1. Hydrologic Implications of 20th Century Warming and Climate Variability in the Western U.S.
Alan F. Hamlet
Prof. Dennis P. Lettenmaier (Chair)
Phd Final Exam
May, 2006

2. Acknowledgements: Western Water Assessment: Committee: Martyn Clark
Dennis P. Lettenmaier (chair)
Deirdre Meldrum (GSR)
Stephen Burges
Daniel Cayan
Richard Palmer
Nathan Mantua
CIG:
Philip Mote
Edward Miles
Adrienne Karpov
Hydro Group:
Andy Wood
Ted Bohn
Kostas Andreadis
Jenny Adam
Family and Friends:
Carys Kresny
Rhys Hamlet
Anya Kresny
Bill Kennedy

3. Background and
Introduction

4. Natural AND human influences explain the observations of global warming best.
Natural Climate Influence
Human Climate Influence
Red: observations of global average temperature
Grey: simulations with a climate model (huge computer program, like weather prediction model only run for hundreds of years)
Natural influence: volcanoes, solar variations – guesses before ~1970, better since then
Human influence: greenhouse gases, sulfate aerosols
it is possible to simulate the climate of the last 100 years, and the conclusion is that humans didn’t matter much before 1960 – the early warming and the cooling were largely natural, and the late-century warming was largely human-caused
All Climate Influences

5. Pacific Decadal Oscillation El Niño Southern Oscillation
A history of the PDO
A history of ENSO
warm
warm
cool

6. Cool Season Climate of the Western U.S.
PNW GB CA
CRB
DJF Temp (°C)
NDJFM Precip (mm)

7. Seasonal Water Balance Naches River 20th Century Climate
More runoff in winter and early spring, less in summer
2040s Scenario
( C + 4% Pcp)

8. At almost every USHCN station, winters warmed
+ signs: warming but not statistically significant

9. Climate change experiments have suggested that in temperature sensitive areas of the West, we should already be able to see the effects of global warming in the historic snow and streamflow records.
Using models we should be able to more fully analyze these changes, as well as other hydrologic effects which are not typically measured.

10. Why Do We Need Model Simulations of the Historic Record?
Longer Record (Avoids problems with decadal variability from 1950 forwards)
Spatial Coverage (high and low elevations not in the observations), river basin scale impacts.
Temporal Resolution (daily time step)
Full suite of hydrologic variables and consistency amongst these variables
Explicit sensitivity analysis for effects of temperature and precipitation

11. Cool Season Precipitation Anomalies Compared to the PDO
-0.845
-0.264
-0.438
-0.053
(Regional to PDO
Correlation R2 )
PNW Trend
CRB Trend

12. Schematic of VIC Hydrologic Model and Energy Balance Snow Model
PNW CA
CRB GB
Snow Model

13. Overview of Research Questions:
How have variations in temperature and precipitation from the early 20th Century on ( ) affected trends in hydrologic variables such as snowpack, volume and timing of runoff and baseflow, seasonal evaporation and soil moisture, and flood risk in the western U.S.?
Is a consistent global warming signal apparent over the western U.S. in this period, and is it possible to make a clear distinction between “natural” variations such as decadal precipitation variability and more systematic effects associated with global warming signals? Are temperature and precipitation different in this regard?
What role do regional climatic regimes and topographic variations play in defining the role of temperature and precipitation variability on hydrologic variations? What areas of the western U.S. are most sensitive to changes in temperature or precipitation changes and why?

14. Research Questions (cont.):
Do the hydroclimatic variations observed in the western U.S. over the 20th century corroborate simulations of climatic changes produced by global climate model scenarios? For instance, is a hypothesis of wetter conditions in the western U.S. due to an intensified global hydrologic cycle born out in the observations? If so, how have these changes affected hydrologic variability?
How do flood risks vary in time and how can these risks be characterized and predicted in the context of interannual and interdecadal climate variability and longer-term variations associated with global warming?

15. Research Topics
Methods for producing long meteorological driving data sets
Effects of observed climate variability on snowpack trends
Effects of observed climate variability on trends in runoff, soil moisture, and evaporation
Evaluating changing flood risks in the context of climate variability and global warming

16. 1) Met Data Processing

17. Problems with Temporal Inconsistencies in Meteorological Records
(S. F. Flathead River at Hungry Horse Dam, MT)

18. Daily Precipitation, Tmax, Tmin
Result:
Daily Precipitation, Tmax, Tmin

19. Comparison of adjusted vs. unadjusted VIC simulations
(S. F. Flathead River at Hungry Horse Dam, MT)
Simulated vs Observed
Root square error

20. Evaluation of Streamflow Simulations of the Colorado River at Lee’s Ferry, AZ

21. Trends in Temperature and Precipitation in the Western U.S.

22. TMAX Regionally Averaged Cool Season Temperature Anomalies 0.74 0.63
0.76
0.62
(Regional to Global
Correlation R2 )
TMAX

23. TMIN Regionally Averaged Cool Season Temperature Anomalies 0.84 0.87
0.94
0.73
(Regional to Global
Correlation R2 )
TMIN

24. Regionally Averaged Cool Season Precipitation Anomalies

25. Trends in Cool Season (Oct-Mar) Precipitation and Temperature
Tmax
Tmin
1916-
2003
DJF Avg Temperature
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)
1947-
2003
DJF Avg Temperature
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)

26. Trends in Warm Season (Apr-Sept) Precipitation and Temperature
Tmax
Tmin
DJF Avg Temperature
1916-
2003
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)
DJF Avg Temperature
1947-
2003
Rel. Trend %/yr
Trend (°C/yr)
Trend (°C/yr)

27. 2) Effects of Temperature and Precipitation Variability on Snowpack Trends in the Western U.S.

28. Overview of Simulation and Analysis
Met Data
Linear Trend
Analysis
VIC
SWE
(warm to cool PDO)
(cool to warm PDO)
with (warm to warm PDO)
Linear Trends:
Base—combined effects of temp and precip trends
Static Precip—effects of temperature trends only
Static Temp—effects of precipitation trends only
Experiments:

29. Trends in April 1 SWE
Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49

30. 1950-1997 relative trends in April 1 SWE vs DJF temperature
Obs
VIC
Obs
VIC
Obs
VIC
Obs
VIC
VIC grid cells (red) and snow courses (blue)
Cascades: west of 120 longitude, south of BC
Rockies: north of central Colorado, includes Wasatch and Uintas in Utah
Dry interior: Nevada, AZ, NM, southern and western Utah, southern Idaho, eastern Wash & OR

31. Overall Trends in April 1 SWE from 1947-2003
DJF avg T (C)
Trend %/yr
Trend %/yr

32. Temperature Related Trends in April 1 SWE from 1947-2003
DJF avg T (C)
Trend %/yr
Trend %/yr

33. Precipitation Related Trends in April 1 SWE from 1947-2003
DJF avg T (C)
Trend %/yr
Trend %/yr

34. DJF avg T (C)
Decadal Variability Doesn’t Explain the Temperature Related Effects to Snowpack
Trend %/yr
DJF avg T (C)
with
DJF avg T (C)
Trend %/yr

35. Trends in the Date of Snow Accumulation and Melt
a) 10 % Accum.
b) Max Accum.
c) 90 % Melt
Change in Date
Change in Date
Change in Date
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date

36. 1916-2003 Effects of Temperature and Precipitation Effects of
a) 10 % Accum.
b) Max Accum.
c) 90 % Melt
Effects of
Temperature
and
Precipitation
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date
Effects of
Temperature
only
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date
Effects of
Precipitation
only
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date

37. 3) Trends in Seasonal Runoff, Evaporation, and Soil Moisture

38. winter flows rise and summer flows drop
As the West warms,
winter flows rise and summer flows drop
Stewart IT, Cayan DR, Dettinger MD, 2005, Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8):
Spring snowmelt timing has advanced by days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

39. June March Trends in simulated fraction of annual runoff in each month
from (cells > 50 mm of SWE on April 1)
March
June
Relative Trend (% per year)

40. Trends in March Runoff
Trends in June Runoff
DJF Temp (°C)
DJF Temp (°C)
Trend %/yr
Trend %/yr

41. Trends in Soil Moisture

42. Trends in Simulated Soil Moisture from 1947-2003
DJF Temp (°C)
April 1
Trend %/yr
July 1
DJF Temp (°C)
Trend %/yr

43. Trends in April 1 SM
Trends in July 1 SM
DJF Temp (°C)
DJF Temp (°C)
Trend %/yr
Trend %/yr

44. 50% WY runoff, 80% max soil moisture recharge, and 50% WY ET
Trends in the Dates of
50% WY runoff, 80% max soil moisture recharge, and 50% WY ET

45. Cumulative Trends in the Date of Hydrologic Events
50% WY Runoff
80% Max SM
50% WY ET
Cumulative Trends in the Date of Hydrologic Events
( ) BR BR BR
DJF Temp (°C)
FPR
FPR
FPR
Effects of
Temp alone
DJF Temp (°C)
FTR
FTR
FTR
Effects of
Precip alone
DJF Temp (°C)
Trend days/50 yr

46. Trends in the “Runoff Ratio” (runoff/precipitation)

47. Effects of Cool Season Precipitation Trends on Trends in the Runoff Ratio
Trend Runoff Ratio
Trend Oct-Mar PCP

48. Temperature Related Downward Trends in Annual Streamflow at The Dalles
Compared with the Effects of Precipitation Variability
Black trace = constant precip
Magenta trace = with precip variability

49. 4) Evaluating Systematic Changes in Flood Risks

50. Evaluating the Hydrologic Model Simulations in the Context of Reproducing Flood Characteristics
Avg WY Date of Flooding OBS
Ln (X100 / Xmean) OBS
Fig 2 Simulated vs Observed date of flooding and ratio of 100 year flood to mean annual flood
Avg WY Date of Flooding VIC
Ln (X100 / Xmean) VIC

51. X100 GEV flood/mean flood Zp Red = VIC Blue = OBS 100-yr 50-yr 20-yr

52. Detrended Temperature Driving Data for Flood Risk Experiments
“Pivot 2003” Data Set
Temperature
Historic temperature trend
in each calendar month
“Pivot 1915” Data Set
1915
2003

53. Trends in January TMIN for a VIC cell in the Cascades

54. Meteorological Records from 1915-2003
Use of a Hydrologic Model with Long Precipitation and Temperature Records
Meteorological Records from
De-trended Temperatures
Observed Precipitation Variability
VIC
Hydrology Model
Variability of Runoff
In Different
River Basin Types
for A Consistent
“Early” and “Late”
20th Century
Temperature
Regime

55. Simulated Changes in the 20-year Flood Associated with 20th Century Warming
DJF Avg Temp (C)
X / X
DJF Avg Temp (C)
Fig year flood A spatial scale
X / X
X / X

56. X / X
X / X
X / X
Fig year flood for A,C,E spatial scales
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X / X
X / X
X / X

57. X100 wPDO / X100 2003 X100 nPDO / X100 2003 X100 cPDO / X100 2003
Fig 7 Warm, neutral, cool PDO 100-year flood
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X100 wPDO / X
X100 nPDO / X
X100 cPDO / X

58. X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003
Fig 8 Warm, neutral, cool ENSO 100-year flood
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X100 wENSO / X
X100 nENSO / X
X100 cENSO / X

59. Effects of Cool ENSO on Flood Risks in Larger Basins
X100 cENSO / X

60. 20-year Flood for “ ” Compared to “ ” for a Constant Late 20th Century Temperature Regime
DJF Avg Temp (C)
X20 ’73-’03 / X20 ’16-’03
X20 ’73-’03 / X20 ’16-’03

61. Summary of Temperature Related Effects
Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West in the 20th century as a result of increasing temperatures.
Temperature-related effects are, in general, organized spatially according to mid-winter temperature regimes.
Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in summer soil moisture.
Flood risks appear to be declining overall due to warming, but the model suggests that flood risks are increasing in many moderate elevation areas where tradeoffs between loss of antecedent snow and increasing basin size favor increasing basin size (typically warmer areas).
Based on scenarios, we expect that the intensity and rates of change of temperature-related effects will increase as global warming progresses in the 21st century.

62. Summary of Precipitation Related Effects
Consistent changes in cool-season precipitation volumes are not apparent in the West. Warm season precipitation, however, seems to be increasing over most of the West.
Changes in cool season precipitation variability are apparent since 1975, but the cause is not yet clear, and it is not possible to say whether these changes are related to global warming, should be considered systematic in nature or not, etc.
Unlike temperature-related effects, precipitation-related hydrologic effects are frequently distributed geographically. (e.g. ENSO variations via storm track effects, or large scale changes in precip. variability affecting the entire West).
Although this study has highlighted some important differences between temperature and precipitation changes and their relation to global warming, a number of important questions remain about how best to represent future precipitation variability and uncertainty in global warming scenarios.