Geography Topic 4 - The storm(flood) hydrograph

What is a storm flood hydrograph

A graph showing the change in discharge caused by a period of rainfall

Discharge is measured in cumecs - m3/s

Here is an example of a storm flood hydrograph

Here is what a flashy and delayed storm hydrograph look like

Factors that affect storm hydrographs

Rock type and geology-impermeable rock means more overland flow

Soil-Well drained sandy soils mean less overland flow

Land use-Urban surfaces and agriculture limit infiltration resulting in high peak discharge, short lag time and a flashy hydrograph

Drainage density-higher density means it is better drained so less chance of flooding

Precipitation/temperature-heavy and prolonged rainfall means soil becomes saturated

Tidal conditions-High tide means water struggles to leave the river system into the sea so more water in DB

Drainage basin area-Large drainage basin means more lag time

Drainage basin shape-Rounded means shorter lag time. Elongated means longer lag time

Slope/gradient-Steeper valley sides means faster overland flow and short lag time

Agriculture affects on water cycle

Irrigation-more evotranspiration in desert area, river levels drop, soil moisture increases

Abstraction-River levels drop, water shortages downstream

Pesticides-Washed into and pollute rivers, poison wildlife

Overstocking with lifestock-Overgraze means less vegetation, compacted ground because of trampling, less infiltration and more runoff

Natural vegetation removed-less interception, more saturated soils so more runoff

Land use affects on water cycle

Urban heat island effect-dark urban surfaces absorb suns energy and radiate heat causing convectional uplift, Causes thunderstorms and heavy rainfall

Sewage into rivers-pollutes river and increases discharge

Drainage systems-Catch rainfall and decrease lag time

Concrete and tarmac-impermeable, no infiltration, no throughflow, increased runoff - flashy storm hydrographs

Reasons for deforestation

Economic benefit - cattle feed, beef, sugar cane and ethanol

How and why do farmers use drainage systems

They use tiles, tubes underground - to remove excess water from the soil profile to help their crops grow

How do the tiles (tubes underground for drainage systems) work

When the water in the soil is higher then the tile, water flows into the tubing which lowers the water table to the depth of the tile over several days. No water is removed if water levels are below the tiles

Advantages and disadvantages of soil drainage

Advantages: Increased productivity and economic outputs

Disadvantages-Increased throughflow, decreased lag time, increased discharge and increased chance of flooding downstream

What happens when nitrogen and phosphor from agriculture enter rivers and ponds

It enriches them so algae and plants grow too fast. This causes them to block sunlight so all organisms in the pond die

What is water abstraction

People pump water from the groundwater store to use. It is most common in areas of low rainfall, high population density, intense agriculture and intense industrial activity

Effects of people taking water out of the ground faster then rainwater replenishes it

Water tables sink, empty wells, intrusion of salt water and ground water contamination