TEMPORAL VARIATIONS IN RIVER DISCHARGE
River regimes → Annual variation in pattern of flow or discharge of a river, measured at a particular point. Water is
supplied from storm flow, overland flow, through flow and groundwater stores
Simple → Period of high and low channel flow, may correspond with seasonal temperature/rainfall change (monsoon or
snowmelt event)
Complex → Especially large river systems with large peaks and troughs throughout the year
FACTORS AFFECTING REGIMES
Physical Characteristics
Basin size
- Influences lag time.
- Large drainage basins → water takes a long time to travel through tributaries or the ground to reach the channel
- Small drainage basin → water has a shorter distance to travel, resulting in a shorter lag time
Basin shape
- Circular drainage basins → all points in the watershed are the same distance from the channel, leading to shorter lag
times & higher peak discharge
- Elongated drainage basins → longer lag times & lower peak discharge as water drains from furthest points of
watershed to the channel
Elevation/slope
- Steeply-sided river valley → faster lag times and higher peak discharge (due to gravity)
- Gently-sloping valleys → longer lag times and lower peak discharge
Rock type
- Permeable rocks → assist percolation
- Porous rocks (sandstone/chalk) → allow water to percolate through pore spaces
- Pervious rocks (limestone) → allow water to travel along joints and bedding planes
- Both types characterise by lack of surface drainage and high infiltration rates
- Impermeable rock (granite, shale, clay) → impedes drainage (restricts percolation) with high rates of overland flow and
surface run off
Soil type
- Controls infiltration rates, soil moisture storage and rate of through-flow
- Sandy soils → high infiltration rates (relatively large air spaces (voids) between particles)
- Clay/Silts → very little through-flow (small pore spaces)
Drainage density
- Total length of streams in a drainage basin divide by area of basin
- Impermeable rock and soils → higher drainage density due to lack of infiltration and percolation meaning water enters
the channel quickly, leading to increased discharge
- Permeable rock and soil → permeable rock and soil types have low drainage density
Rainfall type
- Amount and duration determine ground saturation
- Long period of rainfall → soil reaches field capacity (saturation capacity) impeding infiltration and causing high levels
of surface run off
- Snow can act as a store (intercepts water) and as a transfer when melting
- Vegetation cover → dense vegetation areas will experience high interception rates, root uptake and
evapotranspiration, reducing discharge levels within the basin
- Tropical rainforests intercept up to 80% of rainfall whilst arable land intercepts less than 10%
, Rainfall intensity
- Heavy rainfall may exceed infiltration capacity of soil → high surface runoff and a rapid increase in discharge
Antecedent conditions
- Weather condition in the period preceding a storm event
- Several weeks of prolonged, heavy rainfall → drainage basin reaches saturation capacity quickly; rise in level of water
table and increasing likelihood of saturation capacity being exceeded
Evapotranspiration rates
- Not constant throughout the year in mid-latitude location (UK)
- High temperatures (summer months) increase rates of evapotranspiration → reduced discharge
- Low temperatures (winter months) reduce evapotranspiration and root uptake/interception (reduced growth rates of
vegetation) → increasing discharge
Human Factors
Urbanisation
- Urbanisation → reduces infiltration to 0 due to use of impermeable surfaces (tarmac, concrete)
- Drains and gutters quickly transport water to the river channel
- Reduced lag time and increased discharge
- Rivers in urban areas characterised by “flashy” hydrography (high flood risk)
Deforestation
- Reduces interception, evapotranspiration and protective canopy layer
- Increased infiltration rates and saturation capacity being reached faster
- Rainfall travels by overland flow → high rates of soil erosion without binding nature of root system
Afforestation
- Planting of trees (often as soft engineering for flood management)
- Increasing infiltration and evapotranspiration rates → reduce river discharge
- Can take many years for trees to mature
Water extraction
- Extraction for industrial or domestic use reduces discharge amount
LINKS
- Unrealistic to examine all factors in isolation as they are all linked
- Weeks of heavy rain may have fallen in a drainage basin with sandy soils and gentle slopes following a dry summer
when water table fell significantly → ground may not reach saturation capacity for many weeks and an intense storm
won’t produce torrents of overland flow and flooding
- Drainage basins tend not to be characterised by only one type of soil and rock (or prescience/absence of urban area) so
will experience differences in infiltration rates
EXAMPLES OF RIVER REGIMES
What?
This river regime is quite complex and fluctuated greatly in 1942. Between April
and July, the river flow was between 1250 and 2500. This is much larger than the
flow of no more than 250 throughout the rest of the year. This same increase
occurred in October of 1942 with river flow jumping up to 2250 at the start of
October. In 1996, however, the river regime was much more steady with only one
increase mid-March, where river flow jumped up to 1500. This increase was
compared to a steady rate of no more than 500 throughout the rest of the year
Why?
During the summer months, snowmelt in the Rocky Mountains caused a
significant rise in water levels. Since 1942, the Hoover Dam has been built,
controlling the discharge throughout the year.
River regimes → Annual variation in pattern of flow or discharge of a river, measured at a particular point. Water is
supplied from storm flow, overland flow, through flow and groundwater stores
Simple → Period of high and low channel flow, may correspond with seasonal temperature/rainfall change (monsoon or
snowmelt event)
Complex → Especially large river systems with large peaks and troughs throughout the year
FACTORS AFFECTING REGIMES
Physical Characteristics
Basin size
- Influences lag time.
- Large drainage basins → water takes a long time to travel through tributaries or the ground to reach the channel
- Small drainage basin → water has a shorter distance to travel, resulting in a shorter lag time
Basin shape
- Circular drainage basins → all points in the watershed are the same distance from the channel, leading to shorter lag
times & higher peak discharge
- Elongated drainage basins → longer lag times & lower peak discharge as water drains from furthest points of
watershed to the channel
Elevation/slope
- Steeply-sided river valley → faster lag times and higher peak discharge (due to gravity)
- Gently-sloping valleys → longer lag times and lower peak discharge
Rock type
- Permeable rocks → assist percolation
- Porous rocks (sandstone/chalk) → allow water to percolate through pore spaces
- Pervious rocks (limestone) → allow water to travel along joints and bedding planes
- Both types characterise by lack of surface drainage and high infiltration rates
- Impermeable rock (granite, shale, clay) → impedes drainage (restricts percolation) with high rates of overland flow and
surface run off
Soil type
- Controls infiltration rates, soil moisture storage and rate of through-flow
- Sandy soils → high infiltration rates (relatively large air spaces (voids) between particles)
- Clay/Silts → very little through-flow (small pore spaces)
Drainage density
- Total length of streams in a drainage basin divide by area of basin
- Impermeable rock and soils → higher drainage density due to lack of infiltration and percolation meaning water enters
the channel quickly, leading to increased discharge
- Permeable rock and soil → permeable rock and soil types have low drainage density
Rainfall type
- Amount and duration determine ground saturation
- Long period of rainfall → soil reaches field capacity (saturation capacity) impeding infiltration and causing high levels
of surface run off
- Snow can act as a store (intercepts water) and as a transfer when melting
- Vegetation cover → dense vegetation areas will experience high interception rates, root uptake and
evapotranspiration, reducing discharge levels within the basin
- Tropical rainforests intercept up to 80% of rainfall whilst arable land intercepts less than 10%
, Rainfall intensity
- Heavy rainfall may exceed infiltration capacity of soil → high surface runoff and a rapid increase in discharge
Antecedent conditions
- Weather condition in the period preceding a storm event
- Several weeks of prolonged, heavy rainfall → drainage basin reaches saturation capacity quickly; rise in level of water
table and increasing likelihood of saturation capacity being exceeded
Evapotranspiration rates
- Not constant throughout the year in mid-latitude location (UK)
- High temperatures (summer months) increase rates of evapotranspiration → reduced discharge
- Low temperatures (winter months) reduce evapotranspiration and root uptake/interception (reduced growth rates of
vegetation) → increasing discharge
Human Factors
Urbanisation
- Urbanisation → reduces infiltration to 0 due to use of impermeable surfaces (tarmac, concrete)
- Drains and gutters quickly transport water to the river channel
- Reduced lag time and increased discharge
- Rivers in urban areas characterised by “flashy” hydrography (high flood risk)
Deforestation
- Reduces interception, evapotranspiration and protective canopy layer
- Increased infiltration rates and saturation capacity being reached faster
- Rainfall travels by overland flow → high rates of soil erosion without binding nature of root system
Afforestation
- Planting of trees (often as soft engineering for flood management)
- Increasing infiltration and evapotranspiration rates → reduce river discharge
- Can take many years for trees to mature
Water extraction
- Extraction for industrial or domestic use reduces discharge amount
LINKS
- Unrealistic to examine all factors in isolation as they are all linked
- Weeks of heavy rain may have fallen in a drainage basin with sandy soils and gentle slopes following a dry summer
when water table fell significantly → ground may not reach saturation capacity for many weeks and an intense storm
won’t produce torrents of overland flow and flooding
- Drainage basins tend not to be characterised by only one type of soil and rock (or prescience/absence of urban area) so
will experience differences in infiltration rates
EXAMPLES OF RIVER REGIMES
What?
This river regime is quite complex and fluctuated greatly in 1942. Between April
and July, the river flow was between 1250 and 2500. This is much larger than the
flow of no more than 250 throughout the rest of the year. This same increase
occurred in October of 1942 with river flow jumping up to 2250 at the start of
October. In 1996, however, the river regime was much more steady with only one
increase mid-March, where river flow jumped up to 1500. This increase was
compared to a steady rate of no more than 500 throughout the rest of the year
Why?
During the summer months, snowmelt in the Rocky Mountains caused a
significant rise in water levels. Since 1942, the Hoover Dam has been built,
controlling the discharge throughout the year.
