This document summarizes the results of using the Hydrologic Simulation Program FORTRAN (HSPF) model to simulate hydrologic impacts in the Chesapeake Bay watershed over 22 years. The model performed reasonably well at larger time scales but tended to overestimate peak flows and underestimate low flows. Statistical analysis found increasing streamflow trends correlated with increasing precipitation and land use changes. Peak flows were typically preceded by low flow years and high median flow years showed greater flow variability.

1. Retrospective analysis of hydrologic
impacts in the Chesapeake Bay watershed
Harsh Beria1,3, Rob Burgholzer2, Venkat Sridhar3
Indian Institute of Technology Kharagpur, India & Summer intern
Virginia Department of Environmental Quality, Richmond, VA
Biological Systems Engineering, Virginia Tech, Blacksburg, VA

2. Chesapeake Bay
➢Largest estuary in the United States.
➢Supports more than 17 million people.
➢Includes part of six states and entire District
of Columbia
➢More than 150 rivers and streams drain into
the Bay.

3. Chesapeake Bay Hydrology
➢300 km long, width ranging from 8-48 km
(Cerco and Cole 1994).
➢Shallow water body with average depth of 8
m (Cerco and Cole 1994).
➢Mean annual flow of about 70,000 cfs.
➢Watershed area of 166,000 square
kilometers.

4. Chesapeake Bay Problems
➢Identified as one of the planet’s first marine dead zone in 1980s, due
to lack of oxygen in water resulting in massive fish kills.
➢Runoff from residential, farm and industrial waste containing high
doses of nitrogen and phosphorus pollutants.
➢Eutrophication resulting in a large algal bloom responsible for the
loss of oxygen from water (Boesch et al. 2001).

5. Chesapeake Bay Program
➢Chesapeake Bay Program initiated in 1983.
➢To reduce the concentration of nitrogen and phosphorus in the
estuarine water.
➢Uses a watershed model Hydrologic Simulation Program FORTRAN
(HSPF) to model streamflow, evapotranspiration and transport of
pollutants (Nitrogen, Phosphorus and its species) and sediments.

6. Hydrologic Simulation Program
FORTRAN(HSPF)
➢Lumped parameter model, capable of conducting watershed scale
studies for a number of varying scenarios (Wu et al. 2006).
➢Requires intensive data to run the simulations (Wu et al. 2006).
➢Divides watershed into separate land and river segments.
➢Uses hourly meteorological data to simulate watershed hydrology.

7. Hydrologic Simulation Program
FORTRAN(HSPF)
Flowchart depicting working of HSPF

8. Objectives
➢Evaluate performance of HSPF through statistical parameters.
➢Understand temporal and spatial trends in streamflow for the entire
watershed, and for the respective basins.
➢Compute streamflow elasticity to characterize the streamflow
response to precipitation.

9. Methodology
➢HSPF uses hourly meteorological records from 7 different stations,
divides watershed into 5-km grid and linearly interpolates the inputs
to the entire watershed.
➢HSPF divides entire watershed into separate land and river segments
and reports streamflow and concentration of pollutants at
downstream end of each stream.
➢Processed simulated flows and calculated volume of water draining
the Bay on a daily timestep.

10. Methodology
➢Evaluated model performance by comparing simulated streamflow
with observed values obtained from USGS website, through NSE, R2
and RSR (Moriasi et al. 2007).
➢Conducted parametric and non-parametric tests to understand
temporal trends in streamflow (1984-2005).
➢Computed streamflow elasticity for the respective basins, and the
entire watershed.

11. Streamflow
Basin Nash Sutcliffe
efficiency
Coefficient of
Determination
(R2)
RSR Feedback
Patuxent 0.62 0.68 0.62 Good
Western Shore -0.4 0.48 1.18 Unsatisfactory
Rappahannock 0.32 0.58 0.82 Unsatisfactory
York 0.83 0.84 0.41 Very good
Eastern Shore 0.6 0.62 0.63 Good
James 0.41 0.52 0.77 Satisfactory
Potomac 0.61 0.7 0.62 Good
Susquehanna 0.85 0.85 0.39 Very good
Entire
watershed
0.58 0.65 0.65 Good

12. Streamflow
➢Out of 52 gaging stations, 11 had negative Nash Sutcliffe efficiency
(NSE), implying poor model performance.
➢Tends to overestimate flow in peak flow month (March), as high as
55% of observed flow.
➢Tends to underestimate flow in low flow month (August), as low as
50% of observed flow.
➢In general, HSPF overestimates flow.

13. Seasonal flow variation
➢Peak flow at start of Spring
in March (132,000 cfs).
➢Low flow at end of
Summer in August (30,000
cfs).
➢Flow increases throughout
Fall and Winters.

14. Annual flow variation
➢Peaks in 1989, 1996 and
2003 (7-year recurrence).
➢High flow years preceded
by Low flow years.
➢In 1996 peak, flow increase
ranges from 75% in James to
201% in Potomac.

15. Annual flow variation
➢In 2003 peak, flow increase
ranges from 79% in
Susquehanna to 540% in
York
➢Susquehanna doesn’t show
abrupt response in flow.
➢Trend line indicates a long
term increase in streamflow.

16. Time series smoothing
➢5-year moving average
plot.
➢Trend line indicates a long
term increase in streamflow.
➢R2 = 0.06 Rappahannock
➢R2 = 0.74 Eastern Shore

17. Flow variability
➢Positive slope of trend line
for annual median flow.
➢Positive correlation
between spread (75th and 25th)
and median.
➢Variance of flow higher for
years with high median flow.

18. Mann Kendall Trend test
➢Null hypothesis of no trend rejected at a significance level (α) of
0.01.
➢All basins had positive S values, indicating an increase in streamflow
over 22 years of simulation.
➢Positive correlation coefficient (R) of 0.78 between increase in
streamflow and increase in precipitation.
➢Plausible reason for increase in streamflow is corresponding increase
in precipitation.

19. Spatial analysis of streamflow
➢Susquehanna River basin contributes
to about 58% of flow, although
accounting for about 43% watershed
area.
➢Potomac River basin contributes
about 19% of flow and accounts for
22% watershed area.
➢James River basin contributes about
13% flow and accounts for 16%
watershed area.

20. Changes in Land Use
➢14% (254,047 ac) increase in urban settlement with about 28%
increase in high intensity urban settlement and 9.8% increase in low
intensity urban settlement.
➢25% (24,237 ac) increase in barren land.
➢1.7% (404,730 ac) decrease in forest cover (decrease in evergreen
and deciduous forests but a slight increase of 0.6% in mixed forest
cover).

21. Streamflow elasticity
Basin
Average streamflow
(cfs)
Average
precipitation (inch)
Q 
QP
Streamflow
elasticity
Rappahannock 1907.37 41.84 0.22
Patuxent 471.8 44.41 1.12
Susquehanna 40400 40.27 1.33
Potomac 13142.54 41.94 1.48
Eastern Shore 1344.22 46.8 1.54
Western Shore 1270.23 42.66 1.88
James 9352.83 42.57 2.31
York 1854.52 43.37 2.35
( ) t
p
t
e median
P P Q



22. Streamflow elasticity
➢Elasticity > 1=> 1% increase in precipitation causes >1% change in
streamflow.
➢Overall elasticity = 1.53, implies that the flow is sensitive to
precipitation.
➢Long term increases in streamflow is due to long term increases in
precipitation.

23. Summary
➢HSPF simulates basin scale hydrology well at a monthly and annual
scale, but not at a daily scale.
➢Parametric and nonparametric tests indicate an increase in
streamflow, due to increase in precipitation pattern and land use
change.
➢Peak flows are preceded by low flows.
➢Years with a high median annual flow has a larger variability in
flow.

24. Thank You