WATER CYCLE Diagrams 1

1 The urban water cycle in your garden

Water Cycle in Urban Catchments
Water is essential for all life on earth. About three billion years ago life began as small microscopic
marine organisms. Nearly 300 million years ago primitive amphibians crawled out of their watery
home on to dry land. Humans appeared about 1 million years ago. Since then our numbers have grown
enormously and our way of life has had profound consequences for the water cycle, particularly in
urban areas.
The water cycle begins with water being evaporated by the sun mainly from the sea, which becomes
vapour and forms into clouds. Some of the cloudy vapour blows inland where it falls as rain before
eventually flowing back to the sea.
The main source of fresh water is rainfall runoff which is widely used to meet human needs. Runoff is
a vital part of long-term water supply and renews all water resources, be they rivers, lakes or reservoirs.
Most plants living on land have roots buried in the soil and these plants absorb life-giving water from
the soil. Nitrogen, phoshporus, calcium and many other essential plant nutrients are found in soils and
are carried up into the stems and leaves by water.
There is a continuous cycle of water on earth and the total amount remains close to a constant. The
driving force is the sun evaporating water from the sea, lakes and plants. Water cycle in soils is
especially important for the growth of plants. When rain falls onto the earth, water is absorbed by soils
and excess water flows over the land into creeks and rivers. Water may evaporate from soils or filter
down through the soil to the water table. Plants absorb water from soils and the water is evaporated
from leaves into the atmosphere, completing the soil water cycle.
Every creek, river and lake is surrounded by a catchment and seperating each catchment is a divide
where water flows on the other side in the opposite direction into the neighboring catchment. The total
catchment should be considered in water management schemes. When possible it is best if management
schemes begin in the upper catchment.

Artificial water cycle
In cities the man made water cycle is an addition to the natural water cycle. The Artificial water cycle
includes: town water supply, watering of parks and gardens, sewage system including the leakage of
sewage into creeks and stormwater drains. Many suburban drains always have a water flow, even
during long droughts because of artificial sources like excessive watering of gardens, washing cars and
washing footpaths.
Precipitation
Precipitation is water falling from the atmosphere to the earth as rain, hail, snow, sleet, fog or
dew.
When rain falls on the earth, it is first intercepted by vegetation which covers the land. A small
proportion of the rain is evaporated directly from plant surfaces. Water is collected in the upper canopy
of many tree species, flowing down the stems and trunk into the soil helping to improve water supply
to trees surrounded by concrete and tar.
During rain, water is stored on the surfaces of leaves and stems. When it ceases to rain water will
continue to drip from trees. Leaf drip helps to even out the intensity of storms and may have a small
effect on reducing flood peaks.

Evaporation
An evaporimeter is used to measure the rate of evaporation directly fom a water surface. In Sydney the
annual evaporation of 1800 mm is higher than rainfall, 1200 mm. The ratio between
evaporation/precipitation is the P/E ratio and is a measure of the water available from rain for plant
growth. In Sydney the annual P/E is 0.67, in summer months P/E is less than 1 and as low as 0.33 and
in winter months evaporation drops to 80 mm/month and the P/E rises to 1.7. This data indicates the
high evaporation in summer months restricts plant growth and homegardens, especially lawns need to
be watered to keep plants growing.
The above data demonstrates evaporation is highly significant in Sydney. The climate in many cities in
the world is different and evaporation is lower especially in the Northern cities of London and New
York.
Water is also directly evaporated from surface soils into the atmosphere. Evaporation is faster from
bare soils than protected soils. Mulches protect soils directly from sunlight and reduce water loss by
evaporation. Mulches also insulate soils, keeping them cooler during summer months and reducing
evaporation. Water evaporation from garden soils is a waste of water and in times of drought should be
prevented.

Transpiration
Growing plants transpire water into the atmosphere when they absorb large volumes of water
from soils, which travels up the roots and stems to the leaves. Water evaporates through
stomates, which are small pores in the leaf surface, into the atmosphere.
Water travelling up the stems transports minerals from soils up to the leaves where organic substances
are manufactured for plant growth. Trees transpire large volumes of water. In Sydney a large gum tree
transpires up to 200 litres of water on a sunny summer day.
Direct sunlight is the driving force of transpiration. Trees with a large leaf area transpire water quickly
and high up in the canopy, winds blow moisture away encouraging faster transpiration. Native trees are
evergreens and transpire water in all seasons, while very little transpiration occurs in deciduous trees
during winter when they lose their leaves.
Deep roots enhance transpiration. Many Australian native trees have very deep roots, up to 40 metres in
favourable soils and often the roots of large trees reach down to the water table. During dry spells,
surface soils dry out and native trees with deep roots continue to grow using water in the subsoil. This
helps to make native trees drought resistant. In suburban Sydney the roots of trees sometimes penetrate
into sewage pipes and are able to survive in the driest droughts.
Quick growing native trees have the ability to transpire 2-4 times more water in a year than the average
annual rainfall.
During the severe drought of the early 2000’s in Australia, the depth to the water table increased.

Unfortunately many native trees died in country landscapes because of a lack of water and the inability
of roots to reach down to deep water tables.
Grasses have shallow roots. Annual grasses have very shallow roots often less than 0.5 metres deep.
Permanent grasses have deeper roots and kikuyu roots can be as deep as 2.5 metres. Lawns with
shallow rooted grasses need to be regularly watered at frequent intervals during hot summer weather.
Many lawn grass species are not native to Australia and are not suited to hot dry summers and need to
be watered regularly to be healthy and green. In drought prone Sydney water can be saved by
replacing thirsty lawns with drought proof native species.
Paths and roads made of concrete or tar restrict water entery into soils, but transpiration can occur from
soils covered by impermeable surfaces if plant roots have spread under the path or road. If a tree
canopy spreads over an impermeable surface, the rate of transpiration will depend on the area of the
canopy and not on the smaller area of permeable surface.
Evapotranspiration is the sum of evaporation from surface soils plus transpiration from plants.
Scientists often study evapotranspiration because it is easier to measure than to separately measure
transpiration and evaporation.

Stormwater
Water which does not infiltrate into soils becomes surface runoff which flows downhill eventually
concentrating in rills, creeks and rivers. A small amount of water is trapped in puddles and
becomes depression storage.
Undisturbed creeks meander through trees, bushes and grasses with flood plains waterholes, riffles and
waterfalls. Swamps purify and store water. Aquatic plants and animals thrive in natural ecosystems.
In urban areas, many original creeks are buried in pipes or bulldozed into straight concrete canals.
Living native ecosystems disappear under the sterile drainage systems of the councils engineers. Fish
have difficulty surviving in the polluted water.
Flash flooding is a problem in suburbs with concrete drains and where soils are covered with
impermeable surfaces. Roads, buildings and other impermeable surfaces prevent water entering soils
and all the water flows into stormwater drains. Natural drainage systems store water in swamps and
flood plains reducing the severity of floods. Creeks have bends and meander through a valley slowing
down the water velocity.
A short time of concentration is another factor increasing flood peaks. The travel time from the most
remote point of the catchment to the outlet is the time of concentration. In urban catchments water
flows faster, the time of concentration is shorter and flood peaks higher, compared to rural or natural
catchments
Water flowing in a concrete gutter flows about three times faster than in a grass waterway. Ecodesigned
waterways which meander increase length of the waterway and reduce the slope. Waterways
incorporating natural features will reduce the velocity of water flow and reduce flood peaks.
Straight, concrete canals are designed to swiftly drain water downhill away from upstream, flooded
areas and can increase flooding downstream. Eco-designed drainage systems remove water slowly
from flooded areas but reduce floods downstream. The design of drainage systems need to have a
balance between these factors.
Landform surrounding creek systems varies greatly, especially when the geology varies. In Sydney
creeks running through areas of Hawkesbury Sandstone geology group, are found in narrow, rocky,
steepsided valleys. The creeks flow swiftly along the steep gradient with riffle zones and small
waterfalls. These creeks have small flood plains, few bends and do not meander.
In Western Sydney many creeks meander through flat flood plains. On the Wianamatta Shale geology
group, a common landform is gently undulating hills and creeks meandering through floodplains.
Along the Nepean River valley there are extensive ancient floodplains dissected by creeks.
Many new housing developments increase flash flooding when swamps and floodplains are filled in
and built on. Culverts and bridges built in the earlier rural landscapes are now often too narrow and act
as a dam during heavy rain, blocking the flow of creeks and causing flooding upstream. Stormwater is
increased in new housing developments because of the extensive areas of impermeable buildings and
road surfaces.
Water sensitive urban design is now used in new suburban developments. Houses, buildings, gardens
and drainage systems are designed to reduce water wastage and stormwater flow. Flooding can be
reduced by reducing concrete paving, installing rainwater tanks and encouraging water infiltration into
soils. Water detention basins temporarily hold and gradually release water after flood peaks. The
severity of droughts can be reduced by harvesting stormwater and using the saved water in the home block.

Water Table and Groundwater
Groundwater is found in saturated subsurface soils or rocks. The upper surface of groundwater
is the water table.
Rainwater entering soils is returned to the atmosphere by evaporation and transpiration and excess
water percolates down to the water table. The water table rises during rainy seasons or falls in dry
periods.
When the water table comes to the surface a spring will form. The water table comes to the surface at
rivers or lakes. The supply of ground water helps to keep rivers flowing when it is not raining. After
heavy rain when the water table rises, intermittent springs may form.
Leakage occurs when water percolates down below the root zone and down to the water table.
The water table depth is equal to the depth down to the water surface in a well. A piezometer can be
used to measure water table depth by drilling a small hole in the soil down to below the water table.
The hole will fill up with water to the water table depth. Using two or more piezometers and measuring
the difference in the water table between different piezometers enables the direction of groundwater
flow to be determined.

Water Catchments
Drainage basin for a stream or lake is the water catchment. A catchment is the area determined
by topography where surface runoff from rain will flow downhill in drainage lines and then into
a stream. The divide or watershed is a ridge separating neighbouring catchments.
Rainwater falling in a catchment follows a number of different processes. Evaporation returns water
back to the atmosphere. Runoff water flows overland down to creeks, streams and rivers and eventually
flows to the sea. Water infiltrating into the soil provides water for plant growth and is returned to the
atmosphere by transpiration. Excess water in soils percolates down to the watertable and eventually
seeps into creeks and rivers.
Water balance in a undisturbed catchment in Sydney region
Interception by plant surfaces 15%
Runoff 15%
Seepage to groundwater and rivers 10%
Evapotranspiration 60%
Total 100%
In natural catchments 70% of annual rainfall is absorbed by soils. The amount of water absorbed by
soils in one rainy day varies. Water absorption is lower if the soil moisture content is high and water
runoff is greater during heavy storms. Water absorption will vary in different parts of a catchment
depending on topography, soil type, vegetation cover and other natural characteristics.
Swamps and bogs store water when it rains and gradually release water into creeks during dry weather.
Water is purified in swamps. Swamps are a very important feature in natural catchments and are often
filled in and built over in urban catchments.
Many creeks and rivers have flood plains. Water flows onto flat land surfaces on the sides of rivers
during heavy storms. Flood plains act as a large water storage resevoir, reducing down stream flooding.
Levee banks built on one side of a river can increase flooding on the opposite side of the river.
Natural catchments are altered in many different ways by people. The first victims are often trees.
Cutting down trees reduces water infiltration into soils increasing surface runoff and flooding. The
water table rises after trees are cut down and transpiration is reduced.
Erosion may be a problem after trees are cut down and farmed with poor management practices used
when growing crops and grazing animals.
Roads alter the water characteristics in a catchment. Roads can act as barriers to natural drainage lines
concentrating water into artificial drains. Water can back up causing flooding behind bridges and
culverts with a small capacity. Water runoff from poorly designed roads can cause erosion.
In towns and cities catchments are greatly altered and many problems may occur including flooding
and pollution.

Water balance in high density urban catchment
Runoff up to 90%
Evapotranspiration and seepage to ground water 10%
Total 100%
Rain falling onto the impermeable sufaces of roads, paths and buildings becomes surface runoff and
flows into the council drainage systems.
Home gardens, streetside nature strips and council parks have permeable surfaces. Water absorbed by
garden soils is used by growing plants and is transpired back into the atmosphere. Excess water in soils
percolates down to the water table.
Natural drainage lines, creeks, swamps, wetlands, billabongs and anabranches are filled in and
destroyed when suburban drainage systems are rebuilt. The natural features of catchments help to even
out waterflow, reducing floods and maintaining flow in dry times.
Councils often used swamps and flood plains as garbage dumps. The garbage was covered with a layer
of soil and a park created. These parks were flat and made ideal football fields and cricket grounds.
Councils called this “land reclamation”. The destruction of swamps and flood plains causes
environmental havoc, especially increased flooding and pollution from the garbage dumps.
When streets and houses are constructed land is levelled, drainage lines filled in and runoff collected in
surface gutters running alongside streets. Gutters on the surface collect little seepage from soils. Urban
drainage is efficient in removing surface runoff and not very effective in draining waterlogged soils.

Floods
Flash floods occur in urban areas. Flood peaks are higher in urban areas because there are more
impermeable surfaces and water flows faster down concrete drains. Urban drainage systems reduce
flooding upstream from the drains and sometimes flooding may be increased in the downstream water
disposal area.
In natural catchments flood peaks are lower and water flow in dry weather is more reliable. Natural
wetlands and swamps help to even out water flow in creeks and rivers. Water flow is slower in
meandering creeks, and rocks and fallen trees act as obstacles. During rain, swamps absorb water and
slowly release water into creeks during dry weather. Water infiltrates into permeable soils and some of
this water seeps down to the groundwater and slowly percolates into creeks and rivers. Flood plains are
filled in and built over increasing flood heights.

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Water Sensitive Urban Design, WSUD
WSUD is complementary rather than antagonistic to the natural water cycle. Suburban areas
should be more compatible with natural hydrological and ecological processes with on-site
collection, treatment and utilisation of water flows.
Many on-site technologies are now used to reduce flooding and pollution. Often pollution loads are
higher in floodwaters and the control of flooding can reduce pollution.
Flooding can be reduced with on-site technology in the upper catchment, reducing the necessity for
concrete pipes and drains designed to carry water downhill with the possibility of causing more severe
flooding. Multiple use technologies are now commonly used. Garden water tanks are a proven way to
save water for the home garden and reduce stormwater flood peaks.
In heavy storms leakage of sewage into stormwater drains commonly occurs. Stormwater harvesting
for irrigating parks is now recommended by the State Government and unfortunately stormwater is
often highly polluted with sewage.