Water can take rapid surface routes or slower underground routes. Soil, rock, wetness and land use determine which routes dominate.
Infiltration is water crossing the ground surface into soil. Percolation is its downward movement through soil and rock, potentially recharging groundwater. Throughflow is lateral movement within the soil towards a channel. Baseflow is the groundwater contribution to river flow. Channel flow then carries water downstream.
Overland flow crosses the ground surface. Infiltration-excess overland flow occurs when rainfall intensity exceeds the rate the ground can absorb it. Saturation-excess overland flow occurs when saturated ground has little remaining capacity to store incoming water, often near rivers or after prolonged rain. Both can occur within one basin.
Open soil structure, connected pores and root channels generally support infiltration. Impermeable layers can redirect water sideways as throughflow. Steep slopes and closely connected channels can shorten travel time; permeable soils and rock can delay water. Underground flow is not always slow: large karst conduits can transmit it rapidly.
Paving prevents infiltration locally, while storm drains connect rapid runoff directly to rivers. Compaction by machinery can reduce infiltration even without concrete. Vegetation removal may reduce interception and alter soil structure. Permeable paving, retention areas and restored wetlands can slow selected pathways, but performance depends on soil, maintenance and storm size.
Step by step
At the surface
Rain infiltrates or begins overland flow.
Below the surface
Water moves down by percolation or sideways as throughflow and groundwater flow.
At the river
Fast and slow contributions combine to determine discharge through time.
Worked example: Two reasons for surface runoff
On a dry compacted track, intense rain can exceed infiltration capacity. Beside a stream after days of rain, even moderate rainfall can fall on saturated soil. Both produce overland flow, but the limiting condition differs.
Watch out for this
If rain intensity is modest, surface runoff cannot occur.
Already saturated ground can generate saturation-excess overland flow even when rainfall is not intense.
Check your understanding
Water moves laterally through the soil above an impermeable layer. What is this pathway?
- Throughflow.
- Infiltration.
- Evapotranspiration.