Rainfall, Infiltration and Surface Runoff

Infiltration is the entry of rainwater into soil, while surface runoff is the portion that flows over the land into streams and rivers. Within a watershed, rainfall is divided among interception, infiltration, evapotranspiration, surface storage, groundwater recharge and runoff. Land-use change modifies this division by altering vegetation cover, soil structure, surface permeability, drainage pathways and sediment supply.

Controls on rainfall-runoff response

A watershed’s response depends not only on rainfall amount and intensity but also on antecedent soil moisture, slope, soil permeability, vegetation and drainage characteristics.

• Runoff increases when rainfall intensity exceeds the soil’s infiltration capacity, producing infiltration-excess overland flow.
• Runoff also occurs when the soil becomes saturated and can store no more water, producing saturation-excess overland flow.
• Vegetation intercepts rainfall, reduces raindrop impact and promotes infiltration through roots, litter and soil organisms.
• Rough and permeable surfaces slow water movement, lengthen the time taken for water to reach channels and generally reduce the flood peak.
• Compacted, bare or impervious surfaces reduce infiltration and deliver water to channels more rapidly, producing a shorter lag time and a higher peak discharge.

Effects of major land-use changes

The effects vary with soil, slope, rainfall, land-management practices and the scale of disturbance, but several broad relationships are well established.

• Deforestation reduces canopy interception and exposes soil to direct raindrop impact.
• Loss of roots and organic matter can weaken soil aggregates, reduce macroporosity and lower infiltration, especially where logging or grazing compacts the ground.
• Reduced evapotranspiration after vegetation removal can leave more water available for runoff and soil saturation.
• Mining commonly removes vegetation and topsoil, exposes loose earth and rock, and creates spoil heaps that supply sediment to runoff.
• Mine roads, machinery and compacted surfaces reduce permeability and create rapid artificial drainage paths.
• Excavations and mine pits may locally store water, so the precise runoff effect depends on mine design, geology and drainage arrangements.
• Urbanisation replaces soil and vegetation with roofs, roads and paved surfaces, sharply reducing infiltration and accelerating runoff through drains.
• Intensive cultivation on slopes, repeated tillage, residue removal and soil compaction can increase runoff and sheet, rill and gully erosion.
• Contour farming, terracing, vegetative cover and other conservation practices can instead slow runoff and improve infiltration.
• Drainage or filling of wetlands removes natural water storage, while occupation of floodplains can obstruct or narrow flood-conveyance pathways.

Erosion, sediment load and river carrying capacity

Land-use disturbance can create a connected chain from hillslope runoff to river-channel change and greater flood hazard.

• Faster and more concentrated runoff has greater erosive power and can detach and transport topsoil, especially on bare slopes.
• Sediment enters streams as suspended load and bed load, along with debris derived from gullies, roads, spoil heaps and unstable banks.
• Where channel velocity or transporting power declines, part of this material is deposited on the bed, banks, bars and floodplain.
• Repeated deposition may cause aggradation, raise the channel bed and reduce the cross-sectional area available to convey floodwater.
• Bank erosion and channel instability can increase where rapid runoff produces larger and more frequent discharge peaks.
• A river’s conveyance capacity depends on channel cross-section, slope and hydraulic roughness; it is therefore not a fixed property.
• Sediment deposition and encroachment can reduce conveyance, but not all incoming sediment is deposited in the same reach because finer material may be transported downstream.
• Reduced infiltration also diminishes groundwater recharge and may weaken dry-season baseflow, even while increasing short-duration storm runoff.
• The combined result can be a quicker watershed response, higher flood peaks, greater sedimentation and more frequent overbank flooding for a given storm.

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