Runoff Flashcards
What storage reservoirs can run off be derived from? Specify their pathways
- GROUNDWATER
→base flow → channel storage → runoff - SOIL MOISTURE → through flow → channel storage → runoff
- SURFACE STORAGE → overland flow → channel storage → runoff
What factors influence a hydro graph?
- catchment size
- Storm characteristics (rainfall intensity and duration)
- slope angles
- shape of catchment
- soil type
- vegetation type
- percentage cover
- degree of urbanisation
- soil moisture
What is base flow?
The normal day to day flow of a river
Major source of base flow is groundwater
Describe throughflow
The movement of water through the unsaturated zone
Occurs when lateral conductivity is greater than vertical conductivity (moves through locally perched water table or naturally occurring pipes)
The rate of soil water movement slowly, dictated by darcy’s law
What factors control the rate of flow in a channel
Channel - where the water flows through to reach the main river
- volume of water present
- gradient of the channel
- resistance to flow experienced at the channel bed (governed by the bed surface: boulders etc will slow water down)
Describe the seasonality of river runoff
The seasonal cycle exists due to the dominant feeding mechanisms of rivers: Rainfall (PLUVIAL) Snow melt (NIVIAL) Ice melt (GLACIAL)
And is influenced by climate and topography
Distinguish between Pardé (1933) unimodal and complex regimes
- Simple
- Complex I
- Complex II
Simple:
1 period of high runoff and 1 period of low runoff each year
Complex I:
4-6 runoff phases per year (usually 2+2)
E.G. large European rivers with high spring runoff due to snow melt as well as high autumn/ winter runoff from rainfall
Complex II:
Worlds largest rivers, characterised by changing regimes along length
Explain the principal factors affecting runoff rates and timing
- Dams and Reservoirs
- Inter-basin transfers (water diverted by tunnel/ aqueduct)
- Groundwater abstraction (taking water form ground source)
- Land use impacts (urbanisation, forestry)
RESERVOIRS - reduced downstream flow variability
INTER-BASIN TRANSFERS - Runoff volumes in neighbouring catchments increased as a result of transfers
GROUNDWATER ABSTRACTION - Can lead to extreme minimum water levels (sinking water tables, empty wells). In many areas of Europe ground water is the dominant source of freshwater
Influences on flood size
Antecedent soil moisture
wetness of soil immediately after rainfall or snow melt
Flood events heavily influenced by volume of rainfall that occurs before flood causing rainfall
Amount of Infiltration and storm runoff highly dependent on saturation of soil
Influences on flood size
Deforestation
Vegetation cover = decrease in severity of flooding
- interception lates slows down water hitting surface → less rainfall available for soil moisture → antecedent soil moisture lower under forrest
- Forrest’s have high organic matter in upper soil layers → this absorbs more moisture → lessens the amount of overland flow but could increase through flow
Influences on flood size
Urbanisation
Impervious surfaces → Infiltration excess overland flow is high
But urban areas have vast networks of gutters of drains leading to river drainage systems
Influences on flood size
River channel alterations
Natural river channels at equilibrium with water within it
When the channel is altered it can affect flood characteristics
Eg rigid structures to decrease flood risk ironically increase it instead
Influences on flood size
Land drainage
Drained land becomes drier than what is expected naturally → less storm runoff
However, the rapid removal of water leads to flood peaks in the river drainage system where normally the water would be slow to leave the land surface
Influences on flood size
Climate change
River channels adjust to changes in flow regime which may,in turn, be influenced by climate
Greater extremes of weather = greater chance of flooding
