FACTORS AFFECTING RIVER DISCHARGE.

River discharge

River discharge= volume of water passing a measuring point/ gauging station in a river in a given time. 

Measured = cubic metres per second (cumecs). 

 Overall discharge - drainage basin = the relationship between preciptiation + storage factors.

Formula for drainage basic discharge:
Drainage basin discharge= precipitation - evapo-transpiration + or - changes in storage

Long term variation=  hydrographs =  annual patterns of flow ( the river regime) in response to climate. 

Short-term variation= flood of storm hyrdograph.

Storm Hydrograph.

• Shows variations in a river's discharge over a short period of time, usually during a rainstorm. 
• Base flow= starting and finishing level of the river.
• Rising limb= rising discharge as storm water enters the drainage basin.
• Peak discharge= highest flow in the channel.
• Receding limb= fall in discharge back to base level.
• Lag time= time delay between maximum rainfall amount and peak discharge.

Factors affecting a river's discharge

Rock and soil type

•  Permeable rocks + soils (e.g sandy soils) = absorb water easily= so surface run-off is rare
•  Impermeable rock + soils (e.g clay soils) =  more closely packed= rainwater can’t infiltrate=  water reaches the river        more quickly
• Pervious rocks (e.g limestone) = allows water to pass through joints + porous rocks (e.g chalk) =  spaces between the rock particles

Land use

• Urban areas=  roads = impermeable =  water can’t infiltrate/percolate = runs into drains= gathers speed + joins rainwater from other drains = quicker spillage into the river.
• In rural areas= ploughing up and down (instead of across) hillsides =  channels which allow rainwater to reach rivers faster = increased discharge= meet its bankful discharge.
• Deforestation = less interception= so rain reaches the ground faster= ground  saturated =surface run-off will increase.

Rainfall

•  Amount + type= will affect a river’s discharge
• Antecedent rainfall = is rain that has already happened= ground already saturated= further rain= surface run-off towards the river
• Heavy continual rain or melting snow= more water flowing into the river

Relief

• Steep slopes = rainwater = runs straight over the surface= quicker= before infiltration can occur.
• Gentle slopes= infiltration is more likely.

Weather conditions

• Hot dry weather = baked soil=  water can’t soak in= surface run off= straight into the river.
• High temperatures= increase evaporation rates from water surfaces + transpiration from plants = reduced discharge 
• Long periods of extreme cold weather= frozen ground= water can’t soak in