5.3 Water Budgets & River Systems

What is the water budget?

The annual balance between precipitation, evapotranspiration & runoff (inputs & outputs)

How is the water budget calculated?

Precipitation = Evapotranspiration + Runoff +/- Changes in storage

How is the water budget useful at a national/regional scale?

Water budgets at a national/regional scale indicate the amount of water available for human use

How is the water budget useful at a local scale?

Water budgets at a local scale inform about available soil water

What is available soil water?

The amount of water that can be stored in the soil and is available for growing crops

How is knowing about available soil water useful?

Knowing available soil water in valuable to users who can use it to identify when and how much irrigation is required

What influences water budget?

Climate type

What are river regimes?

The annual variation in the discharge/flow of a river at a particular point

How is a river regime measured?

In cumecs (cubic metres per second)

What influences the character of a river's regime?

Size of river, amount/seasonality/intensity of precipitation, climate, permeability & porosity of geology/soils, type of vegetation cover & human activities/regulation of discharge

What do storm hydrographs show?

The changes in river discharge around the time of a storm

What is plotted on a storm hydrograph?

The occurrence of a short period of rain over the drainage basin & the subsequent discharge of the river

What is the rising limb?

Increase in river discharge as rain flows into water

What is peak discharge and when is it reached?

The highest amount of water discharge within the period of time measured; reached after peak rainfall as water takes time to move over/through ground into the river

What is lag time?

The delay between peak rainfall and peak discharge

What is the falling limb?

The decrease in river discharge as the river returns to its normal level

What is base flow?

Normal discharge of the river

What creates variation in storm hydrographs?

The nature of the rainfall event, the physical characteristics of individual drainage basins & the level of urbanisation around a drainage basin

What are 'flashy' hydrographs?

Hydrographs with steep rising limbs, a high peak discharge & a short lag time

What are 'flat' hydrographs?

Hydrographs with gentle rising limbs, a low peak discharge & a long lag time

How does construction work affect hydrological processes?

Removal of vegetation cover exposes soil which is then replaced by tarmac/concrete, which are impermeable & increase surface runoff

How does a high density of buildings affect hydrological processes?

High density of buildings mean that precipitation falls on rooves & then enters gutters & drainpipes

How do drains & sewers affect hydrological processes?

Drains & sewers reduce the distance & time rainwater travels before reaching a stream/river channel, reducing lag time

How does channelising rivers affect hydrological processes?

Channeling urban rivers with embankments can prevent some flooding, but when they do occur they are more devastating

How do bridges affect hydrological processes?

Bridges restrain the discharge of floodwaters and act as local dams, prompting upstream floods

Why have planners become more important in managing impacts of urbanisation on flood risk?

Many settlements are naturally prone to flooding due to location, many people live in urban places and therefore need protection, & a large amount of money is invested in urban property

What does flood risk management involve?

Strengthening stream/river embankments, putting emergency flood procedures into place & steering urban development away from high-risk areas

What type of weather & climate creates a flashy hydrograph?

Intense storm that exceeds soil infiltration capacity, rapid snowmelt as temperatures rise quickly & low evaporation rates due to temperatures

What type of weather & climate creates a flat hydrograph?

Steady rainfall less than soil infiltration capacity, slow snowmelt as temperatures rise slowly & high evaporation rates due to temperatures

What type of geology creates a flashy hydrograph?

Impermeable rocks such as granite which restrict percolation & encourage rapid surface runoff

What type of geology creates a flat hydrograph?

Permeable rocks such as limestone which allow percolation & limit rapid surface runoff

What type of soil creates a flashy hydrograph?

Soils with low infiltration rate (0-4mm/hr) such as clay soils

What type of soil creates a flat hydrograph?

Soils with high infiltration rate (3-12mm/h) such as sandy soils

What type of relief creates a flashy hydrograph?

High, steep slopes that promote surface runoff

What type of relief creates a flat hydrograph?

Low, gentle slopes that allow infiltration & percolation

What size of basin creates a flashy hydrograph?

Smaller basins

What size of basin creates a flat hydrograph?

Larger basins as it takes more time for water to reach gauging stations

What shape of basin creates a flashy hydrograph?

Circular basins create shorter lag times

What shape of basin creates a flat hydrograph?

Elongated basins tend to have delayed hydrographs

What drainage density creates a flashy hydrograph?

High drainage density means more streams & rivers per unit area, so water moves faster to the measuring point

What drainage density creates a flat hydrograph?

Low drainage density means fewer rivers & streams per unit area so water is more likely to enter the ground and move slowly through the basin

What is drainage density?

The total length of streams in a drainage basin divided by the area of the basin

What sort of pre-existing basin conditions creates a flashy hydrograph?

Basin already wet from previous rain, meaning low levels of interception & more rapid movement through the system

What sort of pre-existing basin conditions creates a flat hydrograph?

Basin dry, low water table, unsaturated soil so high infiltration/percolation

What sort of vegetation creates a flashy hydrograph?

Bare/low density, deciduous in winter, meaning low levels of interception & more rapid movement through the system

What sort of vegetation creates a flat hydrograph?

Dense, deciduous in summer means high levels of interception & a slower passage through the system; more water lost to evaporation from vegetation surfaces

What human activity creates a flashy hydrograph?

Urbanisation producing impermeable concrete & tarmac surfaces; deforestation reducing interception; arable land & downslope ploughing

What human activity creates a flat hydrograph?

Low population density, few artificial impermeable surfaces, reforestation increasing interception & pastoral, moorland & forested land