Comparison of base flows to selected streamflow statistics representative of 1930-2002 in West Virginia

Abstract

Base flows were compared with published streamflow statistics to assess climate variability and to determine the published statistics that can be substituted for annual and seasonal base flows of unregulated streams in West Virginia. The comparison study was done by the U.S. Geological Survey, in cooperation with the West Virginia Department of Environmental Protection, Division of Water and Waste Management. The seasons were defined as winter (January 1–March 31), spring (April 1–June 30), summer (July 1–September 30), and fall (October 1–December 31). Differences in mean annual base flows for five record sub-periods (1930–42, 1943–62, 1963–69, 1970–79, and 1980–2002) range from -14.9 to 14.6 percent when compared to the values for the period 1930–2002. Differences between mean seasonal base flows and values for the period 1930– 2002 are less variable for winter and spring, -11.2 to 11.0 percent, than for summer and fall, -47.0 to 43.6 percent. Mean summer base flows (July–September) and mean monthly base flows for July, August, September, and October are approximately equal, within 7.4 percentage points of mean annual base flow. The mean of each of annual, spring, summer, fall, and winter base flows are approximately equal to the annual 50-percent (standard error of 10.3 percent), 45-percent (error of 14.6 percent), 75-percent (error of 11.8 percent), 55-percent (error of 11.2 percent), and 35-percent duration flows (error of 11.1 percent), respectively. The mean seasonal base flows for spring, summer, fall, and winter are approximately equal to the spring 50to 55-percent (standard error of 6.8 percent), summer 45to 50-percent (error of 6.7 percent), fall 45-percent (error of 15.2 percent), and winter 60-percent duration flows (error of 8.5 percent), respectively. Annual and seasonal base flows representative of the period 1930–2002 at unregulated streamflow-gaging stations and ungaged locations in West Virginia can be estimated using previously published values of statistics and procedures. Introduction Streamflow can be separated into discharge from overland runoff and discharge from groundwater. Base flow is the portion of streamflow contributed by groundwater discharge. Generally, base flows are greater in wetter seasons than in dryer seasons because more water accumulates and is released from groundwater. Knowledge of climatic, seasonal, and monthly differences in base flows can assist scientists and water-resource managers in understanding the capacity of groundwater storage in watersheds and the ability of a stream to maintain flows during droughts. Streamflow statistics have been computed for streamflow-gaging stations, and equations have been determined to estimate streamflows at ungaged locations in West Virginia for the period 1930 to 2002 (Wiley, 2006, 2008; Wiley and Atkins, 2010a). Equations for estimating base flows could be determined using similar methods, but a simpler and lower cost method for estimating base flows is already available if published streamflow statistics can be used as surrogates for base flows. This study, conducted in cooperation with the West Virginia Department of Environmental Protection, Division of Waste and Water Management, investigated the climatic, seasonal, and monthly variability of base flows at 15 selected long-term streamflow-gaging stations, documented the development of relations between base flows and published streamflow statistics, and determined surrogate statistics (the published statistics that can be substituted for base flows) to be used to estimate annual and seasonal base flows at other streamflow-gaging stations and at ungaged locations. The results of this study are representative of the period 1930–2002 and are relevant only to West Virginia, but the procedures presented in this report can be used to determine substitute streamflow statistics that can be used to estimate base flows in other regions. This report presents the procedures used to estimate base flows for 1930–2002. The climatic, seasonal, and monthly 2 Comparison of Base Flows to Selected Streamflow Statistics Representative of 1930–2002 in West Virginia variability of base flows at 15 long-term streamflow-gaging stations is discussed. Relations between mean annual and mean seasonal base flows, and between mean annual and 50-percent duration flows, are shown in illustrations. Relations between mean seasonal base flows and seasonal duration flows are also shown in illustrations. Differences between mean annual base flows and annual streamflow statistics are listed in tables. Base flows for the 15 long-term streamflow-gaging stations are provided in an Appendix as supplementary information. Description of Study Area West Virginia can be differentiated into three physiographic provinces (fig. 1), the Appalachian Plateaus, Valley and Ridge, and Blue Ridge (Fenneman, 1938). The movement of air masses across the State allows identification of two climatic regions (fig. 1), separated by a line defined as the Climatic Divide (Wiley and others, 2000; Wiley and Atkins, 2010b). Generally, the part of the State west of the Climatic Divide is in the Appalachian Plateaus Physiographic Province; altitudes in the Appalachian Plateaus range from about 2,500 to 4,861 ft (NAVD 88) at Spruce Knob along the Climatic Divide to about 550 to 650 ft along the Ohio River. The part of West Virginia east of the Climatic Divide is in the Valley and Ridge Physiographic Province, except for the extreme eastern tip of the State, which is in the Blue Ridge Physiographic Province. Altitudes decrease eastward from the Climatic Divide to 274 ft at Harpers Ferry in the Eastern Panhandle (U.S. Geological Survey, 1990, 2006; National Oceanic and Atmospheric Administration, 2006a). The Appalachian Plateaus Physiographic Province consists of consolidated, mostly siliciclastic sedimentary rocks that have a gentle slope from southeast to northwest near the Climatic Divide and are nearly flat-lying along the Ohio River. One exception is in the northeastern area of the province (west of the Climatic Divide), where the rocks are gently folded and some carbonate rock crops out (Fenneman, 1938). The rocks in the Appalachian Plateaus Physiographic Province have been eroded to form steep hills and deeply incised valleys. Drainage patterns are dendritic. The Valley and Ridge Physiographic Province in West Virginia consists of consolidated carbonate and siliciclastic sedimentary rocks that are folded sharply and extensively faulted (Fenneman, 1938). Northeast-trending valleys and ridges parallel the Climatic Divide. Drainage patterns are trellis. The Blue Ridge Physiographic Province within West Virginia consists predominantly of metamorphosed sandstone and shale (Fenneman, 1938). The province has high relief between mountains and wide valleys that parallel the Climatic Divide. Drainage patterns are trellis. The climate of West Virginia is primarily continental, with mild summers and cold winters. Major weather systems generally approach from the west and southwest, although polar continental air masses of cold, dry air that approach from the north and northwest are not unusual. Air masses from the Atlantic Ocean sometimes affect the area east of the Climatic Divide and less frequently affect the area west of the Climatic Divide. Generally, tropical continental masses of hot, dry air from the southwest affect the climate west of the Climatic Divide. Tropical maritime masses of warm, moist air from the Gulf of Mexico affect the climate east of the Climatic Divide more than west of the Climatic Divide. Evaporation from local and upwind land surfaces, lakes, and reservoirs also provides a source of moisture that affects the climate of the State (U.S. Geological Survey, 1991; National Oceanic and Atmospheric Administration, 2006a). Annual precipitation averages about 42 to 45 in. statewide with about 60 percent received from March through August. July is the wettest month, and September through November are the driest months. Annual average precipitation in the State generally decreases northwestward from about 50 to 60 in. along the Climatic Divide to about 40 in. along the Ohio River; precipitation ranges from about 30 to 35 in. east of the Climatic Divide to about 40 in. in the extreme eastern tip of the State. Greater precipitation along and west of the Climatic Divide is a consequence of the higher elevations along the Divide and the orographic lifting of weather systems generally approaching from the west and southwest. Annual average snowfall follows the general pattern of annual average precipitation, decreasing northwestward from about 36 to 100 in. along the Climatic Divide to about 20 to 30 in. along the Ohio River. East of the Climatic Divide, annual average snowfall ranges from 24 to 36 in. (U.S. Geological Survey, 1991; Natural Resources Conservation Service, 2006; National Oceanic and Atmospheric Administration, 2006a, 2006b).