HOO322324_D48CH
SCHOOL OF ENERGY, GEOSCIENCE, INFRASTRUCTURE & SOCIETY
Urban Studies
D48CH CATCHMENT HYDROLOGY
Semester 1 – 2020/21
Question 1
The water cycle, also commonly known as the hydrologic cycle, involves the unending
circulation of water as it takes a course from Earth’s oceans to the surrounding atmosphere,
and back down to the land in the form of precipitation. There is a total of six steps in this
cycle, each of which are affected and influenced by different factors. The first step in the
hydrologic cycle is precipitation.
Precipitation refers to any water that freezes or liquidises in the atmosphere and then falls
back down to Earth. Types of precipitation include rain, snow, hail, glaze ice, dew, and frost.
The origin of rain, snow and hail involves the process of cloud formation caused from
supersaturation. The term ‘supersaturation’ refers to the state where a solute contains
more of a solute than can be dissolved in it. This often leads to some of the solute detaching
or separating itself from the solution. In order to achieve supersaturation, the relationship
between temperature and water needs to be changed. For example, orographic uplift is a
process where water, or air containing water, that moves onto land starts moving up
mountains. As the top of the mountain is colder than the bottom, the rising, warm air in
water cools down, causing supersaturation to occur. Everything that occurs in clouds below
a zero-degree line is referred to as a warm cloud process, as water contained in it stays in
liquid form. As the process of nucleation, that appears above it, starts to occur more rapidly,
1
, HOO322324_D48CH
big pieces of hail start to form. Due to their weight, they fall back down into the warm cloud
process, where they either start to melt or gain an additional layer of water. The wind blows
them back up and they gain another successive layer, and this process continues until they
get so large and heavy that they are unable to be blown back up and instead fall to the land.
There are different instruments for measuring precipitation, such as pluviometers and
tipping buckets (near surface measurement techniques). An intensity, duration, and
frequency (IDF) curve helps define a relationship between expected rainfall intensity given
its duration with respect to the frequency of events. We are able to compare how fast and
how often rain falls and compare it to historical records.
The second stage in the cycle is infiltration. Infiltration occurs when water enters a surface
and then moves downwards. This rate of infiltration is influenced by porosity, as when there
is increased permeability there is faster infiltration, slope, where a steeper slope prevents
infiltration and there is an inverse relationship, and soil saturation, another inverse
relationship as if there is low soil saturation, a lot of infiltration can occur, and if there is
high soil saturation, waterlogging takes place and there is less infiltration. The size
distribution of the solids that exist in soils consist of three major components: clay (small
particles), silt (medium particles) and sand (large particles). Macropores, such as sands or
larger materials, are more porous and therefore enable a higher rate of infiltration where
water is transported to deeper parts of the soil profile, whereas there is less infiltration
where there are micropores, such as clays and silts and they are smaller and less permeable.
The climate can also have an impact on infiltration, as soils during winter may freeze,
preventing much water from infiltrating. A Hyetograph (Hortonian [infiltration] curve)
displays the relationship between water inputs and infiltration through time. Infiltration is
highest at the beginning of a storm and gradually decreases as the soil becomes more
saturated.
Groundwater is the water that has infiltrated and accumulated beneath the surface in a
saturated (phreatic) zone. It occurs in open spaces within sediment or in natural fractures,
such as rocks. Once the water has reached the saturated zone below the water table, it is
referred to as groundwater. An aquifer is a body of permeable rock which is able to contain
or transfer groundwater. The water slowing layer below and above an aquifer is called an
2
SCHOOL OF ENERGY, GEOSCIENCE, INFRASTRUCTURE & SOCIETY
Urban Studies
D48CH CATCHMENT HYDROLOGY
Semester 1 – 2020/21
Question 1
The water cycle, also commonly known as the hydrologic cycle, involves the unending
circulation of water as it takes a course from Earth’s oceans to the surrounding atmosphere,
and back down to the land in the form of precipitation. There is a total of six steps in this
cycle, each of which are affected and influenced by different factors. The first step in the
hydrologic cycle is precipitation.
Precipitation refers to any water that freezes or liquidises in the atmosphere and then falls
back down to Earth. Types of precipitation include rain, snow, hail, glaze ice, dew, and frost.
The origin of rain, snow and hail involves the process of cloud formation caused from
supersaturation. The term ‘supersaturation’ refers to the state where a solute contains
more of a solute than can be dissolved in it. This often leads to some of the solute detaching
or separating itself from the solution. In order to achieve supersaturation, the relationship
between temperature and water needs to be changed. For example, orographic uplift is a
process where water, or air containing water, that moves onto land starts moving up
mountains. As the top of the mountain is colder than the bottom, the rising, warm air in
water cools down, causing supersaturation to occur. Everything that occurs in clouds below
a zero-degree line is referred to as a warm cloud process, as water contained in it stays in
liquid form. As the process of nucleation, that appears above it, starts to occur more rapidly,
1
, HOO322324_D48CH
big pieces of hail start to form. Due to their weight, they fall back down into the warm cloud
process, where they either start to melt or gain an additional layer of water. The wind blows
them back up and they gain another successive layer, and this process continues until they
get so large and heavy that they are unable to be blown back up and instead fall to the land.
There are different instruments for measuring precipitation, such as pluviometers and
tipping buckets (near surface measurement techniques). An intensity, duration, and
frequency (IDF) curve helps define a relationship between expected rainfall intensity given
its duration with respect to the frequency of events. We are able to compare how fast and
how often rain falls and compare it to historical records.
The second stage in the cycle is infiltration. Infiltration occurs when water enters a surface
and then moves downwards. This rate of infiltration is influenced by porosity, as when there
is increased permeability there is faster infiltration, slope, where a steeper slope prevents
infiltration and there is an inverse relationship, and soil saturation, another inverse
relationship as if there is low soil saturation, a lot of infiltration can occur, and if there is
high soil saturation, waterlogging takes place and there is less infiltration. The size
distribution of the solids that exist in soils consist of three major components: clay (small
particles), silt (medium particles) and sand (large particles). Macropores, such as sands or
larger materials, are more porous and therefore enable a higher rate of infiltration where
water is transported to deeper parts of the soil profile, whereas there is less infiltration
where there are micropores, such as clays and silts and they are smaller and less permeable.
The climate can also have an impact on infiltration, as soils during winter may freeze,
preventing much water from infiltrating. A Hyetograph (Hortonian [infiltration] curve)
displays the relationship between water inputs and infiltration through time. Infiltration is
highest at the beginning of a storm and gradually decreases as the soil becomes more
saturated.
Groundwater is the water that has infiltrated and accumulated beneath the surface in a
saturated (phreatic) zone. It occurs in open spaces within sediment or in natural fractures,
such as rocks. Once the water has reached the saturated zone below the water table, it is
referred to as groundwater. An aquifer is a body of permeable rock which is able to contain
or transfer groundwater. The water slowing layer below and above an aquifer is called an
2
