Water balance + Hydrographs

How can the balance of a river level be described?

Dynamic (constantly changing). This means river levels rise and fall depending on precipitation.

Explain surplus in a river.

Precipitation is greater than evapotranspiration

The soil refills with water (soil moisture recharge)

The soil becomes saturated (soil moisture surplus)

Explain deficit in a river.

Evapotranspiration is greater than precipitation

The soil dries out (soil moisture utilisation)

The soil becomes fully dry (soil moisture deficit)

In what season does deficit in rivers usually occur?

Summer.

In what season does surplus in rivers usually occur?

Winter.

What does a water balance graph look like?

What is the other name for water balance?

Water budget.

What is the equation to find precipitation?

Precipitation = streamflow + evapotranspiration ± change in storage

What is a hydrograph?

A graph used to display changes in different factors during and after a storm event.

Define baseflow.

The level of the water in a river when it doesn’t rain.

Define discharge.

The volume of water in a river.

What is one Cumec equal to?

1 m³s^-1

What is the rising limb?

The period of time when river discharge is rising (on a hydrograph).

What is the falling limb?

The period of time where river discharge is falling (on a hydrograph).

What is peak rainfall?

The highest precipitation during a storm event.

What is peak discharge?

The highest discharge during a storm event.

What is lag time?

The time between peak rainfall and peak discharge.

What is bank full discharge?

The level of discharge where a river’s banks are completely full.

What is storm flow?

The amount of water produced by a storm event.

What is a flashy hydrograph?

A hydrograph with a high peak discharge and a short lag time, which usually results in a flood.

What is a subdued hydrograph?

A hydrograph with a low peak discharge and a long lag time, which usually results in little to no flooding.

Flood Hydrograph:

How does gradient affect lag time and discharge?

Water flows and the river quicker with a steeper gradient

Therefore, drainage basins with steeper sides have flashier hydrographs

Water moves quicker both in throughflow and overland flow

What is antecedent rainfall?

Rainfall that occurs before a storm event.

How does antecedent rainfall affect lag time and discharge?

If the drainage basin is already saturated, there is a lower infiltration capacity

This causes an increase in overland flow

Overland is the fastest transfer of water overall, so lag time is significantly reduced

This results in a flashy hydrograph

How does the soil/rock type affect lag time and discharge?

Impermeable rock causes increased overland flow and reduced throughflow and infiltration

This leads to a flashy hydrograph

Surfaces baked hard or frozen behave similarly to impermeable rock types

Drainage basins underlain by porous rocks such as sandstone have more subdued hydrographs

How does vegetation affect lag time and discharge?

More vegetation increases interception

This slows rainwater movement to the ground and river channel

Water is also lost through transpiration, which reduces the final discharge

This creates a subdued hydrograph

How does the amount and intensity of precipitation affect lag time and discharge?

Heavy rainfall in a short time increases discharge

This results in a flashier hydrograph

How does the type of precipitation affect lag time and discharge?

The lag time for snow > the lag time for rain

This is because snow must take the time to melt before entering the channel

If snow melts rapidly, peak discharge could still be high

What defines a high drainage density?

A basin with a high drainage density has a lot of surface streams acting as tributaries to the main river.

How does drainage density affect lag time and discharge?

Basins with a higher drainage density have flashier hydrographs

All the water will arrive at the main river at roughly the same time, increasing peak discharge

How does basin size affect lag time and discharge?

Large drainage basins catch more precipitation

This means they have higher peak discharge

Small basins have shorter lag times because the water doesn’t travel as far

How does deforestation affect lag time and discharge?

Deforestation reduces interception rates, so water hits the ground directly

A lack of roots increases infiltration

This leads to rapid overland flow and flashy hydrographs

Soil exposed by deforestation leads to erosion and sedimentation

This reduces bank full capacity and increases flood risk

How does afforestation affect lag time and discharge?

Afforestation increases interception

Infiltration is decreased by roots

This leads to a more subdued hydrograph

Afforestation is a useful flood preventer

How does agriculture affect lag time and discharge?

Ploughing breaks up topsoil and increases infiltration, and causes a subdued hydrograph

Terracing on hillsides interrupts overland flow and throughflow, and causes a subdued hydrograph

Grass crops increase infiltration, and cause a subdued hydrograph

Furrows can act as tributaries, and cause a flashier hydrograph

Dense animal population compacts the soil, increasing overland flows and causing a flashier hydrograph

How does urbanisation affect lag time and discharge?

Impermeable urban areas reduce infiltration

Many urban developments are on flat flood plains

Vegetation in cities is decreasing

Urban areas cause a flashier hydrograph

How do flood alleviation schemes affect lag time and discharge?

Soft engineering flood management schemes aim to reduce flashiness in a hydrograph

Afforestation increases infiltration and interception

This slows water’s progress to the river and subdues changes in discharge

How does water abstraction affect lag time and discharge?

Water abstraction reduces baseflow

This means more water must reach the channel before it reaches bank full capacity

This reduces flooding, but is more likely to cause, for example, drought