8.5 Floods and Droughts
8 min read•august 14, 2024
Floods and droughts are extreme water events that shape our planet. These natural hazards can devastate communities, disrupt ecosystems, and cause widespread economic losses. Understanding their causes and impacts is crucial for effective management and adaptation.
is intensifying floods and droughts globally. Rising temperatures alter , leading to more frequent and severe events. Land use changes, like and , also play a role by modifying how water moves through landscapes.
Hydrologic Processes of Floods and Droughts
Water Cycle and Its Influence on Floods and Droughts
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- The water cycle, or hydrologic cycle, is the continuous movement of water on, above, and below the Earth's surface
- Involves processes such as evaporation, transpiration, condensation, precipitation, infiltration, and , which influence the occurrence of floods and droughts
- Evaporation from water bodies and land surfaces, along with transpiration from plants, transfers water to the atmosphere
- Condensation forms clouds and precipitation, which returns water to the Earth's surface
- Infiltration allows water to enter the soil and recharge groundwater, while runoff flows over the land surface and contributes to streamflow
Factors Affecting Flood and Drought Occurrence
- Floods occur when water levels exceed the capacity of natural or artificial channels, resulting in the inundation of normally dry areas
- Can happen due to excessive rainfall (monsoons), snowmelt (spring floods), or the failure of water control structures like dams or levees (Oroville Dam spillway failure, 2017)
- Droughts occur when there is a prolonged period of below-average precipitation, leading to water shortages and reduced streamflow, soil moisture, and groundwater levels
- Can be meteorological (low precipitation), agricultural (insufficient soil moisture), hydrological (reduced streamflow and groundwater), or socioeconomic (water supply-demand imbalance) in nature
- , such as the saturation level of soils and the water content of snowpack, can affect the likelihood and severity of floods and droughts
- Saturated soils have less capacity to absorb additional rainfall, increasing runoff and risk
- Low soil moisture and snowpack levels at the onset of a dry period can exacerbate conditions
- The timing, intensity, and duration of precipitation events, as well as the spatial distribution of rainfall, play a crucial role in determining the occurrence of floods and droughts
- High-intensity, short-duration rainfall events (thunderstorms) can cause flash floods, while persistent, moderate rainfall (monsoons) can lead to riverine flooding
- Uneven spatial distribution of rainfall can result in localized floods or droughts, even within the same region
Factors Influencing Flood and Drought Severity
Climate Change and Variability
- Climate change is altering precipitation patterns and increasing the frequency and intensity of extreme weather events, such as heavy rainfall and prolonged dry spells, which can exacerbate flood and drought risks
- Warmer temperatures lead to increased evaporation and atmospheric moisture-holding capacity, fueling more intense precipitation events (Hurricane Harvey, 2017)
- Shifting precipitation patterns can cause some regions to experience more frequent and severe droughts (California drought, 2011-2017)
- Natural climate variability, such as the -Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), can affect regional precipitation patterns and the likelihood of floods and droughts over multi-year time scales
- El Niño events often bring increased rainfall to the southern United States and Peru, while causing droughts in Australia and Indonesia
- The PDO can modulate the impacts of ENSO and influence long-term drought patterns in western North America
Land Use and Watershed Characteristics
- Land use changes, such as urbanization, deforestation, and agricultural practices, can modify the hydrologic response of a , affecting infiltration rates, runoff, and the capacity of the landscape to store and release water
- Urbanization increases impervious surfaces (roads, buildings), reducing infiltration and increasing runoff, which can exacerbate flooding (Houston floods, 2016)
- Deforestation reduces evapotranspiration and soil moisture retention, increasing runoff and flood risk while potentially intensifying droughts (Amazon rainforest)
- The characteristics of a watershed, including its size, shape, slope, and drainage density, influence the speed and volume of water flow and the potential for floods and droughts
- Smaller, steeper watersheds respond more quickly to rainfall and are more prone to flash floods
- Larger, flatter watersheds have slower water flow and more storage capacity, which can attenuate flood peaks but also prolong drought conditions
- The presence and condition of water management infrastructure, such as dams, levees, and irrigation systems, can influence the occurrence and severity of floods and droughts by regulating water storage and release
- Dams can provide flood control and water storage for drought mitigation, but their management can also contribute to downstream flooding or water shortages (Three Gorges Dam, China)
- Aging or poorly maintained infrastructure can increase the risk of failures and exacerbate flood and drought impacts (Oroville Dam spillway failure, 2017)
Impacts of Floods and Droughts
Economic and Infrastructure Consequences
- Floods can cause damage to infrastructure, including buildings, roads, bridges, and utilities, leading to economic losses and disruption of services
- Floodwaters can inundate homes and businesses, causing structural damage and loss of property (Mississippi River floods, 1993)
- Transportation networks can be impacted, with roads and bridges washed out or submerged, disrupting travel and commerce (Hurricane Katrina, 2005)
- Utilities such as power plants, water treatment facilities, and communication systems can be damaged or disrupted, leading to cascading impacts across sectors
- Droughts can reduce agricultural productivity, leading to crop failures, livestock losses, and food shortages, which can have severe economic and social consequences, particularly in regions that rely heavily on agriculture
- Reduced water availability can lead to decreased crop yields, increased irrigation costs, and loss of income for farmers (, 1930s)
- Livestock production can be impacted by lack of water and forage, leading to animal losses and increased feed costs (Australian Millennium Drought, 1997-2009)
- Food shortages and price increases can result from reduced agricultural output, disproportionately affecting low-income and food-insecure populations
Ecological and Public Health Impacts
- Both floods and droughts can have significant ecological impacts, altering habitat conditions, species distributions, and
- Floods can disrupt aquatic ecosystems, displacing fish and other organisms, and damaging riparian vegetation (Yangtze River floods, 2020)
- Droughts can stress plant communities, reduce biodiversity, and increase the risk of wildfires, which can have long-lasting impacts on ecosystems (Amazon rainforest fires, 2019)
- Floods and droughts can pose public health risks, such as the spread of waterborne diseases, contamination of water supplies, and the exacerbation of respiratory illnesses due to poor air quality during droughts
- Floodwaters can carry pathogens and contaminants, increasing the risk of diseases such as cholera, typhoid, and hepatitis A (Pakistan floods, 2010)
- Drought conditions can concentrate pollutants in water sources, leading to increased exposure to toxins and heavy metals (Flint water crisis, 2014-2019)
- Dry, dusty conditions during droughts can worsen air quality and exacerbate respiratory illnesses such as asthma and allergies (California drought, 2011-2017)
- The socioeconomic impacts of floods and droughts are often disproportionately felt by vulnerable populations, such as low-income communities, rural areas, and developing countries, which may have limited resources and capacity to cope with and recover from these events
- Poverty, lack of infrastructure, and limited access to resources can amplify the impacts of floods and droughts on these communities (East African droughts, 2011-2012)
- Recovery and rebuilding after floods and droughts can be slower and more challenging for disadvantaged populations, exacerbating existing inequalities (Hurricane Maria in Puerto Rico, 2017)
Managing Flood and Drought Risks
Integrated Water Resource Management and Infrastructure
- Implementing (IWRM) approaches that consider the interdependencies between water, land, and related resources can help balance competing demands and improve resilience to floods and droughts
- IWRM involves coordinated development and management of water, land, and associated resources to maximize economic and social welfare without compromising the sustainability of ecosystems
- Stakeholder engagement, cross-sectoral collaboration, and adaptive management are key components of IWRM (Murray-Darling Basin Plan, Australia)
- Investing in water infrastructure, such as flood control measures and drought mitigation strategies, can help reduce the impacts of floods and droughts
- Flood control measures include levees, retention basins, and floodways that can help contain and divert floodwaters away from vulnerable areas (Dutch Delta Works)
- Drought mitigation strategies involve water storage projects (reservoirs), efficient irrigation systems (drip irrigation), and water reuse and recycling (Singapore's NEWater)
- Improving water use efficiency through conservation measures, such as leak detection, water-saving appliances, and drought-tolerant landscaping, can help reduce water demand and increase resilience to droughts
- Leak detection and repair in water distribution systems can significantly reduce water losses (Tokyo's leak detection program)
- Promoting water-efficient appliances and fixtures (low-flow toilets, showerheads) can decrease residential and commercial water consumption
- Encouraging the use of drought-tolerant plants in landscaping can lower outdoor water use and maintain green spaces during dry periods (xeriscaping in the American Southwest)
Preparedness, Planning, and Adaptation
- Developing and implementing early warning systems and can help communities better prepare for and respond to flood and drought events, minimizing loss of life and property damage
- Flood warning systems that monitor rainfall, streamflow, and water levels can provide timely alerts to at-risk populations (Bangladesh Flood Forecasting and Warning Centre)
- Drought monitoring and early warning systems that track precipitation, soil moisture, and water supply can help trigger conservation measures and emergency response (U.S. Drought Monitor)
- Emergency response plans that outline evacuation procedures, sheltering, and resource allocation can improve coordination and reduce the impacts of floods and droughts (California's Standardized Emergency Management System)
- Incorporating climate change projections and scenarios into water resource planning and management can help anticipate and adapt to future flood and drought risks
- Using downscaled climate models and hydrologic simulations to assess the potential impacts of climate change on regional water resources (Colorado River Basin Supply and Demand Study)
- Developing flexible and adaptive water management strategies that can accommodate a range of future climate conditions and uncertainties (California Water Resilience Portfolio)
- Promoting sustainable land use practices, such as preserving wetlands, maintaining riparian buffers, and implementing soil conservation measures, can help mitigate the impacts of floods and droughts by improving water retention and reducing erosion
- Wetland restoration and conservation can provide natural flood storage and improve water quality (Everglades restoration, Florida)
- Riparian buffers along streams and rivers can slow runoff, trap sediments, and provide habitat for wildlife (Chesapeake Bay riparian buffer initiative)
- Soil conservation practices, such as contour farming, terracing, and cover cropping, can reduce erosion and improve soil moisture retention during droughts (Loess Plateau restoration, China)
- Engaging stakeholders, including government agencies, local communities, and the private sector, in collaborative decision-making and risk-sharing can help build social and institutional capacity to manage and adapt to flood and drought risks
- Participatory planning processes that involve diverse stakeholders can lead to more equitable and sustainable water management solutions (Mekong River Commission)
- Public-private partnerships can leverage resources and expertise to invest in water infrastructure and promote innovation in flood and drought management (Miami-Dade County's Water and Sewer Department)
- Community-based initiatives, such as local water user associations and watershed councils, can foster stewardship and build resilience to floods and droughts at the grassroots level (Colorado Watershed Assembly)
Key Terms to Review (31)
Agricultural drought: Agricultural drought refers to a specific type of drought that occurs when there is insufficient soil moisture to support the growth of crops, leading to reduced agricultural productivity. This form of drought is crucial as it directly impacts food production and can lead to food shortages and economic instability, highlighting the relationship between climate conditions and agricultural sustainability.
Antecedent moisture conditions: Antecedent moisture conditions refer to the pre-existing soil moisture levels and overall hydrologic state of a region before a precipitation event occurs. These conditions significantly influence how much rainfall will lead to runoff, flooding, or saturation of the soil. A region with high antecedent moisture conditions may experience increased flooding potential during subsequent rain events, while dry conditions can lead to drought and limited water availability.
Biodiversity loss: Biodiversity loss refers to the decline in the variety and variability of life on Earth, including the reduction of species populations, habitat destruction, and extinction of species. It can result from various environmental changes and human activities, leading to significant impacts on ecosystems and the services they provide. Addressing biodiversity loss is crucial for maintaining ecological balance and resilience against environmental changes.
Climate change: Climate change refers to significant and lasting changes in the average temperature, weather patterns, and other atmospheric conditions on Earth, largely driven by human activities such as burning fossil fuels and deforestation. This term encompasses both global warming, which is a rise in Earth's average surface temperature due to greenhouse gas emissions, and the broader changes in climate systems that result from these increases in temperature.
Deforestation: Deforestation is the process of clearing or removing forests or trees from land, often for agricultural, urban development, or logging purposes. This significant environmental issue leads to habitat loss, decreased biodiversity, and disruptions in ecological balance, affecting various aspects of the environment.
Drought: Drought is an extended period of deficient rainfall relative to the statistical multi-year average for a specific region, leading to water shortages and adverse effects on ecosystems, agriculture, and human health. It can occur naturally or be exacerbated by human activities, such as climate change and land use changes, impacting water availability and increasing vulnerability to other environmental challenges.
Drought index: A drought index is a numerical scale used to quantify the severity of drought conditions based on various environmental factors, such as precipitation, temperature, and soil moisture. This index helps in assessing how dry an area is compared to its normal conditions and can guide water management decisions, agricultural practices, and disaster preparedness efforts.
Drought relief: Drought relief refers to the various measures and strategies implemented to assist communities and ecosystems affected by prolonged periods of insufficient rainfall, leading to water shortages and agricultural challenges. It often includes government interventions, humanitarian aid, and sustainable practices aimed at restoring water supplies, supporting farmers, and addressing food insecurity. Effective drought relief seeks not only to alleviate immediate suffering but also to build resilience against future droughts.
Dust Bowl: The Dust Bowl refers to a severe environmental disaster that occurred during the 1930s in the Great Plains region of the United States, characterized by extreme drought and dust storms. This phenomenon not only devastated agricultural production but also caused significant economic hardship for farming communities, leading to mass migration and long-lasting impacts on land management practices.
Ecosystem functions: Ecosystem functions refer to the natural processes and interactions within an ecosystem that sustain and regulate the environment. These functions include nutrient cycling, energy flow, and the regulation of populations, contributing to the overall health and stability of ecosystems. Understanding these functions is crucial for managing ecosystems, especially during extreme weather events such as floods and droughts, which can disrupt these processes.
El Niño: El Niño is a climate pattern characterized by the periodic warming of sea surface temperatures in the central and eastern Pacific Ocean. This phenomenon has far-reaching impacts on weather and climate across the globe, influencing rainfall patterns, atmospheric circulation, and oceanic conditions.
Emergency response plans: Emergency response plans are structured guidelines and strategies designed to manage and mitigate the impacts of sudden and unforeseen disasters or emergencies, ensuring a coordinated and effective response. These plans outline roles, responsibilities, resources, and procedures to be followed during events like floods or droughts, aiming to protect lives, property, and the environment while facilitating recovery efforts. By anticipating potential hazards and organizing resources effectively, these plans play a crucial role in disaster preparedness and resilience.
Flash flood: A flash flood is a rapid and intense flooding event that occurs within six hours of heavy rainfall, typically in areas with steep terrain or poor drainage systems. This type of flood is characterized by a sudden surge of water that can overwhelm rivers, creeks, and other water bodies, leading to significant hazards for people and infrastructure. The quick onset of flash floods can make them particularly dangerous, as they often occur without warning and can lead to devastating impacts.
Flood: A flood is an overflow of water that submerges land that is normally dry, often caused by excessive rainfall, rapid snowmelt, or other factors that lead to a sudden rise in water levels. Floods can lead to significant damage to property, infrastructure, and the environment, making them a major concern in the study of hydrology and climate patterns.
Floodplain zoning: Floodplain zoning is a land-use planning technique that regulates development in areas prone to flooding, aiming to minimize the risks and damages associated with flood events. By designating specific zones based on flood risk, this approach helps protect both the environment and human life by restricting or guiding the types of activities and constructions that can occur in these vulnerable areas. Effective floodplain zoning incorporates scientific data about flood frequencies and potential impacts, making it a critical aspect of flood risk management.
Great Mississippi Flood: The Great Mississippi Flood of 1927 was one of the most destructive floods in U.S. history, primarily affecting the Mississippi River Basin and causing widespread devastation across several states. This catastrophic event resulted from excessive rainfall and poor management of river levees, leading to the inundation of thousands of homes and displacement of millions of people, showcasing the significant impacts of floods on communities and ecosystems.
Habitat destruction: Habitat destruction refers to the process by which natural habitats are altered or eliminated, leading to the loss of biodiversity and the degradation of ecosystems. This phenomenon can be caused by human activities such as mining, agriculture, urban development, and natural disasters, and it significantly impacts wildlife populations and the balance of ecosystems.
Hydrograph: A hydrograph is a graphical representation of the flow of water in a river or stream over time, showing how discharge changes during different conditions such as storms or droughts. It helps in understanding the relationship between rainfall, runoff, and river discharge, which is crucial for managing water resources and predicting floods and droughts.
Hydrological drought: Hydrological drought occurs when there is a significant decrease in water availability in lakes, rivers, and groundwater reservoirs over an extended period. This type of drought is often related to long-term deficiencies in precipitation, leading to lower streamflows and diminished water supplies that can impact ecosystems, agriculture, and human consumption.
Inefficient irrigation systems: Inefficient irrigation systems refer to methods of watering crops that result in significant water loss, either through evaporation, runoff, or deep percolation below the root zone. These systems often fail to deliver the optimal amount of water needed for crops, leading to reduced agricultural productivity and heightened vulnerability during periods of drought or floods.
Integrated water resource management: Integrated water resource management (IWRM) is a holistic approach that promotes the coordinated development and management of water, land, and related resources to maximize social and economic welfare without compromising the sustainability of vital ecosystems. This approach recognizes the interconnectedness of different water uses and stakeholders, emphasizing the need for comprehensive strategies to address challenges such as floods and droughts.
Meteorological drought: Meteorological drought refers to a prolonged period of abnormally low precipitation relative to the historical average for a specific region. This type of drought is primarily assessed by measuring rainfall deficits over time, leading to water shortages that can affect agriculture, water supply, and ecosystems. Meteorological drought can escalate into more severe types of droughts, impacting various sectors such as agriculture, water resources, and energy production.
Monsoon: A monsoon is a seasonal wind pattern characterized by a significant shift in wind direction, bringing heavy rainfall during certain months, primarily in tropical and subtropical regions. These winds play a critical role in influencing weather patterns, agricultural practices, and water supply, particularly in areas where the intensity of rain can lead to floods or droughts.
Precipitation patterns: Precipitation patterns refer to the distribution and variability of rainfall, snow, sleet, and other forms of moisture that fall from the atmosphere over a specific area and time period. These patterns are crucial for understanding water availability, ecosystem health, and climate dynamics, as they influence not only the occurrence of floods and droughts but also agricultural productivity and water resource management.
Runoff: Runoff refers to the movement of water, usually from precipitation, over the land surface towards rivers, lakes, and oceans. This process plays a crucial role in the hydrological cycle, influencing water availability and quality, while also contributing to various environmental phenomena, including floods and droughts. Runoff can transport sediments, nutrients, and pollutants, impacting ecosystems and biogeochemical cycles.
Severe drought: Severe drought is an extended period of significantly below-average precipitation, resulting in water shortages that affect agriculture, ecosystems, and human populations. This phenomenon can lead to critical challenges, including crop failure, water scarcity, and increased vulnerability to wildfires. The impacts of severe drought are often exacerbated by climate change and can have long-lasting effects on the environment and economy.
Socioeconomic drought: Socioeconomic drought refers to the situation when the demand for a particular resource, especially water, exceeds its supply due to prolonged dry conditions, leading to significant economic and social impacts. This concept highlights the relationship between meteorological droughts and the effects they have on human communities, agriculture, and the economy, stressing that it's not just about the lack of water but also about how that lack affects people and society.
Soil erosion: Soil erosion is the process by which the top layer of soil is worn away and transported from one location to another, often due to water, wind, or human activity. This phenomenon can lead to significant land degradation, loss of fertile soil, and disruption of ecosystems. Factors such as heavy rainfall, drought, and poor land management practices can exacerbate soil erosion, affecting agricultural productivity and environmental health.
Urbanization: Urbanization is the process by which an increasing percentage of a population comes to live in urban areas, resulting in the growth of cities and towns. This phenomenon often leads to changes in land use, economic activities, and social dynamics as people migrate from rural regions to urban centers in search of better opportunities. The effects of urbanization can have significant implications on environmental factors such as water management during floods and droughts, the health of ecosystems, and public health challenges due to increased pollution and density.
Water scarcity: Water scarcity refers to the lack of sufficient available water resources to meet the demands of water usage within a region. This situation can arise due to a combination of factors, including physical water shortages, increased demand from population growth, and poor management of existing water supplies. Water scarcity can have serious implications for agriculture, health, and economic development, especially in areas experiencing extreme weather events like floods and droughts.
Watershed: A watershed is an area of land where all the water that falls within it drains into a common outlet, such as a river, lake, or ocean. This region plays a crucial role in the water cycle by collecting precipitation and channeling it into water bodies, thereby influencing water quality and availability in surrounding areas. Understanding watersheds is essential for managing water resources, predicting floods and droughts, and preserving ecosystems.