Water Resources Notes

Water Resources Notes

The Hydrologic Cycle

  • Definition: The hydrologic cycle describes the continuous movement of water through the atmosphere, hydrosphere, lithosphere, and biosphere.
  • Components: Involves processes such as evaporation, condensation, precipitation, and runoff.
  • Key Statistics:
    • Over 97% of the Earth's water is found in oceans, which dominate evaporation and precipitation globally.
    • More than 70% of the Earth's surface is covered by oceans.

Water Balance

  • Evaporation vs. Precipitation:
    • Over land, precipitation exceeds evaporation + transpiration.
    • Over water, evaporation exceeds precipitation.
  • Advection: Each year, land gains 8% more water than oceans lose, which is replenished by runoff.

Moisture Balance of the Atmosphere

  • Evaporation Sources:
    • Open water bodies, soil, and plant surfaces contribute to evaporation.
  • Transpiration: Plants release water that evaporates into the atmosphere, acting as a cooling mechanism.
  • Actual Evapotranspiration (ACTET):
    • The actual amount of water that would evaporate/transpire under optimal moisture conditions.
    • Factors that influence ACTET include energy from the atmosphere, temperature, and solar intensity.
  • Potential Evapotranspiration (POTET):
    • Represents the water demand under optimum conditions.

Precipitation

  • Definition: Precipitation is the primary source of water supply in the hydrologic cycle.
  • Measurement: Mean Annual Precipitation (1961-1990) is often reported in millimeters, with various ranges indicating levels of rainfall diversity globally.

Water Budget

  • Components of Water Budget:
    • Input: Runoff to streams, gravitational water.
    • Expenditures: Infiltration and percolation.
  • Computation: The water budget can be analyzed using frameworks like ACTET, POTET, and PRECIP relations, noting conditions for water supply and demand.

Drought

  • Definition: A drought is a period of below-average precipitation, affecting water supply levels and is defined against climate normals.
  • Types of Drought:
    • Meteorological Drought: Abnormally low precipitation.
    • Agricultural Drought: Insufficient moisture for crop and pasture growth.
    • Hydrological Drought: Decrease in water supplies and streamflows.
    • Socioeconomic Drought: Impacts on economics and community needs.

Groundwater

  • Importance: Groundwater is a significant source of freshwater, constituting over 22% of global freshwater supplies available for use.
  • Aquifers: Porous rock layers that store groundwater, replenished slowly over time via surface water.
  • Types of Groundwater:
    • Younger and Older Water: Refers to groundwater that has been recharged relatively recently versus that which has been trapped for long periods.
  • Notable Aquifers:
    • The Ogallala Aquifer, spanning multiple U.S. states, is a critical water source for agriculture. Water levels in this aquifer exhibit significant declines over recent decades due to extensive use.

Water Use in Texas

  • Water Source Breakdown (example percentages):
    • Irrigation: 45.1%
    • Municipal: 35.3%
    • Other sectors: 19.6%
  • Global Water Withdrawal Statistics: Highlighting the diverse withdrawal rates by region indicating potential stress on water resources.

Global Water Stress Projections (2050)

  • Categories of Stress:
    • Extremely High (>80%)
    • High (40-80%)
    • Medium to High (20-39%)
    • Low to Medium (10-19%)
    • Low (
  • Significance: These projections highlight urgent needs for water management strategies to mitigate water scarcity risks.