The water cycle involves the continuous circulation of water through different parts of the Earth system. The NOAA National Weather Service's JetStream Max project offers descriptions of each of the ten processes illustrated in the image.
- In response to Earth’s warming oceans and atmosphere, precipitation patterns are changing. Across the nation, the amount of rain falling in the heaviest precipitation events is increasing, and climate models suggest this trend will continue. Models also suggest total precipitation will increase in northern states and decrease in the Southwest.
- Based on results from climate models, scientists project an increase in the frequency of flooding along inland waterways in many areas of the United States.
- Models also indicate an increase in the length of dry periods across most of the United States. As higher temperatures lead to greater evaporation and earlier snowmelt, the threat of seasonal drought will increase. In some regions, changing conditions and increased demand will challenge the reliability of municipal water supplies.
- Water, energy, and land systems interact in many ways. Climate variability and change impact these systems and their interactions. The links between these systems mean they are susceptible to cascading effects from one system to the next, which in turn impacts communities and businesses.
- Decreases in the amount of water available to the natural environment pose threats to the viability of land and aquatic ecosystems.
Changes in the water cycle
The water cycle, encompassing all processes that move water through different parts of the Earth system, is naturally dynamic over time. As climate warms, the rate of each process responds by increasing or decreasing across different regions and over varying time scales. A potential result of these changes is an increase in the frequency and severity of events when locations have too much or too little water at one time.
Water has obvious connections to other sectors of society. The results of too much water (flooding) or too little water (drought) can wreak havoc on homes, businesses, infrastructure, ecosystems, and economies. Both flooding and drought present challenges for human and natural systems across all sectors.
Precipitation and streamflow
Patterns of when, where, in what form, and how much precipitation falls are changing. Over the past century, the intensity and frequency of the heaviest rainfall events have increased significantly across most of the contiguous United States, and scientists expect this trend to continue in the future. As climate changes, scientists project that average annual precipitation will increase across the northern states, and decrease in the southern states, especially the Southwest.
Streamflow patterns are also changing. For instance, declines in spring snowpack and earlier snowmelt are changing the timing of peak streamflow in the West. Over the last century, peak flows have moved earlier in the year. This change has the potential to stress the water supply of communities whose storage facilities are optimized for the gradual melting observed in the past. As warming continues, the portion of precipitation that falls as rain rather than snow is likely to increase, leading to further changes in natural systems and socioeconomic sectors.
Evaporation and drought
Evaporation of water from soil, plants, lakes, rivers, and oceans (all together referred to as evapotranspiration) is a major component of the water cycle. Air temperature, solar radiation, wind, humidity, and water availability all influence the rate of evapotranspiration. In turn, the rate of evapotranspiration affects soil moisture, groundwater recharge, and surface water runoff. Soil moisture is an important factor because it moderates air temperature through evaporation. Climate projections indicate soil moisture will decline in the future, with pronounced changes expected in the Great Plains, Southwest, and Southeast.
As a result of increased potential for evaporation, climate scientists project that the duration of dry spells will increase. Scientists also see evidence that short-term (seasonal or shorter) droughts will become more intense in most regions, and longer-term droughts will intensify in large areas of the Southwest, southern Great Plains, and Southeast.
Municipal water supply
Changes in precipitation patterns may challenge the reliability of water supplies for homes and businesses in some areas of the United States. In order to deliver water through all seasons of the year, water utility companies already employ an array of storage and delivery strategies. Bounded by legal constraints and challenged by issues such as aging infrastructure or population growth, some water supply systems can become inflexible, and therefore vulnerable to a changing climate.
Water for agriculture
The continued availability of irrigation for crops and landscaping is also vulnerable to climate change, particularly in the Southwest where irrigation accounts for the highest volume of water used. Projected increases in temperature and potential evapotranspiration, accompanied by decreases in soil moisture, will challenge this already-dry area with increased demand for water. Demand for water will also grow with population, as people migrate to Sun Belt states for better weather.
Projections for a warmer climate point to a decrease in water quality. During floods, above-average streamflow increases the amount of sediments and pollutants in water. Under drought conditions, persistently low streamflow rates also reduce water quality. Increasing precipitation intensity, along with the effects of wildfires and fertilizer use, increase sediment, nutrient, and contaminant loads of surface waters used by downstream water users and ecosystems.
Along the coast, sea level rise, storm surges, and changes in the use of surface and groundwater can compromise the sustainability of freshwater aquifers and wetlands. As sea level rises, saltwater mixes with freshwater at the surface and underground. As humans withdraw more freshwater from rivers, salty water encroaches on land where freshwater once flowed to the sea. In many coastal cities, sewer systems and wastewater treatment facilities sit on low-lying land along waterways or at sea level, putting these facilities at risk from rising seas and storm surges.
Water and energy
In addition to water used by agriculture and municipal and natural systems, energy production also requires water. Water turns turbines at hydroelectric dams, produces steam in thermoelectric power plants, and cools energy-producing equipment by absorbing waste heat in nuclear plants.
Correspondingly, water production and distribution requires energy. Extracting water from rivers and aquifers, conveying it to storage facilities, treating it, distributing it, and collecting it as wastewater all take energy. Producing energy and water also requires land, which can reduce the extent of land available for agriculture and forestry. Competition for water, energy, and land will increase as climate changes. Increasing population and a growing economy can also intensify this competition.
Water for ecosystems
Humans depend on the natural environment to survive. Ecosystems—communities of living organisms and the environments they inhabit—provide a range of resources and services. Ecosystems on land provide benefits such as food, building materials, outdoor recreation, and wildlife habitat, as well as protection from events such as flooding, storm surge, and wildfire. Aquatic ecosystems—the natural environment surrounding streams, rivers, lakes, estuaries, oceans, and groundwater—also provide ecosystem services. They store water, regulate water quality, support fisheries, provide recreation, and carry water and materials downstream.
Too little water or too much water can both damage ecosystems by reducing their biodiversity and their capacity to provide services for humans. Receiving too little water is a significant threat for ecosystems: on average, humans use more than 40 percent of the renewable supplies of freshwater in more than a quarter of all U.S. watersheds. Freshwater withdrawals are even higher in the arid Southwest. This practice reduces the range of climate variability and change that affected ecosystems can tolerate, ultimately reducing the resilience of communities and businesses that depend on the services these ecosystems provide.
To learn more about impacts of climate change and variability on water resources, visit the subtopic pages:
Excerpted and adapted from the report Climate Change Impacts in the United States: The Third National Climate Assessment (Chapter 2: Our Changing Climate, Chapter 3: Water Resources, and Chapter 10: Energy, Water, and Land Use) and the Water Resources/Climate Impacts and Adaptation Examples webpages published by the U.S. Environmental Protection Agency.