9.4: Future Geographies - Water Resources and Climate Change

Surface water and Runoff

Changes to the patterns of current precipitation and those projected for the future on account of climate change are discussed in earlier chapters. The kind of precipitation is already changing, as northern regions are experiencing more rain than snow. Total snow cover has on average decreased in both hemispheres.The variability of precipitation is changing as well. Many subtropical regions have been or are gripped in devastating droughts. Precipitation is but one element of the hydrologic cycle, that will be affected by climate. The impact of climate change on water resources will have profound effects on ecosystems and humans.

The impact of climate change on river flows and lake levels resulting depend on changes in the amount, type, and timing of precipitation. Changes in evaporation rates also affect river and lake levels. Global scale climate and hydrological models like the one shown in Figure \(\PageIndex{1}\) project runoff increases in the wet tropics and high latitudes, and decreases in the mid-latitudes and some parts of the dry tropics. Most studies show that rising temperatures lead to changes in the seasonality of river flow due to changes in the type of precipitation. Higher winter flows occurring in the European Alps, Scandinavia, the Himalayas, western, central, and eastern North America will result from more precipitation falling as rain and earlier snow melt. The frequency of heavy precipitation events are expected to increase in an ever warming world for most regions. This will likely increase the threat for flash flooding and urban flooding.

Figure \(\PageIndex{1}\): Runoff model projections of large-scale relative changes in annual runoff for the period 2090-2099 relative to 1980-1999. (CC BY-SA 4.0; Courtesy Michael Ritter in The Physical Environment).

 Figure \(\PageIndex{1}\) is a global map showing percentage changes in water availability or hydrologic conditions across different regions of the world. The map is divided into small colored grid cells, with colors representing increases or decreases according to the legend at the bottom.

The color scale ranges from −40% to +40%:

• Dark red and orange indicate large decreases.
• Yellow and light colors indicate smaller decreases.
• Light blue to dark blue/purple indicate increases.
• White or very light areas indicate little change.

Global patterns - Regions showing decreases (red, orange, and yellow):

• Much of the southwestern United States and northern Mexico shows decreasing values.
• The Mediterranean region, including southern Europe and North Africa, contains extensive areas of decline.
• Large portions of the Middle East show substantial decreases.
• Parts of southern Africa display strong reductions.
• Areas of Australia also show decreased values.
• Portions of western South America and other subtropical regions exhibit declines.

Regions showing increases (blue and purple):

• Much of Canada and northern North America displays increasing values.
• Large portions of northern Europe and northern Asia, including Siberia, show increases.
• Parts of eastern North America exhibit increases.
• Several tropical and high-latitude regions display moderate to strong increases.

Spatial pattern - The map reveals a broad contrast between:

• Many subtropical and arid regions, which show declines.
• Many higher-latitude regions, which show increases.

This creates a general pattern in which northern, cooler regions experience greater water availability, while many already-dry regions become drier.

The map illustrates projected or observed changes in water availability across the globe. It highlights regional differences in the hydrologic cycle, showing where water resources may increase and where shortages may become more severe.

The figure demonstrates that changes are not distributed uniformly. Some regions are projected to experience increased precipitation, runoff, or water availability, while others may face increased drought stress and reduced water resources. This type of map is commonly used to examine the impacts of climate variability and climate change on global water supplies.

Ground water

Ground water levels are maintained when aquifers are adequately recharged. How this occurs varies with soil conditions, vegetation and precipitation intensity and timing. Generally speaking, if precipitation intensity is greater than the infiltration capacity of the soil, most water will runoff and not percolate down into the ground water zone. However, more frequent and larger precipitation events projected for some humid region may result in decreased groundwater recharge as the infiltration capacity of the soil is exceeded more often. In semi-arid and arid regions, groundwater level may rise with high-intensity storms as rainfall is ability to infiltrate downward before evaporating.

Soil moisture

The amount of moisture held in the soil depends on when and how much precipitation and evaporation occurs at a place. The resulting changes to the spatial patterns of soil moisture is expected to be quite similar to that of changes in precipitation. Though the amount of change is not well established, projections generally agree on the kind of change that will occur. A reduction in soil moisture in the subtropics and Mediterranean regions will accompany a decrease in precipitation. As shown in Figure \(\PageIndex{2}\) projected decreases in snow cover at high latitudes will also result in less soil moisture. Soil moisture will likely increase in East Africa and central Asia where increases precipitation is expected to occur.

Figure \(\PageIndex{2}\): Fifteen-model mean changes in soil moisture. Stippled areas are those where 80% of models agree on sign of change. (CC BY-SA 4.0; Courtesy Michael Ritter in The Physical Environment).

 Figure \(\PageIndex{2}\) is a world map showing projected percentage changes in water availability, with colors representing decreases or increases relative to current conditions.

A color scale at the bottom ranges from −25% to +25%:

• Dark red to orange represent decreases in water availability.
• Yellow indicates little change.
• Light green to dark blue represent increases in water availability.

Areas projected to become drier - These regions generally show decreases ranging from about 5% to more than 25%.  Large regions shown in red and orange indicate declining water availability, including:

• The southwestern United States and parts of Mexico.
• Much of the Mediterranean region, including southern Europe.
• Parts of North Africa and the Middle East.
• Portions of southern Africa.
• Large areas of Australia.
• Some parts of western South America.

Areas projected to become wetter - In some locations, increases exceed 20–25%. Regions shown in green, blue, and dark blue indicate increasing water availability, including:

• Much of Canada and northern North America.
• Parts of the northeastern United States.
• Large areas of northern Europe and Russia.
• Portions of central and eastern Africa.
• Areas of South Asia and parts of Southeast Asia.

Stippled regions - Many areas contain a pattern of black dots (stippling), indicating locations where projections are more consistent or where there is stronger agreement among climate-model results.

The figure illustrates how changes in climate may alter the global distribution of water resources. It highlights regions that may experience greater water availability and regions that may face increasing water stress. The map emphasizes that changes in precipitation, evaporation, and runoff are expected to vary geographically, potentially affecting agriculture, ecosystems, water supplies, and drought risk around the world.

Water supply

Population growth, economic development and land use change largely determine the stress put on water supplies. Climate change will be an additional factor that affects stressed water resources. Glacier meltwater provides a source of water for many living in mountainous underdeveloped countries. Recent loss of glacier ice has or will place these people at risk. Peruvian glaciers have lost more than 20% of their mass in the past 35 years, resulting in a 12 percent decrease in runoff to the country’s coastal region, where 60 percent of Peru’s population live.

Agriculture, industry, and urban populations across the earth depend on major river systems to transpiration, irrigation, and drinking water. Millions of acres of farmland and tens of million people rely on the Colorado River system for water. Impoundment of water behind large dams regulate the flow and delivery of water supplies. The timing of when regulated flows deliver water is based on the experience of the past 100 years. Scheduled delivery of water may be missed 60 to 90 percent of the time as human-induced climate change continues to make the Colorado River basin drier.