Approximately 71% of Earth’s surface area is covered by water.
Ocean and Freshwater Distribution: Understanding the balance between oceans and freshwater is critical for water resource management.
Definition: The cyclical movement of water through various reservoirs.
Processes involved in the hydrologic cycle include:
Evaporation: 86% from oceans, 14% from land.
Precipitation: 78% over oceans, 22% over land.
Ground infiltration: Water penetrates the ground, replenishing aquifers.
Surface runoff: Water flowing over the land surface back to oceans and rivers.
Transpiration: Water vapor released by plants into the atmosphere.
Figure 9.1 illustrates the hydrologic cycle comprehensively.
22% of all precipitation falls over land:
Evapotranspiration: Over half of this is returned to the atmosphere.
Runoff: Remainder flows to the sea.
High precipitation levels due to:
Orographic lifting: Mountains forcing air to rise.
Gulf moisture: Warm, moisture-laden air influencing rainfall.
Rain shadow effect: Dry areas on the leeward side of mountains.
Definition: Combination of evaporation and transpiration.
Evaporation: Movement of water molecules from surfaces into the air.
Transpiration: Water exits plants through stomata.
Importance: Over half of all precipitation is returned to the atmosphere by this process.
Definition: Precipitation that does not return to the atmosphere.
Breakdown:
95% flows as overland flow.
5% as subsurface groundwater flow.
Accounts for 8% of the entire hydrologic cycle.
Represents the hydrologic cycle at a specific site.
Inputs: Precipitation measured via rain gauge.
Outputs: Include:
Actual Evapotranspiration (ACET): Net outward water movement from land/plants.
Potential Evapotranspiration (POTET): Water that could evaporate under optimal conditions.
Measured using evaporation pans or lysimeters.
Surplus occurs with excessive precipitation; deficit when demand exceeds supply.
Hygroscopic water: Tightly bound to particles, unavailable for plants.
Capillary water: Held between soil particles, accessible to roots.
Gravitational water: Excess water draining to deeper layers.
Influences plant growth and ecosystem health.
Example: Loam: ideal mixture of sand, silt, and clay.
Definition: Insufficient water to meet demand.
Types:
Meteorological drought: Low precipitation, high temperatures.
Agricultural drought: Impact on crop yields due to soil moisture changes.
Hydrologic drought: Decreased stream flow and reservoir levels.
Socioeconomic drought: Economic impacts due to water scarcity.
Largest reservoir of fresh water available to humans.
Major source for 50% of U.S. population.
Covers approximately 4 km beneath Earth's surface.
Groundwater moves slowly through open pores or fractures.
Porosity: Open space percentage in regolith/rock (average <20% but can be 50%).
Permeability: Measure of fluid passage ease, varies across materials.
Intersects land surface at lakes and springs, mimics topography.
Influences recharge and discharge dynamics based on seasonal factors.
Recharge: Process where precipitation contributes to saturation zone.
Discharge: Occurs at surface locations where aquifers release water.
Supply consists of both surface and groundwater sources.
In regions like New England, supply exceeds usage; the Western U.S. faces shortages.
Daily average: 4200 BGD precipitation with significant amounts returning to the atmosphere via ACTET.
Understanding this budget is critical for resource management and planning.
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