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- 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

ass essment of climate change impacts on the Chesapeake Bay was conducted, in parallel, using an earlier version of the Phase 5 Chesapeake Bay Watershe d Model and tools developed for EPA’s BASINS 4 system including the Climate Assessment Tool (CAT). F lows and associated

nutrient and sediment loads were assessed in all river basins of the Chesapeake Bay with t hree key climate change scenarios reflecting t he 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 ( GCM s) , 2 scenarios from

the Intergovernmental P anel 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

wa tershed, 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 indicatio n 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://w ww.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 clim ate 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 se ries 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 increase s 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 a ffect flow and associated nutrient and sediment loads in the watershed depends on the hydrologic balance between preci pitation 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. CurrentDraft Appendix E

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- 2 September 24 , 2010 estimates of the medians of the di fferent 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 hig h 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 pr ecipitation thunderstorm events.

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 a nd they are also generally depressed in the winter high load period of nitrogen ( Figure 2).Draft Appendix E

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- 3 September 24 , 2010 Figure E

- 2. Average annual time series of total nitrogen loads ( mil lions 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% an d flash 30% precipitation conditions (Figure E

- 3).Draft Appendix E

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- 4 September 24 , 2010 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

unde r the flash 10% and flash 30% conditions that are episodic and flashy in nature ( Figure E

- 4).Draft Appendix E

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- 5 September 24 , 2010 Figure E

- 4. Average annual time series of total suspended solids loa ds ( 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 pre cipitation, 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.