Prepared in cooperation with the Maine Department of Transportation Modeled Future Peak Streamflows in Four Coastal Maine Rivers Scientific Investigations Report 2013–5080 U.S. Department of the Interior U.S. Geological Survey Cover. Photograph of Ducktrap River near Lincolnville, coastal Maine, April 4, 2009. Modeled Future Peak Streamflows in Four Coastal Maine Rivers By Glenn A. Hodgkins and Robert W. Dudley Prepared in cooperation with the Maine Department of Transportation Scientific Investigations Report 2013–5080 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2013 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. 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Suggested citation: Hodgkins, G.A., and Dudley, R.W., 2013, Modeled future peak streamflows in four coastal Maine rivers: U.S. Geological Survey Scientific Investigations Report 2013–5080, 18 p., http://pubs.usgs.gov/sir/2013/5080/. iii Contents Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................2 Observed Peak Flows in Maine and New England .........................................................................2 Observed Flood Frequency Changes in Maine ................................................................................2 Projected Future Climate and High Streamflows in the Northeastern United States ...............3 Description of the Study River Basins ........................................................................................................3 Physiography, Land Cover and Use, and Surficial Geology ...........................................................6 Climate ....................................................................................................................................................6 Streamflow .............................................................................................................................................6 Methods...........................................................................................................................................................7 Basin Rainfall-Runoff Models .............................................................................................................7 Flood Frequency Analyses ..................................................................................................................9 Computing Magnitude of Changes ..................................................................................................10 Historical Modeled Peak Flows Compared to Observed Values .........................................................10 Historical Modeled Flood Frequency Compared to Observed Values ................................................10 Changes in Peak Flows on the Basis of Changes in Air Temperature and Precipitation ................11 Changes in Flood Frequency on the Basis of Changes in Air Temperature and Precipitation .......12 Causes of Modeled Changes in Peak Flows and Flood Frequency .....................................................13 Summary and Conclusions .........................................................................................................................15 References Cited..........................................................................................................................................16 Figures 1. Map showing location of rivers selected for study in coastal Maine .................................4 2. Map showing the Pleasant and Narraguagus River Basins in the Eastern Coastal Climate-Response Region, and the Sheepscot and Royal River Basins in the Southern Coastal Climate-Response Region, Maine ..............................................................5 3. Graph showing historical median monthly streamflows for the four study basins in coastal Maine ................................................................................................................................7 4. Schematic diagram of the operational design of the Precipitation-Runoff Modeling System (PRMS) .............................................................................................................................8 Tables 1. Historical modeled annual daily maximum peak flows compared to historical annual daily maximum peak flows. .........................................................................................10 2. Differences between historical modeled peak flows with 50-percent and 1-percent annual exceedance probabilities (AEPs) and observed peak flows with 50-percent and 1-percent AEPs ....................................................................................................................11 3. Annual daily maximum peak-flow changes based on changes in precipitation and air temperature ...........................................................................................................................12 4. Fifty-percent annual exceedance probability peak-flow changes based on changes in precipitation and air temperature ........................................................................................13 iv 5. One-percent annual exceedance probability peak-flow changes based on changes in precipitation and air temperature ........................................................................................14 6. Annual daily maximum snowpack water-equivalent changes based on changes in precipitation and air temperature ............................................................................................15 Conversion Factors and Datum Multiply By To obtain Length inch (in) 2.54 centimeter (cm) inch (in) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Flow rate cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8 Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Abbreviations AEP Annual exceedance probability AOGCM Atmosphere-ocean general circulation model MaineDOT Maine Department of Transportation PRMS Precipitation-Runoff Modeling System USGS U.S. Geological Survey Modeled Future Peak Streamflows in Four Coastal Maine Rivers By Glenn A. Hodgkins and Robert W. Dudley Abstract the four basins when temperature is increased by 7.2°F. If temperature is held constant, 26 to 38 percent increases in To safely and economically design bridges and culverts, peak flow result from a 15-percent increase in precipitation. it is necessary to compute the magnitude of peak streamflows The largest decreases in peak flows at the four basins result that have specified annual exceedance probabilities (AEPs). from 15-percent decreases in precipitation combined with Annual precipitation and air temperature in the northeastern temperature increases of 10.8°F. The largest increases in United States are, in general, projected to increase during peak flows generally result from 30-percent increases in the 21st century. It is therefore important for engineers and precipitation combined with 3.6 °F decreases in temperatures. resource managers to understand how peak flows may change In many cases when temperature and precipitation in the future. This report, prepared in cooperation with the both increase, small increases or decreases in annual daily Maine Department of Transportation (MaineDOT), presents maximum peak flows result. For likely changes projected modeled changes in peak flows at four basins in coastal Maine for the northeastern United States for the middle of the 21st on the basis of projected changes in air temperature and century (temperature increase of 3.6 °F and precipitation precipitation. increases of 0 to 15 percent), peak-flow changes at the four To estimate future peak streamflows at the four basins coastal Maine basins in this study are modeled to be evenly in this study, historical values for climate (temperature and distributed between increases and decreases of less than 25 precipitation) in the basins were adjusted by different amounts percent. and input to a hydrologic model