🌈Earth Systems Science Unit 6 – Hydrologic Cycle & Freshwater Systems

Key Concepts and Definitions

• Hydrologic cycle the continuous movement of water on, above, and below the surface of the Earth
• Evaporation the process by which water changes from a liquid to a gas or vapor
• Transpiration the process by which water is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere
  • Accounts for about 10 percent of the moisture in the atmosphere
• Condensation the process by which water vapor in the air is changed into liquid water
  • Occurs when the air becomes saturated and cannot hold any more water vapor (reaches dew point temperature)
• Precipitation water released from clouds in the form of rain, freezing rain, sleet, snow, or hail
• Infiltration the movement of water from the ground surface into the soil
• Percolation the movement of water past the soil sub-surface into the groundwater system
• Runoff the variety of ways by which water moves across the land, including both surface runoff and channel runoff

Components of the Hydrologic Cycle

• Atmosphere where water is stored as water vapor and transported by wind currents
  • Contains about 0.001 percent of total water on Earth
• Biosphere includes water that is stored in and used by plants and animals
• Lithosphere the solid portion of the Earth, including land surface, rocks, and soils
  • Stores groundwater in aquifers and other underground reservoirs
• Hydrosphere the combined mass of water found on, under, and above the surface of the Earth
  • Includes oceans, lakes, rivers, glaciers, and groundwater
• Cryosphere portion of Earth's surface where water is in solid form (ice and snow)
  • Found in polar regions, mountain glaciers, and sea ice
• Oceans the largest reservoir of water on Earth, containing about 97 percent of total water
• Groundwater water stored beneath the Earth's surface in soil pore spaces and rock fractures
  • Accounts for about 30 percent of freshwater on Earth

Water Distribution on Earth

• Oceans contain approximately 97% of Earth's water (saltwater)
• Glaciers and ice caps hold about 2% of Earth's water in the form of ice (freshwater)
• Groundwater accounts for roughly 0.6% of Earth's water (freshwater)
  • Majority of liquid freshwater on Earth is stored as groundwater
• Surface water (lakes, rivers, wetlands) represents about 0.02% of Earth's water (freshwater)
• Atmosphere holds approximately 0.001% of Earth's water as water vapor
• Biosphere (water in living organisms) contains about 0.00004% of Earth's water
• Soil moisture accounts for around 0.005% of Earth's water
• Distribution of water on Earth is not uniform and varies greatly by region

Freshwater Systems and Their Importance

• Freshwater systems include rivers, lakes, wetlands, and groundwater
• Provide essential ecosystem services (water purification, flood control, habitat for biodiversity)
• Support human activities such as agriculture, industry, and domestic use
• Rivers transport water, nutrients, and sediment across landscapes
  • Play a crucial role in shaping Earth's surface through erosion and deposition
• Lakes store freshwater and regulate local climate
  • Provide recreational opportunities and support aquatic ecosystems
• Wetlands act as natural filters, removing pollutants and excess nutrients from water
  • Serve as critical habitat for many plant and animal species
• Groundwater is a vital source of drinking water for many populations
  • Helps maintain streamflow and support ecosystems during dry periods

Processes Driving the Water Cycle

• Solar energy drives evaporation and transpiration, converting liquid water into water vapor
• Atmospheric circulation transports water vapor around the globe
• Cooling of air leads to condensation, forming clouds and precipitation
• Gravity drives the movement of water from higher to lower elevations (runoff, infiltration, groundwater flow)
• Infiltration and percolation move water from the surface into the soil and groundwater
• Groundwater flow transfers water through aquifers and contributes to streamflow (baseflow)
• Evapotranspiration (evaporation + transpiration) returns water from the land surface and vegetation to the atmosphere
• Water storage in various reservoirs (oceans, glaciers, lakes, groundwater) influences the timing and magnitude of water cycle processes

Human Impacts on Freshwater Resources

• Overexploitation of groundwater leads to aquifer depletion and subsidence
• Dam construction alters river flow, sediment transport, and aquatic habitats
• Urbanization increases surface runoff and reduces infiltration, leading to increased flood risk
• Agricultural practices (irrigation, fertilizer use) can lead to water depletion and pollution
  • Eutrophication occurs when excess nutrients stimulate algal blooms, depleting oxygen in water bodies
• Deforestation reduces evapotranspiration and alters local water balances
• Water pollution from industrial, agricultural, and domestic sources degrades water quality
• Invasive species can disrupt aquatic ecosystems and alter water cycle processes
• Climate change is affecting precipitation patterns, glacial melt, and sea level rise, impacting freshwater availability

Climate Change and the Hydrologic Cycle

• Rising temperatures lead to increased evaporation and atmospheric water vapor content
  • Clausius-Clapeyron relation: a 1°C increase in temperature leads to a ~7% increase in atmospheric water vapor capacity
• Changes in precipitation patterns, with some regions experiencing more frequent and intense droughts or floods
• Accelerated melting of glaciers and ice caps, contributing to sea level rise and altering seasonal water availability
• Shifts in the timing and magnitude of snowmelt runoff, affecting water supply and ecosystem dynamics
• Increased frequency and intensity of extreme weather events (hurricanes, heavy rainfall)
• Warming water temperatures in lakes and rivers, impacting aquatic ecosystems and water quality
• Saltwater intrusion into coastal aquifers due to sea level rise and groundwater overexploitation
• Feedback loops between the water cycle and other Earth system components (carbon cycle, land surface processes)

Water Management and Conservation Strategies

• Integrated water resources management (IWRM) a holistic approach considering social, economic, and environmental factors
• Water efficiency measures (low-flow appliances, leak detection, industrial recycling)
• Sustainable agricultural practices (drip irrigation, conservation tillage, crop selection)
• Protection and restoration of wetlands, floodplains, and riparian zones
• Managed aquifer recharge (MAR) intentionally recharging aquifers with surface water or treated wastewater
• Water pricing and market-based instruments to incentivize conservation
• Rainwater harvesting and stormwater management to reduce runoff and increase local water availability
• Desalination of seawater or brackish groundwater to increase freshwater supply in water-scarce regions
  • Requires significant energy input and can have environmental impacts (brine disposal)
• Public education and awareness campaigns to promote water conservation and stewardship