U.S. research dealing with climate change impacts on hydrological extremes Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington for presentation at European Geophysical Society Nice April 7, 2003

1) The future as indicated by climate models Increasing T -> increased atmospheric moisture -> increased P Hence increased risk of hydrologic extremes

source: Ziegler et al, J. Clim, 2003

A generally accepted hypothesis regarding acceleration of the global water cycle: “According to model predictions, the most significant manifestation of climate change would be an acceleration of the global water cycle, leading to … a general exacerbation of extreme hydrologic regimes, floods and droughts” (NASA Global Water and Energy Cycle solicitation, 2000). “There is evidence that suggests that the global hydrologic cycle may be intensifying, leading to an increase in the frequency of extremes” (Hornberger et al, USGCRP water cycle science plan) Climate models generally project an acceleration in the rate of global water cycling and an increase in global precipitation … (Morel, GEWEX News, 2001)

2) The situation as indicated by observations over the last ~ century Increased in mean and “extreme” P over much of continental U.S. except winter But no apparent changes in floods (although many upward trends in low flows over much of the country)

(from Lins and Slack, 1999)

“Since 1910, precipitation has increased by about 10% across the contiguous United States. The increase in precipitation is reflected primarily in the heavy and extreme daily precipitation events. For example, over half (53%) of the total increase of precipitation is due to positive trends in the upper 10 percentiles of the precipitation distribution.” (Karl and Knight, BAMS, 1998)

Percent contribution of upper 10th percentile daily precipitation to annual total, averaged over U.S. from Karl and Knight, 1998

from Karl and Knight, 1998

from Karl and Knight, 1998

From Easterling et al, BAMS, 2000

Groisman et al (2001) In three of five regions of the eastern two-thirds of the contiguous U.S., a significant increase in the frequency of “very heavy” precipitation events (> 101.6 mm/day) occurred during the 20th century. The return period of “very heavy” precipitation events changed during the past century in the Midwest from 10 to 7 years, in the South from 4 to 2.7 years, and in the Northeast from 26 to 11 years.

3) Is there any relationship between trends in heavy precipitation and (lack of) trends in floods?

Source: Groisman et al, BAMS, 2001

“In the Eastern half of the United States we found a significant relationship between the frequency of heavy precipitation and high streamflow events both annually and during the months of maximum streamflow. An increase of spring heavy precipitation events over the eastern United States indicates with high probability that during the 20th century an increase of high streamflow conditions has also occurred.” Groisman et al, BAMS, 2001

4) Trends in number of “great floods” exceeding Tr = 100 yrs in basins of size > 105 km2 (Milly et al, Nature 2002) More floods in second half of 20th century than in first half; statistically significant even accounting for record differences Detectability consistent with GFDL GCM for very large rivers given observed 20th century CO2 increase and associated warming

source: Milly et al, 2001

Conclusions Results of U.S. studies seemingly inconsistent, but based on statistical analysis, number of trends in annual maximum flood is barely larger than would be expected by chance (and probably not field significant) Natural variability is large enough to obscure fairly large changes, suggests aggregation and/or compositing approaches (but these tend to complicate interpretation) Some of the apparent inconsistencies may well have to do with attempts to perform “simple” time series type approaches to a complicated nonlinear process (issues e.g. with spatial scale of precipitation-runoff interactions and their variability with season, antecedent conditions, temporal signature of extreme precipitation, and surface conditions Is it really possible to make more progress on the problem without a dynamic modeling approach?

REFERENCES Easterling, D.R., J.L. Evans, P. Ya. Groisman, T.R. Karl, K.E. Kunkel, and P. Ambenje, 2000. Observed variability and trends in extreme climate events: A brief review, Bull. Amer. Meteorol. Soc. 81, 417-425. Groisman. P., T.R. Karl, D.R. Easterling, R.W. Knight, P.F. Jamason, K.J. Hennessy, R. Suppiah, C.M. Page, J. Wibig, K. Fortuniak, V.N. Razuvaev, A. Douglas, E. Forland, and P.-M. Zhai, 1999. Changes in the probability of heavy precipitation: Important indicators of climatic change, Climatic Change 42, 243-283. Groisman, O., R.W. Knight, and T.R. Karl, 2001. Heavy precipitation and high streamflow in the contiguous United States: Trends in the 20th century, Bull. Amer. Meteorol. Soc. 82, 219-246. Karl, T.R., and R.W. Knight, 1998. Secular trends of precipitation amount, frequency, and intensity in the United States, Bull. Amer. Meteorol. Soc., 79, 231-241. Karl, T.R., R.W. Knight, and N. Plummer, 1995. Trends in high frequency climate variability in the twentieth century, Nature 377, 217-220. Lins, H.F., and J.R. Slack, 1999. Streamflow trends in the United States, Geophysical Research Letters 26, 227-230. Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth, 2002. Increasing risk of great floods in a changing climate, Nature 415, 514 – 517. Ziegler, A.D., J. Sheffield, and others, 2003. Detection of intensification in global- and continental-scale hydrological cycles: Temporal scale of evaluation, Journal of Climate 16, 535-547.