Current Trends in the Hydrologic Cycle David R' Legates, Ph'D', CCM Center for Climatic Research Uni

1
Current Trends in the Hydrologic CycleDavid R.
Legates, Ph.D., CCMCenter for Climatic
ResearchUniversity of DelawareNewark, Delaware
19716
2
DISCLAIMER

• The Delaware State Climatologist does not
  represent the Executive Branch of Government and
  does not speak for the Governor or any other
  State Agency.

3
DISCLAIMER

• Recent media coverage of events associated with
  the subject of climate change has generated some
  confusion as to the role of the State
  Climatologist.
• Governor Minner February 13, 2007
• But Minner said that reports of Legates work
  with private groups and privately backed
  publications disputing climate change had
  generated some confusion.
• NewsJournal February 22, 2007

4
What is the Scientific Evidence for Changes in
Flood and Drought Frequencies?
5
Human Perception of Floods

• Flood Streamflow above a threshold
• Depends on
• Amount of precipitation
• Timing and form of precipitation (snowmelt, soil
  antecedent moisture conditions)
• Urbanization (more runoff, less infiltration)
• Channelization of streams and rivers (levees,
  dredging activities, bank reinforcement)

6
Talleyville Delaware 1954
7
Talleyville Delaware 2002
8
Examination of streamflow percentiles
for non-urbanized, non-channelized basins in the
United States over the 20th Century Trends are
most prevalent in the annual minimum to median
flow categories and least prevalent in the annual
maximum categoryHydrologically, these results
indicate that the conterminous US is getting
wetter, but less extreme. Lins and Slack (1999)
9
Percentage of the United States Experiencing
Wetness
10
(No Transcript)
11
Trends in terrestrial precipitation estimates
over the past century (New et al., 2001)
12
Precipitation Trends by Latitude Bands

• 60N to 80N Land 0.8/decade
• 40N to 60N Land 0.7/decade
• 20N to 40N Land 0.1/decade
• 0 to 20N Land -0.4/decade
• 0 to 20S Land 0.1/decade
• 20S to 40S Land 0.5/decade
• 40S to 60S Land -0.1/decade

(New et al., 2001)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
What about Droughts?
20
Human Perception of Droughts

• Hydrologic Drought Streamflow or reservoir
  levels that fall below a threshold
• Depends on
• Amount (lack) of precipitation
• Urbanization (increased water demand)
• Increase in water-intensive activities (e.g.,
  industrial, residential, agricultural)

21
Source National Drought Mitigation Center,
University of Nebraska-Lincoln
22
2000 Years of Drought in the Central US
The droughts of the 20th century have been
characterized by moderate severity and
comparatively short duration, relative to the
full range of past drought variability.
Woodhouse and Overpeck (1998) 2000 Years of
Drought Variability in the Central US
23
(No Transcript)
24
Changes in the Palmer Drought Severity Index
US National Assessment, 2001
25
Current Trends in the Hydrologic CycleDavid R.
Legates, Ph.D., CCMCenter for Climatic
ResearchUniversity of DelawareNewark, Delaware
19716