2.1.4 Precipitation and excess runoff within the water cycle

What is a river regime

The annual variation in river discharge over a year shown by changes in flow

What factors influence river regimes

Climate basin size and shape geology vegetation relief drainage density and human activity

How does climate affect river regime

Through precipitation amount seasonality temperature evapotranspiration and snowmelt

How does basin size affect river regime

Larger basins respond more slowly with lower peak flows smaller basins respond quickly with flashier flows

How does geology influence river regime

Permeable rocks increase infiltration and baseflow impermeable rocks increase surface runoff and flashy regimes

How does vegetation affect river regime

Increases interception infiltration and evapotranspiration reducing peak discharge

What is a simple river regime

A regime with one main annual peak in discharge driven by a single dominant factor

What causes a simple river regime

Strong seasonality such as snowmelt or wet and dry seasons

What is a complex river regime

A regime with multiple peaks or irregular discharge due to several interacting factors

Why do large rivers often have complex regimes

They drain areas with different climates precipitation types and human influences

What is a storm hydrograph

A graph showing river discharge response to a single rainfall event

What is baseflow

The normal background flow of a river supplied by groundwater

What is the rising limb

The increase in discharge as runoff reaches the river channel

What controls the steepness of the rising limb

Rainfall intensity basin slope geology land use and antecedent soil moisture

What is peak discharge

The maximum river discharge reached during a storm event

What is lag time

The time between peak rainfall and peak discharge

What does a short lag time indicate

A rapid response to rainfall and higher flood risk

What is the falling limb

The decrease in discharge as water drains from the basin back to baseflow

What is storm flow or direct runoff

The portion of discharge directly caused by the rainfall event

What climatic factors affect storm hydrographs

Rainfall type amount duration intensity temperature and evapotranspiration

How does rainfall intensity affect hydrographs

High intensity rainfall exceeds infiltration causing steep rising limbs and high peaks

How does antecedent soil moisture affect hydrographs

Saturated soils reduce infiltration leading to higher peak discharge and shorter lag time

What is orographic rainfall

Rainfall formed when moist air is forced to rise over high relief

Why does orographic rainfall cause heavy rainfall on windward slopes

Rising air cools condenses and produces prolonged rainfall

What is frontal rainfall

Rainfall formed when warm air is forced to rise over colder denser air at a фронт

What is convectional rainfall

Rainfall caused by surface heating leading to rising warm air and thunderstorms

What is adiabatic cooling

Cooling of rising air due to expansion as pressure decreases

What is the dry adiabatic lapse rate

About 9.8 degrees Celsius per 1000 metres for unsaturated air

What is the saturated adiabatic lapse rate

About 5–7 degrees Celsius per 1000 metres due to latent heat release

What is the collision coalescence process

Rain formation in warm clouds as droplets collide merge and fall

What is the Bergeron Findeisen process

Rain formation in cold clouds where ice crystals grow at the expense of supercooled droplets

What is saturation excess overland flow

Runoff that occurs when soil is fully saturated and cannot absorb more water

What conditions cause saturation excess overland flow

Prolonged rainfall low evapotranspiration impermeable soils and shallow water tables

What is infiltration excess overland flow

Runoff that occurs when rainfall intensity exceeds infiltration capacity

Where is infiltration excess flow most common

Urban areas compacted soils arid regions and during intense storms

How does urbanisation increase runoff

Impermeable surfaces prevent infiltration and speed water into rivers

Why is excess runoff important for flooding

It rapidly increases river discharge and flood risk