How can storm hydrographs and lag times be explained by physical factors?

Understanding how a river responds to a storm is essential when studying flooding. One of the key tools geographers use is a storm hydrograph, which shows how the discharge (volume of water in a river) changes over time following a period of rainfall.

What is a Storm Hydrograph?

A storm hydrograph is a graph that plots two things:

• Rainfall (mm) over time (usually shown as bars).
• River discharge (cumecs – cubic metres per second) over the same time (shown as a line).

Key Features of a Storm Hydrograph:

• Peak Rainfall – the time of maximum rainfall.
• Lag Time – the time delay between peak rainfall and peak discharge.
• Rising Limb – how quickly the river’s discharge increases.
• Peak Discharge – the highest volume of water in the river.
• Falling Limb – how quickly the discharge decreases after the storm.
• Base Flow – the normal flow of the river before and after the storm.

Why Do Storm Hydrographs Vary?

The shape of a storm hydrograph (steep or gentle rising limb, short or long lag time) depends on a range of physical factors in the drainage basin. Let’s explore them.

Geology (Rock Type)

Impermeable rocks (e.g. granite, slate, clay) do not allow water to infiltrate.

• Result: more surface runoff, a shorter lag time, and steeper rising limb.

Permeable rocks (e.g. chalk, limestone) allow water to percolate through cracks.

• Result: slower runoff, longer lag time, and gentler hydrograph.

Example: A river flowing over impermeable granite will respond rapidly to heavy rainfall, increasing flood risk.

Soil Type

Impermeable soils (e.g. clay) become quickly saturated and produce more runoff.

• Result: shorter lag time and higher peak discharge.

Permeable soils (e.g. sandy soils) absorb more rainfall, reducing surface runoff.

• Result: longer lag time and lower flood risk.

Slope (Relief)

Steep slopes lead to rapid surface runoff as water has little time to infiltrate.

• Result: fast rising limb, short lag time, and potentially high flood peak.

Gentle slopes allow more infiltration and slower runoff.

• Result: gentle hydrograph and lower peak discharge.

Upland areas (e.g. Pennines) often produce flashy hydrographs due to steep terrain and shallow soils.

Drainage Basin Shape and Size

Small, circular basins respond more quickly as rainfall reaches the river at the same time.

• Result: short lag time, sharp peak discharge.

Large, elongated basins spread out the water flow.

• Result: longer lag time, lower peak discharge.

Drainage density (how many tributaries feed the main river) also matters: higher density = faster response.

Antecedent Conditions (Previous Weather)

Wet ground from previous rainfall means the soil is already saturated.

• Result: less infiltration, more runoff, shorter lag time.

Dry ground can absorb more water.

• Result: longer lag time, reduced peak discharge.

However, extremely dry, compacted soil can act like impermeable rock, increasing runoff.

Why This Matters

Understanding these physical factors helps predict flood risk and manage rivers. Urban planners, emergency services and flood defence teams all use this knowledge to plan ahead.