basins in the Fort Union coal region of Montana and North Dakota. The two streams. Hay Creek and West Branch Antelope Creek, are ephemeral. Most of the volume and peak discharges are due to snowmelt runoff. Little rainfall runoff occurs, and volume and peak discharges for this runoff are relatively small compared to those for snowmelt runoff. Suspended-sediment concentrations for snowmelt runoff ranged from 4 to 325 milligrams per liter for the Hay Creek and West Branch Antelope Creek watersheds. At the outflow site of the Hay Creek watershed, the dominant dissolved constituents in runoff are magnesium and sulfate; at the outflow site of the West Branch Antelope Creek watershed, they are calcium, magnesium, bicarbonate, and sulfate. The U.S. Geological Surveys Precipitation-Runoff Modeling System was calibrated for both watersheds for the snowmelt runoff. The model was not calibrated for rainfall runoff because of insufficient runoff. Sensitivity analyses indicated the model was most sensitive to the values of snow correction for daily precipitation at precipitation gages, emissivity of the air for longwave radiation, and maximum available water-holding capacity of the soil profile. Testing of several watershed delineations showed that, for well-defined snow distribution, 23 units adequately defined the variability in runoff in the Hay Creek watershed, and 36 units adequately defined the variability in runoff in the West Branch Antelope Creek watershed. INTRODUCTION In response to the U.S. Department of the Interiors call for leasing nomination of Federal coal land, eligible tracts were submitted by mining concerns for consideration of their leasing potential. The U.S. Bureau of Land Management has the responsibility of evaluating the leasing applications for mining of Federal coal. Their evaluation must address potential environmental impacts of mining, which include those related to hydrology. The U.S. Office of Surface Mining Reclamation and Enforcement (1977) provisions outline impacts in terms of the probable hydrologic consequences of the mining and reclamation operations both on and off the proposed permit area and the reasonable assessment of the probable cumulative impacts of mining. These impacts include changes in flow regimes, flood peaks and volumes, sediment yields, water quality, soil-water relations, and water-balance relations for watersheds before, during, and after mining. Purpose and Scope In 1977, the U.S. Geological Survey began an investigation in cooperation with the U.S. Bureau of Land Management on the surface-water hydrology of two watersheds in the Fort Union coal region in eastern Montana and western North Dakota (fig. 1). The purpose of the investigation was to provide a means for U.S. Bureau of Land Management personnel and others to assess the impacts on surface-water hydrology due to changes in land use. The objectives were to: (1) Determine premining hydrologic conditions in two small, representative watersheds and, thus, provide historical data with which to compare the magnitude of changes during and after mining, and (2) develop and calibrate a watershed model that could be used to make reasonably accurate projections of effects on surface-water hydrology resulting from the various land treatments required for mining and reclamation. The complexity of watershed hydrology limits the capability of analyzing all the seasonal changes. Hines and others (1975) discussed the importance of recognizing critical periods. First, in many watersheds there exists a particular period, controlled by cyclical hydrologic events, for which data analysis and model application can be aimed, thereby reducing the components of a watershed model. Second, the recurrence probability of a seasonal hydrologic event commonly can be evaluated statistically because these events often recur on a cyclical basis. This probability can be related to the results of the model predictions. Third, data needs for calibration of the watershed model are reduced. Analysis is greatly simplified if sampling is needed only for a short period of the year. Because of these advantages, the criticalperiod rationale has been used for data analysis and model application. One purpose of this report is to document the kinds and quantity of data required and methods used to adequately define the surface-water hydrology of a watershed in the Fort Union coal region. The report defines the major hydrologic processes and the factors that affect them in regard to highand low-flow conditions, erosion, and chemical quality. The report also contains the analysis of hydrologic processes and factors used to determine the critical runoff periods for the Hay Creek and West Branch Antelope Creek watersheds (fig. 1). The controlling processes in the Fort Union coal region vary from other areas. A discussion of these processes and many basic concepts is included to help the reader understand their importance in the Fort Union coal region. Another purpose of the report is to provide documentation and calibration of a digital watershed model under premining conditions. The hydrologic system of the Hay Creek and West Branch Antelope Creek watersheds was simulated with the Precipitation-Runoff Modeling System developed by Leavesley and others (1983). Calibration of the model was performed for snowmelt runoff. The model was not calibrated for rainfall runoff because of the lack of rainfall runoff.