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.
Contact: GS-W-VArmd_webmaster@usgs.gov
U.S. Department of Interior,
U.S. Geological Survey
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