Significant changes in water quantity and quality are evident across the country. These changes, which are expected to persist, present an ongoing risk to coupled human and natural systems and related ecosystem services. Extreme precipitation events are projected to increase in a warming climate and may lead to more severe floods and greater risk of infrastructure failure in some regions.

Chapter 3 of the Fourth National Climate Assessment

 

 

(a) Groundwater aquifers found throughout the U.S. The colors on this map illustrate aquifer location and geology: blue colors indicate unconsolidated sand and gravel; yellow is semi-consolidated sand; green is sandstone; blue or purple is sandstone and carbonate‐rock; browns are carbonate-rock; red is igneous and metamorphic rock; and white is other aquifer types. {Figure source: USGS available through USGCRP, provided by Aris Georgakakos, Georgia Institute of Technology}. (b) Ratio of groundwater withdrawals to total water withdrawals from all surface and groundwater sources by county. {Data from USGS 2005}.

Water security in the United States is increasingly in jeopardy. Ensuring a reliable supply of clean freshwater to communities, agriculture, and ecosystems, together with effective management of floods and droughts, is the foundation of human and ecological health. The water sector is also central to the economy, contributing significantly to the resilience of many other sectors, including agriculture, energy, urban environments, and industry. The health and productivity of natural aquatic and wetland ecosystems are also closely linked to the water sector (Source: Chapter 3 of the Fourth National Climate Assessment).

In the United States, groundwater provides more than 40% of the water used for agriculture (irrigation and livestock) and domestic water supplies.  Groundwater use for irrigation has increased substantially since about 1900 and in some areas has exceeded natural aquifer recharge rates.For example, in the High Plains Aquifer, the largest freshwater aquifer in the contiguous United States that supports an important agricultural region, the rate of groundwater withdrawal for irrigation is nearly 10 times the rate of natural recharge, resulting in large groundwater depletions (see Figure to the right). Groundwater pumping for irrigation is a substantial driver of long-term trends in groundwater levels in the central United States. In many parts of the United States, groundwater is being depleted due to increased pumping during droughts and concentrated demands in urban areas. Increasing air temperatures, insufficient precipitation, and associated increases in irrigation requirements will likely result in greater groundwater depletion in the coming decades. The lack of coordinated management of surface water and groundwater storage limits the Nation’s ability to address climate variability. Management of surface water and groundwater storage and water quality are not coordinated across different agencies, leading to inefficient response to changing climate (Source: Chapter 3 of the Fourth National Climate Assessment).

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. 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 (Source: U.S. Climate Resilience Toolkit).

Guiding Concepts
  1. How are human and natural components of the hydrologic cycle changing?
  2. How can communities and water managers plan for uncertain future conditions?
  3. How will changing water resources affect food, energy, ecosystems, and human health?

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Updated on November 5, 2019