Stormwater | Climate and Stormwater

Coping with change

By Gary L. Oberts

By now we all have heard of the climate changes that are imminent as a result of our input of various pollutants into the earth’s environment. In all of prior time on this earth, nothing that humankind has done, except the insanity of atomic bombs, has presented such a grave threat to existence as we know it.

But the outlook need not all be bleak. We are an ingenious people who have adapted to many changes in the past and always come out wiser and stronger. Such is the view we can take when looking at our role as those entrusted with handling runoff in what seems to be an uncertain future.

My attempt here is not to educate on the nature of climate change; others are far more qualified to do that. My goal simply is to take some collected thoughts on what likely will happen with the management of rainfall and snowmelt runoff as we head into one of the most uncertain futures we have ever seen and propose some ways to deal with it. I have tried to take consensus from the fine researchers and climate scientists who have defined the future and use it as a starting point for what we as stormwater managers must face. As front-line players, we will be looked upon to find answers to water-quantity, -quality, and -supply problems likely to interrupt our lives. Stormwater management is but one of many important fields that will need to develop this adaptive capacity.

Climate Change Setting
The most definitive outlook for the future comes from the Intergovernmental Panel on Climate Change (IPCC), which revised its outlook in early 2007 (2007a, 2007b). Many others, from Al Gore in An Inconvenient Truth to the Union of Concerned Scientists and the Ecological Society of America (2005) and many, many other prestigious climate researchers have weighed in on what might happen in the future if we do not act soon. The common theme in essentially all of these forecasts is that what previously had been somewhat frightening news about the future has taken a turn for the worse. That is, we are accelerating the warming and all of the secondary impacts at an alarming pace and perhaps might hit a point of no return. In fact, the increased greenhouse gas emissions of the past will continue to influence our climate even if we reduce future emissions to those of the year 2000 (IPCC 2007a).

For purposes of this publication, the IPCC scenario for change is assumed and explored in terms of managing stormwater in response. Take for example the temperature changes reflected in Figure 1 from the latest IPCC Working Group I Fourth Assessment Report (2007a) on climate change by the end of this century. Ignoring the bottom line that holds causative factors steady at 2000 levels but sees an increase from past emissions, these scenarios show that the global climate is likely to change from a minimum of 2.0ºF to as much as 11.5ºF depending on how responsive we are to the need for limits on many of the contaminants we release today.

What is important to remember is that it is not just the temperature change we have to deal with; it is also the secondary effects that result from this change. The IPCC and many others have explained the “trickle down” impacts that we will see as the climate continues to warm. In many respects, it is these secondary effects we as stormwater managers have to deal with.

What to Expect as the Climate Warms
The IPCC (2007a, 2007b) has predicted the following impacts of global warming by the end of the 21st century:

It is “virtually certain” that there will be warmer and fewer cold days and nights and warmer and more frequent hot days and nights.
It is “very likely” that there will be more frequent warm spells and heat waves and heavy precipitation with an increased frequency of large events.
It is “likely” that droughts (areal) and tropical cyclones will increase and that sea level will rise.

The changes in stormwater that result from the climate shifts can be ascertained from the above set of IPCC findings. Table 1 summarizes the IPCC (2007a, 2007b) update, plus additional input from the Union of Concerned Scientists and the Ecological Society of America series on regional climate impacts (2003). Note that the impacts of climate change on biosystems, although of utmost concern, are not addressed because of the nature of this article. Details on the implications of changes listed in Table 1 follow the table.


	Figure 1

Temperature
Article after article in professional journals and local papers report that the last reporting period (whatever that time increment may be) was the hottest, warmest, or most severe since records have been kept. It seems that few months, years, or seasons go by without this new claim attached. As this seems to be accelerating, there does not seem to be an end in sight. The National Arbor Day Foundation has even redrawn its Hardiness Zone Map to reflect national changes of warmer conditions marching northward. Figure 1 shows the IPCC (2007a) outlook for the future temperature increases under various scenarios ranging from a minimum of 2.0ºF to a maximum of about 11.5ºF, depending on the level of remedial action taken. Clearly, the outlook is for continued change.

Increase in temperature has a number of impacts that stormwater managers must keep in mind. The increased evaporation of water and the atmosphere’s ability to hold more water results in increased humidity, which can breed extreme weather conditions. Higher temperature on the land surface translates into warmer runoff and thermal conditions that adversely impact some aquatic life (like trout), although it might favor other, perhaps less desirable, life (like nuisance rough fish).


	Figure 2

Figure 2, from the Wisconsin State Climatology Office, shows what is happening to surface waters as exemplified by the long-term ice cover changes being experienced on Lake Mendota in Madison, WI. Although the duration of ice cover appears to have been decreasing slowly over the last 150 years, the three shortest cover durations have occurred in the last 10 years. Reduced ice cover impacts the normal winter behavior of cold climate lakes and can alter winter evaporation, seasonal stratification, and algal production. In recent years, the St. Paul Winter Carnival has even had to postpone some of its frozen lake events because of unsafe ice conditions.

Some benefits may result from a longer growing season, such as increased crop production and even a chance for stormwater managers to get a good cover crop established over exposed soils. However, an increased growing season also can result in some adverse water-related impacts from increased water demand, increased agricultural land disturbance, increased fertilizer and pesticide use, and longer vector (mosquito and crop pest) seasons.


	Figure 3

Precipitation
The increase in global warmth drives precipitation patterns that are reflecting a changing Earth. Figure 3 is a general depiction of the changing annual precipitation graph modeled for the northern Midwest by the Union of Concerned Scientists and the Ecological Society of America (2005). Although certainly not applicable to all of North America, it does reflect changes expected for many areas affected by changing climate (additional regional variation is addressed in a later section). The graph illustrates the increase (green ellipses) in spring and winter precipitation and the decrease (single ellipse) in summer and fall for two different periods as compared to data from the late 1900s. The behavior depicted in this figure is the driving force behind the discussion that follows. Please note, however, that this graphic does not include extreme wet and dry periods that occurred in 1890–1935 for this part of North America. If added, these periods might have shown a little more deviation in the historic conditions (black) line.

In the northern US, a new mix of winter precipitation with more rain, sleet, and rain-on-snow might increase by 15% to 40% while summer rains decrease by 15%. The implications of this change in temperature-driven precipitation are many when stormwater management is the concern. Almost all outlooks for the future include a warning that the number and severity of extreme events will increase, possibly doubling in many parts of the country. Extreme events vary in character from high-intensity rainfall cells accompanying fronts to tropical storms that inundate coastal areas before moving inland to continue dumping large volumes of rain. The absence of full-winter ice cover also could result in increased lake effect snows for areas already vulnerable to this climatologic phenomenon where air passing over relatively warmer large bodies of water picks up moisture and dumps it in large volumes when it hits land. The past winter’s massive storms seen in upstate New York and the upper peninsula of Michigan could become more common if the Great Lakes remain ice-free and relatively warm.

These changing temperature-precipitation patterns mean that stormwater managers will need to redefine precipitation and runoff statistics presented in such commonly used tools as intensity-duration-frequency (IDF) graphics and precipitation frequency relationships that are the basis for hydrologic calculations and design. Runoff patterns from these events also will need to be examined to redefine response to changing antecedent moisture conditions, snow moisture content, and event distribution and intensity (that is, more rainfall occurring during these fixed periods of time). For example, the precipitation frequency distribution used over much of the northern US was developed by the US Weather Bureau and published as Technical Publication 40 (Hershfield 1961). The input data used to calculate the frequencies were collected during the approximately 30 years prior to its publication (a relatively dry period that began during the Dust Bowl years) and not revised in the following nearly 50 years of climatic change. Since the 1930s in Minnesota, there has been a doubling in the number of intense storms, with another doubling expected this century (UCS-ESA 2005, summary for Minnesota). Is it prudent for stormwater managers to rely on Technical Publication 40 as a reliable predictor of current or future rainfall frequencies? How about the use of runoff models that rely on runoff coefficients that might not accurately reflect the land surface response to certain precipitation amounts?

Managers also will need to revisit current flood and peak storage facilities to determine whether more capacity or a change in operation is warranted. Emergency response and services similarly will need to be examined in light of the findings of the new flood potential calculated in recalibrated runoff hydrologic behavior models. The IPCC has called this “proactive climate change risk management adaptation planning” (IPCC 2007b).

In the same context, the intensity and duration of droughts is changing all across the continent. This means that soil moisture will be depleted, annual recharge will decrease, and runoff from hardened dry soil surfaces could increase. This latter point is especially important when considering the previous paragraphs on extreme events; that is, much less water will soak into the ground and more will run off, possibly exacerbating flooding.

Runoff
Probably no topic is more important to stormwater managers than runoff. The previous two sections describe a changing temperature and precipitation paradigm to which we all must adapt. The extreme variation in wet and dry cycles will not go away, so we must rise to the occasion and do what needs to be done to protect a society that might be unaware of the dangers. The IPCC states that “by mid-century, annual average river runoff and water availability are projected to increase by 10%–40% at high latitudes and in some tropical areas, and decrease by 10%–30% over some dry regions at mid-latitudes and in the dry tropics, some of which are presently water-stressed areas” (IPCC 2007b). It emphasizes that in some places and in particular seasons, however, changes will certainly differ.

There are many impacts that must be considered when runoff changes are evaluated. Perhaps the most important change is that runoff amounts as a fraction of rainfall are likely to increase. In areas where winter precipitation is going to increase, more runoff from frozen ground and more liquid precipitation during the winter season means more runoff. In areas where summer rainfall will decrease and where extreme events translate into high-intensity events on drought-hardened soils, watershed runoff might increase, depending on where it is routed. That is, increased site runoff might be routed to dry ponds, pools, and wetlands where it will not travel farther down the watershed, or possibly it will be routed directly to connected streams, which would result in increased watershed yield. The secondary impact of more water running off is less infiltration, which translates during the year into decreased baseflow as less water is stored in the shallow groundwater zone and less available water supply as groundwater recharge to deeper aquifers decreases. The public will need to be educated on this runoff behavior to learn that low-flow and high-flow conditions could both occur within a fairly short period of time.


	Figure 4

The combination of extreme events and droughts means that water level fluctuations will be commonplace as storage areas (ponds, wetlands, floodplains) change very quickly from the dry, exposed conditions seen in Figure 4 to flooded, high-water conditions that typically follow big melt or rainfall events. Some might view the condition in Figure 4 as desirable, because it appears as though an abundance of storage is available. However, in many cases such as this, the exposed soils are indicative of a shallow storage volume that can become quickly overwhelmed by a significant event and then just as quickly dry out again as dry weather sets in for an extended period. This behavior can be a suitable one, but it must be part of a deliberately designed system and not left to a past design that is not evaluated relative to changing climate conditions.

As previously mentioned, models and relationships used to predict runoff behavior will need to be changed to reflect conditions in a changing world. Predictive models used to estimate such things as flows paths under high flow, water level rise in a storage pond, and the volume of runoff expected for particular rainfall frequencies must be changed to adequately protect the public.

Water Quality
The behavior described above for runoff quantity means that changes also will occur for runoff water quality. Rapid high flow generally means that erosion occurs when streambanks, slopes, and exposed soils wash into moving water. The buildup of various pollutants on urban surfaces during extended dry spells results in highly loaded runoff of such pollutants as heavy metals (traffic, industrial output), nutrients (organic debris, fuels), and various organics (PAHs, oil and grease) being washed off when high-volume events occur.


	Figure 5

One of the biggest impacts of changed winter climate is the probable use of additional salt to keep roads ice-free. The increase of rain during winter and frequent melt periods mean that icing conditions will become more common, resulting in additional salt applications. In many parts of both the US and Canada, increased levels of sodium and chloride are already being detected in groundwater, streams, and lakes. A recent move in the Twin Cities metropolitan area of Minnesota toward the use of 100% salt rather than a salt-sand mix is disturbing. This change is simply to avoid the public works cleanup that sand has presented in the past. Sacrificing future drinking-water supplies and lake quality to avoid spring cleanup seems to be a very poor investment in our future. Claims of increased efficiencies and lower salt use clash with the reality of salt piles on roadsides (Figure 5) and white coatings on urban streets and parking lots for the entire winter period. Clearly, we need to practice wiser salt management.

The impact on lake and stormwater pond water quality likely will be detrimental due to longer periods of stratification and increased water temperature. Stratification leads to depleted oxygen levels and subsequent release of pollutants from bottom sediments. In colder regions, the stratification not only will be temperature driven but also will be chemically driven by increased inflows of dense, saline runoff during snowmelts. Already, many northern lakes are seeing a marked increase in chloride levels, and stormwater ponds already contain extremely elevated salt levels. Longer periods of warmer temperatures also will result in the proliferation of nuisance algal growth in nutrient-rich lakes and ponds, and more lakes will become nutrient rich because of increased pollution washoff and decreased flushing. Generally dry conditions also will lower lake levels. Periodic large inflows might occur during extreme events, but evaporation during warmer days and nights and increased pollution loads likely will offset the increase in volume. Stormwater managers dealing with lakes will need to pay particular attention to runoff treatment, effectively removing pollutants while keeping the volume of water moving into the receiving lake.

Water Supply
Nowhere is the impact of increasing population on the North American continent threatening to be harder felt than water supply. Changing climatic conditions promise to make this problem more severe. The dry western part of the continent looks as though it will continue to experience periods of severe drought during extended periods of generally dry weather. Less precipitation directly translates into less available water. In the western US, less buildup of snowpack in the mountains means less opportunity for effective capture of future water supplies in the extensive reservoir system. The fact that precipitation is changing from snow to more rain, however, means that the volume likely will be there but must be captured with a different approach to reservoir management focusing on flashier and shorter winter runoff capture.

In many parts of the US and Canada where groundwater provides the primary source of drinking water, the impact of less recharge certainly will be seen. With an increase in the amount of runoff relative to the water soaking into the ground, an increasing population will have less groundwater at its disposal at a time when it needs an increased volume. Competition for this source also will increase as drier conditions translate into increased irrigation demand for crops and green grass. Water conservation and new use efficiencies will be essential. Stormwater managers can assist in this effort by implementing infiltration practices and cleaning runoff and routing it to areas where it can soak into the ground.

Adapting Our Stormwater Practices
The discussion above lists many impacts that we most certainly will be seeing as our world warms and precipitation patterns respond to the change. Stormwater managers will be on the front line in trying to cope with these changes and continue maintaining the quality of life the public has come to expect. Following are some suggestions for how we can begin to change our approaches to stormwater management to adapt to this changing world.

Low-Impact, Sustainable Development
The days when it made sense to develop a parcel of land and whisk the water away are long gone. No longer can we treat water as a waste product in need of disposal. The previous discussion should have taught us all that every drop of water falling to Earth is a valuable investment in our future. The IPCC notes that sustainable development, as defined by the Brundtland Commission, can reduce vulnerability to climate change (IPCC 2007b). The role of stormwater in achieving this future can be significant.

The approach to future development throughout the continent should be one where water is intercepted where it falls and soaked into the ground as close to that site as possible. Natural drainage systems should be maintained; wetlands and floodplains should be preserved and restored if they can be; low-water-demanding vegetation should be planted instead of manicured Kentucky bluegrass. These elements are all part of a concept commonly called low-impact development (LID) or any number of similar terms, such as better site design, conservation development, design with nature, or even sustainable development. In short, they all attempt to minimize the stormwater impact of changing pervious surfaces to impervious surfaces. Adopting this ethic for development also will go a long way toward minimizing many of the stormwater impacts of a changing climate.

LID principles also can be applied in retrofit situations where good stormwater management was not applied in the original development. Reducing the coverage of pavement (less heating and less runoff), reintroducing native vegetation (less water demand, better water treatment, more carbon sequestration), and localized infiltration (soil moisture replenishment, groundwater recharge) are all retrofit approaches that can begin to replace antiquated development.

Runoff Treatment Facilities (Structural BMPs)
There will, of course, always be a need for structural facilities to hold, regulate, and treat runoff. For both new and retrofit situations, new designs will be needed to deal with the behavior described previously. The dramatic swings from drought to extreme events will challenge all of us as water levels fluctuate in these facilities as we have never before seen. At a minimum, some kind of operable outlet should be built into every storage facility so that water can be saved during periods of shortage and released in response to extreme events. The capture of more runoff during dry spells of course has implications on downstream biota and stream health that must be considered. Extended periods of low flow and even drying of streams and lowering of pools and wetlands must always be part of an environmental assessment accompanying changes in operating strategy.

Seasonal storage in colder climates should be designed into storage ponds so that winter runoff from rain-on-snow and melt events can be captured instead of sent downstream. This storage volume would be on top of a layer of ice but at a lower storage elevation. This approach also reduces the size of the winter pool that is subject to greatly reduced water quality because of salt inflows. Similarly, winter capture of rain-on-snow events in western water supply watersheds will be essential so that this volume is not lost from the annual reservoir water budget.

New research on the ability of bioretention (University of New Hampshire Environmental Research Group, Norwegian University of Science and Technology, Luleå University of Technology, Water Environmental Research Foundation) to intercept and absorb runoff during snowmelt events is showing that these facilities have a great deal of promise to hold and infiltrate water when it would otherwise flow into our receiving waters. Infiltration of snowmelt can even occur under well-designed and -managed infiltration best management practices (BMPs). The use of subgrade, insulated treatment systems also can prove to be an effective solution to increased runoff when the surface might be too frozen to provide a similar option.

Model and Precipitation Database Revisions
As previously noted, use of hydrologic models built with precipitation assumptions from 50 years ago will not serve us well in a changing climate. Models built as recently as 10 to 20 years ago are probably woefully inadequate to prepare us for what likely is to happen today. The common complaint that “we had our third 100-year storm this summer” should tell us that a change in how we view precipitation patterns is needed.

Updated precipitation frequency and IDF data are essential to reflect the effects of today’s response to climate change. Continued revisions should occur at least every 10 years as we proceed through this century of change.

In a similar manner, the runoff models that we have prepared to describe stormwater behavior should reflect the changes documented above so that we do not underdesign our facilities. The possibility of structural failure and the public health impacts that could result put the onus on the stormwater management community to stay on top of this need.

Water Quality
Essentially all of the impacts of climate change noted in this article carry with them a threat of decreased water quality. Erosion and increased buildup and washoff of pollutants are the primary drivers of this degradation. Increased thermal impacts from warmed runoff, warmer lakes and ponds, and shifts in ecological behavior result.

Managing stormwater to mitigate the increased load beckons us to those BMPs that best can address the challenge. In addition to the move toward LID discussed above, improved pollution prevention is something that can help. Smarter use of chemicals, such as salt, fertilizers, and pesticides, and better pollution control and waste recycling and treatment will keep pollutants from exposure to runoff. The increase in vectors and longer warm seasons will make this element of stormwater programs all the more important.

Rapidly stabilizing exposed and erosive soils is basic to every stormwater control program, but effective implementation will be more important than ever as we face drought conditions followed by extreme events and increased rain-on-snow during the winter. This approach is also critical in coastal areas that will experience new erosion events as sea levels rise, coinciding with increased tropical storm frequency and population increases, and as protective wetlands, broad sand beaches, and coastal barriers wash away.

Finally, we will need to redefine water-quality behavior as climatic conditions change. Current documentation of water-quality behavior might not adequately describe conditions that unfold in the future. Data collection programs should be stronger than ever, not discontinued as too expensive.

Water Supply
Water-supply expectations of the past will need to be re-evaluated to ensure that future demand is met. The availability of water unfortunately is going to decrease as the demands of an increasing population and competing uses increase.

Stormwater practices should change in focus from routing water quickly away toward either capturing and soaking water into the ground or routing it (after pre-treatment) to a water-supply reservoir. Operation paradigms must be revisited to see if those of today will meet the demands of tomorrow. Changes could be needed in the way reservoirs are managed to capture runoff from a snowpack versus rain-on-snow or extreme rainfall events. Attention to infiltration practices that encourage recharge of groundwater should occur over those that route water quickly away to a receiving water body.

Natural Drainage System
The natural drainage system will play more of an important role in water management than ever before in our modern history. The dramatic way in which precipitation will occur and runoff will result places the wetlands, floodplains, storage pools, and drainageways of this system squarely on the front line. Preserving this system where it exists and restoring it where it has been lost should be a high priority for stormwater managers everywhere. Commensurate with this is an accurate description of how these features will behave. They need to be managed safely such that the public is protected adequately. In addition, the habitat they provide should be evaluated to see if any management changes or intervention is needed.

Part of the natural drainage system typically includes forest cover. The thermal cooling and water-quality enhancements (buffer) provided by this cover are two more reasons to make sure that forest cover is included in stormwater management efforts.

Conclusions
As rapid changes in precipitation and runoff patterns occur over this century, stormwater managers must gear up and lead a change in our adaptive capacity. We need to look at basic design input parameters defined by climate and re-evaluate current and future designs to make sure they are adequate for higher, flashier runoff. Changes in water-quality behavior, water-supply availability, and drainage system character are likely to impact the public in a way that only stormwater managers can mitigate. We must step up to the challenge, assume the worst (even if it does not ever happen), and protect the public.

References
Hershfield, D.M. 1961. Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years. Technical Publication 40. US Weather Bureau.

IPCC. 2007a. Climate Change 2007: The Physical Science Basis—Summary for Policymakers. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Available at http://www.ipcc.ch/.

IPCC. 2007b. Climate Change 2007: Climate Change Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Available at http://www.ipcc.ch/.

Union of Concerned Scientists and Ecological Society of America. 2005. Confronting Climate Change in the Great Lakes Region. An update of the 2003 report by regional climate experts for the UCS-ESA. Available at http://www.ucsusa.org/greatlakes/glchallengereport.html.

Gary L. Oberts, P.G., is a senior environmental analyst with Emmons & Olivier Resources Inc. in Oakdale, MN.

SW September 2007

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