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

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.

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.

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).

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.

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.

Hydrologic Simulation Program FORTRAN(HSPF)

Flowchart depicting working of HSPF

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.

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.

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.

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

➢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.

Streamflow

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.

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.

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.

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

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.

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.

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.

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).

Streamflow elasticity

Basin Average streamflow

(cfs)

Average

precipitation (inch)

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

( )tp

t

Q Q Pe median

P P Q

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.

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.

Thank You