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Analysis of Long-Term Hydrologic Records in the

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1 Analysis of Long-Term Hydrologic Records in the
Chesapeake Bay Watershed Karen C. Rice1,2 and Douglas L. Moyer1 1U.S. Geological Survey 2University of Virginia

2 Previous Research: Rice and Hirsch (2012)
Discharge trends in the Chesapeake Bay watershed: 7-day low flow, mean flow, and 1-day high flow. Spatial difference in some of these metrics between North and South in the Chesapeake Bay watershed.

3 Dividing line was approximately the Maryland/Pennsylvania border
Previous Research Dividing line was approximately the Maryland/Pennsylvania border

4 Mean 1-Day Maximum Runoff
North South Rice and Hirsch (2012)

5 Bulletin of the American Meteorological Society, 1998

6 Observed Change in Very Heavy Precipitation
Percent increase in heaviest 1% of all daily events from 1958 to 2012 [in U.S. Climate Assessment (2014), from Karl et al. (2009)]

7 How are changes in P being manifested in Q?
Objective Use Historical Records in Precipitation (P) and Discharge (Q) to Compare Trends in the Chesapeake Bay Watershed How are changes in P being manifested in Q?

8 Linear Regression Trends
27 Watersheds Precipitation: (PRISM data, monthly) Discharge: (USGS gages, daily)

9 Precipitation Discharge NORTH SOUTH

10 Percent Change from 1927 to 2014 Percent Change
Site Number, North to South

11 Specific Quantiles in P and Q

12 Methods For each of the 27 watersheds: Analyze annual distribution of:
monthly total precipitation daily mean Q Record P (cm) and Q (cfs) that corresponds to each decile (0th, 10th, 20th, …100th) for each year Determine the slope of the linear regression line for the whole period for each decile

13 Q that corresponds to 30th decile for each year
at site Slope = ; t-ratio = 2.98; p-value = y = *Year

14 Northern Sites first, followed by Southern Sites
Comparison of Trends in P and Q by Decile Northern Sites first, followed by Southern Sites

15 NORTH Very low flow Mid-flow Storm flow Number of Sites Decile

16 Northern Sites Trends 0th decile Precipitation Discharge

17 Northern Sites Trends 10th decile Precipitation Discharge

18 Northern Sites Trends 20th decile Precipitation Discharge

19 Northern Sites Trends 30th decile Precipitation Discharge

20 Northern Sites Trends 40th decile Precipitation Discharge

21 Northern Sites Trends 50th decile Precipitation Discharge

22 Northern Sites Trends 60th decile Precipitation Discharge

23 Northern Sites Trends 70th decile Precipitation Discharge

24 Northern Sites Trends 80th decile Precipitation Discharge

25 Northern Sites Trends 90th decile Precipitation Discharge

26 Northern Sites Trends 100th decile Precipitation Discharge

27 SOUTH Very low flow Mid-flow Storm flow Number of Sites Decile

28 Southern Sites Trends 0th decile Precipitation Discharge

29 Southern Sites Trends 10th decile Precipitation Discharge

30 Southern Sites Trends 20th decile Precipitation Discharge

31 Southern Sites Trends 30th decile Precipitation Discharge

32 Southern Sites Trends 40th decile Precipitation Discharge

33 Southern Sites Trends 50th decile Precipitation Discharge

34 Southern Sites Trends 60th decile Precipitation Discharge

35 Southern Sites Trends 70th decile Precipitation Discharge

36 Southern Sites Trends 80th decile Precipitation Discharge

37 Southern Sites Trends 90th decile Precipitation Discharge

38 Southern Sites Trends 100th decile Precipitation Discharge

39 Number of Significant Positive Trends in each Decile
North South Decile Decile

40 From North to South: Linkage in P & Q decreases
Trends in P have lower slopes Fewer significant P & Q trends Significance of trends decreases Fewer significant trends in P&Q deciles

41 P ≠ Q Watershed Storage Sponge like: Lag times, travel times Land use:
Urban, Ag, Forest Wetlands Dams Withdrawals Snow pack and timing of snowmelt Antecedent conditions & ET P ≠ Q

42 Number of sites with the probability that a 99th percentile precipitation event results in a 99th percentile discharge event Soil moisture less than the median value Soil moisture greater than the median value P ≠ Q

43 Trends in Runoff Ratio (Q/P)
North South P ≠ Q

44 What about the period specific to the WSM?

45 Daily Mean Discharge: Site 01531500
Q = -31, *year Q = -167, *year

46 Significant slopes at 0th decile Significant slopes at 10th decile
Slope of Linear Trendline Slope of Linear Trendline Site Number

47 Significant slopes at 20th decile Significant slopes at 30th decile
Slope of Linear Trendline Slope of Linear Trendline Site Number Site Number

48 Significant slopes at 40th decile Significant slopes at 50th decile
Slope of Linear Trendline Slope of Linear Trendline Site Number Site Number

49 Significant slopes at 60th decile Significant slopes at 70th decile
Slope of Linear Trendline Slope of Linear Trendline Site Number Site Number

50 Significant slopes at 80th decile Significant slopes at 90th decile
Slope of Linear Trendline Slope of Linear Trendline Site Number Site Number

51 Significant slopes at 100th
decile Slope of Linear Trendline Site Number

52 Why is the number of significant slopes
different between and ? 139 (47%) 14 (4.7%)

53 Definition of the quantitative power of a linear trend test:
Power = f (b/s * n1.5), where b is the trend slope, s is the standard deviation of the error, and n is the number of observations.

54 Long-Term Observations Necessary
“In a nonstationary world, continuity of observations is critical.” (Milly et al., 2008, Science) “The idea here is that we must use long-term hydrologic observations to help us evaluate, on an ongoing basis, how the changing atmosphere is changing hydrologic processes.” (Milly et al., 2015, Water Resour. Res.)

55 Diverse Watershed Complicating Factors: Land Use Wetlands Dams
Withdrawals Hurricanes Atmospheric Forcings

56 Summary 1) P ≠ Q 2) North ≠ South 3) Trends in 1985-2014 ≠ 1927-2014
Analyses have not been reviewed or approved by USGS; please do not cite or quote

57

58 the ability to detect a linear trend?
Is the flow variability in the upper deciles so high that it is masking the ability to detect a linear trend? Addressed by: log10 transforming ranking non-parametric Spearman’s rho

59 Q that corresponds to 30th decile for each year
at site Parametric: Linear Regression Non-parametric: Spearman rho y = *Year Slope = ; t-ratio = 2.98; p-value = Spearman rho = ; p-value

60 Comparison of Methods Trends 0th quantile North South

61 Comparison of Methods Trends 10th quantile North South

62 Comparison of Methods Trends 20th quantile North South

63 Comparison of Methods Trends 30th quantile North South

64 Comparison of Methods Trends 40th quantile North South

65 Comparison of Methods Trends 50th quantile North South

66 Comparison of Methods Trends 60th quantile North South

67 Comparison of Methods Trends 70th quantile North South

68 Comparison of Methods Trends 80th quantile North South

69 Comparison of Methods Trends 90th quantile North South

70 Comparison of Methods Trends 100th quantile North South

71 Summary of Comparison of Four Methods
Number of Significant Slopes for each Quantile *Two of the slopes are negative

72 Plots of Individual Deciles
NS NS NS NS NS Sig. Sig. Sig.

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