water Flashcards
(17 cards)
ITCZ
The Earth consist of six cells of circulating air, which form the globe’s climate control.
For the Northern Hemisphere (the same is true for the south, just in opposite directions):
Hadley Cell - Air rises at The Doldrums, travels upwards, then sinks as it meets the cooler air of the Ferrel Cell. At this meeting point, precipitation tends to occur. The air then travels southwards, heating up as it does. It will then have heated sufficiently to rise up at the Doldrums, commencing the cycle again.
Polar Cell - Cold air sinks near the Arctic Circle, cooling and condensing to form precipitation over northern latitudes. The air then travels southwards, heating until it meets warm air from the Ferrel Cell. The air then rises, causing dry conditions for the land beneath, and then travels northwards, cooling as it does.
Ferrel Cell - The middle cell of the ITCZ (tends to be at a mid-latitude location). The air circulation is determined by the Hadley and
Polar cells either side, similar to a cog system.
A drainage basin
an open subsystem operating within the closed global hydrological
cycle. It’s defined as an area of land drained by a river and its tributaries with a boundary (known as the watershed), usually composing of hills and mountains. The basic flows, inputs and outputs are shown below:
Inputs to the Drainage Basin – Precipitation
▪ Seasonality – In some climates (such as monsoon and Mediterranean) there are strong
seasonal patterns of rainfall. Therefore the time of year determines the precipitation input
within the drainage basin
▪ Variability - sudden or long term changes to the climate can happen, which would affect
precipitation levels and so the drainage basin as a whole.
→ Secular Variability – long term (for example as a result of climate change trends)
→ Periodic Variability – annual, seasonal or monthly context
→ Stochastic Variability – random factors like localisation of thunderstorm
▪ Latitude - The location of the drainage basin has a major impact on climate, and so the
volume and type of precipitation falling. In most cases, the higher the latitude from the
Equator, the colder the climate, and so snowfall occurs more often than rainfall. Also, at
latitudes where air cells converge (ITCZ), the climate will be categorised by the rise or
fall of air -
Convectional
Often a daily occurance. The morning heat warms the ground, which
in turn heats low-level moisture (from plant dew or surface stores). This moisture
evaporates and rises. As the air rises, it cools and the moisture within will condense, to
form rain and in turn (as more moisture accumulates) rainfall. In tropical climates,
convectional rainfall is most common; within tropical rainforests, rainfall occurs
mid-morning before the temperature rises too high for condensation to occur.
Frontal
Where two air masses meet, a wedge can occur of hot air within
cold air - this is called a depression. At the front (were the two air masses meet), warm
moist air is forced to rise above the cold air mass, causing the water moisture within to
cool and condense, to form cyclonic precipitation. Depressions are very common to
the UK - approx 100 depressions hit the UK each year
Relief/Orographic
When warm, moist air (often travelling onto land from sea) meets
land of high relief (e.g. hills), the air mass is forced to rise above the hill to continue
travelling. As it rises, the air mass cools and the moisture within condenses, to form
clouds and rainfall. Orographic rainfall depends on the relief and location of the land
immediate after the sea - many coastal landscapes are too cold, low lying or hills are
set too far inland for relief rainfall to occur.
Interception
Interception is the direct intervention of plants’ leaves in changing the direction or temporarily
stopping precipitation as it falls to the surface. Any moisture retained by the surface of the
leaf (interception store) is generally greatest at the start of storms. A plant’s interception
capacity varies depending on the type of vegetation.
Infiltration
The movement of water from the surface into the soil. The infiltration capacity is the
maximum rate at which water can be absorbed by the soil, and can be affected by:
▪ Soil Composition – Sandy soils have higher infiltration rates compared to clay.
▪ Previous precipitation - The saturation of soils will reduce infiltration rates, hence
surface runoff increases after long, intense periods of rainfall.
▪ Type and amount of vegetation - dense root growth can inhibit the infiltration of water,
and interception of plants’ leaves will delay infiltration (never stopped, as water will never
permanently remain on the leaf).
▪ Compaction of soils will reduce the infiltration rate.
▪ Relief of land – sloped land will encourage more runoff, therefore less infiltration as a
direct result.
Surface Runoff
Water flows overland, rather than permeating deeper levels of the ground.
Overland flow occurs faster where the gradient of land is greater. Surface runoff if the
primary transfer of water to river channels, hence heavily influencing their discharge -
Moderate/Fast
Throughflow
w - Water moves through the soil and into streams or rivers. Speed of flow is
dependent on the type of soil . Clay soils with a high field capacity and smaller pore
spaces have a slower flow rate . Sandy soils drain quickly because they have a lower
field capacity, larger pore spaces and natural channels from animals such as worms.
Some sports fields have sandy soils, to reduce the chance of waterlogged pitches, but this
may also increase the flood risk elsewhere - Moderate/Fast
Percolation
- Water moves from the ground or soil into porous rock or rock fractures
(deeper bedrock and aquifers). The percolation rate is dependent on the fractures that
may be present in the rock and the permeability of the rock - Slow
Groundwater Flow
- The gradual transfer of water through porous rock, under the influence
of gravity. Water can sometimes become trapped within these deeper layers of bedrock,
creating aquifers and long water stores for the drainage basin - Slow
Stores
Soil Water - Water stored in the soil which is utilised by plants - Mid-term
Groundwater - Water that is stored in the pore spaces of rock - Long-term
River Channel - Water that is stored in a river - Short-term
Interception - Water intercepted by plants on their branches and leaves before reaching the
ground - Short-term
Surface Storage - Water stored in puddles, ponds, lakes etc. - Variable
The water table is the upper level at which the pore spaces and fractures in the ground
become saturated. It is used by researchers to assess drought conditions, health of
wetland systems, success of forest restoration programmes etc.
Factors influencing the Drainage Basin
Physical Factors
➢ Climate – influences
amount of rainfall and
vegetation growth.
➢ Soil Composition –
influences rate of infiltration
and throughflow.
➢ Geology – affects
percolation and
groundwater flow
➢ Relief – steeper gradients
of land will encourage faster
rates of surface runoff
➢ Vegetation – affects
interception, overland flow
➢ Size – larger basins collect
more precipitation generally
➢ Cloud seeding – substances dispersed into the
air to provide something for condensation to occur
on in
example: Was used in China right before 2008
Beijing games to try and reduce pollution levels
HUMAN FACTORS
➢ Deforestation – Less vegetation means less
interception, less infiltration, more overland flow
leading to more flooding, cycle speeds up
➢ Afforestation – More vegetation means
interception, less overland flow, more
evapotranspiration
➢ Dam construction – Dams reduce downstream
river flow and discharge, increase surface stores so
more evaporation
Example: Lake Nasser behind Aswan dam in Egypt
– 10-16 billion m3
water loss from Nile
➢ Change in land use – Infiltration is 5 times faster
under forests compared to grasslands. Converting
land to farmland means less interception, increased
soil compaction and more surface runoff
➢ Ground water abstraction – When water is
taken out faster that the water is recharged,
groundwater flow decreases and the water table
drops
Example: In China, groundwater irrigates 40% of
farmland whilst 70% of drinking water comes from
groundwater
➢ Irrigation – Drop in water tables due to high water
usage.
Example: Aral Sea in Kazakhstan shrank in 1960s
due to farmers using the water to grow cotton
➢ Urbanisation – Impermeable surfaces reduce
infiltration, increase surface runoff, river discharge
increase. Cycle speeds up like
Storm Hydrographs
Storm hydrographs represent the variation in discharge within a short period of time (days,
rather than years). Before a storm begins, the main supply of water to the river is through
groundwater or base flow. However, as a storm develops, infiltration and surface runoff will
increase which causes a greater throughflow.
Features of storm hydrographs include:
▪ Rising limb – The increase of river
discharge, not necessarily instantaneously
after precipitation.
▪ Peak flow - The maximum discharge,
delayed after maximum precipitation has
occurred.
▪ Lag time – The time delay between peak
rainfall and peak discharge
▪ Falling limb – As the storm precipitation
levels decrease, discharge will in turn
decrease over time.
▪ Base flow – Eventually, the discharge
returns to its normal level
Physical factors influencing Hydrographs
Flashy Storm Hydrograph Subdued Storm Hydrograph
Description of
hydrograph
Short lag time
High peak
Steep rising limb
Long lag time
Low peak
Gently sloping rising limb
Weather/Climate Intense storm which exceeds
infiltration capacity of soil
Rapid snow melt as
temperatures rise above 0o
suddenly
Low evaporation rates due to
low temperatures
Steady rainfall which is less
than the infiltration capacity of
soil
Slow snow melt as
temperatures rise very slowly
High evaporation rates due to
high temperatures
Rock type Impermeable rocks like granite
which encourage rapid surface
runoff
Permeable rocks like limestone
which allow for infiltration and
reduce runoff
Soil Low infiltration rate High infiltration rate
Relief High and steep slopes → More
runoff
Low and gentle slopes → Less
runoff
Basin size Usually small basin Usually large basin
Vegetation Low density vegetation, less
interception and more rapid
movement of water
High density vegetation, more
interception, more
evapotranspiration
Pre-existing conditions
(Antecedent conditions)
Basin already wet from previous
rainfall
Basin dry
Low water table
