What this lesson teaches
I can explain natural and human influences on the pathways water takes through a drainage basin.
Syllabus 9173, Topic 2.1, 2. Drainage Basin Hydrology. How do pathways in the drainage basin system vary in the tropics?
Make a guess
Light rain falls for three days on ground that is already saturated. Can water run over the surface?
- No. Light rain soaks in.
- Yes. Saturated ground cannot take in more.
- No, because runoff happens on paved ground.
- Not unless the rain turns heavy.
Show the answer
Yes. Saturated ground cannot take in more.
Overland flow happens when rain falls faster than ground can absorb it, or when the ground is already full of water.
Water can take fast routes over the surface or slower routes underground, depending on soil, rock, wetness and land use.
Pathways are the routes water takes through a basin. Infiltration is water soaking into the soil. Percolation is water moving down through soil and rock, refilling groundwater. Throughflow is water moving sideways through soil towards a river. Baseflow is groundwater feeding a river. Channel flow carries water downstream.
Overland flow runs over the ground surface. It happens when rain falls faster than the ground can absorb it. It also happens when the ground is already saturated and cannot hold more water, often near rivers or after long rain.
Open soils, connected pores and root channels help water soak in. A hard layer below can push water sideways as throughflow. Steep slopes speed water up; permeable rock slows it down. In limestone caves, underground water can move fast.
People change pathways. Paving stops infiltration, and storm drains rush water to rivers. Machines pack soil down. Clearing plants reduces interception. Permeable paving and restored wetlands can slow water down.
- Infiltration
- Water soaking into the soil.
- Throughflow
- Water moving sideways through soil.
- Baseflow
- Groundwater feeding a river.
Step by step
At the surface
Water soaks in or runs off.
Below the surface
Water moves down or sideways underground.
At the river
Fast and slow flows combine.
Worked example: Two causes of surface runoff
On a dry, packed track, heavy rain falls faster than the ground absorbs it. Beside a stream after days of rain, even light rain lands on saturated soil. Both cause overland flow, for different reasons.
Watch out for this
If rain is light, there can be no surface runoff.
Already saturated ground can produce runoff even in light rain.
Check your understanding
Water moves sideways through the soil above a layer it cannot pass. What is this?
- Evapotranspiration.
- Throughflow.
- Infiltration.
Show the answer
Throughflow.
Yes. It moves sideways through the soil.