Hydrology

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What is the rough estimate for the quantity of water on Earth?
332.5 million m2 - this remains constant. Would form a sphere with a diameter of 860 miles (much smaller than the moon).
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What proportion of the world's water is fresh or saline?
Saline in oceans- 96.5%; other saline-0.9%; freshwater-2.5%
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Where is freshwater stored?
1.2% on surface/other freshwater; 30.1% in ground water; 68.7% in glaciers and ice caps
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What is a reservoir?
Location of water e.g. lake, ocean, river
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What is flux?
Movement of water between reservoirs
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Define the hydrological cycle
Closed loop of the flux of water
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What is the hydrology budget for any reservoir?
Mass in (Mi) - Mass out (Mo) = Sm (storage mass) (all in Kg)
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What is Sv?
Measurement for flux - 10^6 m^3/s
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Where does most evaporation occur?
Over oceans
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What is the water storage of the atmosphere?
0.001% global water
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How is the atmosphere divided up? (Starting from the lowest section)
Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere
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Where does weather occur?
Top of troposphere- up to 10km
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What is the importance of the ocean in the hydological cycle?
Huge store- largest- 1,338,000,000 km3. Where most evaporation takes place (90%). Ocean mass is not fixed- changes significantly over time in response to glaciations and inter-glacial warming.
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How is water stored on the land?
Surface run-off and rivers. - Store varies considerably spatially and temporally. Controlled by meteorology and geology- where rivers can occur, Groundwater - 1.7% of Earth's water. Aquifers= groundwater reservoirs that provide water.
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What is the water table?
Surface of the groundwater
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What are rivers?
Meeting of surface water and groundwater- flux.
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Define evaporation
The transformation/phase change of liquid water to water vapour (gas) requiring energy
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What is condensation?
The transformation (phase change) of water vapour (gas) to liquid water which releases energy
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What is transpiration?
The transfer of water vapor to the atmosphere from vegetation (evaporation from stomata)
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Define runoff
Movement of water overland and in streams and rivers to the oceans- both a flux and storage.
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What is the movement of water from the surface into the groundwater?
Infiltration
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What is a drainage basin?
The area drained by a river and all its tributaries to one point in an ocean
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What is a watershed?
The divide between drainage basins
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What are river basins?
Collect water from a drainage basin
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Describe the catchment area of the River Ouse?
N.Yorkshire moors, E. Yorkshire dales. Ends at the Humber estuary.
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What are endorheic basins?
River basins that don't end up in the sea e.g. Caspian sea- largest lake in the world
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What is the largest drainage basin in the world?
Amazon
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To what extent does run-off vary in space?
Immenselly- precipitation can vary from 0- 6m /yr
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Give some examples of how precipitation is changing over time?
Amazon, Mediterranean, Syria= drying. UK=wetter
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What river parameters are usually calculated?
Mean depth (area/top width); Hydraulic radius (area/wetted perimeter)
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How do you calculate discharge?
Area=depth X width; Discharge= Area X velocity- m3/s
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What does a hydrograph show?
A measure of river flow over time (either height/stage or flow/discharge)
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When do rivers flood?
After discharge exceeds the capacity of the channel?
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Why are there non-linear responses in hydrographs between rainfall and river discharge?
Lag time between rain falling and travelling over land to rivers
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Why do we look at rivers?
Major impacts on society e.g. flooding
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What is the difference between perennial and ephemoral rivers?
Perennials- flow all year. Ephemoral- may dry up during particular seasons/periods of the year,
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Why is it important to study rivers?
They are a key transfer of water and nutrients to systems and a key source of water to human populations (ancient civilisations developed along rivers; habitats for flora and fauna; transport routeways; and need to be monitored as a hazard risk.
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What are fractal patterns?
The same pattern is found as a system is zoomed in upon
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Why does a fractal process occur?
Chaos theory- small/tiny change leads to another change and another and this continues exponentially
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Why do these fractal processes just happen in river systems?
They are self-organising
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How does velocity change within a river? Why?
Flow of water= slowest on beds and banks; Fastest just below the surface. Why? Friction - anywhere the river touches an object- drag.
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What sediment transport mechanisms are there?
Bedload (Saltation- bounced along) (Traction-rolled/sliding); Dissolved + suspended load
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What is the name of the lift force that acts on sediment grains generated by moving liquid over something?
Bernouli force
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How do you calculate bed shear stress?
Bed shear stress= density of water (kg/m3)(1000) x drag coefficient (mean velocity)^2
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When does erosion or deposition of grains occur?
If bed shear stress (Tb) = more/same as critical stress (Tc)= erosion. If it is less=deposition
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Why does velocity have to be high to erode either large or tiny grains?
Large- heavier mass to move; Tiny- tend to stick together
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What happens to sediment when a river emerges from a mountain? Why?
Spreads out on to floodplain and spreads out- depth decreases- velocity slows- sediment is deposited- alluvial fan.
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What characteristic do all alluvial fans have?
Dynamic- not stable through time
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What are the main types of river channel patterns found? Which are the more common/rare forms?
Meandering; braided (common); straight; anastromosing- lots o river channels crossing over each other (rare)
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Describe some features on meandering rivers. Why do these occur?
Cut-bank- erosion takes place on outside- water flows fastest. Point bar- deposited sediment due to velocity slowing on inside of bends
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Why do meanders shift across floodplains?
Respond to pulses of water during flooding events. Over time- sinuosity changes
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What is formed when a meander is cut off? How does this happen?
An ox-bow lake forms. As bends migrate- they may increase in sinuosity- become more tightly curved. During floods rivers flow over point bar instead of the bend- chute channel. Over time:old bend fills with sediment. Chute channel=new main channel
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Define braided rivers
Complex multichannel systems of low sinuosity which commonly migrate across the flood plain
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Where are braided channels commonly formed?
Mountainous regions
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What is avulsion?
Shift to a new channel- evolution of channel
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What are some of the characteristics of braided channels?
Wider shallow channels; many channels separated by bars (multithread); less fine grained material in channel; weak erodible banks, channel full of bar with lots of cutting and filling of channels
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What controls the difference between whether meandering or braided channels occur?
Physical control equation (Stream power(W)= p(density of water/1000kg/m3) x gravitational constant (9.81m/s2) x discharge (m3/s) x slope
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Why do meandering rivers tend to transport finer grain sediment?
Meandering rivers- lower stream power- lower shear stress- lower bedlock- more cohesive walls. Braided- higher stream power-coarser load; shallower- need higher shear stress.
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What does the Bradshaw model show?
How river qualities such as discharge, channel depth, load particle size etc change from upsteam to downstream
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What is the problem with the bradshaw model?
Too simplified- doesn't capture the complexity of river systems and how other factors outside the fluvial system boundary can affect a river e.g. change in topography due to tectonic activity
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Can you give an example of an environment where rivers are flowing over constantly changing topography?
Himalayas- Collision of continental plates- folding-thrust faults stack-rivers continue to flow
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Why do mountains tend to erode?
They are steep and unstable
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What controls sediment characteristics in bedrock rivers?
Characteristics of the sediment source/input (amount, size, distribution, lithology etc); transport and abrasion processes along the channel
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Why does bedrock become more rounded and smaller in the Bradshaw model?
Chipping (2 pebbles collide-chip off angles); Crushing (of small pebbles between larger); Cracking (weaken pebbles); Grinding(slowly erode down); Splitting(break along lines of weakness)
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Why might pebble fining not occur down a river?
Other inputs of sediment further downstream introducing more angular material downstream. Mixed sediment system.
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What is tectonic activtity?
Land movement in response to plate movement
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What is the profile of a stable long-river according to the Bradshaw model
Steep gradient near source leading to a gentler gradient near the mouth- equilibrium
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How do rivers respond to tectonic activity? Change its erosion rate to match the new profile
Uplift= increase erosion-rivers incise into bedrock- fluvial incision. If uplift stops-erosion decreases- long river profile becomes more gentle.
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Why is the Bradshaw model effective in the UK?
No tectonics
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What is a knickpoint? How does it show past tectonic activity?
A discontinuity in a long river profile. Knickpoint migrates changing the long river profile. You can measure profiles- look for major knickpoints- earthquake markers- see how long ago it occurred from how far it migrated
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What does the stream power law (SPL) calculate?
Equations used to calculate the rate of erosion of a river into its bed
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Describe the "tool and cover" effect of sediment?
Increase amount of sediment in a river-erosion increases- sediment used to erode bedrock. Too much sediment- covers bedrock- sediment eroded instead- no erosion of the bedrock.
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When can you tell if the land beneath a river is moving rapidly?
Lots of knickpoints=very active fault. Mostly concave profiles= less frequent earthquakes
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What else can generate a knickpoint aside from earthquakes?
Rapid sea level change- change the level where the river enters the sea- knickpoint will migrate upstream
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Why are the himalayas being eroded faster than they are being built up?
Himalayas- affect atmospheric circulation=monsoon- 3m rain/yr in a short period to the South. Uplift=2-4mm/yr but erosion causes a particle flux of 1 billion tonnes / yr. Eroded at same rate or more than uplift.
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Why is the tibetan plateau getting bigger?
Erosion= < uplift - no rain= little erosion. Plateau is getting wider.
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Why are there migmatites near the surface in the Himalayas despite being normally found deep in the mantle?
Erosion is leading to focused exhumation. Decreased load = popping up of land.
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What happens to the land below rivers?
Unloading= local rebound/uplift
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What is a numerical model?
Mathematical equations written as code inputted into computer
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What is discretisation? How is this done?
Turning continuous equations into discrete ones as computers need one value made up of 0 & 1. Values are assigned to cells (single unit of space in the computer model)
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Why is the resolution chosen for a model important?
Needs to be an appropriate level of detail without taking up too much time to run
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What are boundaries and what defines these?
Edges of the model (in space and time) defined by math-based boundary conditions
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Why are the majority of models called forward? What is the opposite of this?
Forward- model starts at time, t, and then runs forward- timesteps. Opposite= inverse models
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What does it mean to discretise the equations used in the model?
Calculate the differences between cells in the model= gradient
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What controls the resolution in a computer model?
The quantity of continuous values used
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Aside from boundaries what is also needed in a computer model?
Starting conditions e.g. weather
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What types of scales are there for processes?
Spatial/Temporal Spatial-small (10 m); medium(100 m);large (>km). Temporal- Short(seconds); medium(hours-days); long (months-years).
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Give an example of a model used to investigate how a landscape feature was formed?
Margin in South East Africa- 2 theories ( escarpment retreat) or (plateau degradation). Van der Beek et, 2002- used Cascade to run both models- tried to match up features calculated with features found in the real landscape.
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Why is Child by Tucker et al, 2001 popular? Are there any shortcomings?
User friendly; includes a wide range of fluvial incision laws and tectonic scenarios but no landsliding algorithms
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Give an example of where the CHILD model has been used?
Attal et al, 2008. - To reproduce the catchment morphology of an area in the central Apennines, Italy-showed how the internal drainage basin was most likely to have formed- knickpoint propogating upstream.
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Card 2

Front

What proportion of the world's water is fresh or saline?

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Saline in oceans- 96.5%; other saline-0.9%; freshwater-2.5%

Card 3

Front

Where is freshwater stored?

Back

Preview of the front of card 3

Card 4

Front

What is a reservoir?

Back

Preview of the front of card 4

Card 5

Front

What is flux?

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