Warming temperatures cause snows to melt earlier in the spring and allow storm systems to generate more rain because warm air holds more moisture than cold air. As a result, seasonal floods are expected to arrive earlier in the spring, and the risk of catastrophic floods is expected to increase. These increases have already been observed in the United States – recent decades have brought more heavy summer rainfall events along with increased likelihood of devastating floods. While no single storm or flood can be attributed directly to global warming, changing climate conditions are at least partly responsible for past trends and the increasing frequency of major flood events.
Because spring is arriving earlier, snow packs are also melting earlier. This means that many streams and rivers are reaching their peak flows earlier in the season, which, coupled with increasing numbers of high flow days, can present greater flood risks. This, in turn, affects the availability of spawning habitats for some fish species. The earlier arrival of spring floods may also contribute to worsening drought by mid-summer, stressing vegetation and wildlife, and increasing the risk of wildfire events. Because rainfall is expected to come less frequently, and via more intense storms, floods are likely to become more common and extreme. Precipitation in the United States increased by 7% over the course of the past century, and is expected to increase even more in the coming decades. Most notable is that fact that the strongest 1% of rain storms increased by nearly 20% over the past decade.
The consequences of earlier and more frequent floods include:
These system changes can significantly alter the existing composition of water-dependent ecosystems and the species they support.
In addition to ecosystem impacts, flooding negatively affects the built environment and communities throughout the United States. The Council of Environmental Quality reports that between 1980 and 2013, the United States suffered more than $260 billion in flood-related damages. Examples of recent impacts include record flooding from excessive rainfall in central and northern Illinois in April 2013 that damaged homes and businesses and caused an estimated $1 billion in losses. President Obama’s 2013 Climate Action Plan directs federal agencies to take appropriate actions to reduce investment risks, and specifically to update flood-risk reduction standards, resulting in FEMA’s introduction of a Federal Risk Management Standard. While flooding poses significant risks to people and property as well as flood-prone ecosystems, it can also present conservation opportunities.
Land trusts are responding to on-the-ground impacts of changing water regimes in various ways. Some groups such as the Elkhorn Slough Foundation are prioritizing acquisitions to restore hydrologic connectivity and “reclaim the floodplains”. Others are implementing adaptation projects such as planting trees to shade streams or increasing tree cover in cities to mitigate negative effects of increasing temperatures. In addition to employing strategic conservation planning to reduce risks and enhance resilience, some land trusts are also supporting efforts to mitigate greenhouse gas emissions to reduce the extent of future climate change.
Land trusts must determine the right planning approach for their organization, however, more and more, conservation organizations are working with their communities to identify opportunities to reduce vulnerabilities and prepare for changing temperatures. Agencies are making similar strides to implement projects that reduce risks and plan for resilience by incorporating adaptation, mitigation, and engagement into their strategic goals and objectives. For example, the U.S. Fish and Wildlife Service’s Strategic Plan for Responding to Accelerating Climate Change identifies seven planning and development goals to support sustainable landscapes. Conservation groups are increasingly partnering with agencies as well as other nonprofits and for-profit organizations to respond to this global challenge at local levels.
Resilience describes the ability of a system to persist through extreme change. By working to identify and reduce potential threats land managers can build resilience and achieve multiple management objectives. Assessing vulnerabilities is a critical step in the strategic conservation planning process that helps land trusts identify key threats to resources. Adaptive management planning enables conservation practitioners to implement interventions to reduce vulnerabilities and to monitor and revise strategies as new information becomes available. There is no one-size-fits-all solution to addressing management challenges associated with warming average temperatures, however, planning that acknowledges vulnerabilities can help land trusts better achieve their conservation objectives.
FEMA's training manual explains that a flood is an event in which an abnormally large amount of water accumulates in areas where it is usually not found. Flooding is determined by a hydrological cycle in which precipitation falls from clouds in the form of rain and snow. When it reaches the ground, the precipitation either infiltrates the soil or travels downhill in the form of surface runoff. Some of the water that infiltrates the soil is taken up by plant roots and transported to the leaves where it is transpired into the atmosphere. Another portion of the ground water gradually moves down to the water table and flows underground until reaching water bodies such as wetlands, rivers, lakes, or oceans. Surface runoff moves directly to surface storage in these water bodies. At that point, water evaporates from surface storage, returning to clouds in the atmosphere.
There are seven different types of flooding: Riverine (main stem) flooding occurs when surface runoff gradually rises to flood stage and overflows its banks. Flash flooding is defined by runoff reaching its peak in less than six hours. This usually occurs in hilly areas with steep slopes and sparse vegetation, but also occurs in urbanized areas with rapid runoff from impermeable surfaces such as streets, parking lots, and building roofs. Alluvial fan flooding occurs in deposits of soil and rock found at the foot of steep valley walls in arid Western regions. Ice/debris dam failures result when an accumulation of downstream material raises the water surface above the stream bank. Surface ponding/local drainage occurs when water accumulates in areas so flat that runoff cannot carry away the precipitation fast enough. Fluctuating lake levels can occur over short-term, seasonal, or multi-year periods, especially in lakes that have limited outlets or are entirely landlocked. Control structure (dam or levee) failure has many characteristics in common with flash flooding.
FEMA offers a variety of mapping products by state and county, and allows users to execute a quick search of addresses or place names. Learn more about coastal flood risks and resources here.
Did you know land trusts are working to mitigate climate change? Learn more.