Water Budgets & River Systems (Edexcel A Level Geography)

Water Budgets

• Drainage basins are self-adjusting systems to create balance

• If one part is 'out', then one or all the others will compensate

• The balance between inputs and outputs is known as the water balance 

• The water balance helps to understand the behaviour of individual drainage basins 

• The water balance can be shown using the formula:

  • precipitation (P) = total runoff (streamflow) (O) + evapotranspiration (E) +/- changes in storage (S)

• The water balance shows how much water is stored in a system

• The general water balance in the UK shows seasonal patterns

• In wet seasons, precipitation is greater than evapotranspiration which creates a water surplus

• Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels

• This means there is a positive water balance

• In drier seasons evapotranspiration exceeds precipitation, as plants absorb water, ground stores are depleted

• This produces a water deficit at the end of a dry season generating a negative water balance

Soil water budget

• Shows the balance between inputs and outputs of a soil store over a year

• The budget depends on soil depth, type, texture and permeability 

• The following is based on a typical UK soil budget

River Regimes

• The volume of water moving past a point in a river per given time is called the discharge – m³ /sec or cumecs

• Discharge is calculated as Q = A x V

  • Q is discharged in cumecs

  • A is a cross-sectional area in m²

  • V is the velocity in m/s 

• There is a discharge relationship within drainage basins

• The level of discharge is influenced by:

  • Rate of precipitation

  • The speed at which water transfers to the river across the drainage basin

• Rivers act as the main conduit within a drainage basin to transfer water within the system

• Knowing a drainage basin's input (precipitation) and calculating a river's discharge (output) shows how much water is stored within a drainage basin at any given time

Hydrographs

• Hydrographs are used to measure discharge

• There are two types of hydrographs:

  • Annual

  • Storm

• Annual hydrographs, also known as a river's regime, show the pattern of seasonal variation that takes place through a drainage basin to river discharge over a year

• Hydrographs are measured in cumecs

• Different conditions in different locations produce different levels of discharge over the course of a year

• They may show marked seasonal peaks and low flows, greatly influenced by changes in precipitation, temperature, vegetation or geology

  • E.g. big swings in discharge in tropical rivers relate to the wet and dry seasons; spring increases often suggest melting snow; and permeable rocks reduce discharge most of the year

Storm Hydrographs

• Storm hydrographs show changes in a river’s discharge during and after a storm

• Usually, they are drawn to show how a river reacts to an individual storm

• They compare two variables - rainfall received during an event in mm and river discharge m3/sec

• Each storm hydrograph has a series of parts

Image showing the terminology of a flood/storm hydrograph. Note that rainfall is always in mm and a bar chart and discharge in cumecs m³/sec as a line graph measured over time (usually hours, but can be days)

• There are 3 ways that water from a drainage basin is transferred:

  • Directly into the channel – not much

  • Surface flow – most often

  • Infiltration  - through and baseflow

• The typical shape of a flood/storm hydrograph can be referred to as either 'flashy' or 'flat'

Factors affecting the shape of the hydrograph