Climate change effects on hydrology, floods, and drought for the United States are discussed in the Climate Science Special Report35,36 and the Third National Climate Assessment.6 Increasing air temperatures have substantially reduced the fraction of winter precipitation falling as snow, particularly over the western United States.37,38,39,40,41,42 Warming has resulted in a shift in the timing of snowmelt runoff to earlier in the year.39,43,44,45,46,47 Glaciers continue to melt in Alaska25,48 and the western United States (Ch. 1: Overview, Figure 1.2d).49,50 Shifts in the hydrological regime due to glacier melting will alter stream water volume, water temperature, runoff timing, and aquatic ecosystems in these regions. As temperatures continue to rise, there is a risk of decreased and highly variable water supplies for human use and ecosystem maintenance.32,51
Additionally, heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 and are projected to continue to increase over this century under both a lower and higher scenario (RCP4.5 and RCP8.5; see Easterling et al. 2017, Key Finding 235). There are, however, important regional and seasonal differences in projected changes in total precipitation.
Higher temperatures also result in increased human use of water, particularly through increased water demand for agriculture arising from increased evapotranspiration (Ch. 10: Ag & Rural, KM 1).52,53 In some regions of the United States, water supplies are already stressed by increasing consumption.12 Continued warming will add to the stress on water supplies and adversely impact water supply reliability in parts of the United States. Over the last 30 years, improvements in water-use efficiency have offset the increasing water needs from population growth, and national water use has remained constant.12 However, without efforts to increase water-use efficiency in rural and urban areas, increased future demand due to warming could exceed future supply in some locations.13
In the United States, groundwater provides more than 40% of the water used for agriculture (irrigation and livestock) and domestic water supplies (Ch. 25: Southwest; Ch. 10: Ag & Rural, KM 1).1,12 Groundwater use for irrigation has increased substantially since about 1900 and in some areas has exceeded natural aquifer recharge rates.54 For example, in the High Plains Aquifer, the largest freshwater aquifer in the contiguous United States that supports an important agricultural region,55 the rate of groundwater withdrawal for irrigation is nearly 10 times the rate of natural recharge, resulting in large groundwater depletions (see Figure 3.2).56,57,58,59 Groundwater pumping for irrigation is a substantial driver of long-term trends in groundwater levels in the central United States.60,61 In many parts of the United States, groundwater is being depleted due to increased pumping during droughts and concentrated demands in urban areas.1 Increasing air temperatures, insufficient precipitation, and associated increases in irrigation requirements will likely result in greater groundwater depletion in the coming decades.62 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.
Changes in climate and hydrology have direct and cascading effects on water quality.63,64 Anticipated effects include warming water temperatures in all U.S. regions, which affect ecosystem health (Ch. 7: Ecosystems), and locally variable changes in precipitation and runoff, which affect pollutant transport into and within water bodies.6,65 These changes pose challenges related to the cost and implications of water treatment, and they present a risk to water supplies, public health, and aquatic ecosystems. Increases in high flow events can increase the delivery of sediment,66,67,68 nutrients,69,70,71,72 and microbial pathogens23,73 to streams, lakes, and estuaries; decreases in low flow volume (such as in the summer) and during periods of drought can impact aquatic life through exposure to high water temperatures and reduced dissolved oxygen.74,75,76 The risk of harmful algal blooms could increase due to an expanded seasonal window of warm water temperatures and the potential for episodic increases in nutrient loading.23,24,77 In coastal areas, saltwater intrusion into coastal rivers and aquifers can be exacerbated by sea level rise (or relative sea level rise related to vertical land movement) (Ch. 1: Overview, Figure 1.4), storm surges, and altered freshwater runoff. Saltwater intrusion could threaten drinking water supplies, infrastructure,78 and coastal and estuarine ecosystems (Ch. 8: Coastal).79,80 Indirect impacts on water quality are also possible in response to an increased frequency of forest pest/disease outbreaks, wildfire, and other terrestrial ecosystem changes; land-use changes (for example, agricultural and urban) and water management infrastructure also interact with climate change to impact water quality.