2.3 Hydrologic cycle and water resources

Water is life's most essential resource, constantly moving through Earth's systems. The hydrologic cycle describes this endless journey, from oceans to clouds, rain to rivers, and back again. It's a complex dance of evaporation, precipitation, and flow that shapes our planet's landscapes and climate.

Understanding the hydrologic cycle is key to managing our freshwater resources. As human activities increasingly impact water availability and quality, we face growing challenges in ensuring sustainable access to clean water for all. Climate change adds another layer of complexity to this vital issue.

The Hydrologic Cycle

Components and Processes

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• Hydrologic cycle represents continuous water movement on, above, and below Earth's surface
• Solar energy drives the cycle causing evaporation from water bodies and transpiration from plants
• Key components include:
  • Evaporation: water changes from liquid to gas
  • Transpiration: water vapor released from plants
  • Condensation: water vapor cools and forms clouds
  • Precipitation: water falls as rain, snow, sleet, or hail
  • Infiltration: water soaks into the ground
  • Runoff: water flows over land surface
  • Groundwater flow: water moves through underground aquifers
• Atmospheric processes facilitate water vapor transport and condensation
  • Air circulation patterns (trade winds, jet streams)
  • Temperature changes (adiabatic cooling)
• Surface water processes involve movement in rivers, lakes, and oceans
  • Overland flow during precipitation events
  • River discharge into oceans
• Subsurface processes include:
  • Infiltration: water enters soil from surface
  • Percolation: water moves deeper into ground
  • Groundwater movement through aquifers and geological formations

Spatial and Temporal Scales

• Hydrologic cycle operates across various scales
• Spatial scales range from local to global
  • Local: small watershed drainage (creek, pond)
  • Regional: river basin or aquifer system
  • Global: ocean currents, atmospheric circulation
• Temporal scales span minutes to millennia
  • Short-term: individual storm events (minutes to hours)
  • Seasonal: monsoon cycles, snowmelt patterns
  • Long-term: glacial cycles, climate change impacts

Water Distribution and Movement

Atmospheric Water

• Water vapor primary form of atmospheric water
• Distribution influenced by:
  • Temperature (warmer air holds more moisture)
  • Pressure (lower pressure allows more evaporation)
  • Air circulation patterns (trade winds, jet streams)
• Atmosphere contains ~0.001% of Earth's total water
• Concentrations vary across regions and altitudes
  • Higher near equator and coastal areas
  • Lower in polar regions and high altitudes

Surface Water

• Comprises ~0.02% of Earth's total water
• Found in various forms:
  • Rivers: flowing freshwater systems
  • Lakes: standing freshwater bodies
  • Wetlands: areas saturated with water (marshes, swamps)
  • Artificial reservoirs: man-made water storage
• Oceans contain ~97% of Earth's water
  • Crucial for global water circulation (thermohaline circulation)
  • Regulate global climate (heat absorption and distribution)

Subsurface Water

• Includes soil moisture and groundwater
• Accounts for ~0.61% of Earth's total water
• Groundwater movement governed by:
  • Hydraulic gradients: difference in water pressure
  • Porosity: amount of void space in rock or soil
  • Permeability: ability of material to transmit water
• Water moves between reservoirs through various processes:
  • Evaporation: surface water to atmosphere
  • Precipitation: atmosphere to surface
  • Infiltration: surface water to groundwater
  • Groundwater discharge: subsurface to surface (springs, seeps)

Water Availability and Quality

Climate Factors

• Precipitation patterns impact water availability
  • Amount, intensity, and frequency of rainfall
  • Seasonal variations (monsoons, dry seasons)
• Temperature affects water availability and quality
  • Higher temperatures increase evaporation rates
  • Warmer water holds less dissolved oxygen
• Evaporation rates influence water loss from surface bodies
  • Higher in arid regions, lower in humid areas

Land Use Impacts

• Urbanization alters surface runoff and infiltration
  • Increased impervious surfaces lead to more runoff
  • Reduced groundwater recharge
• Deforestation affects water cycle
  • Reduced transpiration and interception
  • Increased soil erosion and sedimentation
• Agricultural practices impact water quantity and quality
  • Irrigation consumes large amounts of water
  • Fertilizer use can lead to nutrient pollution (eutrophication)
• Industrial activities affect water resources
  • Water pollution from chemical discharges
  • Excessive withdrawals for manufacturing processes

Natural and Anthropogenic Factors

• Geological factors influence water chemistry and quality
  • Rock composition affects mineral content
  • Weathering processes release ions into water
• Topography and soil characteristics impact water movement
  • Slope affects runoff speed and erosion potential
  • Soil texture determines infiltration rates
• Global climate change alters water availability and quality
  • Changing precipitation patterns (more intense storms)
  • Glacial melt affects river flow regimes
  • Sea-level rise threatens coastal aquifers (saltwater intrusion)

Freshwater Resources and Management

Major Sources and Uses

• Freshwater sources include:
  • Rivers: provide flowing water for various uses
  • Lakes: store large volumes of freshwater
  • Glaciers: store freshwater as ice
  • Groundwater aquifers: underground water reservoirs
• Primary uses of freshwater globally:
  • Agriculture: 70% (irrigation, livestock)
  • Industrial processes: 20% (manufacturing, cooling)
  • Domestic consumption: 10% (drinking, sanitation)

Water Scarcity and Challenges

• Water scarcity affects over 40% of global population
  • Physical scarcity: insufficient water resources
  • Economic scarcity: lack of infrastructure for access
• Overexploitation of groundwater leads to:
  • Aquifer depletion: water table drawdown
  • Land subsidence: ground surface sinking
  • Saltwater intrusion in coastal areas
• Water pollution threatens freshwater quality
  • Agricultural runoff (fertilizers, pesticides)
  • Industrial discharge (heavy metals, chemicals)
  • Inadequate sanitation (pathogens, organic waste)

Management Strategies

• Transboundary water management challenges
  • Shared water resources across political boundaries
  • Require international cooperation and agreements (Nile Basin Initiative)
• Climate change impacts exacerbate management challenges
  • Altered precipitation patterns
  • Increased water demand due to higher temperatures
• Sustainable water management approaches:
  • Water conservation: efficient irrigation, leak detection
  • Water recycling: wastewater treatment and reuse
  • Desalination: converting seawater to freshwater
  • Integrated watershed management: holistic approach to water resource planning