Draft Appendix E – Chesapeake Bay TMDL for Nutrients and Sediment

E-1 September 24, 2010

Appendix E. Summary of Initial Climate Change Impacts on the Chesapeake Bay 

Watershed Flows and Loads

The potential effects of climate change have not been explicitly accounted for in the current 

Chesapeake Bay TMDL allocations, beyond application of a 10-year hydrologic period, due to 

staff resource and time constraints combined with known limitations in the current suite of 

Chesapeake Bay models to fully simulate the effects of climate change. A preliminary 

assessment of climate change impacts on the Chesapeake Bay was conducted, in parallel, using 

an earlier version of the Phase 5 Chesapeake Bay Watershed Model and tools developed for 

EPA’s BASINS 4 system including the Climate Assessment Tool (CAT). Flows and associated 

nutrient and sediment loads were assessed in all river basins of the Chesapeake Bay with three 

key climate change scenarios reflecting the range of potential changes in temperature and 

precipitation in the year 2030. The three key scenarios came from a larger set of 42 climate 

change scenarios that were evaluated from 7 Global Climate Models (GCMs), 2 scenarios from 

the Intergovernmental Panel on Climate Change (IPCC) SRES (Special Report on Emissions 

Scenarios) storylines, and 3 assumptions about precipitation intensity in the largest events. These 

42 climate change scenarios were run on the Phase 5 Watershed Model of the Monocacy River 

watershed, a subbasin of the Potomac River watershed in the Piedmont region, using a 2030 

estimated land use based on a sophisticated land use model containing socio-economic estimates 

of development throughout the watershed. The results provide an indication of likely 

precipitation and flow patterns under future potential climate conditions.

Downscaling of GCM temperature and precipitation data sets were provided by Consortium for 

Atlantic Regional Assessment (http://www.cara.psu.edu/). Weather data reflecting each climate 

change scenario were created by modifying a 16 year period of historical data of precipitation 

and temperature from 1984 to 2000. The climate change simulation provided for low, medium, 

and high climate change effects projected out to 2030. 

The Susquehanna River basin covers almost half the Chesapeake watershed and has a major 

influence on flows and loads to the Chesapeake. The Susquehanna River responses were 

examined with an annual average time series of flows and loads reported as a percent difference

of the 2030 climate scenarios to the 2000 Base Scenario. Generally flows were seen to decrease 

in the climate change scenarios despite the higher climate change precipitation inputs. 

Decreased flows were due to the increased estimates in temperature which, in turn, increased the 

simulated evapotranspiration in the Susquehanna River watershed.

In the Chesapeake Bay watershed, the 2030 estimated temperatures are about 1.5 degrees 

centigrade higher over the current temperatures. This estimate is relatively consistent in the 

different GCMs and has a high degree of certainty. Estimated precipitation increases among the 

seven global climate models are about 2% over current conditions, especially at higher rainfall 

events, and this is estimated with a moderate degree of certainty. How these temperature and 

precipitation increases affect flow and associated nutrient and sediment loads in the watershed 

depends on the hydrologic balance between precipitation and evapotranspiration.

Temperature increases tend to increase evapotranspiration in watersheds and this can offset 

increases in precipitation. This seems to be the case in the Chesapeake Bay watershed. Current 
Draft Appendix E – Chesapeake Bay TMDL for Nutrients and Sediment

E-2 September 24, 2010

estimates of the medians of the different scenarios run have an annual average flow, nitrogen, 

and phosphorus load decrease of -6.0%, -1.6%, and -2.1% respectively. Because sediment loads 

increase with higher rainfall events, the median of the nine scenario estimates for sediment is for 

an increase of 4.9%. Figures E-1 through E-4 show annual average time series of flow, nitrogen, 

phosphorus and sediment loads for the three climate change scenarios compared to the 2000 

Base Scenario.

For all three scenarios, flow is decreased in the high flow winter period, though for two of the 

scenarios, summer flows are higher (Figure E-1). This may be due to the “flash 30%” and “flash 

10%” precipitation conditions used in these scenarios as summer precipitation is characterized by 

short term, high precipitation thunderstorm events. 

Susquehanna Average Monthly Flow

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month CFS

2000 Base

Max (ECHM - B2)

Median (HDCM - B2)

Min (CSIRO - A2)

Figure E-1. Average annual time series of flow (cubic feet per second) for the 2000 Base Scenario and the 

maximum, median, and minimum climate change scenarios.

Total nitrogen loads follow the overall flow conditions and they are also generally depressed in 

the winter high load period of nitrogen (Figure 2). 
Draft Appendix E – Chesapeake Bay TMDL for Nutrients and Sediment

E-3 September 24, 2010

Susquehanna Average Monthly Total Nitrogen

0

5

10

15

20

25

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Million Pounds TN

2000 Base

Max (ECHM - B2)

Median (HDCM - B2)

Min (CSIRO - A2)

Figure E-2. Average annual time series of total nitrogen loads (millions of pounds per year) for the 2000 Base 

Scenario and the maximum, median, and minimum climate change scenarios.

The total phosphorus time series is similar to total nitrogen but is somewhat more responsive to 

episodic high flows in the two flash 10% and flash 30% precipitation conditions (Figure E-3).
Draft Appendix E – Chesapeake Bay TMDL for Nutrients and Sediment

E-4 September 24, 2010

Susquehanna Average Monthly Total Phosphorus

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Million Pounds TP

2000 Base

Max (ECHM - B2)

Median (HDCM - B2)

Min (CSIRO - A2)

Figure E-3. Average annual time series of total phosphorus loads (millions of pounds per year) for the 2000 

Base Scenario and the maximum, median, and minimum climate change scenarios.

In the Chesapeake Bay watershed, the concentration of TSS can increase three orders of 

magnitude from low flow to extreme high flow conditions, particularly in the larger rivers. 

Combined with higher flows, the higher TSS concentrations generate estimates of TSS loads 

under the flash 10% and flash 30% conditions that are episodic and flashy in nature (Figure E-4). 
Draft Appendix E – Chesapeake Bay TMDL for Nutrients and Sediment

E-5 September 24, 2010

Susquehanna Average Monthly Total Suspended Solids

0

200

400

600

800

1000

1200

1400

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Month Million Tons TSS

2000 Base

Max (ECHM - B2)

Median (HDCM - B2)

Min (CSIRO - A2)

Figure E-4. Average annual time series of total suspended solids loads (millions of tons per year) of the 2000 

Base Scenario and the maximum, median, and minimum climate change scenarios

Overall, the model findings show the potential range of response of flows and loads to climate 

change, at least over a relatively short planning horizon of 20 years. If the historic and model 

trends hold true with respect to precipitation trends increasing in the larger events, and if 

estimated increases in evapotranspiration with higher temperature outweigh estimated 2030 

increases in precipitation, then flow and nutrient loads in the Chesapeake Bay should experience 

relative declines on an annual average basis. However, the increased precipitation and its related 

flows may increase sediment loads.