Topic 5: The water cycle and water insecurity
What are the processes operating within the hydrological cycle from global to local scale?
The global hydrological cycle:
A system is any set of interrelated components that are connected together to form a working
whole.
• characterised by inputs, stores, processes (or flows) and outputs
A closed system occurs when there is a transfer of energy but not matter between the system
and its surroundings (the inputs come from within the system).
• The Hydrological cycle
The hydrological cycle is a closed system because all the water is continually circulated
through the stores and there is a constant amount of water in the system.
Inputs, outputs, stores and flows
Inputs:
- Precipitation
Outputs:
- Evaporation from water surfaces
- Evaporation and transpiration (evapotranspiration) from vegetation
- River discharge
Stores:
- Groundwater
- Interception
,- Soil water
- Rivers
Flows:
- Through-flow
- Infiltration
- Groundwater
- Base flow
- Overland flow
- Stem flow
- Channel flow
An open system receives inputs from and transfers output of energy and matter to other
systems.
• The Drainage basin
The global water budget is the annual balance of water fluxes and the size of the water stores
water fluxes are the rate of flow between stores
The water stores have different residential times, but the constant circulation, albeit at variable
speeds, means that water is generally considered a renewable resource.
• Fossil water is an exception; this is water that has been contained in an undisturbed
place, usually as groundwater in an aquifer
Residence time is the average time a water molecule will spend in a store.
If a water is stagnated in an area for a long period of time it may become polluted.
,The drainage basin
A drainage basin is an area of land that is drained by a river and its tributaries, ands separated
from neighbouring drainage basins by a ridge of high land called a watershed or divide.
• A drainage basin is an open system. As it is linked to other systems by inputs and
outputs and involves a number of linked processes and stores.
Physical factors affecting drainage basin inputs.
Precipitation “type, frequency, distribution”:
The highest precipitation inputs to drainage basins are found in the tropics, due to the
Intertropical Convergent Zone (ITCZ), as this leads to convectional precipitation. The relief may
also affect the levels of precipitation due to orographic precipitation.
Physical factors affecting drainage basin flows
Interception:
Interception is the process by which raindrops are prevented from falling directly on to the
ground surface by the presence of a layer of vegetation. The water later reaches the soil via
stem flow (water flowing down the vegetation stems to the ground below)
The undergrowth may intercept again some of the water falling from the canopy, secondary
interception.
Some of the water will return to the atmosphere via evapotranspiration (known as
interception loss)
The vegetation type and cover also influences the amount of interception: the denser types of
vegetation, such as coniferous forests, intercept more rainfall than sparser deciduous forest.
, Infiltration and throughflow:
Infiltration is the movement of water vertically downwards through pores in the soil, and the
infiltration capacity is the maximum rate at which the soil can absorb precipitation.
o The rate of infiltration varies by the water which is already in the soil - degree of
saturation.
Direct runoff (overland flow) : Water flowing over the surface of the ground is known a direct
runoff or overland flow.
There are two types:
• Saturated overland flow - Saturated overland flow occurs when water accumulates in
the soil until the water table reaches or ponds on the surface, forcing further rainwater
to run off the surface.
• Infiltration-excess overland flow - Infiltration-excess overland flow occurs when the
rainfall intensity exceeds the infiltration capacity, so the excess water flows over the
ground surface.
Percolation and groundwater flow: When infiltrating water reaches permeable underlying
bedrock, it will continue to move slowly downwards into the rock by the process of percolation.
Until it reaches a groundwater storage or an aquifer.
The rate of percolation and groundwater flow will depend on the permeability of the rock,
which is linked to its porosity.
Groundwater flow will increase with porosity and perviousness impermeable rock as well as the
angle of rock strata and gravity
What are the processes operating within the hydrological cycle from global to local scale?
The global hydrological cycle:
A system is any set of interrelated components that are connected together to form a working
whole.
• characterised by inputs, stores, processes (or flows) and outputs
A closed system occurs when there is a transfer of energy but not matter between the system
and its surroundings (the inputs come from within the system).
• The Hydrological cycle
The hydrological cycle is a closed system because all the water is continually circulated
through the stores and there is a constant amount of water in the system.
Inputs, outputs, stores and flows
Inputs:
- Precipitation
Outputs:
- Evaporation from water surfaces
- Evaporation and transpiration (evapotranspiration) from vegetation
- River discharge
Stores:
- Groundwater
- Interception
,- Soil water
- Rivers
Flows:
- Through-flow
- Infiltration
- Groundwater
- Base flow
- Overland flow
- Stem flow
- Channel flow
An open system receives inputs from and transfers output of energy and matter to other
systems.
• The Drainage basin
The global water budget is the annual balance of water fluxes and the size of the water stores
water fluxes are the rate of flow between stores
The water stores have different residential times, but the constant circulation, albeit at variable
speeds, means that water is generally considered a renewable resource.
• Fossil water is an exception; this is water that has been contained in an undisturbed
place, usually as groundwater in an aquifer
Residence time is the average time a water molecule will spend in a store.
If a water is stagnated in an area for a long period of time it may become polluted.
,The drainage basin
A drainage basin is an area of land that is drained by a river and its tributaries, ands separated
from neighbouring drainage basins by a ridge of high land called a watershed or divide.
• A drainage basin is an open system. As it is linked to other systems by inputs and
outputs and involves a number of linked processes and stores.
Physical factors affecting drainage basin inputs.
Precipitation “type, frequency, distribution”:
The highest precipitation inputs to drainage basins are found in the tropics, due to the
Intertropical Convergent Zone (ITCZ), as this leads to convectional precipitation. The relief may
also affect the levels of precipitation due to orographic precipitation.
Physical factors affecting drainage basin flows
Interception:
Interception is the process by which raindrops are prevented from falling directly on to the
ground surface by the presence of a layer of vegetation. The water later reaches the soil via
stem flow (water flowing down the vegetation stems to the ground below)
The undergrowth may intercept again some of the water falling from the canopy, secondary
interception.
Some of the water will return to the atmosphere via evapotranspiration (known as
interception loss)
The vegetation type and cover also influences the amount of interception: the denser types of
vegetation, such as coniferous forests, intercept more rainfall than sparser deciduous forest.
, Infiltration and throughflow:
Infiltration is the movement of water vertically downwards through pores in the soil, and the
infiltration capacity is the maximum rate at which the soil can absorb precipitation.
o The rate of infiltration varies by the water which is already in the soil - degree of
saturation.
Direct runoff (overland flow) : Water flowing over the surface of the ground is known a direct
runoff or overland flow.
There are two types:
• Saturated overland flow - Saturated overland flow occurs when water accumulates in
the soil until the water table reaches or ponds on the surface, forcing further rainwater
to run off the surface.
• Infiltration-excess overland flow - Infiltration-excess overland flow occurs when the
rainfall intensity exceeds the infiltration capacity, so the excess water flows over the
ground surface.
Percolation and groundwater flow: When infiltrating water reaches permeable underlying
bedrock, it will continue to move slowly downwards into the rock by the process of percolation.
Until it reaches a groundwater storage or an aquifer.
The rate of percolation and groundwater flow will depend on the permeability of the rock,
which is linked to its porosity.
Groundwater flow will increase with porosity and perviousness impermeable rock as well as the
angle of rock strata and gravity
