Water Resources Study Notes

Definition: The hydrologic cycle is the continuous movement of water through the atmosphere, hydrosphere, lithosphere, and biosphere, characterized by processes such as evaporation and precipitation.
Key Components:
- Evaporation and Precipitation: These forces constantly circulate water.
- Water Distribution: Over 97% of the Earth's water is in the oceans, which dominate both evaporation and precipitation.
- Surface Coverage: More than 70% of the Earth’s surface is covered by oceans, emphasizing their central role in global precipitation and evaporation.

Water Budget Over Land:
- Precipitation exceeds evaporation plus transpiration.
Water Budget Over Water:
- Evaporation exceeds precipitation.
Advection: Over land, 8% of water is gained each year while oceans lose this same percentage, which is replenished by runoff.

Visualization: Atmospheric water vapor levels can be represented in millimeters (mm). A sample graph shows values ranging from 0 to 64 mm.

Components of Water Movement:
- Total input: 100 units
- Cloud formation: 66
- Precipitation: 78
- Evaporation: 20
- Atmospheric advection of water vapor: 12
- Evapotranspiration: 14
- Runoff: 8

Source and Output:
- Input:
- Evaporation (ocean): 86%
- Evapotranspiration (land): 14%
- Output:
- Precipitation (ocean): 78%
- Precipitation (land): 22%
- Total: 100% in/output balance.

Definition: ACTET is the amount of water that is actually evaporated or transpired from the land surface, which includes:
- Evaporation Sources:
- Open water
- Soil
- Plant surfaces
- Transpiration:
- Process by which plants release water vapor, contributing to atmospheric moisture.
- Acts as a cooling mechanism.

Definition: POTET refers to the maximum amount of water that could potentially evaporate or transpire under optimal moisture conditions when adequate precipitation and soil moisture are present.
Factors Influencing POTET:
- Determined by atmospheric energy availability:
- Latitude (influences solar angle, length of day)
- Temperature
- Formula Representation:
- $POTET = ext{demand}$

Definition: Represents the supply of water in the hydrologic cycle.
Comparison:
- PRECIP vs. POTET:
- PRECIP denotes the water supply, while POTET signifies the demand for that water.

Data Representation: A graph displays mean annual precipitation values ranging from 0 mm to over 2500 mm, with a scale of 1:120,000,000 indicating areas of varying precipitation levels.

Inputs:
- Precipitation
Expenditures:
- Interception, percolation of surface water into groundwater, throughfall, evaporation from land and water, runoff to streams, overland flow.
Soil-Moisture Zones:
- Gravitational infiltration leads to percolation into groundwater zones.

Regional Variations: A map illustrates annual river runoff variability across climates, with regions of less than 50 mm to over 1000 mm shown.

Definitions:
- Field Capacity: The maximum amount of water that soil can hold against the force of gravity.
- Wilting Point: The point at which plants can no longer extract sufficient water from the soil.
- Soil Hydrology: Breakdown includes water available and unavailable to plants.

Illustrative Data: Graph presenting trends in soil moisture over time for locations like Kingsport, Tennessee, versus overall precipitation and ACTET levels.
Indicators: Includes surplus (precipitation greater than ACTET), deficit (POTET greater than ACTET), and recharge (precipitation greater than POTET).

Definition: Occurs when precipitation is less than expected or necessary, indicating a supply-demand imbalance relative to climate normals.
Nature: Drought is viewed as a naturally recurring phenomenon within the global climate system.
Types of Drought:
- Meteorological
- Agricultural
- Hydrological
- Socioeconomic

Date Reference: Data from March 24, 2026, highlighting the drought impact types and intensity levels from short-term impacts (less than 6 months) to long-term impacts (greater than 6 months).

Definition and Source: Groundwater constitutes the largest potential freshwater source, accounting for over 22% of freshwater. The majority is found in aquifers which are characterized as permeable rock layers that allow groundwater flow.
Linkage to Surface Water: Groundwater sources are critically linked to surface water levels, contributing to recharge processes over extensive time frames.

Visual Representation: Diagrams illustrate groundwater flow directions, distinguishing between shallow and deep wells, indicating local and distant recharge areas.

Location: Covers states including Wyoming, Colorado, Texas, and more.
Water-Level Change: Variability in water levels across regions with indicators of declines over time.

Trends from 1950 to 2024: Trends indicate rising and falling water levels in several aquifers like the Ogallala and Edwards Aquifer.

Usage Distribution: Water sources divided by sector:
- Irrigation: 45.1%
- Municipal: 35.3%
- Other: 19.6%

Global Annual Withdrawals: Totaling 3414 km³, with notable distribution across different regions:
- Canada: 45 km³ (1.3%)
- USA: 448 km³ (13%)
- Asia: 2007 km³ (59%)
- Europe: 476 km³ (14%)
- Africa: 149 km³ (4%)
- Oceania: 24 km³ (0.7%)

Classification: Areas classified based on levels of groundwater scarcity ranging from low to extremely high levels.

Forecast: Classification of global water stress levels projected for 2050, ranging from low (<10%) to extremely high (>80%).