Unit 3 - Physical Systems and Sustainability (9GEO01)
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Topic 5 – Water Cycle and Water Insecurity Notes:
EQ1 (5.1-5.3): What are the processes operating within the hydrological cycle from global to local scale?
5.1:
System a set of things working together as parts of a mechanism
Closed system a natural physical system that doesn’t allow the transfer of matter in or out of the system
Causes of the global hydrological system:
Solar energy evaporation caused by the sun
Gravitational potential energ y the potential energy of a massive object in relation to another subject due to
gravity
o Water flows downhill due to GPE
o Sea level = base level (water rests here)
Water budget the inflows to any water system or area equal to its outflows plus the change in storage during
a time interval (takes into account all the water the in stores and flows)
Stores these are stocks of water, places where water is held (e.g., oceans, glaciers surface runoff,
atmosphere)
Fluxes the measurement of the rate of flow between the stores (the greatest fluxes occur over the oceans)
Processes the physical factors which drive the fluxes of water between stores
Residence Time:
The average time a water molecule will spend in the reservoir or store.
It impacts the turnover within the water cycle
Atmospheric water has the shortest residence of 10 days
The most significant feature of the budget is that only 2.5% of it is freshwater; the rest is in oceans.
Non-renewable - Fossil Water: Ancient, deep groundwater made from pluvial (wetter) periods in the geological
past
Non-renewable - The Cryosphere: Made up of those areas of the world where water is frozen into snow or ice
Hydrology of polar and tropical rainforest regions:
Polar Regions: Tropical Rainforest Regions:
Marked seasonal differences Limited seasonal differences
High pressure – sinking air Low pressure – rising air
Long residence time Short residence time
Less rainfall (less than 100mm) Lots of rain fall (up to too 2500mm)
Dry and cold Wet and warm
Limited vegetation Dense vegetation
Orographic + frontal precipitation Convectional rainfall
ITCZ:
Intertropical convergence zone
The area of intense heating moves north and south of the equator
Most of the world’s rainfall is created here
Area of low pressure
Biggest flows
Thermohaline Circulation:
, The way which heat is transferred around the global oceans
Colder denser and more saline water sinks (polar regions)
Sinking water draws in replacement water from warmer tropics
Warm tropical water moves to north/south and is replaced by cold water that is drawn up from the ocean
bottom
5.2:
Water insecurity the lack of a reliable source of eater or lack of quantity to meet the needs of the local human
population and environment
Water stresses the lack of freshwater resources to meet the standard water demand (physical and economic
scarcity)
Inputs:
The main input is precipitation:
1. Form: rain, snow or hail. Clearly, with snow, entry of water into the drainage system will be delayed.
2. Amount: this will affect the amount of water in the drainage basin and the fluxes within it.
3. Intensity: the greater the intensity, the greater the likelihood of flooding.
4. Seasonality: this is likely to result in the drainage basin system operating at different flow levels at different
times of the year.
5. Distribution: this is significant in very large drainage basins, such as the Nile and the Ganges, where tributaries
start in different climate zones.
Flows:
1. Interception: the retention of water by plants and soils which is subsequently evaporated or absorbed by the
vegetation.
2. Infiltration: the process by which water soaks into, or is absorbed by, the soil.
3. Percolation: similar to infiltration, but a deeper transfer of water into permeable rocks.
4. Throughflow: the lateral transfer of water downslope through the soil
5. Groundwater flow: the very slow transfer of percolated water through pervious (permeable) or porous rocks.
6. Surface runoff: the movement of water that is unconfined by a channel across the surface of the ground. A.k.a.
overland flow.
7. River or channel flow takes over as soon as the water enters a river or stream; the flow is confined within a
channel.
Outputs:
1. Evaporation: the process by which moisture is lost directly into the atmosphere from water surfaces, soil and
rock.
2. Transpiration: the biological process by which water is lost from plants through minute pores and transferred to
the atmosphere.
3. Discharge (channel flow): into another, larger drainage basin, a lake or the sea.
Factors affecting flows in a drainage basin:
Factor: Impact:
Snow covered slopes Flow concentrated in spring / summer
Steep slopes Less infiltration
Permeable rock increase infiltration
Impermeable rock Less infiltration
High drainage density High surface runoff
Low drainage density Low surface runoff
Rural land use More infiltration and absorption
, Urban land use More surface runoff
Use of reservoirs Reduces surface runoff increase evaporation
Large drainage basin More water flow through systems
Forested slopes Increase interception
Factors affecting infiltration:
Duration of rainfall
Antecedent soil moisture (water from the previous event)
Oil porosity
Vegetation cover
Raindrop size
Slope angle
Saturated overland flow (no space) the flow of water occurring on the ground surface when there is excess
water due to the soil being too saturated
Infiltration excess overland flow (space) the flow of water occurring on the ground surface where there is
enough space in the soil, but the water is falling too fast drops are too large and pores are blocked
Types of rainfall:
Orographic Warmer moist Atlantic air from the southwest is forced to rise as it reaches the western
uplands
As it rises it cools over the high ground producing heavy rain over the western and northern
parts of the UK
Frontal Forms over low-pressure areas when warmer moist air from the southwest meets colder
polar air from the north
The warmer air is forced to rise over the denser cold air forming rain along both warm and
cold fronts
Both West and East Coast of the UK
Convectional Typical of the eastern and south-eastern UK in the summer dream periods of high
temperatures.
The rainfall is created is often intense and associated with electrical storms and Thunder
Physical factors affecting drainage basin:
Inputs
o Precipitation
Flows
o Interception
o Percolation and groundwater flow
o Direct runoff (overland flow)
o Infiltration and throughflow
Outputs
o Evaporation and transpiration
o Channel flow
Human factors affecting drainage basin:
Farming and irrigation
Deforestation
Abstraction
Construction of dams
Building river defences
Overpopulation
Pollution from industry
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