The drainage basin as an open system

Overview

Drainage basin and infiltration, percolation and throughflow.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a drainage basin, and what marks its boundary?

   Model answer

   A drainage basin is the area of land drained by a river and all of its tributaries. Its boundary is the watershed, the ridge of higher land separating one basin from the next; precipitation falling on one side of the watershed drains into one basin, precipitation on the other side drains into the neighbouring basin.

2. 2

   Why is a drainage basin described as an open system rather than a closed one?

   Model answer

   Because both energy and matter cross its boundaries. Water enters as precipitation and leaves as evapotranspiration and as river discharge to the sea, and solar and gravitational potential energy enter and are lost. A closed system would exchange only energy with its surroundings, keeping all its matter inside; a drainage basin does not, so it is open.

3. 3

   What are the inputs to the drainage basin system?

   Model answer

   Precipitation in all its forms - rain, snow, hail, sleet, dew and fog drip - is the main input of matter, along with the dissolved minerals and sediment it carries. Energy inputs are solar radiation, which drives evaporation, and gravitational potential energy from the relief of the basin, which drives all downslope movement of water.

4. 4

   What are the outputs from a drainage basin?

   Model answer

   Evapotranspiration, which is evaporation from open water, soil and surfaces combined with transpiration from vegetation; channel discharge of water and its sediment and dissolved load out of the basin mouth to the sea or lake; deep percolation of groundwater into aquifers below or beyond the basin; and energy lost as heat.

5. 5

   Name the stores in a drainage basin system.

   Model answer

   Interception storage on leaves and stems, surface storage in puddles, ponds and lakes, soil moisture storage in the pore spaces of the soil, groundwater storage in the saturated rock below the water table, channel storage in the river itself, and vegetation storage as water held inside plants. Snow and ice on the surface are also a temporary store.

6. 6

   Name the transfers, or flows, that move water through a drainage basin.

   Model answer

   Throughfall and stemflow move intercepted water to the ground; infiltration moves water from the surface into the soil; percolation moves it from the soil down into the underlying rock; throughflow moves it laterally through the soil towards the channel; groundwater flow moves it slowly through the rock to the channel; overland flow moves it across the surface; and channel flow carries it out of the basin.

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