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Does more extreme rainfall mean more flooding? Answer: Not always

Posted on 15 September 2010 by ltryhorn

Guest blog post by Lee Tryhorn and Stephen Shaw.

An extremely challenging aspect of present-day climate research is associated with the prediction of regional climate change impacts. That’s what everyone wants to know – how will climate change affect me? People are not directly affected by the global mean temperature. They care about the temperature, rainfall, and wind where they are. This blog post is the first in a series aimed at exploring what the local impacts to climate change might look like in different areas of the globe.

A common sound bite associated with climate change is that with the expected increase in extreme rainfall events, we can expect more flooding. Recent work at Cornell University looking at inland flood risk in New York State suggests that this is not always the case.

Non-coastal floods occur when rivers or streams overflow, spilling into the adjacent land. This type of flooding is already a huge issue for New York State. Many original settlements were concentrated along rivers, and much of the state’s infrastructure reflects these early patterns of development. From 1994-2006 many regions of the State experienced more than four FEMA-designated flood disasters, with one county experiencing seven! (See Figure 1). Currently, flood damage costs an average of $50 million a year. Consequently, any changes to flooding associated with climate change are of great interest to the people of New York.

Figure 1. Number of FEMA-declared flood disasters in New York counties (Source: compiled from FEMA website).

Documented increases in annual rainfall (Figure 2a), as well as higher rainfall intensities (Figure 2b), are often used as justification for predictions of an increased likelihood of flooding. For some parts of the globe, particularly urban areas and those with steep topography, this direct link between precipitation and flooding is quite obvious. Urban areas have impervious surfaces, less vegetation, and compacted soils that minimize the ability of soil to store water. Similarly, areas with very steep slopes are likely to experiences increases in flash floods following increases in rainfall intensity. In these areas, increased precipitation translates directly to increased runoff.

Figure 2a. Annual precipitation (mm) for Ithaca, New York. The figure was created from precipitation data obtained from the Northeast Regional Climate Center.

Figure 2b. Graph of the number of extreme precipitation events (over 50mm in 48 hours) per year in New York State. The figure was created from precipitation data from 46 stations obtained from the Northeast Regional Climate Center.

However, for most of New York State, the picture is more complex.

To better understand flood processes in New York State, linkages between rainfall, streamflow, and snowmelt were examined in three regions of the state. As it turns out, less than 20% of the largest streamflows are actually caused by the largest rain events! (See Table 1). Most of the largest rainfall events in this region occur between May and October. During this time of year, moisture-laden air from the south reaches New York State and often causes two-day rainfall amounts that exceed 125mm. However, these events are usually counteracted by high soil-water storage capacity associated with drier soil conditions and lowered water tables (common during the warmer summer months).

% Annual Max Discharge Events Occurring When:

Watershed

2-Day Annual Max Rainfall

3-Day Annual Max Snowmelt

May to October

Ten Mile River

15

10

14

Fall Creek

20

20

5

Poultney River

17

9

10

Table 1. Summary of causative conditions of annual maximum discharges on three watersheds representative of conditions in New York State.

The very largest rainfall events do tend to be associated with the largest flood events, but these rainfall events are often associated with hurricanes or other relatively rare weather patterns that are difficult to accurately predict in the future.

Most (around 60%) of the highest streamflow events in New York result from moderate rainfall events (25-75mm two-day events) on very wet soils (limited rainfall storage capacity). In the future, higher temperatures are likely to dry the soils out due to increased evaporation. So even though the rainfall amounts of the largest storms are likely to increase, the resulting streamflows (and therefore flooding) are uncertain, as there may be a buffering effect from possible increases in soil dryness.

Ultimately, despite the projected increases in extreme rainfall events, it remains uncertain whether flooding will actually increase in New York State with climate change.

New York State is a unique regional situation and it is inappropriate to take a single situation like this and extrapolate it to a global level. This is a common problem in climate science communication. As we further explore climate change impacts at the regional and local level we can expect similar results - that is, a more nuanced, complex picture of change.

These findings are taken from Shaw et al. 2010. Chapter 4: Water Resources. ClimAID: The New York State Adaptation Assessment. This report will be publicly available later this year. A journal article on the same topic is also currently under review.

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Comments

Comments 1 to 10:

  1. Very interesting post. It is too bad that it cannot be extrapolated generally.

    Do many floods get caused by rapid melt of snow in spring? That might occur more often in an AGW world. My impression is that rapid spring melt has happened in the US midwest, but I have not seen any clear linkage to AGW.
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  2. Several points:

    1. More dams are being built which means less flooding of lowlands.

    China, Brazil and probably India and hopefully Pakistian have/will make more dams which will reduce flooding. One hopes that various green agendas about non-dam building are weighed up with the human impacts, such as in current areas of Pakistan.

    2. The idea that both droughts and floods will increase with global warming is too simplistic, and ignores the fact that in some or many areas, an increase in one of these partially cancels out an increase in the other.

    One doesn't walk from a desert straight into a wall of jungle. With increased warming, drought and flood-prone regions may merge closer together, which is a net positive effect. Distinctions between rainall zones - eg deserts to savannah to rainfoest may become less abrupt.

    Some areas of marginal rainfall will become less prone to drought with more rainfall regionally, and some areas with more floods will become less prone to flood with more droughts regionally-that is- some of the effects will cancel out and the net effect from global warming will be positive in these areas.

    3. Flooding in marginal drought/dry regions, such as in most of Australia, actually produce a net benefit. It is only in regions which are already prone to high rainfall that increased flooding generally produces net negatives. However with more dams (eg Brazil, China), this can be largley mitigated.
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  3. Precipitation is one of the interesting twists in AWG - it could mean a positive feedback (water vapour and clouds with heat retention) or a negative feedback (clouds with increased albedo and increased snow cover at higher latitudes).

    A nicely nuanced post.
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  4. @thingadonta: "The idea that both droughts and floods will increase with global warming is too simplistic, and ignores the fact that in some or many areas, an increase in one of these partially cancels out an increase in the other."

    How do you arrive at your conclusion that this will happen in "many areas"? Do you have research to back this up or is this your opinion?

    "With increased warming, drought and flood-prone regions may merge closer together, which is a net positive effect."

    So, suffering from two natural disasters instead of one is *better*? You have to realize that droughts and flooding usually don't take place during the same time of the year...

    I'm sorry, but you are being naively optimistic, here. That's an awfully big risk you're taking with the lives and livelihoods of millions of people.
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  5. thingadonta. I'm not so sure about Australia's great benefits from drought and flooding. The places where this works are the places where it has always worked. The occasional extensive flooding of north Queensland leading to the occasional filling of Lake Eyre being the classic example.

    In other regions which have been taken over for crop production, drought leads to water repelling surface soils. Isolated heavy rainfall events can then lead to local flooding and erosion rather than replenishing the surface moisture or the water table. I'm no expert, but I'd not be surprised if similar results eventuated elsewhere. Intermittent heavy rainfall is no substitute for steady seasonal precipitation.

    This might be merely a feature of Australia's thin and depleted soils. It might also be a consequence of inappropriate past farming practices in other areas as well. I very much doubt that the optimistic it'll-all-balance-out-in-the-end approach is very helpful.
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  6. @thingadonta:
    '3. Flooding in marginal drought/dry regions, such as in most of Australia, actually produce a net benefit. It is only in regions which are already prone to high rainfall that increased flooding generally produces net negatives. However with more dams (eg Brazil, China), this can be largley mitigated.'

    NO flooding causes erosion, silting up of wet lands, washing away creek and river banks and causing millions of dollars damage.

    That is why most of Australia has poor soils.
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  7. Criscanaris (#3)

    The precipitation, when it is absorbed by the soil represents a

    negative feedback because the water vapor is the driver of climate.
    It happens that our soil is sealed by the accumulation of ammonia from

    the decomposition of organic matter (soil science) which is a natural

    phenomenon plus the manufacture of fertilizers based on ammonium.
    See this presentation:

    Greenhouse effect, the bigger the better.


    We must pay attention to the rate of absorption of water by soil (mm /

    h)
    because the rains have not only increased speed in millimeters per 48

    hours.
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  8. thingadonta (#2)

    Here in the Amazon (rainforest) we live with the two situations.
    Floods and drought due to soil sealing to 15 cm deep, which extends up to 50 cm.
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  9. Very interesting post and topic. I look forward to future installments.
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  10. Excellent. I appreciate the precision of this post, which takes care not to over- or understate the case.

    But it would be helpful to know more about the background and qualifications of the authors, so readers can judge for themselves the credibility. (A bit of bio info would also e useful when citing this rebuttal in a debate or interview.)

    Ideally, the brief bio (could be one line) would appear right with the post, or perhaps in a link within the post.
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