Rainfall Variability and River Floods

A seasonal rainfall deficit means that total rainfall over a season and reporting area is below its long-period average. It does not imply that every place or river basin received little rain. A flood occurs when concentrated rainfall generates river discharge greater than the channel can safely carry.

Why the rainfall deficit can conceal flood-producing rain

Seasonal rainfall is an aggregate across time and space, whereas a river responds to rainfall within its drainage basin. Heavy rain over a small but hydrologically important upstream catchment may therefore coexist with deficient rainfall across the larger region.

• Administrative rainfall units and river-basin boundaries often do not coincide, so a district or state average may poorly represent rainfall feeding a river.
• A few intense spells can produce floods even when long dry intervals keep the seasonal total below normal.
• Rainfall near headwaters or over tributary catchments can raise downstream discharge far from the area receiving the rain.

How concentrated rainfall becomes a river flood

When rainfall intensity exceeds the soil's infiltration capacity, more water becomes surface runoff and reaches streams rapidly. Tributaries collect this runoff and create a high flood peak in the main river.

• Steep slopes, thin or saturated soils, sparse vegetation and impermeable urban surfaces increase rapid runoff.
• Simultaneous flood peaks from several tributaries can combine and sharply increase the main river's discharge.
• A downstream area may flood despite little local rain because the river carries water received upstream.

Conditions that amplify flooding

Flood severity depends not only on rainfall volume but also on the basin's condition and the river's carrying capacity.

• Antecedent soil saturation reduces water absorption and increases runoff from a subsequent storm.
• Reduced channel capacity, drainage congestion and obstructions can raise water levels for the same discharge.
• Settlement on floodplains increases exposure, turning a hydrological flood into a larger disaster.