Andrew Simon 1 and Lauren Klimetz 1,2 1 USDA-ARS National Sedimentation Laboratory, Oxford, MS

1. National Sedimentation Laboratory Trends in Precipitation and Stream Discharge over the Past Century for the Continental United States Andrew Simon1 and Lauren Klimetz1,2 1USDA-ARS National Sedimentation Laboratory, Oxford, MS 2Dept. Of Civil and Environmental Engineering, U. of Tennessee, Knoxville, TN andrew.simon@ars.usda.gov

2. Background and Context The National Sedimentation Laboratory has been conducting research in support of U.S. EPA to determine “background” or “reference” rates of suspended-sediment transport in the continental United States. • “Reference” defined as a “stable” (not static) channel where sediment delivered from upstream can be transported through the reach without the channel aggrading, degrading widening or narrowing over a period of years; • Because of differences across the country in climate, topography, geology and land use, a regional approach was required; • Level III Ecoregions were used.

3. Level III Ecoregions and Available Data

4. Example: EPA Region 8

5. Example “Reference” Yield

6. However… • Questions raised about how current flow and transport conditions may be different from historical, sampling periods; • In Oklahoma, a mean-annual precipitation difference between wet and dry periods of 33% led to a 100% difference in runoff, which in turn led to a 183% difference in sediment yield. (Garbrecht, 2008); • Results not surprising as more sediment moves at higher discharges; • The question becomes do wet periods cause transport ratings to shift ?... AND/OR • Do they de-stabilize channel systems?

7. Transport Ratings Stable Ratings Unstable Ratings

8. Precipitation Data Source: U.S. Historical Climatology Network (USHCN) operated by the National Climatic Data Center (NCDC) of NOAA; http://www.ncdc.noaa.gov/oa/climate/research/ushcn/ushcn.html Details: • More than 1,000 station nationwide • Monthly data to nearest 0.01” • Does not differentiate between rainfall and snowfall

9. Discharge Data Source: U.S. Geological Survey gauging station records http://nwis.waterdata.usgs.gov/nwis/dv Details: • More than 6,000 stations nationwide • Used more than 2,900 (with minimum 30 suspended-sediment samples and associated instantaneous discharge) • Mean-daily data to three significant figures

10. Summary of Data

11. Precipitation(mm) • Dust Bowl Era • Wet period in 1990’s • All but ER 42 increasing

12. Water Yield(m3/s/km2) Shows combined effects of changes in precipitation and anthropogenic influences • All Ecoregions except ER 46* and ER 48 show less discharge per unit area • Indication of significant human impacts

13. Annual Departure from Long-Term Mean Water Yield(%) • Trends for all Ecoregions show decrease, except ER 46* and 48 • Most secondary peaks around 1970 • Less discharge per unit area

14. Water Retention: Dam Construction Summary 1970-1980 • Reached peak retention per unit area • Farm Bill led to significant upstream retention

15. Potential Human Impacts • construction of thousands of dams • agriculture • logging • surface mining • coal-bed methane production • channelization • irrigation, tile drainage

16. Decadal Trends (Wet/Dry Periods) Precipitation Water Yield (m3/s/km2) By 1970-1980, Ecoregions 17, 42 and 43 have the most dams

17. Water Yield Per Unit of Precipitation

18. Summary of Changes Over Past 100 Years Note: * denotes that data period started during the 1920’s or 1930’s (Dust Bowl Era), resulting in higher than expected increases. Change in precipitation Change in water yield over Change in water over 100 year period 100 year period yield per unit Level III Ecoregion precipitation (in 3 2 (in mm) (in percent) (in percent) (m /s/km ) percent) 15 Northern Rockies 85.9 15.7 -0.00026 -1.63 -26.4 17 Middle Rockies 4.01 1.04 -0.01066 -68.9 -81.8 42 Northwestern Glaciated Plains -7.74 -2.04 -0.00045 -20.6 -84.9 43 Northwestern Great Plains 24.7 6.35 -0.00131 -79.1 -74.4 46 Northern Glaciated Plains 11.1 2.09 0.00082 3043* 1070* * 48 Lake Agassiz Plain 45.5 8.94 0.00261 353 0

19. National Context Seasonal Breakdowns: Winter: December – February Spring: March – May Summer: June – August Autumn: September - November

20. Seasonal Changes in Precipitation: 100 Years Spring Summer Winter Autumn

21. Change in Precipitation (Percent)

22. Change in Precipitation (mm)

23. Seasonal Changes in Water Yield: 100 Years Spring Summer Autumn Winter

24. Change in Water Yield (Percent)

25. Change in Water Yield Per Unit Precipitation Indicative of Human Impacts

26. Median Suspended-Sediment Concentrations

27. Median Suspended-Sediment Yields

28. Summary and Conclusions • EPA 8: Although increased precipitation, significant reductions in water yield in the four western ecoregions due to human impacts; well below average since the 1970s-80s when most dam construction in the region was completed. • EPA 8: Changes in precipitation combined with human-induced changes have caused dramatic shifts in discharge rates such that historically diverse ecoregions now provide similar amounts of water (per unit drainage area) to streams (ie. Middle Rockies similar to the Northwestern Great Plains). National Context • Most of the U.S. experiencing more precipitation than 100 years ago. • Dramatic shifts in timing: More in autumn, much less in winter nationwide. • Although numerous regions are experiencing significantly more precipitation, discharge per unit area has been drastically reduced in many areas, particularly west of the Mississippi River. • The drastic reductions in water yield can be largely attributed to human impacts given the general increase in precipitation in these regions. This probably has significant effects on geomorphic and ecological processes, and habitat.