What this lesson teaches
I can explain natural and human influences on water storage in a drainage basin.
Syllabus 9173, Topic 2.1, 2. Drainage Basin Hydrology. How does water storage in the drainage basin system vary in the tropics?
Make a guess
A forest canopy is already dripping wet from a morning storm. A second storm arrives in the afternoon. What changes?
- Nothing, because canopies hold the same each storm.
- More rain reaches the ground beneath.
- The canopy holds twice as much water.
- The trees absorb the extra rain.
Show the answer
More rain reaches the ground beneath.
A wet canopy has little space left to intercept water. More of the second storm drips through or runs down the stems.
How much water a store can hold depends on its space, recent rain and human use.
Interception storage is water held on leaves and branches. Some evaporates. The rest drips or runs down stems to the ground. A canopy that is already wet can hold less.
Soil moisture is water held in spaces in the soil, above the water table. Depth, texture, roots and recent rain affect how much it holds. Clay holds a lot of water but lets it through slowly. Porosity (how much water a material can hold) and permeability (how easily water passes through) are different properties.
Groundwater fills pores and cracks in rock below the water table. Permeable rock stores water and releases it slowly to rivers. A deep aquifer may barely react to one storm, but can drop over years if pumping exceeds refilling.
Channel storage is the water in rivers and streams at any moment. It rises when water levels rise. Dams add reservoir storage and change flows downstream.
People change storage. Cutting forests removes interception. Paving stops water refilling the ground. Irrigation adds soil water but can drain rivers or aquifers. Reservoirs can keep space for floods or store water for dry seasons. The effects depend on timing, management and which store is being measured.
- Interception
- Water held on leaves and branches.
- Groundwater
- Water in rock below the water table.
- Channel storage
- Water in rivers at any moment.
Step by step
Name the store
Canopy, soil, groundwater or channel?
Check starting conditions
How much space is left after recent rain?
Trace the result
A full store passes the next storm on quickly.
Worked example: A storage trade-off
A reservoir is kept nearly full to secure water before a dry season. Now it has little space to hold an unexpected storm. The same dam serves two competing goals.
Watch out for this
The most porous rock always lets water through fastest.
Water needs connected openings to flow. Tiny or unconnected pores store water but let it through slowly.
Check your understanding
Which sign best shows groundwater storage falling over several years?
- A steady fall in the water table as pumping exceeds refill.
- Wet leaves and soil after a week of showers.
- A low river level in the dry months of one year.
Show the answer
A steady fall in the water table as pumping exceeds refill.
Yes. It shows long-term depletion.