Retrospective analysis of hydrologic impacts in the Chesapeake Bay watershed

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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 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 ( ). Computed streamflow elasticity for the respective basins, and the entire watershed.

11 Streamflow Basin Patuxent Western Shore Rappahannock York Eastern Shore James Potomac Susquehanna Entire watershed Nash Sutcliffe Coefficient of efficiency Determination (R2) RSR Feedback Good Unsatisfactory Unsatisfactory Very good Good Satisfactory Good Very good 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 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 Rappahannock Patuxent Susquehanna Potomac Eastern Shore Western Shore James York Average streamflow Average (cfs) precipitation (inch) Streamflow elasticity

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