2.1.3 Part 3: Storm Hydrograph Components

What is a storm (flood) hydrograph?

A graph showing short-term variations in river discharge (over hours or days) in response to a specific rainfall event.

Define baseflow on a hydrograph.

The normal, steady background discharge of a river, sustained by groundwater flow and deep soil moisture.

Define the rising limb.

The initial steep increase in river discharge on a hydrograph as runoff reaches the channel.

Define peak discharge.

The maximum volume of water flowing through the river channel during or after a storm event.

Define lag time.

The time delay between the peak of the rainfall event and the peak discharge of the river.

Define the falling (recession) limb.

The descending curve on a hydrograph as storm water drains away and discharge returns to baseflow.

What is storm flow (direct runoff)?

The portion of river discharge directly produced by a storm, calculated by subtracting baseflow from total discharge.

What does a short lag time and steep rising limb indicate?

A "flashy" river catchment with rapid surface runoff and high flood risk.

Part 4: Climatic Drivers & Hydrograph Response

How does snowfall affect hydrograph lag time compared to rainfall?

Snowfall delays runoff, creating a long lag time until temperatures rise and trigger snowmelt.

What rainfall intensity rate typically leads to infiltration-excess overland flow?

Heavy rainfall exceeding 30 mm per hour.

How do cold winter temperatures increase surface runoff?

Water freezes in upper soil pores, blocking infiltration and forcing meltwater across the surface.

How does high summer evapotranspiration affect a storm hydrograph?

It reduces the volume of water reaching the river, increasing lag time and flattening the peak discharge.

Define antecedent soil moisture conditions.

The level of saturation already present in the soil prior to a new rainfall event.

What type of overland flow occurs when soil is fully saturated before a storm?

Saturation-excess overland flow.

What named UK storm sequence in February 2020 demonstrated extreme antecedent saturation?

Storm Dennis, which caused severe flooding because soils were already saturated from Storm Ciara days earlier.

How do deciduous trees affect summer hydrographs in the Exe catchment?

Transpiration exceeds 300 liters per tree per day in summer, reducing available runoff and flattening hydrograph peaks.

What occurred on the River Exe in January 2014 due to antecedent conditions?

Prolonged winter rainfall caused complete soil saturation, driving rapid peak discharge and localized flooding near Exeter.