River discharge

 = amount of water flowing down a river at any one time
 measured in cubic metres of water per second (cumecs)
 changes over time, depending on amount of precipitation, evapotranspiration and
nature of drainage basin itself
Factors affecting river discharge
 Climatic factors: major impact on drainage basin system operation. Discharge
responds to changes in precipitation input and output changes of evaporation and
transpiration.
 Precipitation: major factor as only input. large amounts of rainfall causes levels to
rise and dry weather leads to falling river levels. type and intensity of rainfall are
important. heavy rain doesn’t have time to infiltrate so increases discharge.
snowfall causes sudden rapid melting and increases discharge.
 Antecedent moisture: if ground is saturated, rain can’t infiltrate so large amounts
of overland flow and increasing discharge rapidly.
 Temperature and evaporation: below freezing, soil is frozen and impermeable. Rain
falls on frozen soil and runs rapidly, increasing discharge. Temp affects evaporation
rates, when high there’s less water reaching the river.
 Transpiration and evapotranspiration: forests reduce discharge. Increased
interception and transpiration means evapotranspiration is more important output
than discharge. Vegetation encourages infiltration and through flow rather than
overland flow. Reduces the speed at which rainfall reaches river, reducing levels.
 Seasonality: temperate areas, season has an important influence. Cold reduces
evaporation rates. Rain in summer means there’s increasing discharge in this
season.
Drainage basin characteristics
 Size and shape: smaller collect less rain and less discharge, respond faster to rapid
inputs. Shape affects flooding. Circular basins respond more promptly to rainfall
inputs and have a higher discharge than long narrow basins.
 Drainage density: total length of surface streams per square km. relates to the
infiltration rate. basins with a low infiltration have more overland flow and higher
drainage density. higher drainage densities respond quicker to rainfall inputs.
 Soil and rock type: impermeable soil and bed rock results in overland flow, faster
process so higher discharge. Permeable and porous use all 3 processes so lower discharge, baseflow keeps summer discharge high, reducing seasonal variations.
Pervious rocks, cracks that water can flow in (granite and limestone). Rocks with
pore spaces between particles means water can soak down (chalk and sandstone).
 Slopes: steep slopes have more overland flow and higher river levels, less time for
rain to infiltrate.
 Vegetation type and land use: forests reduce discharge as increased interception,
so greater evaporation and increased transpiration so water removed before river.
Pasture land allows rainfall to soak into the ground but may have less
evapotranspiration, increasing discharge. Floodplains are fertile and have
machines on them, squashes soil, reducing infiltration, increasing overland flow
and river discharge.
Hydrographs
 Hydrograph: graph showing how river’s discharge changes over time
 Annual hydrograph: graph showing how a river’s discharge changes over the
course of one year is called an annual hydrograph
 Storm hydrograph: graph showing how a river’s discharge changes over a short
period of time, responding to a single input of rainfall
Annual Hydrograph
 Useful when hydrologists study the responses of a river to its environment
 River dart: Dartmoor to Devon (short). peaky hydrograph, typical of rivers in west
Britain. Lots of overland flow as river responds quickly to frequent inputs of rainfall.
River likely to experience flash floods. Peaky because high rainfall and frequent
rainfall, typical of hilly areas in western Britain; impermeable soil and bedrock as
Dartmoor is made of granite encourage overland flow; grass covered drainage
basin with little forest to slow down and absorb rainwater.
 River Avon: flows across Salisbury plain, a chalk upland in south Britain. smooth
hydrograph with low and infrequent flood peaks. little overland flow but more base
flow. river responds to seasonal rainfall changes and evaporation rates. unlikely
flooding. Low hydrograph as rainfall is low in south England and permeable soil and
bedrock.
Storm hydrograph
 Discharge: amount of water in the river channel. varies over
time and is result of rainwater flowing into channel via
overland flow, through flow and base flow.
 Cumecs: cubic metres of water per second. the unit of
measurement for river discharge.
 Approach segment: discharge before the rainstorm
 Rising limb: discharge rises steeply after the storm, mostly due to overland flow
 Bankfull discharge: when the river channel is completely full. if the river rises
further there will be a flood
 Peak discharge: the highest level that a river reaches during a flood
 Lag time: the time between the max. rainfall and peak discharge. drainage basins
with a lot of overland flow will have a short lag time.
 Falling limb (receding): when river levels fall after the peak discharge. its less steps
than the rising limb as through flow is now reaching the river
 Stormflow: stream discharge after a rainstorm, produced by a combination of
overland flow and then through flow. it’s overland flow that makes the greatest
contribution to the flood peak

 Base flow (groundwater): stream discharge produced by water seeping from the
bedrock. very slow process
Influences on shape of storm hydrograph
• Precipitation: prolonged rainfall leads to saturated ground and lots of overland flow.
intense rainfall means infiltration capacity is exceeded, even in permeable basin,
produces overland flow. snowfall can’t flow as frozen. melts when depression brings
more precipitation. snow melts from warm air and rain so 2 lots of precipitation
reach river. ground under snow is frozen so water runs over surface, producing
rapid and massive input into water.
• Temperature: summer is warm so high evaporation. Winter is cold and low
evaporation, more precipitation. Frozen ground leads to lots of overland
• Vegetation: forests encourage interception, evapotranspiration and infiltration.
Forest areas have smaller flood peaks
• Seasonality: similar rainfall inputs has different effects on hydrography at different
times of the year
• Soil and rock type: permeable reduces overland flow, enhance through flow and
base flow. Impermeable enhances surface run off
• Basin relief: steep slopes and high relief in basin gets water to the river quickly and
creates high flood peak
• Urbanisation: tarmac/concrete increases overland flow, water gets to river faster.
gutters/drains speed up through flow.