Overview
This lecture explains what watersheds and drainage basins are, describes their structure, and explores factors affecting water movement and quality in these areas.
Definition of Watersheds and Drainage Basins
- A watershed is an area of land that drains all streams and rainfall to a common outlet, such as a reservoir or river mouth.
- Watershed size can range from very small (a footprint) to large areas draining into major rivers or bays.
- The term “watershed” can be used interchangeably with “drainage basin” or “catchment.”
- Ridges or hills separating two watersheds are called drainage divides.
- Watersheds include surface water (lakes, streams, reservoirs, wetlands) and underlying groundwater.
- Larger watersheds contain multiple smaller watersheds within them.
Importance of Watersheds
- Watersheds determine the streamflow and water quality of a river, influenced by natural and human activities in the drainage area.
How Watersheds Work
- Precipitation falling within a watershed will generally flow to the outflow point, but not all water leaves the watershed.
- Streamflow out of a watershed depends on several factors, including precipitation, infiltration, evaporation, transpiration, storage, land cover, and water use.
Key Factors Affecting Water Movement
- Precipitation: Main source of water for streamflow; not all precipitation becomes runoff.
- Infiltration: Water soaks into the soil, contributing to groundwater or seeping into streams; depends on soil type, soil saturation, land cover, and slope.
- Soil Characteristics: Clayey or rocky soils absorb less water, leading to more surface runoff.
- Soil Saturation: Saturated soils cannot absorb more water, resulting in increased runoff.
- Land Cover: Impervious surfaces (roads, buildings) increase runoff and can cause flooding.
- Slope: Steep slopes lead to faster runoff compared to flat areas.
- Evaporation: Water returns to the atmosphere; affected by temperature and weather.
- Transpiration/Evapotranspiration: Plants absorb groundwater and release it into the air, slowing runoff and aiding infiltration.
- Storage: Reservoirs can store water, affecting downstream streamflow patterns.
- Human Water Use: Withdrawals for irrigation, industry, and public supply reduce available streamflow.
Watershed Analogy
- A swimming pool with a cover acts like a watershed, with all falling rain flowing to the lowest point, illustrating basic watershed behavior.
Key Terms & Definitions
- Watershed — An area of land draining water to a common outlet point.
- Drainage Basin — Another term for watershed.
- Drainage Divide — Elevated land separating different watersheds.
- Surface Water — Water on the land surface (lakes, rivers, wetlands).
- Groundwater — Water stored underground in soil and rock.
- Infiltration — Process of water soaking into soil.
- Runoff — Water flowing over land into streams.
- Impervious Surface — Human-made surfaces preventing water absorption (e.g., pavement).
Action Items / Next Steps
- Review water cycle concepts, including precipitation, evaporation, and infiltration.
- Explore case studies of local watersheds and identify human impacts.
- Complete any assigned readings on surface water and watershed management.