Stormwater
Working Group Report
This report provided content for the
Wisconsin Initiative on Climate Change Impacts first report,
Wisconsin’s Changing Climate: Impacts and Adaptation,
released in February 2011.
Stormwater Management in a Changing Climate:
Managing High Flow and High Water Levels
in Wisconsin
WICCI Stormwater Working Group
June 2010
The WICCI Stormwater Working Group
Water resource management is a complex and many layered process, demanding a
comprehensive and inclusive approach to assessing potential impacts from climate change.
Water resource management, beginning with an understanding of our existing resources (data
acquisition), involves planning, design, construction and operation of infrastructure and
associated systems to serve and protect society's water needs. Water resource "managers"
includes local government officials, state regulators, engineers, planners and utility operators.
The WICCI Stormwater Working Group members, all of whom are involved in some aspect of
water resource management, represent a subset of the audience for this report.
Co-Chairs Kenneth W. Potter, UW-Madison, Civil and Environmental Engineering
David S. Liebl, UW-Madison, Engineering Professional Development, and UW-Extension
Members Jim Bachhuber*
Jeremy Balousek
Ken Bradbury
Kurt Calkins
Pat Eagan
Rick Eilertson
Greg Fries
Keith Haas*
Mike Hahn*
Kevin Kirsch
Najoua Ksontini
Mike Martin
Paul McGinley*
Rob Montgomery
Ned Paschke
John Ramsden*
Tom Sear
Jon Schellpfeffer*
Mike Schwar*
Rodney Taylor
Eric Thompson
Bill Walker
John Walker*
Bob Watson*
AECOM
Dane County Land Conservation Division
Wisconsin Geological and Natural History Survey
Columbia County Land & Water Conservation
UW-Madison, Engineering Professional Development
City of Fitchburg Engineering
City of Madison Stormwater Utility
City of Racine Water & Wastewater Utility
Southeast Wisconsin Regional Planning Commission
WI-DNR Runoff Management Section
WI-Department of Transportation
Milwaukee Metropolitan Sewerage District
UW-Stevens Point
Montgomery Associates Resource Solutions
UW-Engineering Professional Development
Natural Resources Conservation Service
SEH
Madison Metropolitan Sewerage District
HNTB
WI-Department of Transportation
MSA Professional Services
WI-Department of Agriculture Trade and Consumer Protection
USGS-Wisconsin Water Science Center
WI-DNR Watershed Management Section Mission The goal of the WICCI Stormwater Working Group is to build capacity within Wisconsin's water
resource management profession to address climate related changes in planning, design and
management of our water resource infrastructure. The Stormwater Working Group has analyzed
the effect of changing climate on Wisconsin's precipitation patterns (rainfall and snowfall), and
the resulting impacts upon high stream flows and surface flooding, high water levels in lakes
and impoundments, and high groundwater levels and soil saturation.
* Internal reviewers
2
Contents
I. Executive Summary ..........................................................................4
II. Introduction ....................................................................................10
III. Design to Manage High Water Conditions ....................................10
IV. Potential Changes In Wisconsin Climate ....................................12
V. Effect Of Potential Climate Changes On High-Flow Conditions ....12
VI. Vulnerability in a Changing Climate ..............................................15
VII. Adapting to a Changing Climate .................................................17
VIII. High Water Adaptation Strategies ..............................................17
IX. Analysis of Historical Precipitation Record ..................................24
X. WICCI Downscaled Global Climate Model Projections ...............32
XI. References ...................................................................................40
Financial assistance for this Sector Applications Research Program (SARP) project was provided by the Climate Program Office of the U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA) pursuant to NOAA Award No. NA09OAR4310138. The statements, findings, conclusions, and recommendations are those of the research team and do not necessarily reflect the views of NOAA, US Department of Commerce, or the US Government. 3
Stormwater Management in a Changing Climate:
Managing High Flow and High Water Levels in Wisconsin
WICCI Stormwater Working Group - Kenneth W. Potter, David S. Liebl - Co-Chairs
with: Zachary Schuster and Vanessa Cottle
June 2010
I. Executive Summary
Climate change in Wisconsin is likely to
increase the severity and frequency of high
flows and high water levels. Our analysis of
downscaled climate projections suggest that
Wisconsin precipitation is trending toward
wetter conditions and more intense rainfall.
Climate models also predict increases in
cold season precipitation and increases in
the ratio of rainfall to snowfall, potentially
increasing the frequency of damaging
flooding from rivers, lakes, and groundwater.
As a result of these changes we expect
increases in the magnitude and frequency of
high flows in streams and rivers, and high
water levels in streams, rivers, lakes and
impoundments
Engineers have traditionally used historical precipitation and runoff data to design and evaluate
infrastructure to manage the risks associated with precipitation to acceptable levels Unless we
modify the planning, design and management of this infrastructure to account for climate
mediated changes in precipitation, we will face greater than expected damages from high flows
and water levels.
This is the first written report of the Wisconsin Initiative on Climate Change Impacts (WICCI) Stormwater Working Group. Members of this group include engineers, planners, utility
operators, local government officials, state regulators, and academic researchers. This report
provides background on the design of infrastructure and management practices used to
manage high water conditions, discusses potential changes in Wisconsin climate based on
historical data and downscaled climate model results, and presents specific adaptation
strategies that recognize the large uncertainties in climate predictions.
The WCCI Stormwater Working Group believes that scientific knowledge about the potential
increase in magnitude and frequency of precipitation is sufficient to warrant immediate changes
in the methods we use to plan, design and manage stormwater-related infrastructure. While the
list of specific climate impacts is long and growing, we focus on three main areas for this report:
4
1) More frequent and severe rural stream and river flooding caused by increased rainfall,
and shifting precipitation patterns that favor more rain during periods of low infiltration
and evapotranspiration.
2) Increased occurrence of inland lake flooding resulting from increased precipitation in
winter and spring.
3) Groundwater flooding caused by rising water tables due to increased cold-weather
precipitation and increased variability in frost conditions.
With respect to the factors affecting high water conditions, WICCI statistically downscaled
climate projections for Wisconsin vary by climate model. However, those projections do support
the following generalizations:
1. Modest increases in the magnitude of intense precipitation events are expected during
the 21st century. For example, averaged over the state, the magnitude of the 100-year,
24-hour storm event (5"-7") is expected to increase by about 11% by the 2046-2065 time
period.
2. Total precipitation and heavy precipitation events are projected to increase
significantly during the winter and spring months of December - April. This combination
of more precipitation and more intense events has the potential to cause more high
water events.
3. The amount of precipitation that occurs as rain during the winter months of December
to March is also projected to significantly increase. Winter rain can create stormwater
management problems (e.g. icing), and increase the risk of high water events during a
season when rainfall does not normally occur in Wisconsin.
Examples of high water impacts: Upland Erosion; Urban Street Flooding; Groundwater flooding.
Unless appropriate adaptation strategies are adopted, we can expect increases in the
frequency and severity of the following high water impacts:

Erosion of slopes during intense rainfall events resulting in high sediment and
phosphorus loads to streams, rivers, lakes and wetlands.

Degradation of aquatic habitat as a result of manure runoff from fields and drain
systems.

Impairment of roadways and bridges washed-out due to high water or slope failure.
5

Groundwater flooding of property and cropland.

Contamination of rural residential wellheads as a result of surface water and
groundwater flooding.

Flooding of urban streets and homes due to inadequate runoff drainage systems.

Failure of impoundments, levees and stormwater detention ponds.

Failure of rain gardens and other biofiltration Best Management Practices (BMPs)
due to prolonged periods of saturated soils.

Stormwater inflow and groundwater infiltration to sanitary sewers, resulting in
untreated municipal wastewater overflowing into to lakes and streams.
The WICCI Stormwater Working Group has identified specific actions that can be taken to build
capacity in Wisconsin to adapt to the challenges of our changing climate. Many of these
adaptation strategies are steps that ought to be taken today as part of the continuing
improvement of the water resource management professions. Many of the specific management
recommendations are good public policy, in any climate.
High Water Adaptation Strategies
Traditional design and management strategies for high water conditions assume that the climate
is not changing. However, analysis of historic climate data and predictions by climate models
indicate that Wisconsin's climate is changing and will continue to change. Unless our design
and management strategies adapt to changing climate conditions, using traditional approaches
will lead to the risk of significant increases in economic and environmental damage.
The WICCI Stormwater Working Group recommends the following adaptation strategies that
can lead to increased societal capacity to minimize risk from high water conditions.
Assessing Site-specific Vulnerabilities We recommend that local units of government be provided the technical and financial
assistance needed to assess and mitigate their vulnerabilities to potential high water conditions
caused by present and future climate.
Closing Regulatory Gaps We recommend that the State of Wisconsin work with municipalities and counties to develop
minimum design and performance standards for the control of the high water impacts of
development. We further recommend that these standards specify that regulatory control
extend to the 100-year storm event and require regular updating with the most recent rainfall
statistics. Consideration should also be given to requiring additional stormwater storage
capacity to account for uncertainties in future rainfalls.
We recommend that Wisconsin Department of Natural Resources develop an approval process
for prior converted croplands that are being removed from agricultural use that will encourage
their restoration and prevent development in flood-prone areas. We also encourage county and
municipal governments to adopt an approval process or place land use controls on development
that occurs on hydric soils in areas that are likely to experience future flooding.
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Climate Monitoring and Modeling We recommend that Wisconsin's climate monitoring network of cooperative weather stations,
stream gauges and groundwater monitoring wells be improved and maintained to provide
continued high quality data to support short and long term climate impact modeling.
Specific information needed to address climate impacts including the following:

Fine scale rainfall data using calibrated National Weather Service precipitation and radar
measurements.

Real time stream-flow data from an expanded United States Geological Survey stream
gauge network.

Groundwater level data from strategically placed observation wells to enable
identification of vulnerability to groundwater flooding.

Detailed understanding of sub-watershed characteristics to improve runoff and flood
modeling.

Geospatial data for drainage districts to identify vulnerability to increased high flows and
ground water levels.

Location of high risk and vulnerable practices in flood-prone areas, such as hazardous
materials and petroleum storage, drinking water wells and septic systems.
Fine-scale Multi-sensor Measurement of Precipitation
Building Technical Capacity We recommend that the state develop and implement a long-term plan for developing
continuous hydrologic simulation models of stream flow for critical watersheds. When
appropriate, the models should be coupled to groundwater models. Participants in such
modeling could include the Wisconsin Geological & Natural History Survey (WGNHS), the U.S.
Geological Survey (USGS), the Southeast Wisconsin Regional Planning Commission, and
private consulting firms.
Research We recommend an investment in research at the state and national level to build capacity and
provide knowledge in the areas of winter/spring hydrology, hydrologic modeling, and decisionmaking under uncertainty for water resource management.
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Stakeholder Action To Build Adaptive Capacity
The WICCI Stormwater Working Group has also identified specific actions that can be taken by
water resource system stakeholders that will lead to an increase in our ability to adapt to our
changing climate.
Regulators  Revise local building standards to address runoff control.

Base design standards on updated rainfall statistics.

Require standby power for buildings with sump pumps to avoid flooding caused by storm
related power outages.

Incentivize behavior change through fees and credits.
Planners  In areas that are internally drained or have hydric soils, coordinate with regulators to
assure that future land use changes do not increase flood vulnerability.

Create or designate new surface flood storage areas (e.g. wetlands) to mitigate high
water impacts.

Use updated models to predict groundwater impacts on development.

Periodically update estimates of high water profiles based on revised rainfall data.

Identify at-risk stream-crossings and develop maintenance and high water contingency
plans.
System Designers  Coordinate the design of sanitary and stormwater systems to minimize high water
impacts.

Identify high hazard areas and apply more stringent design criteria.

Anticipate groundwater impacts on bio-infiltration best management practices (BMPs).

Increase wastewater system peak flow management capacity, and minimize stormwater
inflow and groundwater infiltration.

Use low-impact design to minimize runoff from newly developed areas.
System Managers  Upgrade urban storm drainage systems based on continuous hydrologic modeling and
climate predictions.

Manage to minimize high flow impacts rather than sediment removal during high storm
flows (e.g. bypass stormwater bio-infiltration BMPs).
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
Assess impacts of high flow events on sewage treatment plant process viability, and
evaluate impacts of bypassing high storm flows around the treatment plant’s biological
processes.

Flood-proof vulnerable buildings and infrastructure.

Build capacity for drinking water quality emergency assessment and response.
Educators  Conduct public and technical education programs on climate impacts and adaptation.

Educate communities about the hazards of building in areas prone to high water.

Educate property owners about sanitary sewer inflow prevention.

Encourage conservation tillage, stream buffers and other low-impact agricultural
practices to minimize rural runoff.
Securing Long-Term Capacity
Building adaptive capacity among this diverse group will require a sustained effort. The water
resource management profession needs organizational support to integrate disciplines,
knowledge and implementation through a multidisciplinary effort comprising academics,
outreach educators, private sector design professionals, municipal engineers and other
resource managers to:

Facilitate communication among water resource management disciplines.

Be a source of credible information for communities, the public and practitioners on
climate change.

Be an authoritative voice to policy makers and the private sector on climate adaptation
strategies.
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Stormwater Management in a Changing Climate:
Managing High Flow and High Water Levels in Wisconsin
II. Introduction
High water conditions have forever presented challenges, and over time society has developed
and refined strategies for managing them. Flood risk is managed by building infrastructure such
as levees, dams, and diversion channels, or by regulating exposure to flood risk through
floodplain management and land use regulation. Stormwater runoff management practices are
used to convey stormwater and mitigate the hydrologic impacts of development, as well as
maintain water quality. Sanitary sewer systems are designed to accommodate additional flows
during storms. Bridges and culverts are built to allow safe passage over streams and rivers
under all but the most extreme flood conditions. Stream banks are armored to prevent bank
erosion. Agricultural practices are used to limit soil erosion, mitigating downstream water quality
impacts.
The methods used to design the infrastructure and management practices associated with
these strategies have been based on historic rainfall and stream flow data. Given the scientific
consensus that the earth's climate is changing as a result of increasing greenhouse gases,
these data are no longer considered representative of future conditions (Milly, et al., 2008).
The WICCI Stormwater Working Group was formed in 2008 to build capacity within the state's
water resource management profession to evaluate the effectiveness of existing infrastructure
and management practices and adapt design methods using the latest available information on
climate change. Members of this group include engineers, planners, utility operators, local
government officials, state regulators, and academic researchers.
This is the first written report of the WICCI Stormwater Working Group. In it we provide
background on the design of infrastructure and management practices used to manage high
water conditions, discuss potential changes in Wisconsin climate based on historical data and
climate model results, and present specific adaptation strategies that recognize the large
uncertainties in climate predictions
III. Design to Manage High Water Conditions
Society's infrastructure is built to manage to acceptable levels the risks associated with excess
precipitation, and has been traditionally designed and evaluated using historical precipitation
and runoff data. Unless the planning, design and management of this infrastructure is modified
to account for climate mediated changes in precipitation patterns, the risk of significant
economic and environmental damage will increase.
Strategies for managing high water conditions are based on either the use of infrastructure that
conveys, stores, or protects against high water, or on plans and regulations that promote or
require avoidance of high water conditions. Conveyance systems (usually channels and pipes)
are designed to safely pass a specified flow of water to a downstream discharge. Storage
systems (such as stormwater basins and flood reservoirs) temporarily detain water, reducing
peak flows to specified levels. Protective measures, such as levees and floodwalls provide
protection up to a specified water level. Land use planning and regulation aim to avoid property
inundation based on one or more specified water level.
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The specification of design flows and levels is normally based on frequency of occurrence (or its
reciprocal, the recurrence interval). On average, a water level with a recurrence interval of 10
years is equaled or exceeded once every 10 years. The choice of the appropriate design
recurrence interval is commonly based on experience and judgment. For example, urban storm
drains are commonly designed to pass the 10-year flow without causing property damage in
adjacent areas. This is based on the implicit assumption that designing for larger events would
be unduly expensive, while designing for smaller events would likely lead to unacceptable flood
damage. Likewise, sediment control structures are often designed to carry the 10-year flow
because most sediment transport occurs at equal or smaller flows. In situations where the
stakes are higher for public safety or economic damage, a more formal design strategy is often
used. For example, major flood mitigation projects are often designed to maximize net
economic benefits, i.e. the difference between the expected reduction in flood damages and the
project cost.
How are flow or level exceedance frequencies determined for a specific location? If relevant
historical flow and/or water level data are available, frequencies are determined by statistical
analysis of the data. When such data are not available, which is typically the case, a numerical
model (e.g. rainfall-runoff model) or statistical model (e.g. a regional regression equation) is
used to produce a simulated flow record based on historical precipitation data. Frequencies are
then estimated from the simulated flow record.
There are two general approaches for using rainfall-runoff modeling to estimate flow and water
level frequencies: the design storm and continuous hydrological modeling. In the design storm
approach, occurrence frequencies are estimated for individual rainfall events, which are then
used as input to a rainfall-runoff model that simulates individual storm events (event model).
The resulting runoff peaks are assumed to have the same frequency as the corresponding
design storm.
In the continuous simulation approach, a continuous historical rainfall record and other
meteorological data are used as input to a rainfall-runoff model that simulates both wet and dry
periods (continuous hydrologic simulation model). Statistical analysis of the model data is then
used to estimate frequencies of peak flows and levels. Use of continuous hydrologic simulation
explicitly accounts for important pre-storm (antecedent) conditions, such as soil moisture and
surface water storages, and should generally give more accurate results than use of the design
storm approach. However, because the design storm approach is much less labor and data
intensive, it is the technique most commonly used in practice.
Rainfall occurrence frequencies used in the design storm method are estimated from historic
rainfall data. This same rainfall and other meteorological data are also used in the continuous
simulation method. Hence the assumption that these data represent future conditions is implicit
in the design of strategies for managing future high water conditions. Climate change has
rendered this assumption invalid. More importantly for Wisconsin (and much of the world), the
expected changes in climate will generally increase the magnitude and frequency of high water
conditions.
Unless our design strategies are adapted to the changing climate, there will be
significant increases in the risk of damages resulting from high water conditions.
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IV. Potential Changes in Wisconsin Climate
The WICCI Stormwater Working Group analyzed both past and predicted precipitation data
(See sections IX and X). Historical precipitation data were obtained from NOAA-National
Climatic Data Center (NCDC) CO-OP weather observation stations. Potential changes in future
precipitation for a range of greenhouse gas emission scenarios were inferred from statistically
downscaled climate projections provided by the WICCI Climate Group.
Analysis of Historic Climate Data - The analyses of long-term Wisconsin precipitation records
indicate that over the last 140 years there have been extended periods of much greater than
average annual and daily precipitation. These periods are distributed throughout the record and
hence neither support nor disprove the hypothesis that the magnitude and frequency of large
rainfall events have increased in Wisconsin as a result of global climate change. Note also that
the data used to develop runoff design standards in Wisconsin (Bulletin 71) are derived from a
period that appears drier than either the earlier or current period of greater precipitation (Figure
2 pg. 26).
Analysis of Downscaled Climate Predictions - With respect to the factors affecting high water
conditions, the downscaled climate projections for Wisconsin vary greatly across climate
models. However, the projections support the following generalizations:
1. Modest increases in the magnitude of intense precipitation events are expected
during the 21st century. For example, averaged over the state, the magnitude of the
100-year, 24-hour storm event (5"-7") is expected to increase by about 11% by the
2046-2065 time period
2. Total precipitation and intense precipitation events are projected to increase
significantly during the winter and spring months from December to April. The
combination of more precipitation and more intense events has the potential to cause
more high water events.
3. The amount of precipitation that occurs as rain during the winter months of December
to March is also projected to significantly increase. This has the potential to cause
stormwater management problems and increases the risk of producing high water
events during a season where such events currently do not normally occur in Wisconsin.
V. Effect of Potential Climate Changes on High-Flow Conditions
How would these potential climate changes affect high water conditions? The answer depends
on characteristics of the water body and its contributing watershed, and on whether the focus is
on flooding or water quality.
Stream and River Flooding - In most watersheds outside urbanized areas, soil conditions
typically are a more important factor than impervious surfaces. Hence the occurrence of a
large flood usually results from large rainfall events over soils that have reduced infiltration
capacity because of soil saturation by previous rainfalls, snow melt or heavy frost. In
Wisconsin, stream and river flooding can occur in all seasons, although the largest floods are
usually in spring and summer.
12
Expected increases in the magnitude and frequency of large rainfall events will very likely
increase flood magnitudes in all Wisconsin stream and rivers, although the amount of increase
will vary greatly. The increase is likely to be the greatest in watersheds that are most vulnerable
to flooding in late winter or early spring when there is the greatest likelihood for increased
rainfall due to climate change. Figure 1 provides information on the seasonal distribution of
large floods in selected gauged Wisconsin watersheds, and give some idea of the most
vulnerable watersheds. Quantifying this vulnerability to flooding is challenging due to the critical
dependence of surface runoff on soil type, frost and soil moisture, and the amount of
transpiration by vegetation.
Figure 1
Seasonal occurrence of top 10% annual peak stream flows in Wisconsin. There is a
statewide spring predominance, with summer peak stream flows mostly in the southwest.
Source: USGS annual flood data. Period of Record: +50 years of record
Graphic Design: E. Murdock
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Lake Flooding - For all but the smallest lakes, flooding generally results from unusually high
rainfall over weeks to months. As in the case for stream and river flooding, increases in
winter/spring rainfall are likely to have the greatest affect on lake flooding. This can be
complicated by the role of frost, which can either increase or decrease runoff and recharge.
Slow-draining lakes and lakes without a natural outlet will be most vulnerable.
Groundwater Flooding - Groundwater drains much more slowly than surface water. High
groundwater results when recharge exceeds drainage over periods of months or years. As with
streams, rivers, and lakes, the expected increases in winter/spring precipitation (when recharge
is unaffected by transpiration) are likely to have the greatest impact on the occurrence of high
groundwater water conditions. And as for runoff, soil type, soil moisture, vegetation and frost
are the critical factors determining the amount of recharge versus runoff However, the
conditions favoring high groundwater recharge may reduce the chances of stream, river, and
lake flooding caused by surface runoff.
Groundwater Flooding at Brodhead,
WI - During 2007-2008, southern
Wisconsin experienced above-normal
precipitation, and some intense rainfall
events. Rising regional groundwater
levels resulting from increased recharge
caused groundwater flooding of
basements and yards. Low-lying
developed areas situated over shallow
groundwater (less than ten feet) and with
poor surface drainage reported
continuously running sump pumps during
this period. Several houses became
uninhabitable due to persistent soil
saturation and flood damage.
Groundwater flooding - Spring Green
Urban Stormwater Flooding- Locally, stormwater causes high water conditions as a result of
heavy rainfall over relatively small areas, and the duration of the initiating storm event is usually
short (minutes to hours). Furthermore, a large portion of an urban contributing watershed is
usually impervious, therefore the peak rate of surface runoff is relatively unaffected by soil
moisture conditions prior to the initiating storm event.
For these reasons, the design of stormwater infrastructure is usually based on single storm
events (design storms). For urban stormwater, changes in the magnitude of rainfall quantiles at
the daily or shorter time scale are the most relevant. It appears that climate change in
Wisconsin will result in modest increases in daily rainfall over the next century, resulting in
greater storm flows in urban watersheds.
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Water Quality - In Wisconsin's urban watersheds, the primary water quality issues have to do
with stormwater runoff or, as is also the case in Milwaukee, combined sewer overflows. In both
cases, changes in the magnitude and frequency of large daily or shorter duration rainfalls are
the most relevant. Climate change in Wisconsin is expected to result in modest increases in
daily rainfall quantiles over the next century, as well as increases in the frequency of large
rainfalls. These changes will require greater investments in stormwater infrastructure for both
new and existing development.
In rural areas, nutrient and sediment runoff from agricultural lands is the most critical water
quality concern. As is the case with urban watersheds, changes in the magnitude and
frequency of large daily or shorter duration rainfalls are most critical. But unlike urban areas,
agricultural lands are particularly vulnerable to large rainfall events that occur in the spring when
soil is bare. Hence nutrient and sediment runoff from agricultural watersheds is likely to
increase as a result of the combined impact of the projected increases in the magnitude and
frequency of large rainfalls and in cold-weather precipitation.
VI. Vulnerability in a Changing Climate
We expect the effects of a changing climate on surface waters and groundwater to have a
gradual but significant impact on society. While intense precipitation events capture our
immediate attention, seasonal shifts in the frequency and timing of smaller events may prove
more costly overall.
Of particular concern is long-lived
infrastructure that is vulnerable to high water
conditions or that protects against the
impacts of high water conditions. The former
includes water supply systems, wastewater
treatment systems, and stream crossings
(bridges and culverts). The latter includes
infrastructure that control floods (e.g., dams
and levees), manages stormwater, and
controls soil and stream bank erosion.
The design of such infrastructure has and
continues to be based on historical
precipitation data. Unless appropriate
adaptation strategies are adopted, we can
expect increases in the frequency and severity
of the following high water impacts:
Reedsburg POTW

Erosion of slopes during intense rainfall events resulting in high sediment and
phosphorus loads to streams, rivers, lakes and wetlands.

Degradation of aquatic habitat as a result of manure runoff from fields and drain
systems.

Impairment of roadways and bridges washed-out due to high water or slope failure.

Groundwater flooding of property and cropland.
15

Contamination of rural residential wellheads as a result of surface water and
groundwater flooding.

Flooding of urban streets and homes due to inadequate runoff drainage systems.

Failure of impoundments, levees and stormwater detention ponds.

Failure of rain gardens and other biofiltration Best Management Practices (BMPs) due to
prolonged periods of saturated soils.

Stormwater inflow and groundwater infiltration to sanitary sewers, resulting in untreated
municipal wastewater overflowing into to lakes and streams.
In summary, our previous investment in public safety and environmental protection risks being
compromised by precipitation impacts that are beyond those anticipated by infrastructure
designers and water resource managers using historic data.
Case Study: The Storms of 2008
Heavy rainfall across parts of southern Wisconsin during June, 2008 overwhelmed stormwater
management infrastructure, causing wide-spread flooding. While not necessarily a result of
climate change, these storms led to a massive increase in nutrient and sediment loading to
surface waters caused by erosion, and:
Stage readings on 38 river gauges
broke previous records;
810 square miles of land was flooded;
Of 2,500 private wells tested, 28% were
contaminated;
161 wastewater treatment plants overflowed
90 million gallons raw sewage;
A total of $34M in flood damage claims
were paid by the Federal Emergency
Management Administration (FEMA).
16
VII. Adapting to a Changing Climate
The Intergovernmental Panel on Climate Change (IPCC) defines adaptation as, “Initiatives and
measures to reduce the vulnerability of natural and human systems against actual or expected
climate change effects." In this report we address adaptation of the built and managed water
resource environment to climate impacts, and recommend an evolution of current design and
management practices to reduce risk and meet the challenges of our changing climate.
While the historical record clearly shows that adaptation to a changing climate is prudent,
recommendations for adaptive measures based on climate projections are less certain. As
noted above, predictions of climate impacts can vary widely across Global Circulation Models
(GCMs). Furthermore, we need better understanding of the effects of some of the impacts (e.g.
increases in the amount and timing of winter/spring precipitation) before specific adaptation
recommendations can be made.
We address this uncertainty by identifying these priorities among our adaptation
recommendations:
a) Strategies that address deficiencies in traditional design practices based on outdated
climate information carry a higher priority than those that are based on anticipated future
conditions described by less certain climate predictions.
b) Adaptation strategies that show a clear cost/benefit to society for either today's
conditions or for the near future climate are preferred.
c) Strategies that increase our capacity to respond as new information about climate
change becomes available are preferable to those that require making large investments
in infrastructure today as a hedge against an uncertain future.
As climate adaptation is implemented, synergies and conflicts among the approaches we
describe are likely to occur. Thus, our recommendations should serve as a starting point for
resource managers faced with the complex and long-term challenges provided by climate
change.
VIII. High Water Adaptation Strategies
Traditional design and management strategies for high water conditions have assumed that the
climate was not changing. However, analysis of historic climate data and predictions by climate
models indicate that Wisconsin's climate has and will continue to change. Unless our design
and management strategies are adapted to changing climate conditions, using traditional
approaches will lead to significant increases in economic and environmental damage. The
WICCI Stormwater Working Group has identified the following adaptation strategies that can
lead to increased societal capacity to minimize risk from high water conditions.
Assessing Site-specific Vulnerabilities - Vulnerability to high water events varies widely
between urban and rural areas, and across the state. The flood damages experienced in
Wisconsin in 2008 exposed many weaknesses in our ability to protect against high water
conditions. While these weaknesses are being addressed in the affected communities, similar
17
vulnerabilities exist in communities across the state. Most units of local government do not have
the capacity to perform the climate vulnerability assessments needed to evaluate the need for
future adaptation requirements.
- We recommend that local units of government receive the technical and financial assistance
needed to assess and mitigate their vulnerabilities to potential high water conditions caused by
today's and future climate.
Steps Toward Building Adaptive Capacity
Building on the experiences of communities having recent intense rainfall, conduct a state-wide
evaluation of vulnerabilities to climate change impacts and develop implementation plans to
mitigate the identified vulnerabilities. Priority areas include:
 Floodplains and surface flooding.
 Areas of hydric soils and groundwater flooding.
 Vulnerable infrastructure.
 Stormwater BMPs.
 Sanitary sewer inflow and infiltration.
 Emergency response capacity.
Case Study - Baraboo R. at I-90/94/39
The flood-damages experienced in Wisconsin
in 2008 exposed many weaknesses in the
strategies in place to protect against high
water conditions. In many cases, these
weaknesses are being addressed. For
example, the Wisconsin Department of
Transportation is conducting a review of
the vulnerability of the entire interstate
highway system as a result of flood-triggered
closures of I-39, I-90, and I-94 at the
Baraboo River in Columbia County.
Engineers will be weighing the costs of
flood-proofing stream crossings and
embankments against the economic costs
of temporarily closures of this important
roadway.
Regulatory Gaps - Even without climate change, there are gaps in state and local regulations
affecting the management of high water conditions. Although the WICCI Stormwater Working
Group did not attempt to systematically identify all such gaps we did identify major gaps in the
regulation of stormwater and wetlands.
- We recommend that the State of Wisconsin work with municipalities and counties to develop
minimum design and performance standards for the control of the high water impacts of
development. We further recommend that these standards specify that regulatory control
extend to the 100-year storm event and require regular updating with the most recent rainfall
18
statistics. Consideration should also be given to requiring additional stormwater storage
capacity to account for uncertainties in future rainfalls.
Without appropriate mitigation practices, urban and suburban development significantly
increase the peak rate and volume of storm runoff, leading to increased risk of flooding in
downstream streams and lakes. In Wisconsin, while there is statewide regulation of the impact
of stormwater runoff on water quality, there are no statewide standards controlling storm runoff
peak flows and volumes. This has been left to counties, towns, and municipalities and as a
result there are a wide range of standards. However, while a few local standards may mitigate
the high water impacts of development, there are many jurisdictions in which the local
ordinances fail to prevent significant increases in downstream flood risk.
- We recommend that WDNR develop an approval process for prior converted croplands that
are being removed from agricultural use, to encourage their restoration and prevent
development in flood-prone areas. We also encourage county and municipal governments to
adopt an approval process, or place land use controls on development on hydric soils in areas
that are likely to experience future flooding.
Wetlands are an essential tool for managing high water, providing flood storage capacity for
overflowing streams and rivers, and precluding runoff that would occur if low-lying areas were to
be developed. By restoring prior converted croplands and preventing the loss of existing
wetlands, we can build capacity for adapting to future high water conditions.
What are Prior Converted Croplands and Hydric Soils? Wetlands that were drained, dredged, filled, leveled or otherwise manipulated (including
the removal of woody vegetation) before December 23, 1985 to enable production of an
agricultural commodity are defined in federal law as prior converted croplands. They
must:
1. Have had an agricultural commodity planted or produced at least once prior to
December 23, 1985, and have not since been abandoned.
2. Do not have standing water for more than 14 consecutive days during the
growing season. (Sites with standing water more than 14 consecutive days are
considered farmed wetlands.)
Hydric soils are formed under conditions of saturation, flooding or ponding for periods long
enough during the growing season to develop anaerobic conditions near the surface. The
presence of hydric soils indicates that an area may be susceptible to flooding by surface
and/or groundwater. Up until 2005, wetlands converted to agricultural use (usually through draining) were exempted
from regulation under the Clean Water Act and state wetland laws, even when agricultural
production ended and the land use changed. Since February 2005, a "prior converted"
determination remains valid as long as the area is devoted to an agricultural use but is voided
when the land use changes.
However, once agricultural production is discontinued, the landowner can continue to maintain
the property’s drainage system (surface and subsurface) to prevent the reestablishment of
wetlands at the site. If drainage is maintained (preventing a return to wetland conditions) the
19
site remains outside the jurisdiction of section 404 of the Clean Water Act, and the landowner
can develop the site. If the drains are not subsequently maintained, this can result in a return to
wetland conditions and flooding of the newly developed area.
Steps Toward Building Adaptive Capacity
WDNR and communities should develop and institute state-wide standards for management of
stormwater peak flows (to complement water quality standards) in developed and developing
areas. This includes reconsidering fundamental design principles in light of climate change and
requiring adoption of the most recent rainfall quantiles from forthcoming NOAA Atlas 14.
The State should reconsider regulations regarding development in areas of potential high
groundwater including prior-converted croplands, historically hydric soils, internally drained
areas and groundwater-dominated lakes.
The WDNR and local communities should evaluate the need for revising floodplain maps in light
of predicted climate change.
Climate Monitoring and Modeling - Our ability to recognize climate change depends on a
state and federally operated monitoring network that collects data on temperature, rainfall,
stream flow and groundwater. While recent analysis of regional climate and rainfall data has
provided insights into changes in climate over the last century, stormwater, floodplain and
wastewater mangers will need better tools for modeling future runoff, soil moisture and
groundwater conditions if they are to adapt to increases in precipitation.
- We recommend that Wisconsin's climate monitoring network be improved and maintained to
provide continued high quality data to support short and long term climate impact modeling.
For example, the resolution (geographic scale) of our weather data needs to be improved to be
able to support both predictive climate modeling and rainfall/runoff impact modeling. Calibrated
radar rainfall data that can account for rainfall amounts at the sub-watershed or catchment level
will allow runoff flows to individual conveyances and BMPs to be estimated, and provide for the
accurate measurement of localized rainfall amounts that is essential for managing stormwater
BMPs. Specific information needed to address climate impacts include:






Fine scale rainfall data using calibrated National Weather Service precipitation and radar
measurements.
Real time stream-flow data from an expanded United States Geological Survey stream
gauge network.
Groundwater level data from strategically placed observation wells to enable
identification of vulnerability to groundwater flooding.
Detailed understanding of sub-watershed characteristics to improve runoff and flood
modeling.
Geospatial data for drainage districts to identify storm and high flow vulnerabilities.
Location of high risk and vulnerable practices in flood-prone areas, such as hazardous
materials and petroleum storage, drinking water wells and septic systems.
20
Steps Toward Building Adaptive Capacity
The state, in cooperation with the federal government, should expand its monitoring program to
provide climate data for supporting decision-making and modeling of high water conditions.
Building Technical Capacity - Traditional design of infrastructure for managing or protecting
against high flows and water levels has mostly been based on design storm analysis, which
neglects the role of antecedent conditions, such as soil moisture, soil frost, and river, lake, and
groundwater levels. Continuous hydrologic simulation can account for these variables, and
provide more realistic assessment of potential climate impacts.
- We recommend that the state develop and implement a long-term plan for developing
continuous hydrologic simulation models of streamflow for critical watersheds. When
appropriate, the models should be coupled to groundwater models Participants in such
modeling could include the Wisconsin Geological & Natural History Survey (WGNHS), the U.S.
Geological Survey (USGS), the Southeast Wisconsin Regional Planning Commission, and
private consulting firms.
Continued and more comprehensive modeling of stream flow to support watershed and
floodplain delineation, combined with an ability to predict antecedent soil moisture and
groundwater conditions is needed for predicting the severity of high water conditions. In recent
years the WGNHS has worked with counties to develop updated regional groundwater models;
but there is no comparable state effort to support the development of surface water models.
Steps Toward Building Adaptive Capacity
DOT and Wisconsin universities should evaluate the methods used for designing roadways,
bridges and stream crossings with respect to potential high water conditions, and consider the
use of a risk-based design methodology for major projects.
Wisconsin universities should develop educational programs to increase literacy among policy
makers and water resource managers about climate change impacts and adaptation and to
support their decision-making.
Wisconsin universities should develop educational programs for engineers, consultants and
managers on the need for and use of continuous hydrologic simulation.
Research - New research in a number of areas of water resources management are clearly
required to support the adaptation strategies discussed above.
- We recommend an investment in research at the state and national level to build capacity and
provide knowledge in the areas of winter/spring hydrology, hydrologic modeling, and decisionmaking under uncertainty for water resource management.
Virtually all of the climate models predict significant increases in the amount of winter/spring
precipitation and the fraction that is rain rather than snow. This could significantly increase river,
lake, and groundwater flooding as well as soil erosion and associated sediment and nutrient
pollution. In Wisconsin, however, winter/spring hydrology is poorly understood. In particular,
there is limited capacity to predict the formation of soil frost and its impact on runoff and ground
water recharge. New research on this topic is critical.
Hydrologic models must be improved to provide useful assessments of climate change impacts.
Of particular importance is the need to be able to distinguish impacts due to climate change
21
from those due to local and regional changes in land use and management. Although there are
research models that can predict surface and groundwater impacts, they are not yet suitable for
operational use.
The design of infrastructure for managing high water and its impacts is currently based on
historical experience and data that are no longer adequate. Furthermore, given the uncertainty
of climate model predictions, it is not possible to simply apply these predictions to the current
design process. Instead, we need to improve our quantitative understanding of the damages
associated with high water, particularly in design problems for which we typically have not made
explicit damage estimates. This improved understanding can then be used to improve the
design process and minimize the risk from climate change.
Steps Toward Building Adaptive Capacity
The scientific community should continue to reduce the uncertainties in predications of climate
change impacts.
The state should support research to determine how predicted changes in fall and spring
temperature and precipitation (and hence in antecedent conditions) will affect flood risk
associated with rivers/streams, lakes, and groundwater in regions of Wisconsin.
The scientific community should:
 Develop tools and build professional capacity so that practitioners can distinguish the
hydrologic effects of local and regional human activities from climate change.

Evaluate and improve strategies for managing high water conditions.

Periodically reevaluate and revise climate and hydrologic design models and criteria.

Improve the capacity to distinguish between the impacts of climate change, and those
caused by land use management decisions.
Stakeholder Action To Build Adaptive Capacity - The WICCI Stormwater Working Group has
also identified specific actions that can be taken by water resource system stakeholders that will
lead to an increase in our ability to adapt to our changing climate.
Regulators:
 Revise local building standards to address runoff control.

Base design standards on more representative updated rainfall statistics.

Require standby power for buildings with sump pumps to avoid flooding caused by storm
related power outages.

Incentivize behavior change through fees and credits.
Planners:
 In areas that are internally drained or have hydric soils, coordinate with regulators to
assure that future land use changes do not increase flood vulnerability.
22

Create or designate new surface flood storage areas (e.g. wetlands) to mitigate high
water impacts.

Use updated models to predict groundwater impacts on development.

Periodically update estimates of high water profiles based on revised rainfall data.

Identify at-risk stream-crossings and develop maintenance and high water contingency
plans.
System Designers:
 Coordinate the design of sanitary and stormwater systems to minimize high water
impacts.

Identify high hazard areas and apply more stringent design criteria.

Anticipate groundwater impacts on bio-infiltration BMPs.

Increase wastewater system peak flow management capacity, and minimize stormwater
inflow and groundwater infiltration.

Use low-impact design to minimize runoff from newly developed areas.
System Managers:
 Upgrade urban storm drainage systems based on continuous hydrologic modeling and
climate predictions.

Manage to minimize high flow impacts rather than sediment removal during high storm
flows (e.g. bypass stormwater bio-infiltration BMPs).

Assess impacts of high flow events on sewage treatment plant process viability, and
evaluate impacts of bypassing high storm flows around the treatment plant’s biological
processes.

Flood-proof vulnerable buildings and infrastructure.

Build capacity for drinking water quality emergency assessment and response.
Educators:
 Conduct public and technical education programs on climate impacts and adaptation.

Educate communities about the hazards of building in areas prone to high water.

Educate property owners about sanitary sewer inflow prevention.

Encourage conservation tillage, stream buffers and other low-impact agricultural
practices to minimize rural runoff.
23
Securing Long-Term Capacity - Building adaptive capacity among this diverse group will
require a sustained effort. The water resource management profession needs organizational
support to integrate disciplines, knowledge and implementation through a multidisciplinary effort
comprising academics, outreach educators, private sector design professionals, municipal
engineers and other resource managers to:

Facilitate communication among water resource management disciplines.

Be a source of credible information for communities, the public and practitioners on
climate change.

Be an authoritative voice to policy makers and private sector on climate adaptation
strategies.
IX. Analysis of Historical Precipitation Record
High water conditions in Wisconsin can result from both short, intense rainfall events and from
large amounts of rainfall over monthly or seasonal periods. Consequently, our analysis of
historical precipitation focused on both yearly precipitation totals and on the frequency and
magnitude of intense precipitation events. We used data from four cities – Madison,
Minneapolis, Green Bay and Milwaukee. These cities provide good spatial coverage of the
region and had complete daily records of 80 years or longer.
Historic Total Seasonal and Annual Precipitation
For each of the four cities, we analyzed annual precipitation totals for the full length of their
precipitation records provided by National Climatic Data Center:
Madison
1869-2008
140 Years
Minneapolis
1891-2008
118 Years
Green
Bay
1897-2008
112 Years
Milwaukee
1928-2008
81 Years
To investigate potential linear trends in annual variations, we first applied the Kendall-Theil
Robust Line test to calculate the average change in annual precipitation per decade, and then
applied the nonparametric Mann-Kendall trend test to check for statistical significance at the
95% confidence level. The results of these calculations (Table 1) do not support the presence
of a statistically significant increasing trend in annual precipitation at the three cities with the
longest records.
24
Table 1
The annual precipitation totals for each city for the period of record are plotted in Figure 2 (pg.
26). Also included on each plot is a five-year moving average for each of the records.
The Madison record (1869 to 2008) provides the most extensive picture of long-term variations.
Note that while high annual precipitation totals of 44 inches were recently recorded in Madison
in 2007 and 2008, a similar wet period was also observed during the 1880s. And the three
highest annual precipitation totals over the entire Madison record were recorded between 1880
and 1884. [Note also the period of record for IWS Bulletin 71.]
The high annual precipitation totals recorded at Madison in the early 1880s are coincident with
high annual average discharges recorded for the Mississippi River (Olsen, 2007) as well as high
lake levels for Lake Superior and Lake Michigan (Quinn and Sellinger, 2006). This suggests a
regional wet period prior to the onset of record keeping for Milwaukee, Minneapolis and Green
Bay.
Historic Magnitude and Frequency of Intense Precipitation Events
We also used the record of daily precipitation to analyze variations in the magnitude and
frequency of intense precipitation events. For this study, an event is any one-day precipitation
total, and an intense event is a daily precipitation total that exceeds a chosen threshold of either
2 or 3 inches. Annual maximum daily precipitation for the four cities are displayed in Figure 3
(pg.27) .
25
Figure 2
26
Figure 3
27
We applied the Kendall-Theil test to calculate the change per decade over the period of record,
and the Mann-Kendall trend test to calculate the statistical significance of those variations
(Table 2). The three longest records – Madison, Minneapolis and Green Bay - show no
statistically significant variations at the 95% confidence level.
Table 2
Frequency of Intense Precipitation Events - Global Circulation Models (GCMs) predict that
climate change will cause an increase in the number of intense precipitation events. For the
Wisconsin historical data we counted the number of 3” inch exceedances for a series of fiveyear time periods for each of the four cities (Figure 4, pg. 29). While the occurrence of five 3”
daily events recorded in Madison between 2004 and 2008 is consistent with a prediction of
increased intense precipitation events, the other three records do not corroborate such a
conclusion.
We also counted the number of events greater than 2” of daily precipitation for the same fiveyear periods (Figure 5, pg. 30). While the Madison and Minneapolis records show frequent 2”
exceedances during the past several decades, they also show earlier periods of high frequency.
Also note that no 2” daily precipitation totals were recorded in Green Bay between 1999 and
2008.
Discussion - The analyses of long-term Wisconsin precipitation records indicate that over the
last 140 years there have been extended periods of much greater than average annual and
daily precipitation. These periods are distributed throughout the record and hence neither
support nor disprove the hypothesis that the magnitude and frequency of large rainfall events
have increased in Wisconsin as a result of global climate change. Note also that the data used
to develop runoff design standards in Wisconsin (Bulletin 71) are derived from a period that
appears drier than either the earlier or current period of record.
28
Figure 4
29
Figure 5
30
"Extreme" Rainfall - Discussions of climate impacts often raise concerns about "extreme"
rainfall or other storm events. While intense rainfalls over short periods can be responsible for
severe damage from flash flooding and erosion, prolonged rainfall of lesser magnitude over a
period of days is more commonly the cause of high water damage.
Figure 6
Between 1950 and 2007, more than one hundred thirty five storm events of greater than 5" were
recorded across the state. Figure 6 shows the spatial distribution of the largest recorded 24hour rainfall >5" for the 102 CO-OP rain gauges with >95% complete data.
Our historic rainfall statistics and downscaled climate projections do not provide information that
allows us to accurately predict the frequency of these events for any single location in
Wisconsin. However, every Wisconsin community should assess its vulnerability to rainfall
events that exceed the 100-year 24-hour design storm.
31
X. WICCI Downscaled Global Climate Model Projections
The WICCI Climate Group has applied the method of statistical downscaling to the precipitation
output from 15 Global Circulation Models (GCMs) to render future projections for Wisconsin's
climate over a system of grid cells with a resolution of 1/10° latitude by 1/10° longitude (~7 miles
by 7 miles). The WICCI Climate Group's statistically downscaled data feature projections for
three time periods: historical hindcasts (1961-2000) used to debias the GCMs, and forecasts for
mid-21st century (2046-2065), and late 21st century (2081-2100).
The basis for the spatial distribution of the GCM projections over the downscaled grid network
was provided by historical climate data recorded at 164 NOAA CO-OP stations between
January 1950 and December 2007. Hydrologic variables such as monthly averages and daily
intensities were spatially interpolated throughout the state, and then used to develop statistical
models for downscaling the future climate projections
GCM climate projections were then downscaled for four different climate scenarios published in
the Special Report on Emissions Scenarios, 2000 (SRES) (Nakicenovic, 2000). These
scenarios provide a geopolitical narrative and predictions about future atmospheric greenhouse
gas concentrations. This study used the SRES A1B scenario, which assumes an increase in
atmospheric carbon dioxide to 720 ppm by the year 2100. Note: projections for the SRES A1B
scenario were available for 14 of the 15 GCMs used by the WICCI climate group for the
statistical downscaling.
WICCI Climate Group downscaled precipitation projections vary significantly between GCMs.
This variability is illustrated in Figure 7, which shows the downscaled projections of the 100year, 24-hour quantile for each model for Madison. For 2010 the predictions range from 5.5" to
9" of rainfall. To address this variability, we adopted the commonly-used practice of averaging
the projections of the 14 GCMs.
Figure
7
32
Understanding Quantiles - A rainfall quantile is the depth of precipitation for a storm of a
given duration and recurrence interval as estimated from observed precipitation data.
Quantiles are used to provide designers with an estimate of the amount of precipitation
expected from a storm of a given duration and recurrence interval probability. For
southern Wisconsin, IWS Bulletin 71 (1992) provides the following quantiles for a rainfall
of 24 hours duration:
2-year
2.78 inches
10-year
4.2 inches
100-year
7.06 inches
These quantiles are based on data recorded between 1907 and 1986, and are used as
both engineering design criteria and for stormwater regulation (See figure 2 pg.26).
Analysis of Downscaled Precipitation Projections
We selected four cities for exploring the spatial distribution of the projections, Madison and Eau
Claire ("inland" cities), Milwaukee and Green Bay (“lakefront" cities). Projections for each city
were obtained from the downscaled grid cell that contains the primary NOAA CO-OP station in
each city. The locations of the cities corresponding grid cell center points are:
Madison
Milwaukee
Eau Claire
Green Bay
43.1° N
44.9° N
44.5° N
43.1° N
89.3° W
91.5° W
88.1° W
87.9° W
Most hydrologic models give one output for a given set of inputs. However, instead of
producing just one precipitation time-series for each model, the WICCI downscaling method
produced daily probability distributions that can be used to calculate precipitation magnitudes
and frequencies or produce simulations. Our analysis of the WICCI downscaled data focused
on projections for several metrics relevant to hydrologic design, including the following:

Magnitude of the 10-year and 100-year, 24-hour storm event.

Annual frequency of 3” of precipitation exceedance.

Average monthly precipitation.

Monthly frequency of 2” precipitation exceedance.

Precipitation as snow and rain during winter and early spring.
Downscaled Magnitude and Frequency of Intense Storm Events
Magnitude of Intense Storm Events - We averaged the 10-year and 100-year, 24-hour
precipitation amounts from 14 downscaled GCMs. Table 3 (pg. 34) presents the calculated
quantiles (in inches) for mid and late 21st century, with the percent change compared to the
historical hindcast period. As a reference, the table also includes the estimated 100-year, 24hour precipitation quantiles from NOAA Technical Paper 40 (TP-40) and ISWS Bulletin 71,
currently used as design standards.
33
Table 3
We used the WICCI downscaled data to determine the projected expected number of annual
exceedances of 3" events for each of the three modeled time periods. (A 3” event is equivalent
to the TP-40 2-year to 5-year 24-hour quantile for much of Wisconsin.) Table 4 shows the
increases in exceedances per year for each of the four cities from the 1961-2000 period to the
2046-2065 period.
Table 4
34
The percent increase in the frequency of 3” exceedances averages 28% for the two inland cities
and 43% for the two lakefront cities. Note, however, that the percent changes in recurrence
interval (the average time interval between exceedances) are more modest. For example, for
Green Bay the change in the recurrence interval of 3" daily rainfalls is from 6.5 years to 4.4
years. The significance of these changes will depend on the magnitude of damages associated
with 3" rainfalls.
Discussion - The projected
increases in the magnitude of
intense rainfall events (represented
by the 100-year quantiles) can be
put into perspective by comparing
them with the quantiles published
in TP-40 and ISWS Bulletin 71 that
are currently used in practice.
Note that we report on maximum
daily values while TP-40 and
Bulletin 71 use 24-hour maximum
values. In order to correct for the
difference between the maximumdaily and 24-hour records,
quantiles for the downscaled data
were calculated by multiplying the
daily quantiles by a value of 1.13.
(Huff and Angel, 1992)
For example, the TP-40 100-year
quantile for Madison is 5.9 inches.
Bulletin 71 (published in 1992)
uses quantiles that are based on
longer and more recent
precipitation records. The Bulletin
71 100-year quantile for Madison is
7.06 inches, which is similar to the
downscaled 100-year, 24-hour
quantile of 6.97 inches for the
1961-2000 historical hindcast period.
This suggests that by adopting the
Bulletin 71 standard for Wisconsin,
design for intense events would
represent current conditions.
Figure 8 shows the percent increase in the 100-year ,
24-hour rainfall quantile from the 1961-2000 period to
the 2046-2065 period, projected onto the WICCI
downscaled grid cells.
The modest changes in the predicted recurrence intervals of 3" events can be adapted to by
periodically revising rainfall quantiles and adopting the revised values for use in engineering
design. However, for design based on somewhat arbitrary standards (e.g., the 10-year event
used for design of urban runoff conveyances), it may be necessary to evaluate the
appropriateness of the standard in light of potentially significant increases in exceedance
frequencies.
35
Downscaled Average Monthly Precipitation and Threshold Exceedances Per Month
While intense, short-duration storm events are usually the cause of high water events in urban
watersheds, longer duration precipitation or precipitation over frozen or saturated soils typically
causes flooding in larger watersheds. The WICCI downscaled projections show Wisconsin
receiving more average precipitation and more intense events during the winter and spring
seasons. To assess changes in monthly and seasonal precipitation we calculated projected
changes for average monthly precipitation and average monthly 2” exceedances.
Average Monthly Precipitation - Figure 9 compares the average monthly precipitation for the
1961-2000 historical hindcast period and the projected 2046-2065 time period, and the percent
change by month. These data indicate that the largest differences in total monthly precipitation
between the historical hindcast period and 2046-2065 are likely to occur during the winter and
spring months. For Madison, the average projected difference in total precipitation during the
period December - April is 13%, with the highest monthly increase (20.6%) in January. Figure
10 (pg. 37) shows the increase in December to April precipitation for Eau Claire is projected to
increases by about 17%.
Figure 9
Threshold Exceedances - We then focused on winter/spring exceedances using a threshold of
2” because such an event has the potential to produce a significant amount of runoff during
frozen or saturated soil conditions. Table 5 (pg. 37) presents the expected number of 2”
exceedances and their corresponding recurrence intervals for the historical hindcast and mid
21st century for Eau Claire, Green Bay, Madison and Milwaukee.
The changes in recurrence intervals highlight the hydrologic implications of the seasonal
increases in threshold exceedances. The results for Madison show the 2” exceedance for the
months of December and March increasing from a monthly recurrence interval of approximately
once every 100 years to once every 30 years. Such increases in frequency, especially during
months when the ground is frequently frozen or saturated, have the potential to significantly
affect the occurrence of seasonal high surface water events and groundwater flooding.
36
Figure 10
Table 5
37
Precipitation as Rain or Snow
Our final analysis was to explore the projected changes in precipitation that falls as rain
versus snow during the winter months of December to March. To do this, we performed a
Monte Carlo analysis using the downscaled daily distributions for precipitation and
temperature to generate a total of 250 time-series for each model. Precipitation on any
given day was assumed to be snow if the daily mean temperature was less than 1.3°C
(34.3°F) and rain if it was greater than 1.3°C. We averaged the results of all the Monte
Carlo trials for each model for each of the three time periods and calculated the changes in
the average monthly precipitation as rain and snow.
The percentages of precipitation as rain for December through March for each of the three
time periods are shown in Figure 11. The figure clearly shows that the proportion of winter
precipitation falling as rain is projected to significantly increase.
Figure 11
Table 6 (page39) provides the average monthly precipitation as rain and the corresponding
monthly percent change versus the 1961-2000 historical hindcast period for Madison. Note
that the rainfall totals were obtained from Monte Carlo simulation, and so may not exactly
match the values reported in the earlier part of this section.
The colder months of January and February show the most noticeable increases in total
rainfall, but the warmer months of November and March also show increases that are more
likely to produce high water events. For example, the monthly precipitation as rainfall for
March is projected to increase from 1.01 to 1.83 inches by the 2046-2065 time period. This
represents a 50.1% increase in the amount of precipitation as rain. As Table 5 (pg. 37)
shows, this increase in rainfall is also likely to include more intense events that have the
potential to cause high water events if the ground is frozen or saturated.
38
Table 6
Summary of Potential Changes in Wisconsin Climate
With respect to the factors affecting high water conditions, the downscaled climate projections
for Wisconsin vary greatly across climate models. However, the projections support the
following generalizations:
1. Modest increases in the magnitude of intense precipitation events are expected during
the 21st century. For example, averaged over the state, the magnitude of the 100-year,
24-hour storm event (5"-7") is expected to increase by about 11% by the 2046-2065 time
period.
2. Total precipitation and intense precipitation events are projected to increase
significantly during the winter and spring months from December to April. The
combination of more precipitation and more intense events has the potential to cause
more high water events.
3. The amount of precipitation that occurs as rain during the winter months of December
to March is also projected to significantly increase. This has the potential to cause
stormwater management issues and increases the risk of producing high water events
during a season when such events currently do not normally occur in Wisconsin.
39
XI. References
Herschfield, D.M., 1961 Rainfall frequency Atlas of the United States, U.S. Department of
Commerce
Huff, F. A. and J. R. Angel, 1992. Rainfall Frequency Atlas of the Midwest (Bulletin 71), Illinois
State Water Survey
Milly, P.C.D., Betancourt, J., Falkenmark, M., Hirsch, R.M., Kundzewicz, Z.W., Lettenmaier,
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