Human Influences on Drainage Basins

Human activities can significantly impact drainage basin systems, altering their natural processes and water balance. Two major human factors influencing drainage basins are:

1. Deforestation: The removal of forest cover has several effects on the hydrological cycle within a drainage basin:
   • Increased volume and rate of surface runoff
   • Reduced interception of rainfall by tree canopies
   • Decreased soil infiltration capacity
   • Altered evapotranspiration rates

Highlight: Deforestation can lead to increased flood risk and soil erosion within a drainage basin.

2. Land use change and urbanization: The transformation of natural landscapes into urban areas has significant impacts on drainage basin hydrology:
   • Creation of impermeable surfaces (e.g., roads, buildings) reduces infiltration
   • Increased surface runoff and faster delivery of water to rivers
   • Altered natural drainage patterns through channelization and storm sewer systems
   • Reduced evapotranspiration due to decreased vegetation cover

Example: Urban areas often experience more frequent and severe flooding compared to rural areas due to the high proportion of impermeable surfaces and modified drainage systems.

Understanding these human influences on drainage basins is crucial for effective water resource management and flood mitigation strategies. By recognizing the impacts of deforestation and urbanization, policymakers and planners can develop more sustainable approaches to land use and water management within drainage basins.

Drainage Basins as Open Systems

Unlike the global water cycle, drainage basins operate as open systems, influenced by external factors. The key components of a drainage basin system include:

1. Inputs: Primarily precipitation
2. Flows: Driven by gravity (e.g., infiltration, surface runoff)
3. Stores: Glaciers, rivers, oceans, plants, soils
4. Outputs: Channel flows to the sea, evapotranspiration

Definition: Drainage basin - An area of land drained by a river and its tributaries.

The open nature of drainage basin systems is evident in their interaction with external influences, such as:

1. Climate patterns affecting precipitation
2. Solar radiation driving evapotranspiration
3. Human activities altering land use and water flow

Vocabulary: Evapotranspiration - The combined process of evaporation from land surfaces and transpiration from vegetation.

The Global Hydrological Cycle

The global hydrological cycle is a complex system that describes the continuous movement of water on, above, and below the surface of the Earth. This cycle operates as a closed system on a global scale, with no external inputs or outputs. Water moves between different stores through various processes, including evaporation, condensation, precipitation, and runoff.

Vocabulary: Hydrological cycle - The continuous movement of water within the Earth and atmosphere.

The hydrosphere, which contains all of Earth's water, is composed of 97% saline water from oceans and only 3% freshwater. Of this freshwater, 69% is frozen in the cryosphere, 30% exists as groundwater, and only 0.3% is available as liquid freshwater in lakes and rivers.

Highlight: Only a small fraction of Earth's water is readily available for human use, making water management crucial.

Factors affecting precipitation and evapotranspiration include:

1. Climate
2. Latitude
3. Altitude
4. Proximity to water bodies
5. Vegetation cover

Example: Tropical rainforests experience high levels of precipitation and evapotranspiration due to their location near the equator and dense vegetation cover.

The impact of deforestation on the hydrological cycle is significant. Deforestation can lead to:

1. Increased surface runoff
2. Reduced infiltration
3. Altered evapotranspiration rates
4. Changes in local and regional precipitation patterns

Definition: Evapotranspiration - The process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.