WCC: Water Cycle - The Water Balance, Runoff Variation and Storm Hydrographs Flashcards
What is soil moisture recharge?
When soil moisture stores can start to fill up again as precipitation exceeds evapotranspiration. (October - January)
What is soil moisture utilisation?
When evapotranspiration exceeds precipitation and so plants have to utilise water from the soil store (April - July)
What is soil moisture deficit?
When there deficiency of soil water as the store has been used up by plants and evapotranspiration exceeds precipitation. There is a risk of drought, plants must adapt to survive and crops must be irrigated. (August - September)
What is soil moisture surplus?
When all the soil water has been recharged to full capacity. Precipitation exceeds evapotranspiration. There is surplus water for plant use, run-off and groundwater recharge. (February - April)
Give the general water balance equation.
P = O + E +- S P = precipitation O = total runoff (streamflow) E = evapotranspiration S = storage (in soil and rock)
What is field capacity?
The maximum amount of water soil can hold
What is field capacity affected by?
The soil, e.g the type and depth
What is river discharge?
The volume of water a river passing a given point at a given time.
Describe and explain the geology of a river with a flashy hydrograph.
Non porous soil and impermeable rock so water doesn’t infiltrate or percolate, encouraging rapid overland flow
Describe and explain the vegetation coverage of a river with a attenuated hydrograph.
High coverage to increase interception and throughfall, reducing the quantity and speed of water reaching the surface and therefore restricting surface runoff.
Possibly deciduous trees.
Describe and explain the drainage basin shape of a river with a flashy hydrograph.
Round as the points where rain falls are all closer to the river, so more water enters the river faster
Describe and explain the relief of the land of a river with a flashy hydrograph.
Steep sided upland river basins reduce infiltration and increase rate of surface runoff
Describe and explain the land use of a river with a flashy hydrograph.
High level of urbanisation as impermeable surfaces cause a lack of infiltration and percolation, increasing the rate of surface runoff.
Most settlements are designed to transfer water as quickly as possible away from human activity to the nearest river.
Describe and explain the stream density in the basin of a river with a flashy hydrograph.
High density as the points where rain falls are all closer to the river or its tributaries, so more water enters the river at once
How does rainfall affect surface runoff?
Intensity and duration - if high the infiltration capacity of the soil is exceeded so runoff is more rapid
Antecedent rainfall - Heavy rain on already saturated soil will produce rapid runoff
4 reasons urbanisation increases runoff
Removal of topsoil and compaction of ground with earth moving machinery during building work reduces infiltration
Building roads increases impermeable surface area
Building drains and sewers rapidly transports water to rivers
Straightening of river channels and lining with concrete leads to faster delivery of water downstream of an urban area and increases flood risk there
What is a river regime?
The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics
What is a flashy river regime?
Where discharge varied lots over a short period of time, due to shape or permeability.
Why is the evapotranspiration in the water balance referred to as ‘potential’ evapotranspiration?
Evapotranspiration increases with temperature. Often the temperature, and therefore the atmosphere’s ability to hold water vapour, is greater than the amount of water available. Potential evapotranspiration is the amount of water that could be evaporated from an area if there was sufficient water available.
What is the water balance?
The state of equilibrium in the drainage basin between the inputs, outputs, and changes in ground storage.
How do rising temperatures and actively growing vegetation in the UK summer impact the water balance?
There may be a moisture deficit
How would a moisture deficit in the UK summer impact on the water table?
The water table would fall as discharge from springs may be needed to replenish river flows
How does high precipitation, low temperatures and less interception and uptake of water by vegetation in the UK winter impact the water balance?
There is usually a water surplus
What does a water surplus lead to?
Wet soil, high river levels and run-off
What does a water deficit lead to?
Dry soil, falling river levels ans possibly drought
Give the equation for river discharge.
Discharge = Area x Velocity
Measured in cumecs
What are the 9 factors that influence runoff percentages between drainage basins? (how much precipitation ends up in the river)
MAYBE DELETE
Rain intensity Climate Vegetation (type and amount) Soil (type and depth) Bedrock permeability Shape of the land Size of the drainage basin Shape of the drainage basin Ground conditions
What is knowledge of discharge important for?
Assessment and management of water resources
Design of water-related structures
Flood warning and alleviation schemes
What are the physical factors affecting runoff variation?
Rainfall - intensity and duration Antecedent rainfall Snowfall Geology - porosity and permeability Vegetation type Drainage basin shape Slope angle
Give 4 ways urbanisation increases runoff
Removal of topsoil and compaction of ground with earth-moving machinery during building work
Building of roads
Building of drains and sewers
Straightening of river channels and lining with concrete
How does rainfall intensity and duration affect runoff?
If both are high the infiltration capacity of the soil is exceeded so runoff is more rapid
How does antecedent rainfall affect runoff?
Heavy rain falling on already saturated soil will produce rapid runoff
How does snowfall and subsequent melting affect runoff?
Heavy snow may store water on the ground surface during a prolonged period of cold weather. Runoff is reduced and water levels in rivers may fall .
When temperatures rise massive amounts of water are released through melting, greatly increasing runoff. Runoff rates may be even quicker if the ground remains frozen and restricts infiltration.
How does geology affect runoff?
More permeable rocks have higher infiltration rates, reducing surface runoff
How does vegetation type affect runoff?
This varies with the season. In summer there are more leaves on deciduous trees, so interception is higher and runoff lower. Conifers produce less variability between summer and winter.
How does drainage basin shape affect runoff?
Rain reaches the river more quickly from a round basin than an elongated basin
How does slope angle affect runoff?
In steep-sided upland river basins the water reaches the river channels much more quickly than in gently sloping lowland river basins.
How does the removal of topsoil and compaction of ground with earth-moving machinery during building work affect runoff?
It reduces infiltration, increasing runoff
How does the building of roads affect runoff?
It increases the impermeable surface area, increasing runoff
How does building drains and sewers affect runoff?
They transport water rapidly to rivers, increasing runoff
How does the straightening of river channels and lining with concrete impact flooding?
It leads to the faster delivery of water downstream of an urban area and increases the risk of flooding there
How can deforestation lead to greater chances of flooding?
Deforestation exposes the soil to greater rates of erosion, leading to the sedimentation of the channel. This reduces the bankfull capacity of a river, leading to a greater chance of flooding.
What is bankfull capacity?
The maximum discharge that a river channel is capable of carrying without flooding
What are the human factors affecting runoff variation?
Urbanisation
Deforestation
Agriculture
Give 4 agricultural factors affecting runoff
Ploughing
Terracing
Grass
Animal density
How does ploughing affect runoff?
It breaks up topsoil allowing greater infiltration, reducing rate of runoff.
Contour ploughing can increase rate of runoff.
Ploughing wet soils can lead to plough pans, inhibiting percolation leading to greater surface flows.
What is contour ploughing?
Creating furrows that run directly down slope, acting as small stream channels
What are plough pans?
Impermeable smears in the subsoil
How does terracing affect runoff?
Terracing on hillsides stops movement of water downhill, reducing rate of runoff.
How do grass crops affect runoff?
Grass crops increase infiltration, reducing rate of runoff
How does animal density affect runoff?
Large numbers of animals on small areas of land can impact soils leading to overland flows
What is a storm hydrograph?
A graph of the discharge of a river over the time period when the normal flow of the river is affected by a storm event
Why are storm hydrographs important?
They can predict how a river might respond to a rainstorm which can help in managing the river
How is base flow represented on a storm hydrograph?
Base flow is the initial discharge, its slow movement meaning it gradually increases throughout the rainstorm event. When all the storm water has passed through, the river returns to its base flow.
What does the rising limb indicate?
How quickly a river responds to a storm event
What part of a storm hydrograph shows the discharge rising?
The rising limb
Why does the discharge start to rise faster than base flow after the start of a storm event?
As storm water enters the drainage basin the river begins to be fed by much more fast-moving water
What is peak discharge?
The point (on a storm hydrograph) when river discharge is at its greatest
What is the lag time?
The time taken from peak rainfall to peak discharge
What What part of a storm hydrograph shows the discharge falling?
The receding limb
What is the line of bankfull discharge on a storm hydrograph?
The point a river will begin to overflow
What kind of response is a river said to have if it experiences a rapid rise in discharge over a short period of time?
Flash responses
How is most of the water entering a river with a flash response?
Through surface runoff
What storm hydrograph characteristics do flashy responses have?
Steep rising limb and small lag time
What storm hydrograph characteristics do attenuated responses have?
Gentle rising limbs and long lag times
Describe and explain the soil water in a river basin with a flashy hydrograph.
Saturated soil with result in higher levels of surface runoff, increasing the speed at which water enters the river.
Describe and explain how drainage basin size affects a river’s storm hydrograph.
A larger drainage basin will have a higher peak discharge but a longer lag time. This is because more rain will fall within the basin, however it will take longer to reach the river.
Smaller drainage basins therefore have smaller peak discharge and shorter lag times.
Give 2 ways humans can attempt to reduce the flashiness of a river’s hydrograph.
Soft engineering flood management schemes
Water abstraction
Describe and explain an example of a soft engineering flood management scheme that can reduce flashiness in a river’s hydrograph.
Afforestation increases interception and infiltration, slowing down the progress of water to the river channel and subduing any changes in discharge.
How can water abstraction reduce flashiness in a river’s hydrograph?
Water abstraction reduces the base flow so more water must reach the channel before it reaches bankfull capacity
What factors affect the shape of storm hydrographs?
Geology Vegetation cover Size of drainage basin Shape of drainage basin Land use Stream density Soil water Relief of the land
Why would the presence of deciduous trees support an attenuated hydrograph?
Tn the summer they can increase rates of transpiration through interception by up to 40%.
