6_WaterResources
Water, Weather, and Climate Systems
Water Resources
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.
The Hydrologic Cycle
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.
Hydrologic Cycle Model
Figure 9.1 illustrates the hydrologic cycle comprehensively.
Surface Water on Land
Distribution of Precipitation
22% of all precipitation falls over land:
Evapotranspiration: Over half of this is returned to the atmosphere.
Runoff: Remainder flows to the sea.
Precipitation in North America
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.
Evapotranspiration
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.
Runoff
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.
Groundwater System
The Water-Balance Equation
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.
Determining POTET
Measured using evaporation pans or lysimeters.
Surplus occurs with excessive precipitation; deficit when demand exceeds supply.
Soil Moisture
Types of Soil Moisture
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.
Soil-Moisture Balance
Influences plant growth and ecosystem health.
Example: Loam: ideal mixture of sand, silt, and clay.
Drought
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.
Groundwater Resources
Groundwater Profile and Movement
Largest reservoir of fresh water available to humans.
Major source for 50% of U.S. population.
Covers approximately 4 km beneath Earth's surface.
Movement and Characteristics
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.
Groundwater Table
Behavior and Characteristics
Intersects land surface at lakes and springs, mimics topography.
Influences recharge and discharge dynamics based on seasonal factors.
Recharge and Discharge
Recharge: Process where precipitation contributes to saturation zone.
Discharge: Occurs at surface locations where aquifers release water.
Water Supply in the U.S.
Overall Supply Patterns
Supply consists of both surface and groundwater sources.
In regions like New England, supply exceeds usage; the Western U.S. faces shortages.
U.S. Water Budget
Daily average: 4200 BGD precipitation with significant amounts returning to the atmosphere via ACTET.
Understanding this budget is critical for resource management and planning.