Storage depends on available space, previous rainfall and human use. A large store is not necessarily an empty store.
Interception storage is water held temporarily on leaves, branches and other vegetation surfaces. Some evaporates; the rest reaches the ground as throughfall or stemflow. Canopy size, leaf condition, rainfall duration and wind change how much can be intercepted. A canopy already wet has less remaining storage capacity.
Soil moisture occupies spaces within the soil above the water table, alongside air where the soil is unsaturated. Depth, texture, structure, roots and previous rainfall affect storage and drainage. Clay can retain much water in small pores while transmitting it slowly; porosity and permeability are not the same property.
Groundwater occupies connected pores and fractures in the saturated zone. The water table marks its upper surface in an unconfined aquifer. Permeable rock can store and transmit water, delaying its release to rivers. A deep aquifer may respond slowly to one storm but decline over years if pumping exceeds replenishment.
Channel storage is the water present in rivers and streams at a given time. It generally increases as water level rises. Dams also create reservoir storage and change downstream releases. These volumes vary through storms, seasons and longer periods.
Deforestation removes interception and changes root activity. Paving limits recharge; irrigation adds soil water but may deplete a river or aquifer. Reservoir operations can reserve space for floods or fill it for dry-season supply. The effects depend on timing, management and the store being measured.
Step by step
Identify the store
Distinguish canopy, soil, groundwater and channel water.
Check starting conditions
Recent rain and water use determine how much space remains.
Trace the consequence
A full store may pass more of the next storm rapidly onwards.
Worked example: A storage trade-off
An illustrative reservoir is nearly full to secure water supply before a dry season. It now has little spare capacity to hold an unexpected storm. The same physical structure serves competing goals; flood reduction depends on its operating level and forecasts.
Watch out for this
The most porous rock always transmits water fastest.
Water must travel through connected openings. Numerous tiny or disconnected pores can store water without allowing rapid flow.
Check your understanding
Which observation best indicates reduced groundwater storage over several years?
- A wet leaf immediately after one shower.
- A persistent fall in the water table where pumping exceeds recharge.
- A high river stage during one storm.