This project was done in cooperation with the
Delaware River Basin Commission , the
Pennsylvania Department of Environmental
Protection, and the
Delaware Department of Natural
Resources and Environmental Control in support of establishing high-flow
TMDLs in the Christina River Basin..
INTRODUCTION
The Christina River Basin drains 565 square miles in southeastern Pennsylvania and northern Delaware.
The basin encompasses the watersheds of four main streams, the Red Clay, White Clay, and Brandywine Creeks
and the Christina River. Streams in the Christina River are used for drinking water supplies,
for recreation, and as receiving waters for discharges of municipal and industrial effluent.
Water quality in some parts of the Christina River Basin is impaired
and does not support designated uses of the streams.
Water-quality problems in the basin identified by previous studies include
elevated levels of suspended sediment, bacteria, nitrogen, phosphorus, heavy metals,
organic compounds, and temperature. Low dissolved oxygen concentrations also exist. Stream
habitat is classified as "poor" in many stream reaches. These conditions are attributed to
both point and non-point sources of contamination.
A multi-agency water-quality management strategy
included a modeling component to evaluate the effects of
point and nonpoint-source contributions of nutrients
and suspended sediment on stream-water quality.
To assist in nonpoint-source evaluation, four
independent models, one for each of the four main
subbasins of the Christina River Basin, were developed
and calibrated by the U.S. Geological Survey (USGS) using the model code Hydrological
Simulation Program-Fortran (HSPF).
The HSPF model simulates streamflow, suspended sediment, and the
nutrients, nitrogen and phosphorus. In addition, the
model simulates water temperature, dissolved oxygen,
biochemical oxygen demand, and plankton as secondary
objectives needed to support the sediment and nutrient
simulations. Water-quality
data for model calibration were collected in each of the
four main subbasins and in small subbasins
predominantly covered by one land use following a
nonpoint-source monitoring plan.
SIMULATION OF STREAMFLOW AND WATER QUALITY IN THE BRANDYWINE CREEK SUBBASIN
The Brandywine Creek is the largest of the
Christina River subbasins and drains an area of 327 square miles.
For the model, the basin was subdivided
into 35 reaches draining areas that ranged from 0.6 to
18 square miles. Three of the reaches contain a regulated reservoir.
Stormflow and base-flow samples were collected during
1998 at six sites in the Brandywine Creek subbasin.
Eleven different pervious land uses and two
impervious land uses were selected for simulation.
Land-use areas were determined from 1995 land-use
data. The predominant land uses in the basin are
forested, agricultural, residential, and urban.
The hydrologic component of the model was run
at an hourly time step and calibrated using streamflow
data for eight USGS stream-
flow-measurement stations for the period of January 1,
1994, through October 29, 1998. Daily precipitation
data for three National Oceanic and Atmospheric
Administration (NOAA) gages and hourly data for one
NOAA gage were used for model input. The difference
between observed and simulated streamflow volume
ranged from -2.7 to 3.9 percent for the nearly 5-year
period at the eight calibration sites. Annual differences
between observed and simulated streamflow generally
were greater than the overall error. For example, at a site
near the bottom of the basin (drainage area of 237 square mile),
annual differences between observed and simulated
streamflow ranged from -14.0 to 18.8 percent, and the
overall error for the 5-year period was 1.0 percent.
Calibration errors for 36 storm periods at the eight
calibration sites for total volume, low-flow-recession
rate, 50-percent lowest flows, 10-percent highest flows,
and storm peaks were within the recommended criteria
of 20 percent or less. Much of the error in simulating
storm events on an hourly time step can be attributed to
uncertainty in the rainfall data.
The water-quality component of the model was
calibrated using monitoring data collected at six USGS
streamflow-measurement stations with variable water-quality
monitoring periods ending October 1998.
Because of availability, monitoring data for suspended-solids
concentrations were used as surrogates for
suspended-sediment concentrations, although suspended-solids
data may underestimate suspended sediment
and affect apparent accuracy of the suspended-sediment
simulation. Comparison of observed to simulated loads
for two to six individual storms in 1998 at each of the
six monitoring sites indicate that simulation error is
commonly as large as an order of magnitude for suspended
sediment and nutrients. The simulation error tends
to be smaller for dissolved nutrients than for particulate
nutrients. Errors of 40 percent or less for monthly or
annual values indicate a fair to good water-quality
calibration according to recommended criteria, with
much larger errors possible for individual events.
Assessment of the water-quality calibration under
stormflow conditions is limited by the relatively small
amount of available water-quality data in the basin.
Duration curves for simulated and reported sediment
concentration at Brandywine Creek at Wilmington,
Del., are similar, indicating model performance is better
when evaluated over longer periods than when evaluated
on individual storm events.
Users of the Brandywine Creek HSPF model
should be aware of model limitations and consider the
following if the model is used for predictive purposes:
flow and water quality for individual storm events may
not be well simulated, but the model performance is
reasonable when measured over longer periods of time;
the observed flow-duration curve for the simulation
period is similar to the long-term flow-duration curve at
Brandywine Creek at Chadds Ford, Pa., indicating that
the calibration period is representative of all but the
highest 1 percent of flow at that site; relative errors in
flow and water-quality simulations are greater for
smaller drainage areas than for larger areas; and
calibration for water quality was based on limited data.
SIMULATION OF STREAMFLOW AND WATER QUALITY IN THE WHITE CLAY CREEK SUBBASIN
The White Clay Creek is the second largest of the
subbasins and drains an area of 108 square miles.
For the model, the basin was subdivided
into 17 reaches draining areas that ranged from 1.37 to
13 square miles. Ten different pervious land uses and two
impervious land uses were selected for simulation.
Land-use areas were determined from 1995 land-use
data. The predominant land uses in the White Clay
Creek Basin are agricultural, forested, residential, and urban.
The hydrologic component of the model was run
at an hourly time step and primarily calibrated using
streamflow data from two USGS streamflow-measurement stations for the
period of October 1, 1994, through October 29, 1998.
Additional calibration was done using data from two
other USGS streamflow-measurement stations with
periods of record shorter than the calibration period.
Daily precipitation data from two National Oceanic and
Atmospheric Administration (NOAA) gages and
hourly precipitation and other meteorological data for
one NOAA gage were used for model input. The
difference between simulated and observed streamflow
volume ranged from -0.9 to 1.8 percent for the 4-year
period at the two calibration sites with 4-year records.
Annual differences between observed and simulated
streamflow generally were greater than the overall error.
For example, at a site near the bottom of the basin
(drainage area of 89.1 square miles), annual differences between
observed and simulated streamflow ranged from -5.8 to
14.4 percent, and the overall error for the 4-year period
was -0.9 percent. Calibration errors for 36 storm periods
at the two calibration sites for total volume, low-flow-recession
rate, 50-percent lowest flows, 10-percent
highest flows, and storm peaks were within the recommended
criteria of 20 percent or less. Much of the error
in simulating storm events on an hourly time step can
be attributed to uncertainty in the hourly rainfall data.
The water-quality component of the model was
calibrated using data collected by the USGS and state
agencies at three USGS streamflow-measurement
stations with variable water-quality monitoring periods
ending October 1998. Because of availability, monitoring
data for suspended-solids concentrations were used
as surrogates for suspended-sediment concentrations,
although suspended solids may underestimate suspended
sediment and affect apparent accuracy of the
suspended-sediment simulation. Comparison of
observed to simulated loads for up to five storms in 1998
at each of the two nonpoint-source monitoring sites in
the White Clay Creek Basin indicate that simulation
error is commonly as large as an order of magnitude for
suspended sediment and nutrients. The simulation error
tends to be smaller for dissolved nutrients than for
particulate nutrients. Errors of 40 percent or less for
monthly or annual values indicate a fair to good water-quality
calibration according to recommended criteria,
with much larger errors possible for individual events.
The accuracy of the water-quality calibration under
stormflow conditions is limited by the relatively small
amount of water-quality data available for the White
Clay Creek Basin.
Users of the White Clay Creek HSPF model
should be aware of model limitations and consider the
following if the model is used for predictive purposes:
streamflow and water quality for individual storm
events may not be well simulated, but the model
performance is reasonable when evaluated over longer
periods of time; the observed flow-duration curve for the
simulation period is similar to the long-term flow-duration
curve at White Clay Creek near Newark, Del.,
indicating that the calibration period is representative of
all but highest 0.1 percent and lowest 0.1 percent of
flows at that site; relative errors in streamflow and
water-quality simulations are greater for smaller
drainage areas than for larger areas; and calibration for
water-quality was based on sparse data.
SIMULATION OF STREAMFLOW AND WATER QUALITY IN THE RED CLAY CREEK SUBBASIN
The Red Clay Creek is the smallest of the Christina River subbasins and drains an
area of 54 square miles. Stormflow and base-flow samples were
collected during 1998 at one site in the Red Clay Creek
subbasin. For the model, the basin was subdivided
into nine reaches draining areas that ranged from 1.7 to
10 square miles . One of the reaches contains a regulated reservoir.
Ten different pervious land uses and two impervious
land uses were selected for simulation. Land-use
areas were determined from 1995 land-use data. The
predominant land uses in the Red Clay Creek subbasin
are agricultural, forested, residential, and urban.
The hydrologic component of the model was run
at an hourly time step and calibrated using streamflow
data from three U.S. Geological Survey (USGS)
streamflow-measurement stations for the period of
October 1, 1994, through October 29, 1998. Daily
precipitation data from one National Oceanic and
Atmospheric Administration (NOAA) gage and hourly
data from one NOAA gage were used for model input.
The difference between observed and simulated streamflow
volume ranged from -0.8 to 2.1 percent for the
4-year period at the three calibration sites. Annual
differences between observed and simulated streamflow
generally were greater than the overall error for the
4-year period. For example, at a site near Stanton, Del.,
near the bottom of the basin (drainage area of 50.2 square miles),
annual differences between observed and simulated
streamflow ranged from -5.8 to 6.0 percent and the
overall error for the 4-year period was -0.8 percent.
Calibration errors for 36 storm periods at the three
calibration sites for total volume, low-flow-recession
rate, 50-percent lowest flows, 10-percent highest flows,
and storm peaks were 20 percent or less. Much of the
error in simulating storm events on an hourly time step
can be attributed to uncertainty in the rainfall data.
The water-quality component of the model was
calibrated using nonpoint-source monitoring data
collected in 1998 at one USGS streamflow-measurement
station and other water-quality
monitoring data collected at three USGS streamflow-measurement
stations. The period of record for water-quality
monitoring was variable at the stations, with an
end date of October 1998 and the start date ranging
from October 1994 to January 1998. Because of
availability, monitoring data for suspended-solids
concentrations were used as surrogates for suspended-sediment
concentrations, although suspended solids
may underestimate suspended sediment and affect
apparent accuracy of the suspended-sediment
simulation. Comparison of observed to simulated loads
for five storms in 1998 at the one nonpoint-source
monitoring site at Wooddale, Del., indicates that
simulation error commonly is as large as an order of
magnitude for suspended sediment and nutrients. The
simulation error tends to be smaller for dissolved
nutrients than particulate nutrients. Errors of
40 percent or less for monthly or annual values indicate
a fair to good water-quality calibration according to
recommended criteria, with much larger errors possible
for individual storm events. Assessment of the accuracy
of the water-quality calibration under stormflow
conditions is limited by the sparsity of available waterquality
data in the basin.
Users of the Red Clay Creek HSPF model should
be aware of model limitations and consider the following
when predictive scenarios are desired: streamflowduration
curves indicate the model simulates stream-
flow reasonably well when evaluated over a broad range
of conditions and time, although streamflow and the
corresponding water quality for individual storm events
may not be well simulated; streamflow-duration curves
for the simulation period compare well with duration
curves for the 57.5-year period ending in 2001 at
Wooddale, Del., and include all but the extreme high-
flow and low-flow events; calibration for water quality
was based on sparse data, with the result of increasing
uncertainty in the water-quality simulation.
SIMULATION OF STREAMFLOW AND WATER QUALITY IN THE CHRISTINA RIVER SUBBASIN
The Christina River subbasin (exclusive of the Brandywine,
Red Clay, and White Clay Creek subbasins) drains an
area of 76 square miles. Stormflow and base-flow samples were collected during
1998 at two sites in the Christina River subbasin.
For the model, the basin was subdivided
into nine reaches draining areas that ranged from 3.8 to
21.9 square miles. Ten different pervious land uses and two
impervious land uses were selected for simulation.
Land-use areas were determined from 1995 land-use
data. The predominant land uses in the Christina River
subbasin are residential, urban, forested, agricultural, and open.
The hydrologic component of the model was run
at an hourly time step and calibrated using streamflow
data from two U.S. Geological Survey (USGS)
stream-flow-measurement stations for the period of October 1,
1994, through October 29, 1998. Daily precipitation
data from one National Oceanic and Atmospheric
Administration (NOAA) meteorologic station and
hourly data from one NOAA meteorologic station were
used for model input. The difference between observed
and simulated streamflow volume ranged from -2.3 to
5.3 percent for a 10-month part of the calibration
period at the two calibration sites. Annual differences
between observed and simulated streamflow generally
were greater than the overall error for the 4-year period.
For example, at Christina River at Coochs Bridge, near
the bottom of the free-flowing part of the subbasin
(drainage area of 21 square miles), annual differences between
observed and simulated streamflow ranged from -6.9 to
6.5 percent and the overall error for the 4-year period
was -1.1 percent. Calibration errors for 36 storm periods
at the three calibration sites for total volume, low-flow-recession
rate, 50-percent lowest flows, 10-percent highest
flows, and storm peaks were within the recommended
criteria of 20 percent or less. Much of the error
in simulating storm events on an hourly time step can
be attributed to uncertainty in the rainfall data.
The water-quality component of the model was
calibrated using nonpoint-source monitoring data collected
at two USGS streamflow-measurement stations
and other water-quality monitoring data. The period of
record for water-quality monitoring was variable at the
stations, with a start date ranging from October 1994 to
January 1998 and an end date of October 1998. Because
of availability, monitoring data for suspended-solids
concentrations were used as surrogates for suspended sediment
concentrations, although suspended-solids
data may underestimate suspended sediment and affect
apparent accuracy of the suspended-sediment simulation.
Comparison of observed to simulated loads for up
to six storms in 1998 at the two nonpoint-source monitoring
sites (Little Mill Creek near Newport and Christina
River at Coochs Bridge, Del.) indicate that
simulation error is commonly as large as an order of
magnitude for suspended sediment and nutrients. The
simulation error tends to be smaller for dissolved nutrients
than for particulate nutrients. Errors of 40 percent
or less for monthly or annual values indicate a fair to
good water-quality calibration according to recommended
criteria; much larger errors are possible for individual
events. Assessment of the water-quality
calibration under stormflow conditions is limited by the
relatively small amount of available water-quality data
in the subbasin.
Users of the Christina River subbasin HSPF
model and HSPF models for other subbasins in the
Christina River Basin should be aware of model limitations
and consider the following if the model is used for
predictive purposes: streamflow-duration curves suggest
the model simulates streamflow reasonably well
when measured over a broad range of conditions and
time although streamflow and the corresponding water
quality for individual storm events may not be well simulated;
streamflow-duration curves for the simulation
period compare well with duration curves for the
58-year period ending in 2001 at Christina River at
Coochs Bridge, Del., and include all but the extreme
high-flow and low-flow events; and calibration for water
quality was based on limited data, with the result of
increasing uncertainty in the water-quality simulation.
Senior, L.A., and Koerkle, E.H., 2003, Simulation of streamflow and water quality in the
Brandywine Creek subbasin of the Christina River Basin, Pennsylvania and Delaware, 1994-98:
U.S. Geological Survey Water-Resources Investigations Report 02-4279, 207 p.
[ PDF file ]
Senior, L.A., and Koerkle, E.H., 2003, Simulation of streamflow and water quality in the
White Clay Creek subbasin of the Christina River Basin, Pennsylvania and Delaware, 1994-98:
U.S. Geological Survey Water-Resources Investigations Report 03-4031,142 p.
[ PDF file ]
Senior, L.A., and Koerkle, E.H., 2003, Simulation of streamflow and water quality in the
Red Clay Creek Subbasin of the Christina River Basin, Pennsylvania and Delaware, 1994-98:
U.S. Geological Survey Water-Resources Investigations Report 03-4038,119 p.
[ PDF file ]
Senior, L.A., and Koerkle, E.H., 2003, Simulation of streamflow and water quality in the
Christina River Basin Subbasin and overview of simulations in other subbasins of the
Christina River Basin, Pennsylvania, Maryland, and Delaware, 1994-98:
U.S. Geological Survey Water-Resources Investigations Report 03-4193,144 p.
[ PDF file ]
MORE INFORMATION ON THE CHRISTINA RIVER BASIN
The Christina River Basin
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