Land Use and Rainfall-Runoff Response

A river basin converts part of its rainfall into streamflow. Upstream deforestation increases the proportion and speed of surface runoff, often producing a larger and earlier downstream flood peak.

Changes in rainfall partitioning

• Removing the canopy reduces interception and temporary storage of rainfall on leaves and branches, so more water reaches the ground quickly.
• Lower vegetation cover and evapotranspiration can leave more water available for runoff and may allow soils to become saturated sooner.
• Loss of roots, litter and soil organic matter reduces infiltration and water storage, especially where clearing, grazing or roads compact the soil.

How the downstream peak rises

These changes shorten the time taken for rainfall to reach streams and alter the basin's flood hydrograph.

• Faster overland flow and quicker tributary response increase the rising limb and can shorten the lag time between rainfall and peak discharge.
• When runoff waves from several upstream sub-basins arrive together, their greater synchronization raises the downstream peak discharge.
• Accelerated soil erosion may deposit sediment in channels and floodplains, reducing channel conveyance and increasing the likelihood of overbank flooding.

Conditions governing the effect

• The influence is generally clearer in smaller catchments, where land-cover changes can strongly affect runoff timing and concentration.
• Steep slopes, compacted soils, forest roads, intense rainfall and prior soil saturation strengthen the response; wetlands and intact floodplains can moderate it.
• During exceptional or prolonged rainfall, even forested soils may become saturated. Forests reduce flood risk but cannot prevent every flood, particularly in large river basins where rainfall distribution, channel capacity and floodplain conditions also matter.