1.1 - Drainage Basins
The concept of a drainage basin as an open system
A drainage basin is a distinct area of land where all water is channelled through a river and its network of tributaries. It operates as an open system, meaning it interacts with its surroundings through inputs and outputs that cross its boundaries.
Characteristics of a drainage basin system
- Open system dynamics - Inputs like rainfall enter the basin, while outputs such as evaporation leave it, demonstrating constant interaction with the external environment.
- Boundary interactions - The edges of the basin define where water enters or exits, influencing how water cycles through the area.
- Hydrological components - The basin includes various elements such as stores (where water is held) and flows (how water moves), which work together to regulate water distribution.
Key inputs, outputs, flows, and stores in drainage basin hydrology
Understanding the components of a drainage basin involves identifying the primary inputs, outputs, flows, and stores that shape the hydrological cycle within it.
Main components of drainage basin hydrology
- Inputs - The chief input is precipitation, which includes forms like rain, snow, hail, and dew, delivering water into the basin from the atmosphere.
- Outputs - Water exits the basin through processes like evaporation (water turning into gas) and transpiration (water loss from plants), collectively termed evapotranspiration, as well as through run-off into larger bodies of water.
- Flows - These are the pathways water takes within the basin, including:
- Infiltration (water soaking into the ground).
- Throughflow (water moving sideways through soil).
- Overland flow (water running over the surface).
- Baseflow (slow movement of groundwater to rivers).
- Stores - Water is temporarily held in various natural reservoirs such as:
- Vegetation (plants and trees).
- Soil moisture (within the ground).
- Aquifers (water-bearing rocks).
- Cryosphere (snow and ice).
The processes of precipitation, interception, and evapotranspiration
The journey of water through a drainage basin begins with precipitation and involves critical processes like interception by plants and loss through evapotranspiration.
Understanding key hydrological processes
- Precipitation dynamics - This is the transfer of moisture from the atmosphere to the earth's surface, occurring as rain, sleet, snow, or other forms. Its total amount and intensity influence whether water runs over the surface or infiltrates into the soil.
- Interception by vegetation - Plants capture raindrops, preventing immediate contact with the ground. During heavy or prolonged rain, leaves can no longer hold water, leading to:
- Throughfall (water dripping to the ground).
- Stemflow (water trickling along branches and stems to the soil).
- Some water remains on leaves and is later evaporated.
- Evapotranspiration mechanisms - This combines evaporation (liquid water turning to gas from surfaces) and transpiration (water loss from plant leaves). Factors affecting evaporation include air temperature, wind speed, and surface type, with bare rocks losing water faster than covered soils. Potential evapotranspiration refers to the maximum water loss possible if water supply were unlimited.
The movement of water through flows like infiltration and overland run-off
Water movement within a drainage basin occurs through various flows, each playing a unique role in how water is transported from one store to another or out of the system.
Types of water flows in a drainage basin
- Infiltration process - Water seeps into the ground, with the infiltration capacity indicating how much water a soil can hold, and the infiltration rate showing how quickly water enters.
- Throughflow movement - Water travels laterally within the soil, often along natural channels or between soil layers.
- Overland run-off occurrence - This happens when rainfall intensity surpasses the soil's infiltration rate or when the soil becomes fully saturated, causing water to flow over the surface.
- Percolation into deeper layers - Water moves downwards through soil and rock into the groundwater zone.
- Baseflow contribution - Groundwater moves towards rivers, requiring the water table to be higher than the river level to create the necessary gradient for flow.
The role of various stores such as vegetation, soil, and aquifers
Stores within a drainage basin act as natural reservoirs, holding water for varying durations and regulating the hydrological cycle.
Major water stores in drainage basins
- Vegetation storage - Plants, especially trees, hold significant amounts of water compared to grasses or crops.
- Soil moisture retention - The capacity to store water in soil depends on porosity (the volume of pore spaces) and permeability (how easily water moves through).
- Aquifer significance - These are underground layers of rock that hold water, serving as the most crucial store for maintaining river flow and regulating the hydrological cycle.
- Cryosphere storage - Comprising snow and ice, this is the largest freshwater store globally, with water potentially locked away for thousands of years.
How were these notes?