Impacts From Floods and Droughts

The number and intensity of floods and droughts in the United States, including associated wind-related events, have increased over the past forty years with concomitant damages increasing as well (U.S. Global Change Research Program, 2018a).Figure 6.1a shows the number of flood and drought-related events since 1980, with Figure 6.1b showing the estimated damages per year associated with the events. What is clear is that the upward trend for damages is beyond the expected year to year variation. What cannot be discerned from these data is the percentage of the damage cost that could have been avoided through better planning, a topic we discuss in Chapter 7.

6.1.1 Flood damages

Floods are part of the natural cycle of weather events and water flows. However, flood damage is a human concept. Early farmers recognized that annual spring

Figure 6.1 (a and b) Number of flood events and the associated damages.Source:

U.S. Global Change Research Program (2018a).

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floods brought nutrients to their cropland, increasing yields. Their cultures adapted to these floods, and generally viewed them as beneficial. However, flooding also killed livestock, ruined crops, damaged dwellings, and impeded river traffic. The extent of damage that is caused depends on what is flooded, flood level and duration, and what is in the water. Ironically, because floodplains are flat, are located conveniently for river transportation, and often have fertile soils, they are attractive sites for establishing towns, cites, and farms. As settlements expanded and more structures were constructed, more flood damage occurred.

One can classify the nature of flood damages by (1) damages associated with natural variation in water levels resulting from having placed crops, livestock, and structures in the wrong place and (2) damages to livelihoods and property resulting from increased water levels resulting from human alteration of natural water flow. The latter is of more interest to us here even though the former too often occurs as a result of human hubris, ignorance, or willingness to gamble with the probability of a flood event. In the former category, we put the unwise construction of homes in drained wetlands or floodplains unprotected or inadequately protected by levees. Levees provide protection in the event of a high water flow event provided that the water does not breach the levee or exceed its height. Levees constructed by other communities can cause water levels to rise for other communities located on the same river. These rising waters can overtop levees built to handle‘natural’flood levels.

As we have repeated throughout this text, water, or anything else for that matter, does not simply disappear. It goes somewhere and furthermore it goes downhill.

And the speed of water matters. Human actions change the speed and pathways of water movement. Altering drainage patterns with barriers, drainage systems, or reservoirs changes how fast water moves and how quickly water arrives in waterways. Changing vegetative covers alters infiltration and evapotranspiration levels which change the proportion of water that moves into the atmosphere versus surface flow. Human intervention includes vegetative change, irrigation and drainage systems, levees, and dams (Chapter 5).

Constructing levees to protect human activities from flooding alters the natural water flows. Conducting activities in floodplains and in areas susceptible to flood damage increases the potential for damage. [Less intensive use, such as leaving the land in parks, pastures, and forests mitigates this risk.] Any upstream activity or modification to the landscape that increases runoff can contribute to the risk of flood damage, with the possible exception of major rainfall events.

Storm surges associated with hurricanes can also cause flooding. Even though they are natural phenomena, their timing or magnitude cannot, at this time, be precisely anticipated. The severity of their impacts can be exacerbated by human actions that can increase their damage, such as degrading or removing coastal wetlands and barrier islands that otherwise would diminish the energy and volume of water associated with these events.

6.1.2 Drought impacts

In the United States the economic costs of droughts amount to some $9 billion per year, generally to agricultural interests (NOAA, 2020). The damages result from a variety of economic insults, including reduced water for drinking, irrigation, and industrial use. River base flow below the depth necessary for barges and other vessels impedes river transportation. Delays increase costs and can cause a diminution of the value of the goods transported. In watersheds where human interventions have reduced subsurface base flow, the likelihood for low flows sufficient to adversely impact human activity is increased. The impact of low flow can be particularly acute when the rights to withdraw water from the steam have been based on stream level from years with high flow.

Droughts may appear to be solely a consequence of climatic variation. However, their severity and frequency are affected by human actions. Land management affects the vulnerability of the soil and the vegetation to periods of low precipitation. Humans exacerbate the impacts of these droughts by how they manage the organic matter in the soil and the vegetation. Even in river systems where extreme deficits of precipitation have not occurred, humans can create situations of water scarcity, generally downstream of dams where water flows can be regulated, by diverting water for irrigation or other uses thereby depriving downstream users of the water. Redirecting water from rivers further serves to reduce base flow, depriving downstream habitats of water raising the potential for ecological damage.

Altered land use in the form of urbanization has increased impervious surface area, leading to more rapid runoff and reduced infiltration, which in turn reduces longer term water storage. Our need to grow crops for food has altered the land cover over vast expanses of arable land. In the United States of the approximately 802 million hectares (3.1 million square miles) of land area, approximately 28%

has been altered by humans for use as cultivated cropland, pastures (22%) or settlements (6%) (Sleeter et al., 2018). Changing the land cover by converting native grass or forest land to crop use, alters the flow of energy, water, and greenhouse gases from the land to the atmosphere. Agricultural lands tend to have more runoff and less soil water storage than land with native vegetation.

Also, agricultural uses often require drainage or irrigation systems, both of which decrease water storage. In droughts, the reduced water storage associated with urban and agricultural land increases the drought’s impacts.

The organic component of soils has declined precipitously since the mid-nineteenth-century with the expansion of agriculture. Degrading soils along with deforestation and conversion of grassland to cropland was a major source of carbon to the atmosphere (Lalet al., 1997). In the agricultural breadbasket of the United States, roughly half the organic matter in cropland soils has been lost, largely as a consequence of tillage practices that exposed the soil to oxidation (Natural Resource Conservation Service, 2013). The soil organic matter holds

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much of the water that sustains growing plants in periods of low precipitation.

Studies have shown with a 1% increase in soil organic matter, the available water holding capacity in the soil increased by 3.7% (Food and Agricultural Organization, 2003). The loss of one half of the 6% carbon from these soils initially contained results in a more than 10% loss of the capacity to store water.