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SOILS AND WATER IN CITIES

WATER CYCLE

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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.

Natural water cycle

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rain, hail, snow and fogs

interception and evaporation from plant surfaces

depression storage(puddles)

runoff

soil infiltration and percolation

transpiration and evaporation

water table and ground water

creeks, brooks, rivers and lakes

swamps, wetlands, billabongs and anabranches

oceans, seas, harbours, bays and gulfs

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, phosphorus, 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 pla Big text nts. 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 separating 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 The urban water cycle in towns and cities differs from the natural water cycle in bushland. 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. In cities additional water is supplied to houses and industry by the town water system. Waste water is disposed of in the sewage system. The water catchment is altered and many creeks are converted to concrete lined drains.

Town Water Supply Potable water is high quality water treated to remove harmful microorganisms and is distributed to houses where people can drink the healthy water. Droughts are common in Australia and storage reservoirs need to be larger than many overseas cities. Sydney's reservoirs need to store more water than the total amount of water needed in several years. Potable water supplied to town water systems is very valuable and should be used wisely. Large storage dams need to be built and valuable land is drowned under water. Town water supplies are treated and this costs money and uses energy.

Water harvesting Town water should only be used when clean, healthy water is needed. A large proportion of water does not need treatment to health standards high enough for safe drinking. Potable water is not needed for watering gardens or flushing toilets. Many industries do not need potable water. The use of potable water directly from taps needs to be reduced and alternate water supplies should be utilized. Suburban water harvesting should be used when high quality potable water is not necessary for health reasons. Falling rain can be harvested directly in the suburbs and gardeners are now encouraged to install rainwater tanks. Industry and Councils can save water by harvesting rainwater runoff for industrial purposes and irrigating parklands. Trees can be irrigated by water harvested from council stormwater drains. Well watered, fast growing trees will reduce floods and store carbon in a greenhouse sink.

Sewage systems Water can leak from sewage pipes into natural drainage lines. Leakage of sewage can cause very serious pollution. Grey water is sewage consisting of water from kitchen, bathroom and laundry. Black water is water from toilets and is the most polluted form of sewage. Water can leak from cracks in old dilapidated sewage pipes and during rainfall water can seep into sewagelines filling the pipes to capacity and forcing sewage to overflow from pipes into natural drainage lines.

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

Failing rain is intercepted by vegetation. A small proportion of the rain is evaporated directly from the plants surfaces. Even during rainfall evaporation occurs, especially from tall trees. Studies on a south coast of NSW forest showed 15% of yearly rainfall was evaporated directly from the forest foliage.

With many tree species water is collected in the upper canopy and flows down the stems and eventually down the trunk into the soil. In many urban situations this process could be of major significance. The supply of water to the roots of a tree can be restricted by impermeable paving surrounding the tree. Water collected by the canopy and flowing down the trunk into the soil will help to increase water infiltration into the soil and to counter balance the restriction of water infiltration by impermeable paving.

During rain, water is stored on the surfaces of leaves and stems. When it ceases to rain water will continue to drip from a tree. This can help to even out a rainstorm and to reduce flood peaks.

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 precipitation/evaporation 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.

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 into atmosphere through stomates, which are small pores in the leaf surface.

Water traveling 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 liters 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.

The rate of transpiration is proportional to the leaf area. Leaf area index (LAI) measures area of leaves and LAI equals 1 when the area of leaves equals land surface area. Trees have a large LAI, up to 6 and deep roots encouraging a high transpiration rate. Transpiration by plants helps to dry out soils. During rain, water will infiltrate more readily into a dry soil. The removal of water by transpiration allows more water to enter soils during rain and this will reduce water runoff and lower flooding.

The rate of transpiration is regulated by the total evaporation. Different plant species transpire at different rates. The ratio between transpiration/total evaporation is often called the crop factor and can be as high as 0.95 for lucerne in Jan(summer). Deciduous trees vary from 0.75 in Jan down to 0.1 in June (winter), when deciduous trees lose their leaves in winter.

Deep rooting plants have the ability to utilize water at greater depths in the soil. This enables these plants to grow and transpire when shallow rooting plants have wilted and ceased to transpire. Deep rooting plants generally have high crop factors. eg lucerne develops very deep roots and has a high crop factor.

Deep roots enhance transpiration. Many Australian native trees have very deep roots, up to 40 meters in favorable 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.

The depth roots penetrate into a soil depends on the availability of water. If no available water is in the subsoil, plants develop a shallow root system. Frequent light watering encourages shallow roots while less frequent heavy watering ensures water penetrates deep into the subsoil and encourages growth of deep roots. Plants with deep roots are more drought tolerant.

Quick growing native trees have the ability to transpire 2-4 times more water in a year than the average annual rainfall.

Grasses have shallow roots. Annual grasses have very shallow roots often less than 0.5 meters deep. Permanent grasses have deeper roots and kikuyu roots can be as deep as 2.5 meters. 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.

In certain situations the removal of trees has resulted in the rise of the water table. This has caused great problems when there is salt in the subsoil. A rising water table brings the salt to the surface and this can kill vegetation and in urban areas salt can cause damage to buildings.

The vegetation covering the land is a very important component of the water cycle. When vegetation is removed or reduced the checks and balances in the water cycle are disrupted. Trees are large, with deep roots having a major role in maintaining a balanced water cycle.



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. Eco-designed 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 cannals 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.

Groundwater Groundwater is found in saturated subsurface soils or rocks. The upper surface of groundwater is the water table.

Leakage of excess water can occur down a soil profile beyond the root zone and into the groundwater. In urban areas impermeable surfaces reduce water reaching the groundwater. Cutting down trees decreases the removal of soil water by transpiration and the groundwater may rise. The groundwater can either rise or fall after urbanization.

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.

Many native gum trees have deep roots reaching down to the water table. 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.

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.

Salting occurs when salts in the subsoil are brought to the surface by a rising water table. Surface salting often kills plants and damage brick buildings. Salty subsoils are common in some parts of Western Sydney.

SOIL WATER

Field Capacity When a soil has drained freely for several days after rain or irrigation, the water content is the field capacity of the soil.

The field capacity is a measure of the soils ability to store water. A sandy soil freely drains and the field capacity is low. A clayey soil holds water and has a high field capacity. A clayey soil can store more water than a sandy soil.

Soil structure effects the field capacity. A structureless clay with no large pores has a very high field capacity. A well structured clay with large pores will drain freely and has a lower field capacity.

Organic matter can absorb water and increases field capacity.

The field capacity of a soil is mainly determined by the pore size distribution. Small pores hold water by capillary forces. Large pores freely drain and do not hold water at field capacity.

Wilting Point When a soil is dry and plants suffer from permanent wilting because they are unable to absorb water, the moisture content is the wilting point of the soil.

When the moisture content is above the wilting point, plants can absorb water from the soil. Below the wilting point, water is tightly adsorbed to clay particles and is unavailable to plants.

Sandy soils have very low wilting points while clays have high wilting points. A clay contains a significant amount of water unavailable to plants.

Available Water The amount of water stored by a soil that can be absorbed by a plant is the available water and is equal to the water held at the field capacity minus the wilting point.

The available water is the water stored by a soil and is useful to plants. Texture, structure and organic matter have an effect on the available water. Clayey soils generally have a higher available water than sandy soils. A well developed structure can increase the available water. Organic matter absorbs water and increases the available water.
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Typical moisture properties of soils

Soil type        Field capacity         Wilting point                 Available water

Sand                     4.0%               1.3%                            2.7%

Loam                    25.5                12.2                            14.3

Clay                      34.3              16.5                            17.8

The available water stored in a soil is often measured as mm of water. Farmers prefer to sow a crop when there is sufficient water stored in the soil to ensure good plant growth. If the amount of stored water is low, the farmer needs to pray for rain. The amount of water available to plants also depends on the depth of the plant roots.

Water storage in soils

Available water                    mm water stored in 10 cm soil (soil bulk density 1.3 g/cc)

3%                                   2.3mm

10%                                  7.7

15%                                   11.5

Soluble salts in soils reduces the availability of water to plants. Salt increases the osmotic pressure of water. High salt content in soils will kill plants.

Depth of plant roots Plants with deep roots can tap a larger reservoir of water are more drought resistant. Plants with a shallow root system can only absorb water from the surface soil and are more susceptible to droughts.

When irrigating enough water should only be applied to wet the soil to a depth slightly deeper than the root zone of the plants. Excess water will drain to a depth below the roots and will not be available to plants.

Roots do not penetrate into dry soils. When watering plants sufficient water should be applied so the water wets the soil to a depth a little greater than the root depth. Plant roots should be encouraged to grow deep roots and to help drought proof the plants.

Crop Root depth Grass

Pastures 0.6 metres              Kikuyu 2.4m

Sorghum 2                        Paspalum 1.5

Lucerne 6 or more metres         Couch 1.5

Wheat, Oates, Barley 3.3         Buffalo 1.0

Maize 1 to 2                     Kentucky bluegrass 0.4

Rice 0.2                         Bent grasses 0.35

Potatoes 0.6 to 2                Poa annua 0.15

Lettuce 0.1 to 0.2

Fruit trees 0.6 to 1.3

Grape vines 0.5 to 1

Gum trees up to 40m

SOIL WATER INFILTRATION

It is well known that urban development increases flooding. In urban areas the coefficient of runoff increases, and the time of concentration decreases. The coefficient of runoff in undisturbed areas can be as low as 0.1, while in inner city suburbs, coefficient of runoff can be as high as 0.9.

During rainfall a high value of coefficient of runoff occurs when most rainfall flows over the land and little water is absorbed by soils. When all the rain flows over the surface and no rain infiltrates into the soil the coefficient of runoff equals 1. If a high volume of water enters the soil the coefficient of runoff is small and approaches 0.

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 water resources be they rivers, lakes or reservoirs. Plants need water to grow and evapotranspiration by plants returns water from the soil into the atmosphere.

Rate of water infiltration The rate at which water enters the soil during rain or irrigation is the infiltration rate.

When rain first enters a dry soil, infiltration is rapid, then decreases as it continues to rain. During the early stages of rainfall, water fills up the dry pores. When the pores are filled with water and the soil is saturated, infiltration slows down until it reaches a constant value.

When it has stopped raining, the surface soil will be saturated with water. Free water in a saturated soil will drain under gravity. After about 2 3 days, free water will have percolated downwards and the soil moisture will reach an equilibrium. When the free water has drained downwards, the moisture is at 'field capacity'.

At field capacity, soil water is held in the soil by several forces. The major part of the water is held by capillary forces. Some water is adsorbed onto the surfaces of soil particles, especially clay particles and organic matter.

Capillary water is the main supply of water to plants. Capillary water may be removed by transpiration by plants and evaporation. If the top soil is dry and the underlying soil is wet, water can move upwards by capillary action.

Above the water table there is a capillary fringe. Water moves upwards from the water table by Capillary forces. In some. soils, especially clays which contain many fine pores, the capillary fringe can be up to one metre above the water table.

During rain the infiltration rate slows down as the soil becomes wet.

Infiltration rates vary greatly between different soils, and typical values are, 100mm/hour during the first hour of rain, decreasing to 10mm/hr after six hours of rain. (figure 2 and table 4)

Peds are small aggregates (1 2mrn ) found in many soils. Pedal soils have a good structure and water flows easily through these soils. Apedal soils have a poor structure and apedal clayey soils have poor water infiltration. Sandy soils generally have high water infiltration and clayey soils have low infiltration rates.

If the intensity of rainfall is constant, and infiltration decreases at a typical rate, there is no runoff during the early stages of the rain event. When the infiltration rate is slower than the rainfall intensity, runoff is expected to occur.

Infiltration rate is controlled by the soil layer which has the lowest hydraulic conductivity. Often the top soil is a loam and has a high hydraulic conductivity. A clayey subsoil with a low hydraulic conductivity will act as a throttle, slowing down the final infiltration rate.

In the field, infiltration can be directly measured using a ring infiltrometer. (Black 1965) A circular tube is forced into the soil, which is ponded with water. At specific time intervals, rate of water flow into the soil is recorded. To reduce variability, the rings need to be fairly large, generally rings should be bigger than 112 meter in diameter. Often a double ring infiltrometer is used to reduce errors from lateral water movement. Variability in these experiments is often high, so it is advisable to carry out a number of replicates.

It is common in many soils for the infiltration rate to decrease by a factor of ten, after six hours of rain. A steady state equilibrium is usually reached before six hours. Infiltration rates vary greatly between different soils, and typical values are, 10Omm/hour during the first hour of rain, decreasing to 1 mm/hr after six hours of rain. (figure 1 and table 1)

Improving Water Infiltration

It infiltration is improved and more water penetrates into the soil, surface runoff is decreased and flooding is reduced.

For rapid infiltration the soil needs large pores which are connected vertically. Large pores are numerous in soils which have a well developed structure and stable aggregates.

The soil surface should be protected from the impact of raindrops. The force of falling raindrops is very high which can dislodge soil particles, breaking down structure and forming a puddle on the soil surface. When the soil is puddled, infiltration is low and when the surface drys, a crust will form. Vigorous plant growth protects the surface soil from raindrops. Mulches also help to protect surface soils. Excessive cultivation which break$ down the surface structure, will increase puddling during rain. Leaving large clods on the surface when cultivating, will reduce puddling. if a soil is cultivated when it is very wet, puddling often occurs during cultivation.

Vigorous plant growth helps to improve infiltration. Plant roots bind soil particles together into aggregates. Deep rooting plants increase infiltration into the subsoil. Decaying organic matter also improves structure. Organic matter can be added by using mulches, green manures and composts.

Earthworms are well known for improving soil fertility by creating a good structure, which is highly permeable. Other soil animals can help permeability. Micro organisms are very important in breaking down organic matter into substances which bind soil particles. Filamentous micro organisms are a powerful bonding force and many fungi help form a stable structure. Soil organisms need a supply of decaying plant material to feed on. Many modern gardening practices are harmful to soil organisms. Earthworms are harmed by many insecticides, fungicides and herbicides. Some fertilisers change the soil pH, which can be harmful to earthworms and micro organisms.

Compaction decreases infiltration. Inappropriate ploughing has produced plough pans in many farming soils. Heavy traffic, people or vehicles can cause compaction. Often it is best to form a path or road, so as to limit the area effected by compaction. Compaction in a garden can be relieved by cultivation or by inserting a fork and wobbling it.

For large expanses of lawn and turf, many special management techniques have been used. In special cases, an artificial soil can be created, consisting of sand overlaying gravel and drainage pipes. This may be necessary on first class sporting fields where vigorous grass growth is required and the field needs to be dry on sporting days.

Different types of machines have been used to re vigorate turf. Infiltration into turfs can be increased by: coring, spiking, 'Verti Draining', 'HydroJecting', grooving and slicing/slotting.

Soil ameliorates can be used to improve structure. In clayey soils which have a high exchangeable sodium %, the application of gypsum will improve structure. On the market there is a number of patented soil conditioners used to improve infiltration. The benefit of these products can vary greatly between products and under different conditions.





SOIL WATER/WATER INFILTRATION