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C
Speaker Abstracts
Global to Regional Perspectives on Intensification of the Hydrologic Cycle:
Implications for Extreme Events
T.G. Huntington, U.S. Geological Survey
Climate warming is expected to intensify or accelerate the global hydrologic cycle,
resulting in increases in rates of evaporation, evapotranspiration (ET), and precipitation and an
increase in the concentration of atmospheric water vapor. The strength of the hydrologic
response, or sensitivity of the response for a given amount of warming, is a critical outstanding
question in hydroclimatology. An assessment of the published record on observations of trends
in various components of the hydrologic cycle and associated variables provides insight into this
question. The weight of evidence from global and regional trends in evaporation, ET, and
atmospheric water-vapor concentration supports an ongoing intensification of the hydrologic
cycle. Global trends in precipitation, runoff, and soil moisture are more uncertain, in part
because of high spatial and temporal variability and lack of consistent, high-quality, long-term
records. Changes in regional ocean salinity indicate possible increasing evaporation at low
latitudes and increasing freshwater inputs (precipitation, runoff, and melting ice) at high
latitudes. Ongoing lengthening of the growing season may contribute to increasing ET rates. The
evidence for an increase in the frequency, intensity, or duration of extreme weather events like
hurricanes and floods is mixed; consequently, regional to global trends remain uncertain.
Understanding Changes in Precipitation and Runoff with a Changing Climate
Kevin E. Trenberth, National Center for Atmospheric Research
The global hydrological cycle and its changes over time are examined in light of
observations and current understanding. A particular focus is on how precipitation changes as the
climate changes and changes in extremes, including risk of flooding and drought. Net changes in
surface evaporation are fairly modest, and a much larger percentage change occurs in the water-
holding capacity as atmospheric temperatures increase (7% per C). In this talk we will examine
the consequences of this, especially noting the differences over ocean, where water supply is
unlimited, and over land. A description will also be given of the understanding of other large-
scale changes in patterns and amount of precipitation, soil moisture, and drought. It is important
to understand not only changes in mean precipitation, but also the intensity, frequency, duration,
and type, and this also applies to the storms that bring precipitation. Understanding these
profound consequences of climate change is especially important for water managers.
24
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Appendix C 25
Is Precipitation Becoming More Intense?
Pavel Groisman, National Oceanic and Atmospheric Administration
An overview of 12-year-long National Climatic Data Center (NCDC) studies of changes
of intense precipitation during the period of instrumental observations will be presented with a
focus on North America. NCDC has created a database of daily and hourly time series of high
scientific quality for use in assessment of changes in precipitation characteristics over the regions
where we have sufficient amount of information to answer the question outlined in the talk’s
title.
Prior to 2005, NCDC constructed various time series of precipitation characteristics and
analyzed their trends. Now (in addition to routine updates of these time series), we have
analyzed the factors that control intense precipitation (e.g., CAPE and land-falling
tropical cyclones trajectories),
assessed the rainfall distribution characteristics (e.g., hourly rainfall rates), their changes,
and their relationships with global and regional surface air temperatures, and
investigated changes in “direct impact” characteristics of precipitation spectra such as
prolonged no-rain periods, fire weather indices, and maximum rainfall intensity.
Our past and ongoing studies (as well as findings by other foreign researchers) embolden our
opinion that in the past several decades over most of the extratropics precipitation became more
intense. However, the changes in intense precipitation also occur with changes in several other
precipitation characteristics and they too deserve our thorough attention.
A Process-Based “Bottom-Up” Approach for Addressing
Changing Flood-Climate Relationships
Katie Hirschboeck, University of Arizona
In response to the unprecedented persistence of extreme drought conditions in the western
United States, some western water managers have moved beyond conventional approaches to
plan for future extreme low flow conditions in innovative ways involving paleo-records,
scenarios, and climate projection modeling. In contrast, flood hazard managers are far more
constrained in developing ways to incorporate climate change information operationally, in part
because of existing flood policy, but also because of the short-term, localized, and weather-based
nature of the flooding process itself. What is needed is information that is presented in an
operationally useful format for flood managers and that describes how changes in the large-scale
climatic drivers of hydrometeorological extremes will affect flooding variability in specific
watersheds. This presentation outlines a framework for linking global climatic change to the
gauged time series of peak flows in individual watersheds. Using a process-sensitive “bottom
up” approach, each individual peak in a gauged record is associated with its flood-producing
storm type and circulation pattern. This approach highlights the underlying physical reasons for
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26 Appendix C
flood variations in specific watersheds, defines how mixed flood distributions and outlier events
may be linked to climate shifts, and challenges the underlying “iid” assumption that flood peaks
are independently and identically distributed. Linking extreme flood events to meteorological
causes driven by shifting circulation features can provide water managers with critical climate-
based interpretive information for how flood probability distributions are likely to respond within
individual watersheds under future climate change scenarios.
The Ghost of Flooding Past, Present, and Future
Harry F. Lins, U.S. Geological Survey
An element of human-enhanced greenhouse theory is that the hydrological cycle will
accelerate. This has led to the hypothesis that extreme events, such as floods and droughts, will
increase in frequency and/or severity. Published studies indicate that precipitation has increased
over the past century, and this increase has been characterized as occurring in “extreme” and
“intense” precipitation. However, empirical studies from North America and Europe find no
evidence of an increase in flood frequency or magnitude during the 20th century, although
increases in low to moderate streamflows have been widely reported. What, then, are the likely
effects of an accelerated hydrological cycle on streamflow in general, and on floods in
particular? This question is considered using data and the published literature with respect to two
issues: What is known about the sensitivity of various return-period floods and annual
precipitation? What is the likely impact of a given percentage change in precipitation on a flow
quantile (e.g., Q100 versus Qmean)? Results indicate that the precipitation sensitivity of mean
streamflow is much greater than that of peak streamflow, and that precipitation sensitivity
decreases as flood return period increases. This suggests that human-induced greenhouse
warming may be more likely to produce noticeable and significant changes in the mean state of
hydrological regimes than in hydrological extremes.
Planning for Non-Stationary Extreme Events: Statistical Approaches
Richard M. Vogel, Tufts University
It is no longer possible to consider streamflow and other hydrologic processes as a
stationary process. Nearly all of the methods developed for the planning, management, and
operation of water resource systems assume stationarity of hydrologic processes. Non-
stationarity can result from a myriad of human influences ranging from agricultural and urban
land use modifications, to climate change and water infrastructure. Most previous work in trend
detection associated with extreme events has focused on the influence of climate change, alone.
This study takes a different approach by exploring flood and low flow trends in watersheds that
are subject to a very broad range of anthropogenic influences. We define a decadal flood
magnification factor as the ratio of the T-year flood in a decade to the T-year flood today. Using
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Appendix C 27
historical flood data across the entire United States we obtain typical flood magnification factors
in excess of 2-5 for many U.S. regions, particularly those regions with higher population densities.
A simple statistical model is developed that can both mimic observed flood trends as well
as the frequency of floods in a non-stationary world. This model is used to explore a range of
flood planning issues in a non-stationary world. Importantly, non-stationarity in both extreme
high and low flows is shown to result from a variety of processes including land use, climate,
and water use, with likely interactions among those processes making it very difficult to attribute
trends to a particular cause. Multivariate regression models are shown to provide a useful tool
for developing the type of conditional forecasts of the moments of extreme events necessary for
planning in a non-stationary world.
Planning in a non-stationary and uncertain world is not a new challenge for engineers,
because the classic “capacity expansion problem” and other planning problems have always
involved both non-stationarity and uncertainty. What is new is the increased variety of sources of
uncertainty and non-stationarity that are now inherent in nearly all water resource planning
problems, making it essential to incorporate non-stationary planning models of the type discussed
here.
Planning for Non-Stationarity and Floods: A Management Perspective
Gerald E. Galloway, University of Maryland
Recent decades have seen a growing increase in flood damages across the nation. A
resultant focus on reducing these flood damages has brought long-neglected attention to the
systematic assessment and improvement of the quality of existing flood damage reduction
structures and pleas for “protection” for areas not now ringed by levees, floodwalls, or other such
structural measures. The specter of climate change has led many agencies, both in the United
States and abroad, to closely examine how they would deal with more frequent and more severe
floods and consider how they might adapt to these future conditions. Flood risk management has
replaced flood damage reduction in the lexicon of federal engineers, and considerable effort is
now focused on both how they might best manage flood risk and how they might communicate
the level of future risk to the public. Given the uncertainties surrounding the calculation of
recurrence intervals, how do managers and engineers decide how high their levees should be and
how structural measures fit with non-structural actions such as zoning, floodproofing,
evacuation, etc.? In 2008, a committee chartered by the Netherlands government recommended
to the Parliament that standards for coastal and riverine defense (recurrence intervals) be raised
by a factor of 10 to deal with the myriad flood and storm uncertainties faced by that nation. What
guidance can be given today to U.S. planners to deal with an uncertain future? They must do
something now, but what should this something be?
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28 Appendix C
Mechanisms for Global Warming Impacts on the Large-Scale
Atmospheric Branch of the Hydrological Cycle
Richard Seager, Columbia University
It is a robust prediction of state-of-the-art climate models that greenhouse gas-induced
global warming will cause the wet regions of the planet (in the deep tropics and the mid to high
latitudes) to get wetter while the subtropical dry zones get drier. It is also projected that the
subtropical dry zones will expand poleward. Here we analyze the 13 models that made available
all the required data to determine the mechanisms responsible for these changes in the
hydrological cycle. The mechanisms are divided into first, thermodynamic ones that only rely on
a change in specific humidity, second, dynamic ones that only rely on changes in the mean
circulation and, third, changes in transient eddy moisture fluxes. Much of the basic pattern of
change in precipitation—evaporation (P-E) is accounted for thermodynamically as humidity rises
in a warmer atmosphere and intensifies existing patterns of moisture transport. However,
changes in circulation are required to explain many changes of P-E in the tropics and, especially,
to explain the poleward expansion of the subtropical dry zones. Increases in poleward transient
eddy heat moisture fluxes also assist in drying the subtropics and moistening the higher latitudes.
Causes of the increased transient eddy fluxes are shown to be complex.
Much of the thermodynamic-induced change in P-E can itself be accounted for simply by
atmospheric warming under fixed relative humidity. The mechanisms for projected drying of
southwestern North America will be analyzed. This region will dry no matter what, but it is also
shown that the character of the tropical Pacific atmosphere-ocean response to increasing
greenhouse gases will determine the relative magnitude of the drying. Recent climate change is
reviewed for evidence of these changes already occurring, but it is concluded that recent trends
have been dominated by large-amplitude natural decadal atmosphere-ocean variability. Near-
term hydroclimate prediction therefore must account for both anthropogenic change and the
evolution of natural modes of variability.
Connecting Global-Scale Variability to Regional Drought:
Mechanisms and Modeling Challenges
Siegfried Schubert, NASA’s Goddard Space Flight Center
Recent research has linked long-term drought (or more specifically extended periods of
reduced precipitation) to a number of factors including slowly varying sea surface temperatures
(SSTs), the influences of the land surface (e.g., atmosphere/soil moisture feedbacks, aerosols,
and vegetation changes), as well as the chance occurrence of extended runs of dry years that can
occur even in the absence of any year-to-year memory in the climate system. The possibility of
predicting long-term drought rests largely on the strength of the SST linkages to the land
component of the hydrological cycle, and of course on our ability to predict the relevant SST
changes. The U.S. CLIVAR (Climate Variability and Predictability) working group on drought
recently initiated a series of global climate model simulations forced with idealized SST anomaly
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Appendix C 29
patterns, designed to address a number of uncertainties regarding the impact of SST forcing and
the role of land-atmosphere feedbacks on regional drought. This talk reviews some of those and
related results, with a focus on the U.S. Great Plains, although the basic mechanisms appear to be
relevant to drought in many other regions of the world. Issues to be addressed include the
seasonality of the global SST response, the impact of soil moisture feedbacks, the potential
predictability associated with SST changes, as well as model deficiencies currently limiting our
ability to simulate and predict long-term drought.
Do We Need to Put Aquifers into Atmospheric Simulation Models?
Evidence for Large Water Table Fluctuations and Groundwater Supported
ET under Conditions of Pleistocene and Holocene Climate Change
Mark Person, New Mexico Tech
Aquifer-atmosphere interactions can be important in landscapes where the water table is
shallow (<2m) and the watershed topography is gentle. Regional climate models that include
aquifer hydrodynamics indicate that between 5 to 20% of evapotranspiration is drawn from the
aquifer. The groundwater-supported fraction of evapotranspiration is higher under drought
conditions, when evapotranspiration exceeds precipitation. The response time of an aquifer to
drought conditions can be long—on the order of 200-500 years—indicating that feedbacks
between these two water reservoirs act on disparate timescales. Analysis of Holocene and late
Pleistocene paleowater table records suggests that water table fluctuations can be as great as 50
m during drought conditions. With recent advances in the computational power of massively
parallel supercomputers, it may soon become possible to incorporate physically based
representations of aquifer hydrodynamics into GCM land surface parameterization schemes. This
may help to improve our predictions of the long-term consequences of droughts on water
resources and climate dynamics.
Breaking the Hydro-Illogical Cycle: The Status of Drought Risk
Management in the United States
Mike Hayes, University of Nebraska-Lincoln
This presentation will focus on drought risk management within the United States given
the context of climate variability, climate change, and extremes. As the last presentation in the
workshop, an attempt will be made to connect comments and issues addressed within previous
presentations and breakout groups. A focus will be placed on drought monitoring, impact
assessment, mitigation, and planning efforts taking place now across the country, and on
suggesting where current efforts need more concentration. The National Integrated Drought
Information System (NIDIS) will also be highlighted. Drought fits well into the enhanced efforts
by the climate community to create and provide “services” and decision support tools. Each
service and tool being designed for drought helps define the “big picture” of drought for policy-
makers and others who need that scale of information. But they also work to localize drought,
putting valuable information in the hands of agricultural producers and community, tribal, and
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30 Appendix C
other grassroots decision-makers—exactly what is needed to boost drought risk management
through the rest of this century.
