Water Budgets & River Systems (Edexcel A Level Geography)

• 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

Graph showing changes in soil moisture stores, on a local scale, over a year

• 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 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'

'Flashy' hydrograph showing short lag time with high peak discharge

‘Flat’ hydrograph with low peak discharge

Factors affecting the shape of the hydrograph

Factor	'Flashy' hydrograph Short lag time, high peak discharge, steep rising limb	'Flat' hydrograph Long lag time, low peak discharge, gentle rising limb
Rock type	Impermeable rock - decreases percolation and increases surface run off	Permeable rock - allow percolation leading to lower surface run off
Soils	Clay soils have a low infiltration rate increasing surface run off	Sandy soils have a high infiltration rate, decreasing surface run off
Weather/climate	Heavy or prolonged rainfall and rapid snowmelt can exceed the capacity of the soil leading to increased surface run off Low evaporation rates increases surface run off	Steady rainfall and slow thaw of snow don't exceed the infiltration capacity of the soil High evaporation rates lead to lower surface run off
Antecedent conditions	Saturated soil so infiltration is low and surface run off greater	Unsaturated soils so infiltration is high and surface run off low
Vegetation	Deciduous plants/trees mean interception levels are lower in winter Lack of vegetation leads to less interception	Deciduous plants/trees mean there are high levels of interception  Greater levels of vegetation leads to higher interception
Drainage basin size	Smaller basins have steep rising limbs and short lag times as water reaches the rivers more quickly	Large basins have longer lag times and gentler rising limbs as the water takes longer to flow through the drainage basin
Human activity	Deforestation reduces interception Urbanisation increases impermeable surfaces Agriculture can increase compaction of the soil and ploughing can increase surface run off	Afforestation increases interception