2.5 The Water Cycle and Earth's Water Resources

The water cycle is Earth's ultimate recycling system, constantly moving water between the atmosphere, land, and oceans. It's driven by solar energy and gravity, shaping our planet's climate and ecosystems. Understanding this cycle is crucial for managing our precious water resources.

Human activities are significantly altering the water cycle through land use changes, pollution, and climate change. These impacts affect water availability, quality, and distribution, highlighting the need for sustainable water management practices to protect this vital resource.

The Water Cycle

Major Components

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• The water cycle, also known as the hydrologic cycle, is the continuous movement of water on, above, and below the surface of the Earth
• Major components of the water cycle include evaporation, transpiration, condensation, precipitation, infiltration, surface runoff, and groundwater flow
• Evaporation is the process by which water changes from a liquid to a gas, primarily from the surface of oceans, lakes, and rivers (oceans account for approximately 90% of global evaporation)
• Transpiration is the process by which water vapor is released into the atmosphere through the leaves of plants (accounts for about 10% of global water vapor)
• Condensation occurs when water vapor cools and changes back into liquid water, forming clouds and fog (typically happens as air rises and cools in the atmosphere)
• Precipitation is the process by which water falls from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail (global average annual precipitation is about 990 mm)
• Infiltration is the process by which water on the Earth's surface enters the soil and becomes groundwater (rate depends on factors like soil type and moisture content)
• Surface runoff is the flow of water over the Earth's surface, which can lead to the formation of streams, rivers, and lakes (occurs when precipitation exceeds infiltration capacity)
• Groundwater flow is the movement of water through the soil and rock layers beneath the Earth's surface (can eventually resurface through springs or seeps)

Processes of Water Movement

Driving Forces

• The sun's energy drives the water cycle by causing evaporation and transpiration, which move water from the Earth's surface into the atmosphere (provides energy for phase changes)
• Gravity plays a crucial role in the water cycle by causing precipitation to fall from the atmosphere and surface runoff to flow downhill (moves water from higher to lower elevations)
• Atmospheric circulation patterns, such as the Hadley Cell and the Ferrel Cell, influence the distribution of moisture and precipitation across the Earth's surface (create global wind patterns)
• Temperature differences between the equator and the poles create convection currents that drive atmospheric circulation and the movement of water vapor (warm air rises at equator, cool air sinks at poles)

Influencing Factors

• The Coriolis effect, caused by the Earth's rotation, deflects atmospheric circulation patterns and influences the direction of prevailing winds, which transport moisture (deflects to the right in N. hemisphere, left in S. hemisphere)
• Topography, such as mountain ranges, can force air to rise, cool, and condense, leading to increased precipitation on the windward side and rain shadows on the leeward side (orographic lifting)
• Land-sea temperature contrasts can create local wind patterns (sea breezes and land breezes) that influence the movement of moisture and the formation of coastal precipitation
• Vegetation cover can affect the water cycle by influencing transpiration rates, intercepting precipitation, and altering surface runoff and infiltration patterns (forests tend to increase infiltration and reduce runoff)

Earth's Water Reservoirs

Major Reservoirs

• Oceans are the largest reservoir of Earth's water, containing approximately 97% of the planet's water supply (total volume of about 1.34 billion cubic kilometers)
• Glaciers and ice caps store nearly 2% of Earth's water in the form of ice, primarily in the polar regions and high-altitude mountain ranges (e.g., Antarctica and Greenland)
• Groundwater, which is water stored in the pores and cracks of soil and rock beneath the Earth's surface, accounts for about 0.6% of the planet's water (30 times more than surface water)
• Surface water, including lakes, rivers, and streams, contains approximately 0.02% of Earth's water (majority found in lakes)

Minor Reservoirs

• The atmosphere holds about 0.001% of Earth's water in the form of water vapor, clouds, and precipitation (amount varies with temperature and humidity)
• Biosphere, which includes water stored in living organisms and their tissues, contains a small fraction of Earth's total water supply (mostly in plants)
• Soil moisture, which is water held in the spaces between soil particles, accounts for a tiny portion of Earth's water but is crucial for plant growth and ecosystem functioning
• Human-made reservoirs, such as dams and artificial lakes, store a relatively small amount of water but can significantly impact local water cycles and ecosystems

Human Impacts on the Water Cycle

Land Use Changes

• Agriculture, particularly irrigation practices, can lead to the depletion of groundwater reserves and alter the natural flow of surface water (e.g., diversion of rivers for irrigation)
• Urbanization and the construction of impervious surfaces, such as roads and buildings, can increase surface runoff and reduce infiltration, leading to increased flooding and decreased groundwater recharge (urban areas have 2-6 times more runoff than natural areas)
• Deforestation can disrupt the water cycle by reducing transpiration, altering precipitation patterns, and increasing soil erosion and surface runoff (can lead to reduced rainfall and increased desertification)

Climate Change and Pollution

• The burning of fossil fuels and the emission of greenhouse gases can lead to climate change, which can affect the water cycle by altering precipitation patterns, increasing the frequency and intensity of droughts and floods, and causing sea-level rise (global average sea level has risen about 20 cm since 1900)
• Water pollution, caused by industrial waste, agricultural runoff, and sewage disposal, can degrade the quality of surface water and groundwater resources, making them unsafe for human consumption and harmful to aquatic ecosystems (e.g., eutrophication and algal blooms)
• Acid rain, caused by the emission of sulfur dioxide and nitrogen oxides from fossil fuel combustion, can alter the pH of surface waters and soils, affecting the water cycle and ecosystem health
• Thermal pollution, caused by the discharge of warm water from power plants and industrial facilities, can alter the temperature and oxygen levels of aquatic ecosystems, disrupting the water cycle and biodiversity

Water Resource Management

• The construction of dams and reservoirs can alter the natural flow of rivers, disrupt aquatic habitats, and affect the water cycle by increasing evaporation and changing sedimentation patterns (there are over 57,000 large dams worldwide)
• Overexploitation of water resources, such as excessive groundwater pumping, can lead to the depletion of aquifers, subsidence of land, and the intrusion of saltwater into freshwater reserves (e.g., the Ogallala Aquifer in the US has declined by over 300 feet in some areas)
• Water diversion projects, such as the transfer of water from one river basin to another, can alter regional water cycles and affect the availability of water resources for ecosystems and human use (e.g., the California State Water Project)
• Water conservation practices, such as drip irrigation, low-flow appliances, and wastewater recycling, can help reduce human impacts on the water cycle and promote sustainable water resource management