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

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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* AECOM Jeremy Balousek Dane County Land Conservation Division Ken Bradbury Wisconsin Geological and Natural History Survey Kurt Calkins Columbia County Land & Water Conservation Pat Eagan UW-Madison, Engineering Professional Development Rick Eilertson City of Fitchburg Engineering Greg Fries City of Madison Stormwater Utility Keith Haas* City of Racine Water & Wastewater Utility Mike Hahn* Southeast Wisconsin Regional Planning Commission Kevin Kirsch WI-DNR Runoff Management Section Najoua Ksontini WI-Department of Transportation Mike Martin Milwaukee Metropolitan Sewerage District Paul McGinley* UW-Stevens Point Rob Montgomery Montgomery Associates Resource Solutions Ned Paschke UW-Engineering Professional Development John Ramsden* Natural Resources Conservation Service Tom Sear SEH Jon Schellpfeffer* Madison Metropolitan Sewerage District Mike Schwar* HNTB Rodney Taylor WI-Department of Transportation Eric Thompson MSA Professional Services Bill Walker WI-Department of Agriculture Trade and Consumer Protection John Walker* USGS-Wisconsin Water Science Center Bob Watson* 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

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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. 

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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:

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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.

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.

Examples of high water impacts: Upland Erosion; Urban Street Flooding; Groundwater flooding.

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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.

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 decision-making under uncertainty for water resource management.

Fine-scale Multi-sensor Measurement of Precipitation

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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.

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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.

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.

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 duringthis period. Several houses became uninhabitable due to persistent soil saturation and flood damage. Groundwater flooding - Spring Green

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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:

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.

Reedsburg POTW

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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).

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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

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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.

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

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.

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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.

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

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. 

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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.

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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 decision-making 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

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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.

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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.

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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.

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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) .

26 Figure 2

27

Figure 3

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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 five-year 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 five-year 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.

29Figure 4

30Figure 5

31

"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 24-hour 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.

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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 100-year, 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

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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 43.1° N 89.3° W Eau Claire 44.9° N 91.5° W Green Bay 44.5° N 88.1° W Milwaukee 43.1° N 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, 24-hour precipitation quantiles from NOAA Technical Paper 40 (TP-40) and ISWS Bulletin 71, currently used as design standards.

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).

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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

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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 maximum-daily 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.

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.

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.

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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.

37

Figure 10

Table 5

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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.

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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.

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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, D.P., and Stouffer, R.J., 2008, Stationarity is dead: Whither water management?: Science, v. 319, no. 5863, p. 573-574 Nakicenovic, N. et al (2000). Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change. Cambridge, U.K.: Cambridge University Press. Available online at: http://www.grida.no/climate/ipcc/emission/index.htm Olsen, R. (2007). Upper Mississippi River System Flow Frequency Study [Powerpoint Slides]. Retrieved from: http://www.iwr.usace.army.mil/inside/products/proj/docs_proj/31May07/UMRSFFS-31May07.ppt Quinn, F.H. and Sellinger, C.E. (2006). A reconstruction of Lake Michigan-Huron water levels derived from tree ring chronologies for the period 1600-1961. Journal of Great Lakes Research 32: 29-39.