1.3 Global water distribution and water balance

Earth's water is distributed across various reservoirs, with oceans holding the lion's share. Only a tiny fraction is freshwater, crucial for human needs. Understanding this distribution is key to managing our limited water resources wisely.

Water balance tracks inputs, outputs, and storage changes in a system. This concept is vital for assessing water availability and sustainability at different scales. By analyzing water balance, we can make informed decisions about water management and conservation.

Global Water Distribution and Reservoirs

Global distribution of water resources

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• Earth's water distributed across several reservoirs
  • Oceans largest reservoir containing ~97% of Earth's water (Pacific, Atlantic, Indian)
  • Ice caps and glaciers second-largest reservoir storing ~2% of Earth's water (Antarctica, Greenland)
  • Groundwater stores ~0.6% of Earth's water
    • Majority is saline with only a small fraction being fresh water (aquifers, springs)
  • Surface water contains ~0.01% of Earth's water (Amazon River, Great Lakes, Lake Baikal)
  • Atmosphere holds ~0.001% of Earth's water as water vapor (clouds, humidity)
• Water moves between these reservoirs through the hydrologic cycle
  • Processes include evaporation, transpiration, precipitation, infiltration, runoff (water vapor, rainfall, snowmelt)

Freshwater vs saltwater availability

• Freshwater resources have low concentrations of dissolved salts (<1,000 mg/L)
  • Essential for human consumption, agriculture, most industrial processes (drinking water, irrigation, manufacturing)
  • Limited availability with only ~2.5% of Earth's water being fresh water
    • Most freshwater locked up in ice caps, glaciers, deep groundwater aquifers (polar regions, mountain ranges)
    • Surface water and shallow groundwater most accessible freshwater resources (rivers, lakes, wells)
• Saltwater resources have high concentrations of dissolved salts (>1,000 mg/L)
  • Primarily found in oceans, seas, some inland water bodies (Pacific Ocean, Dead Sea)
  • Not suitable for direct human use without desalination (reverse osmosis, distillation)
  • Abundant with ~97.5% of Earth's water being saltwater

Water Balance and Sustainability

Water balance components and scales

• Water balance accounts for water inputs, outputs, storage changes within a system over time
  • Based on the principle of mass conservation: $inputs = outputs ± change in storage$
• Components of water balance:
  1. Precipitation (P): water input from atmospheric moisture (rain, snow, hail)
  2. Evapotranspiration (ET): water output through evaporation and plant transpiration
  3. Runoff (R): water output as surface flow (streams, rivers)
  4. Groundwater flow (G): water input or output through subsurface flow (aquifers)
  5. Change in storage (ΔS): change in water stored within the system (soil moisture, groundwater, surface water)
• Water balance equation: $P = ET + R + G ± ΔS$
• Spatial scales for water balance analysis:
  • Local: individual sites or small catchments (farm, forest stand)
  • Regional: larger watersheds or river basins (Mississippi River Basin, Amazon Basin)
  • Global: entire Earth's surface

Water balance for sustainability assessment

• Water availability compares water inputs with outputs and storage changes
  • Assessed by comparing precipitation, groundwater recharge with evapotranspiration, runoff (water budget)
  • Water stress occurs when water demands exceed available supplies (droughts, overuse)
• Sustainable water management balances water use with available supplies for long-term availability and ecosystem health
  • Requires understanding water balance components and their variability over time
  • Strategies include:
    1. Improving water use efficiency (drip irrigation, low-flow fixtures)
    2. Enhancing water storage and distribution infrastructure (reservoirs, pipelines)
    3. Protecting and restoring natural water sources (wetlands, forests)
    4. Implementing water conservation measures and demand management (tiered pricing, education)
• Water balance analysis helps identify:
  • Areas of water surplus or deficit (wet vs dry regions)
  • Seasonal or interannual variability in water availability (monsoons, El Niño)
  • Impacts of land use changes, climate change, or other factors on water resources (deforestation, global warming)
  • Opportunities for water resource development or conservation (dams, rainwater harvesting)