Base-Flow Characteristics of Streams in the Valley and Ridge, Blue Ridge, and Piedmont Physiographic Provinces of Virginia and other Mid-Atlantic States

Donald C. Hayes, and David L. Nelms, U.S. Geological Survey, 1730 East Parham Road, Richmond, VA 23228


Population growth within the Valley and Ridge, the Blue Ridge, and the Piedmont Physiographic Provinces of Virginia has led to concerns about the allocation of surface-water flow and the increase in demands on the ground-water resources. Various hydrologic studies in Virginia have (1) described the base-flow characteristics of streams, (2) identified regional differences in these flow characteristics, and (3) described, where possible, the potential surface-water and ground-water yields of basins on the basis of the base-flow characteristics. Streamflow data were collected and low-flow characteristics computed (annual minimum average 7-consecutive-day flow for 2-year and 10-year recurrence intervals) for 254 continuous-record streamflow gaging stations and 461 partial-record streamflow gaging stations throughout Virginia. The continuous-record data were analyzed by means of historical mean daily discharge data, and the partial-record data were analyzed by means of correlation of discharge measurements to mean daily discharge data. The State was divided into eight regions on the basis of physiography and geographic grouping of residuals computed in regression analysis.

Additional base-flow characteristics were computed for streams in the Valley and Ridge, the Blue Ridge, and the Piedmont Physiographic Provinces of Virginia as part of the Appalachian Valley and Piedmont Regional Aquifer-System Analysis study. The provinces were separated into five regions: (1) Valley and Ridge, (2) Blue Ridge, (3) Piedmont/Blue Ridge transition, (4) Piedmont northern, and (5) Piedmont southern. Various flow statistics, which represent streamflows predominantly composed of base flow, were determined for 217 continuous-record streamflow-gaging stations and for 192 partial-record streamflow-gaging stations. Variability of base flow was represented by the logarithm of the ratio of the 50-percent exceedance discharge to the 90-percent exceedance discharge on the streamflow duration curve (base-flow variability index). Effective recharge rates also were calculated.

Median values for the various flow statistics range from 0.15 cubic foot per second per square mile for the 90-percent exceedance discharge on the streamflow-duration curve to 0.61 cubic foot per second per square mile for mean base flow. The 50-percent exceedance discharge on the streamflow-duration curve is an excellent estimator of mean base flow for the Piedmont/Blue Ridge transition region and Piedmont southern region, but this value tends to underestimate mean base flow for the remaining regions. The base-flow variability index ranges from 0.07 to 2.27, with a median value of 0.55. Effective recharge rates range from 0.07 to 33.07 inches per year, with a median value of 8.32 inches per year.

 

Differences in the base-flow characteristics exist between the five regions. The median discharges for the Valley and Ridge, the Blue Ridge, and the Piedmont/Blue Ridge transition regions are higher than those for the Piedmont regions. The flow statistics are consistently higher and the values for base-flow variability are lower for basins within the Piedmont/Blue Ridge transition region relative to those from the other regions, whereas the basins within the Piedmont northern region show the opposite pattern. Results from statistical analysis indicate that the regions can be ranked in terms of base-flow characteristics from highest to lowest as follows: (1) Piedmont/Blue Ridge transition, (2) Valley and Ridge and Blue Ridge, (3) Piedmont southern, and (4) Piedmont northern. The base-flow variability index shows an opposite relation and ranks the regions from lowest to highest in the same order.

 

Group rankings of the base-flow characteristics were used to designate the potential surface-water yield for the regions. An approach developed for this investigation assigns a rank for potential surface-water yield to a basin according to the quartiles in which the values for the base-flow characteristics are located. Both procedures indicate that the Valley and Ridge, the Blue Ridge, and the Piedmont/Blue Ridge transition regions have moderate-to-high potential surface-water yield, and the Piedmont regions have low-to-moderate potential surface-water yield.

In order to indicate potential ground-water yield from base-flow characteristics, aquifer properties for 51 streamflow-gaging stations with continuous-record streamflow data were determined by methods that use streamflow records and basin characteristics. Areal diffusivity ranged from 17,100 to 88,400 feet squared per day, with a median value of 38,400 feet squared per day. Areal transmissivity ranged from 63 to 830 feet squared per day, with a median value of 270 feet squared per day. Storage coefficients, which were estimated by dividing areal transmissivity by areal diffusivity, ranged from approximately 0.001 to 0.019 (dimensionless), with a median value of 0.007.

 

The median value for areal diffusivity decreases as potential surface-water yield of the basins increases. Areal transmissivity generally increases as storage coefficient increases; however, basins with low potential surface-water yield generally have high values of areal transmissivity associated with low values of storage coefficient over a narrow range relative to those from basins designated as having moderate-to-high potential surface-water yield. Although the basins with high potential surface-water yield tend to have comparatively lower values for areal transmissivity, storage coefficients generally are large when compared to those from basins with similar values of areal transmissivity but different potential surface-water yield.

 

Aquifer properties were grouped by potential surface-water yield and were related to hydrogeologic units categorized by large, medium, and small well yields for the Valley and Ridge Physiographic Province and for the Blue Ridge and the Piedmont Physiographic Provinces. Generally, no trend is evident between areal diffusivity and the hydrogeologic units. Some of the high values of areal diffusivity are associated with basins predominantly underlain by hydrogeologic units with small well yields, especially basins with a low potential surface-water yield. Areal transmissivity and storage coefficient tend to decrease, as expected, as more of the basin is underlain by the hydrogeologic unit with small well yields in the Valley and Ridge Physiographic Province. 

 

A similar trend is indicated for the hydrogeologic unit with medium well yields in the Blue Ridge and the Piedmont Physiographic Provinces. Areal transmissivity and storage coefficient tend to increase, which is unexpected, as more of the basin is underlain by the hydrogeologic unit with small well yields in the Blue Ridge and the Piedmont Physiographic Provinces. The base-flow characteristics of a basin may provide a relative indication of the potential ground-water yield, but other factors need to be considered, such as geologic structure, lithology, precipitation, relief, and the degree of hydraulic interconnection between the regolith and bedrock.

 

Baseflow characteristics also were computed for 221 additional continuous-record streamflow gaging stations from North Carolina, Maryland, Delaware, Pennsylvania, and New York as part of the Mid-Atlantic Integrated Assessment study. Regional clusters of the group rankings of the base-flow variability index are evident spatially. Analysis of the data plots suggests that the Piedmont/Blue Ridge transition region extends into central and western North Carolina. Other regional clusters suggested by data analysis are within the Appalachian Plateaus in southwest Virgina and Pennsylvania and Fall Zone areas in eastern Virginia and possibly eastern North Carolina.


Hayes, D.C., and Nelms, D.L.,  Base-Flow Characteristics of Streams in the Valley and Ridge, Blue Ridge, and Piedmont Physiographic Provinces of Virginia and other Mid-Atlantic States  


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