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The aim of this paper is to study and compare the rain water harvesting system employed in Melbourne and Adelaide in Australia for preserving sustainability of water. The focus should also be given on the variation of the climate, daily water balance model and other.

The specific objectives of this report are to

  • Evaluate saving of water through rainwater harvesting in Melbourne and Adelaide;
  • determine the reliability of the undertaken strategy;
  • examine the effects of the variation in the climatic condition of Australia;
  • assess the amount of water
Background

Water scarcity is a problem that is already affecting over 40% of the global population (Reporter, 2018). This problem is expected to rise because of population growth, global warming and climate change in general (Adugna, et al., 2018). Cities, which are already hosting about 55% of the world’s population (and this is expected to reach 68% by 2050) (Meredith, 2018), are the most affected as the increasing urban population puts pressure on water reserves. The reserves are also already stretched due to too much waste and too little rain (SBS News, 2018).

Australia is the driest continent on earth (Preston, 2008) hence the need for alternative water harvesting systems and saving measures in the country is inevitable. Some of these include: water conservation, water recycling, rainwater harvesting and desalination. This research focuses on the potential of rainwater harvesting system in Melbourne and Adelaide.

Australia is the world’s driest inhabited continent, characterized by the lowest volume of water in rivers, smallest area of perpetual wetlands and lowest run-off. Its climate is slowly drying and has very high variability (Australian Government, 2018). Th highly variable climate is due to atmospheric variation and different currents. The country has regular cycles of floods and droughts that result in extremely variable water storage volumes especially in its major dams (van der Sterren, et al., 2010). Human activities are continuously exerting pressure on marine environments in the country, making water crisis a national problem.

Melbourne and Adelaide are among Australian cities that are feeling water stress (Wright, 2017). Within a decade, these cities could experience water crisis. Water scarcity s when there is <100 m3 deficit per capita per annum while water stress is when there is <1,700 m3 deficit per capita per annum (Taffere, et al., 2016). Melbourne’s water storage now stands at 58.4% from 62.1% the previous year, while Adelaide’s water storage is at 46.2% from last year’s 56.6% (Australian Government - Bureau of Meteorology, 2018). The continuing water shortage in Melbourne and Adelaide is mainly caused by population growth and climate change (Chalkley-Rhoden, 2017). Therefore the need for alternative water harvesting system in the two cities is inevita

 Problem Statement

Water plays a pivotal role in people’s day-to-day activities. Every government wishes to supply citizens with adequate water for domestic, agricultural and industrial use. Adequate water supply has positive impacts on people’s quality of life, environmental conservation and economic growth. However, Australia, which is the world’s driest continent, is facing water shortage problem. Melbourne and Adelaide are among the most affected cities. This problem is mainly caused by climate change and the rapidly increasing urban population (Lee, et al., 2017).

Problem Statement

As climate change intensifies and urban population continues to grow, water demand in both cities will soon outstrip supply. As a result, water crisis is looming. However, this problem can be alleviated by exploring alternative water sources. This study proposes to investigate the potential of rainwater harvesting systems for increasing water supply in Melbourne and Adelaide. The study will entail carrying out comprehensive investigation so as to create a water balance model that can determine if rainwater harvesting systems is a reliable alternative for water shortage in the two cities. 

This study aims at investigating the potential of rainwater harvesting systems in Melbourne and Adelaide, so as to determine if these systems can be used to alleviate water scarcity in the two cities. This will be achieved by creating individual water balance models for Melbourne and Adelaide.   

The specific objectives of the study are:

  • To evaluate and predict the potential of rainwater harvesting in Melbourne and Adelaide
  • To determine the reliability of rainwater harvesting systems in Melbourne and Adelaide
  • To assess the effects of Australia’s climatic condition variation on the efficiency and reliability of rainwater harvesting systems in Melbourne and Adelaide
  • To assess the amount of water used in Melbourne and Adelaide and determine if rainwater harvesting systems are a feasible alternative for water scarcity
  • To create a water balance model for the cities of Melbourne and Adelaide.
  • Research Justification

Water is very important to life (Alexe & Toma, 2013); (Popkin, et al., 2010). According to the United Nations, people have a right to safe and clean drinking water. The water also plays a key role in attaining other human rights such as livelihoods and food (Hall, et al., 2014). However, factors such as population growth, inefficient water practices and climate change have resulted to global water scarcity. Australia is among the most affected countries (considering also that it is the driest continent on earth).

The focus of this paper is on two Australian cities: Melbourne and Adelaide. These cities are already experiencing water shortage and water crisis is looming. In the near future, water demand will surpass water supply. In order to protect the lives of residents, protect the environment (biodiversity and ecosystems) and promote economic growth of Melbourne and Adelaide, the need for alternative water sources cannot be overemphasized. This research will be examining the potential of water harvesting systems as an alternative source of water for Melbourne and Adelaide. The paper will use software to create a model that can be used to estimate the volume of rainwater that can be harvested using different rainwater harvesting systems. This model can be used by the federal government, state governments, local governments, companies and individuals to determine the most appropriate type and size of water harvesting systems they can use to generate more water for various uses in Melbourne and Adelaide. Most of the previous studies did not consider the specific characteristics and variations of Melbourne and Adelaide,

Objectives

instead they created general models for Australia, which does not provide accurate estimates for specific cities. This is because rainwater harvesting is a site-specific source of water (Stout, et al., 2017). It is also worth noting that the variability of Australian climatic conditions is very high hence the need to create specific rainwater harvesting system models for each city. Findings from this research can be used by policymakers in Melbourne and Adelaide to develop appropriate infrastructure and plans that will promote use of rainwater harvesting systems thus increasing water supply in the two cities and reducing overreliance on mains supply. 

Melbourne Rainfall

Melbourne is located at an altitude of 35 m above sea level. Its climate is classified as temperate oceanic (warm and temperate), and is known for its high variability due to geographical location of the city. The city is located to the south hence it is affected by westerly winds’ flow. These winds bring low pressure systems in different months of the year, which causes climate variations. According to Koppen-Geiger climate classification system, the climate of Melbourne is Cfb – a west coast marine climate (Denk, et al., 2013). Cfb represents a generally humid climate with a sort dry summer. During mild winters, heavy precipitation occurs due to presence of mid-latitude cyclones (PhysicalGeography.net, 2018). Koppen classification system is classifies climate based on regions’ characteristics seasonalities and other multiple variables (Chen & Chen, 2013). 

The climograph of Melbourne is as shown in Figure 1 below. The graph shows that Melbourne receives significant amount of rainfall all year round, including the direst month. The month with the lowest rainfall is February and March, which receives 44 mm each while the month that receives the highest rainfall is October with 71 mm. The average annual rainfall is 666 mm while the average monthly rainfall is 55.5 mm. Generally, Melbourne does not receive very abundant rainfall but the rainfall it receives is well distributed over the months (Climatestotravel.com, 2018). The difference of rainfall received during the wettest and driest months is 27 mm.

Figure 1: Climograph of Melbourne (Climate-Data.Org, (n.d.)b)

The warmest month of the year is February with an average temperature of 20.3°C (February is also the month that receives the lowest rainfall) while the coldest month is July with an average temperature of 9.4 °C (July is also the month that receives the highest rainfall). The difference of temperature between the warmest and coldest months is 10.9 °C.

Melbourne Rainfall

Adelaide is located at an altitude of 49 m above sea level. Its climate is classified as Mediterranean climate (mild, warm and temperate). The city is the driest of all capital cities of Australia (World Weather and Climate Information, 2016). The summer months receive less rainfall than the winter months. The summer months of Adelaide are dry and warm while the winter months are wet and mild. The summer usually runs from December to February, the winter runs from June to August while spring runs from September to November. According to Koppen-Geiger climate classification system, the climate of Adelaide is Csa – hot dry-summer Mediterranean climate. Csa climate is mainly characterized by cool, wet winters and hot, dry summers, and is located between latitude 30 °N and 45 °N, and the south of equator (Encyclopedia Britannica, 2018).

The climograph of Adelaide is as shown in Figure 2 below. From the graph, it is seen that Adelaide receives low rainfall during summer and some significant amount of rainfall during winter. The month with the lowest rainfall is February, which receives 15 mm each while the month that receives the highest rainfall is July with 76 mm. The average annual rainfall is 536 mm while the average monthly rainfall is 44.7 mm. Generally, Adelaide does not receive abundant rainfall. Also, most of the rainfall is received during winter whereas summer months receive very little rainfall. The difference of rainfall received during the wettest and driest months is 61 mm.

Figure 2 Climograph of Adelaide (Climate-Data.Org, (n.d.)a)

The hottest months of the year are January and February with an average temperature of 22.1°C (January and February are also the months that receive the lowest rainfall) while the coldest month is July with an average temperature of 10.8 °C (July is also the month that receives the highest rainfall). The difference of temperature between the hottest and coldest months is 11.3 °C.

From the climographs of Melbourne and Adelaide above, it shows that Melbourne receives more rainfall than Adelaide. This is because the average annual rainfall and average monthly rainfall of Melbourne is 666 mm and 55.5 mm respectively while the average annual rainfall and average monthly rainfall of Adelaide is 536 mm and 44.7 mm respectively. This means that with all other factors held constant, the potential of rainwater harvesting is expected to be high in Melbourne than in Adelaide.

literature Review

Water is a very essential natural resource for human, animal and plant life (Friedrich, 2015); (Rietveld, et al., 2016). However, the water is gradually becoming scarce due to factors such as population growth, economic growth and climate change (Amos, et al., 2016); (Traboulsi & Traboulsi, 2017). Rainwater harvesting is a technique of collecting and storing rainwater (Abdulla & Al-Shareef, 2009); (Tamaddun, et al., 2018). This method is considered to be ancient but it is very important even today (Martin, et al., 2015); (Manikandan, et al., 2011). As a matter of fact, rainwater harvesting is considered a viable option for reducing the gap between water demand and supply (Lani, et al., 2018), in many cities across the world (Anon., 2013). It is an inexpensive source of water in rural and urban areas (Al-Houri, et al., 2014).

Adelaide Climate

Implementation of rainwater harvesting systems has significantly increased among Australian households in recent years (australian Building Codes Board, 2016). Historically, federal, state and local governments in Australia discouraged harvesting and use of rainwater. However, this has changed over the recent years and many governments are now encouraging rainwater harvesting and offering rebates to individuals and companies installing rainwater tanks (Productivity Commission, 2011). Studies have also shown that majority of households are installing rainwater harvesting systems in their homes so as to save water and reduce overreliance on mains supply.

From the current trends, it is expected that uptake of rainwater harvesting systems in Australia will continue growing. This is because rainfall across the Australian continent is continuing to reduce and climatic conditions varying significantly hence the need for alternative water sources is going to increase. Other factors that will promote uptake of rainwater harvesting systems include: adoption of building regulatory requirements that encourage rainwater harvesting; changes in pricing of water; and public awareness and education about recent droughts that have prompted people to shift to rainwater harvesting so as to ensure steady supply of water.   

Components of a rainwater harvesting system

The main components of a rainwater harvesting system are a roof catchment, filter (for removing runoff’s initial fraction), cistern (storage tank) and a pump (Sample, et al., 2013). The roof collects water by the help of gutters that channel the water to downspouts. The water is then passed through a filter and then piped to the storage tank. The water can then be directed to the end users by gravity or propelled by a pump, depending on the design of the rainwater harvesting system and the location of the end user.

The rainwater storage tanks are available in different designs (shapes and sizes), depending on factors such as roof area, local rainfall characteristics, water demand, and availability or reliability of mains water. These tanks can also be made from different materials, including coated corrugated iron or metal, concrete, tiles and polypropylene. The best rooftops suitable for rainwater harvesting are those made of corrugated iron or metal, followed by those made from tiles ad lastly concrete. Some storage tanks can also have a water treatment system.

Functions of rainwater harvesting systems

A rainwater harvesting system has 6 key functions. These are:

Water collection: the system is used to collect or harvest rainwater from the roof using gutters. The amount of water collected is directly proportional to the rainfall intensity and surface area of the roof.

Conclusion

Water transport: after collecting the water, the rainwater harvesting system transports it via downspouts and piping, towards the storage tank.

Debris removal: the system also removes debris from the water by filtering. This is done by passing the collected water through a filter or series of filters. The filtering is used to pre-clean the water.

Water storage: the rainwater harvesting system also has a storage tank for storing the collected water. The storage tank can be underground, on surface or at an elevated point.

Water pressurization: depending on the type of rainwater harvesting system, it may have a pump for pressurizing the water so that it can be supplied to the final destination where it is intended to be used.

Water disinfection: if the harvested rainwater is to be used for drinking, it must be treated first (Abbasi, 2011). Therefore the rainwater harvesting system may have a water treatment system to disinfect the water and make it potable (suitable for drinking).

Types of rainwater harvesting systems

There are different types of rainwater harvesting systems. Some of these include the following:

Direct-pumped (submersible) system: this is the commonest type of rainwater harvesting system, mainly for domestic properties. In this system, the pump is installed in the underground water storage tank. The harvested rainwater is then pumped directly from the tank to various appliances such as WCs. The storage tank is connected to the mains water that feeds it when it is about to run dry so as to maintain water supply.  

Indirect-pumped (suction) system: this system is similar to the submersible system only that the pump is not located within the underground storage tank, instead it is located in a control unit inside a utility room. The control unit is also connected to the mains water supply that maintains water supply when the tank runs dry. Therefore water from the mains water supply is not fed into the storage tank first but it is directly pumped to the water appliances.

Indirect gravity system: this system has a header tank (high level tank). First, the harvested rainwater is pumped to the header tank. This water is then supplied to the outlets through gravity only. In this system, the pump is only used when filling the header tank otherwise flow of water from the tank to the appliances is by gravity. Additionally, the mains water is directly fed to the header tank, instead of the main harvesting tank.

Indirect pumped system: this system is similar to the indirect gravity system only that the internal tank does not have to be at an elevated point but can be placed at any level since water supply to the outlets is not by gravity. Instead a pressurized supply is provided by a booster pump. The system can be used in multi-storey buildings.

Gravity only system: this system does not have a pump and therefore no use of energy at any point. The system operates through gravity only. In this system, the collection tank and filter are located above all water outlets. This is the most energy-efficient rainwater harvesting system (Rainharvesting Systems Ltd, 2018).

Types of rainwater storage tanks

Rainwater harvesting systems have tanks for storing the harvested rainwater. The tanks are available in different sizes and shapes, and are made of different materials. The most common types of rainwater tanks include the following:

Concrete tanks: these tanks are made of concrete hence they are generally heavy, strong and durable. Because of their heaviness, concrete tanks are usually installed underground. This makes it expensive to excavate the ground but also helps in saving space in the garden. When installing these tanks, it is important to know where the utilities are located, such as septic tanks. There are two major types of concrete rainwater tanks: monolithic-pour tanks concrete and ferro-concrete tanks.

Monolithic-pour concrete tanks are those that are prefabricated in a factory and transported to the site for assembling, or poured in place. Ferro-concrete tanks are those that are made by spraying a special concrete mixture directly on a metal frame. Concrete tanks are also susceptible to leaching of lime from the concrete into the tanks especially when the tanks are still new. The leaching can cause the water to become alkaline making it unsuitable for most uses, such as irrigation, cooking, bathing, etc. However, this can be prevented by using a lining on the internal surface of the concrete tank. An example of a concrete rainwater tank is shown in Figure 3 below

Figure 3: A concrete rainwater tank (Rainharvest.co.za, 2010)

Metal tanks: these tanks are less expensive in comparison with concrete tanks. They are lightweight and installed above the ground. The metal tanks are also easy to install. Fabrication of metallic water storage tanks is usually done using galvanized steel. The main disadvantage of steel galvanized tanks is that they are susceptible to rusting. When they are exposed to moisture, these tanks can rust thus affecting the quality of stored water. This problem can be prevented by ensure proper galvanization of the tank to make it rust-proof. To avoid the problem of galvanized steel, many manufacturers are using alternative metals such as aluminium. An example of a metal water storage tank 

Polythene tanks: these are the most popular rainwater storage tanks because of their low cost and they are very light hence can be transported from one place to another and installed easily. The tanks can also be moulded into different shapes and sizes with ease. If these tanks are exposed to extreme sunlight levels, they are likely to be affected by corrosion. These tanks are usually molded around a central steel mould during manufacturing hence they usually come in standard sizes and shapes (mostly circular). Examples of polythene tanks are shown in

Bladder tanks: these tanks are characterized by a flexible membrane that makes it possible to install in small space or an irregular space. When water enters the bladder, the flexible membrane expands, and when water is drawn from the bladder, it contracts. Installation of these tanks require an underground framework so as to prevent movement of the bladder tanks. These tanks can also be installed vertically, such as in a frame against a wall or fence. Generally, balder tanks are the most flexible rainwater storage tank alterative but they have the most limited storage capacity. Therefore these tanks are a viable options for people with small plots or few household members or low water demand. Examples

Fiberglass tanks: these tanks are made from fiberglass, which is resistant to rust. Fiberglass is a polymer comprising of densely woven plastic strands reinforced with glass fibers. The reinforcement ensures that the fiberglass is very strong. The fiberglass tanks are stronger than most of the metal tanks. The fiberglass tanks are suitable for storing high volumes of rainwater as their high tensile strength enables them withstand high pressure of large water volume. However, the fiberglass tanks are usually more expensive than most of the other tanks (Regenerative Corporation, 2017). An example of a fiberglass tank is shown in Figure 7 below (Rainwater Rsources, 2013)

Round corrugated tanks: these tanks are very strong and usually in a round shape. There are two main categories of round corrugated tanks: small round tanks and large round tanks. The small round tanks usually have a storage capacity ranging from 1,000 liters to 9,500 liters, while the large round tanks usually have a storage capacity ranging from 10,000 liters to 46,800 liters. An example of a round corrugated tank is shown in Figure 8 below (Auzzie Wrinkly Tin Tanks, 2018).

Slimline water tanks: these tanks are mainly used for storing rainwater intended for industrial use. The slimline tanks are available in different shapes and sizes, making them suitable for households with varied water needs. An example of a slimline tank is as shown in Figure 9 below

As clean water continues to become scarce worldwide, water bills are increasing gradually. Water demand has also increased over the years and many households and businesses are spending significant amount of their income and revenue on water bills. This has driven both households and companies to turn to rainwater harvesting as an alternative source of water for domestic and commercial uses. The rainwater harvesting systems have both advantages and disadvantages. Some of these are discussed below

Advantages

Some of the key advantages of rainwater harvesting systems include the following:

Saves money: the scarcity of water has resulted to increased water bills in many cities, including Melbourne and Australia. The harvested rainwater can be used in different ways depending on its quality and quantity. Irrespective of how it is used, rainwater reduces the overall water consumption thus lowering water bills.

Saves a valuable resource: water is a very valuable natural resource that human beings cannot do without. Harvesting rainwater means saving it from being wasted. Instead of the rainwater being washed away as runoff or being absorbed into the ground, it gets collected and used in homes, farms or business premises. Therefore rainwater harvesting is a way of preventing wastage of rainwater.

Improves biodiversity and ecosystems: rainwater harvesting systems reduces drawing of water from rivers, seas, oceans and dams. It means that water from these sources will be available for use by plants and aquatic animals. Thus rainwater harvesting helps in making water available for animals and plants thus preserving and improving the environment.

Improves quality of health and life: water is life and human beings cannot survive without it. Harvested rainwater can be used for washing, cleaning, cooking, irrigation, industrial activities, etc. This water can also be treated and used for drinking. As a result, rainwater harvesting systems makes people’s lives more comfortable and happier (Owusu & Teye, 2015). 

Increases property value: homes installed with rainwater harvesting systems costs higher because they guarantee water availability. Therefore rainwater harvesting systems is a form of investment for homeowners. Once the system is installed, the value of the house goes up.

Enhance water reliability: with the right size of water storage and high collection efficiency, a rainwater harvesting system can guarantee water available throughout the year. It is normal to experience water shortages from mains supply due to factors such as low water levels in reservoirs (especially during dry seasons), periodic maintenance of municipal water distribution system, repairs of municipal water supply system, contamination of water from mains supply, etc. But a rainwater harvesting system collects maximum water when it rains and stores it in a storage tank for use, especially during dry seasons. This ensures steady supply of water at all times.

Reduce stress on mains supply: rainwater harvesting systems are not only beneficial to end users but also to governments. These systems reduce reliance on mains supply because people can use rainwater harvested and stored in storage tanks. As a result, the mains supply system can perform at its maximum efficiency and scheduled maintenance can be carried out without much interference or outcry from the public.

Reduces carbon footprint: processing and distribution of water requires a significant amount of energy (BlueBarrel, LLC., 2018). The rainwater harvesting system does not require energy as water is harvested and used on site without processing or distribution. Therefore the system saves the energy thus reducing pollution and emission of greenhouse gases that are associated with energy production. Therefore rainwater harvesting system is an environmentally friendly source of water that helps in protecting the environment.

 Disadvantages

High initial cost: the cost of purchasing a rainwater harvesting system and installing it may be higher than expected. The cost of bigger and more efficient systems is also usually higher thus the owner will have to spend a significant amount of money so as to install a sizeable and efficient system.

Maintenance costs: rainwater harvesting systems require continuous maintenance so as to ensure that they perform efficiently and also to prevent contamination of the harvested water. The system usually requires frequent checking, including cleaning of the storage tank.

Risk of water contamination: if the rainwater harvesting system is not maintained properly, the water may get contaminated (Lee, 2015). If this happens, the water will require treatment before it can be used, which increases the cost. But the risk of contamination can be prevented through use of the right material and proper maintenance of the system (Amin & Alazba, 2011).

Limited water storage: the amount of rainwater harvested is limited by the average annual rainfall received in that particular area. Even if water demand increases drastically, the amount of rainwater harvested will still remain the same (Rinkesh, 2018). This means that the capacity of the rainwater harvesting system is largely limited by the amount of rainfall in the area. 

Reduces stormwater runoff, flooding and erosion: rainwater harvesting systems are also useful in reducing occurrences such as flooding, erosion and stormwater runoff (City of San Diego, 2018). These occurrences usually have devastating effects, including displacement of people and property destruction.

Estimation of Harvested Rainwater Volume

Rainwater harvesting potential is dependent on several factors including location, elevation, roof surface area (size), type of roof, evaporation rake, leakages, etc. The two fundamental variables that are needed for the calculation of the amount of rainwater that can be harvested from a rooftop are the rooftop surface area and the average annual rainfall for that specific area. Since the rainwater cannot be collected 100%, it is recommended to include a collection efficiency, depending on the type of rainwater harvesting system used. Therefore the estimated amount of rainwater that can be harvested in Melbourne and Adelaide per m2 of rooftop surface area is calculated using equation 1 below (assuming a collection efficiency of 90% – 0.9) (Centre for Ecological Sciences, (n.d.)):

Rainfall harvested per year (m3) = rooftop surface area (m2) x average annual rainfall (m) x collection efficiency ………….. (1)

Melbourne:

Roof surface area = 1 m2

Average annual rainfall = 666 mm = 0.666 m

Rainfall harvested per year = roof surface area (m2) x average annual rainfall (m) x collection efficiency

= 1 m2 x 0.666 m x 0.9 = 0.5994 m3

= 599.4 liters of rainwater per year.

Adelaide:  

Roof surface area = 1 m2

Average annual rainfall = 536 mm = 0.536 m

Rainfall harvested per year = roof surface area (m2) x average annual rainfall (m) x collection efficiency

= 1 m2 x 0.536 m x 0.9 = 0.4824 m3

= 482.4 liters of rainwater per year.

The collection efficiency above takes into consideration the efficiency of the rainwater harvesting system, leaks, absorption and evaporation.

From the above estimations, it shows that Melbourne has a higher potential of rainwater harvesting than Adelaide. The difference between the rainwater that can be harvested in Melbourne and Adelaide is 11,700 liters per m2 per year. It is important to note that these calculations are only used to demonstrate how to estimate the amount of rainwater that can be harvested in Melbourne and Adelaide. They do not represent the data collected from this study. The actual data and values to be used in this paper will be obtained from relevant agencies, departments and other participants specified in the methodology section.

After knowing the amount of rainwater that can be potentially harvested per year, the next step is to estimate the amount of water usage per household. This can be determined by looking at the individual household water bills or water meters.

 Water Balance Model

Establishing a balance between water demand/consumption and supply is not easy. This is due to variable parameters such as amount of rainfall received, population growth and varying water needs. Water balance models are used to estimate the amount of water supplied and that of water demanded. This study will create water balance models for Melbourne and Adelaide. The water balance models will be used to estimate the percentage of water supplied by the rooftop rainwater harvesting systems in Melbourne and Adelaide, in comparison with the total water demand in the region. Therefore this model will determine the percentage of water consumption that is accounted for by the rainwater harvesting systems.

The first water balance models were developed in 1940 by Thornthwaite (Thapa, et al., 2017). Most of these models were used for water balance analyses and simulating rainfall runoff. Water balance models are created using the following parameters: rainfall data, rainfall loss factor, rooftop surface area, rainwater demand, available storage volume and overflow of storage tank (Imteaz, et al., 2011).

After determining domestic water demand in the two cities, it will be compared with the volume of potential rooftop rainwater harvested in the two regions. These two sets of data will be used to create a water balance model. The water balance model is basically used to determine the gap between water demand and water supply, by taking into account the water demand (outflow), the volume of rainwater harvested (inflow), and the amount of water remaining (storage) (Stout, et al., 2017). When the difference between water demand and harvested rainwater is known, it becomes easy to determine the type and size of rainwater harvesting systems that can be used to boost water supply.

A water balance model can be used to determine the potential and reliability of rainwater harvesting systems in an area (Cowden, et al., 2008). In this study, the water balance model will be used to estimate the percentage of water demand that can be met by the rooftop rainwater harvesting systems in Melbourne and Adelaide. The model can create a graph showing the percentage of water demand that is provided by the rooftop rainwater harvesting systems. An example of such graph is shown in Figure 10 below (Adugna, et al., 2018)

Figure 10: Example of water balance model

From the water balance graph in Figure 10 above, it shows that rooftop rainwater harvesting systems contribute relatively low percentage of water demand in the study area. In January, the rainwater harvesting systems accounted for only 0.07% of the total water demand, in February it was 0.17%, in March it was 0.36%, in April it was 0.57%, in May it was 0.61%, in June it was 0.90%, in July it was 2.30%, in August it was 2.17%, in September it was 1.02%,

in October it was 0.19%, in November it was 0.10%, and in December it was 0.07%. On average, the rooftop rainwater harvesting systems account for about 0.71% of the total water consumption in the area. From this model, it can be concluded that the study area receives high rainfall during the months of July and August. This model also reveals that most of the water demand in the area is met by the mains supply. Therefore the potential and reliability of rooftop rainwater harvesting systems are still low in the area.

Water balance model can be used to establish ways on how to improve the potential of rainwater harvesting. For example, using graph in Figure 3 above, the volume of rainwater harvested can be improved by increasing the number and size of rooftop rainwater harvesting systems. The government can make it mandatory for property owners to install rooftop rainwater harvesting systems or come up with other strategies of promoting installation of rainwater harvesting system, such as incentives, tax reliefs, subsidies, etc. Since the amount of rainfall received in an area cannot be changed because it occurs naturally, the potential of rainwater harvesting systems can only be improved by increasing the size of rainwater harvesting systems (size of roof, size of storage tanks, etc.) and the collection efficiency. This requires proper planning, starting from the design of building roofs that can capture maximum amount of rainwater.

Water balance models can also be used to determine the months of the year when the volume of harvested rainwater exceeds water consumption or demand. During these months, the excess water can be stored in available storage facilities and use later when water demand exceeds the volume of rainwater harvested by the rainwater harvesting systems.

The water balance model created in this study can be used by the Victoria state government and the South Australia state government for various projects, such as: improving the water supply infrastructure (changing the design, establishing the right time for maintenance, etc.), establishing the right type of rooftop rainwater harvesting systems for the residents, establish the right approach of water rationing, develop appropriate strategies of preventing runoff (if needed), etc. The water balance models created in this paper will be very useful in improving water supply in the two cities by capitalizing on rainwater harvesting system

Melbourne is the capital city of Victoria state and it is the state’s administrative, business, recreational and cultural hub. The area of Greater Melbourne area is 9,992.5 km2. The region has a population of about 4.9 million, with private or residential dwellings approximated to be 1.832 million (City of Melbourne, 2018). The greater area of Melbourne has 23 federal divisions, as shown in Figure 11 below. The average annual rainfall of Melbourne is 666 mm. There

Adelaide is the capital city of South Australia State, with an estimated area of 3,257.7 km2. The area has a population of about 1.34 million, with private dwellings approximated to be 700,000 (Australian Bureau of Statistics, 2017). The map of Adelaide is as shown in Figure 12 below. The average annual rainfall of Adelaide is 536 mm.

Recent satellite images of Melbourne and Adelaide will be obtained from Google Earth. The purpose of obtaining satellite images is to use them for digitalizing the rooftops of all buildings in these study areas using Google Earth’s polygon tool. To complete the digitalizing process, building rooftops will be traced directly above the satellite imagery so as to differentiate rooftops of buildings from other objects in the area. This process will produce digitalized maps of Melbourne and Adelaide containing thousands of boundaries. The files shall be saved KMZ files with each file representing building rooftops of Melbourne and Adelaide

Calculation of Roof Surface Area

Estimation of the amount of rooftop rainwater that can be harvested is also influenced by the total surface area of the buildings’ rooftops in the area. Therefore the total rooftop surface area of buildings in Melbourne and Adelaide will be calculated. Geographical Information System (GIS) is one of the most important hydrological modelling tools today because of the capacity it has in handling digital data and information. In this research, ArcGIS’s georeferencing and digitalization will be used. The process will start by converting the KMZ files for Melbourne and Adelaide into shape files. This will then be followed by defining the coordinate system for every shape file so as to enable computation of areas of the digitalized building rooftops. The calculation of building rooftops surface areas will be done automatically using ArcGIS model’s geometry calculation tool.

ArcGIS geometry tool has several formulas for calculating area, depending on the shape of the rooftop. Some of the formulas include the following:

Area of rectangular roof = length x width

Area of circular roof = πr2 (where r = radius of the circular roof).

Area of square roof = side x side

Area of trapezoid roof =  (where h = height, a and b are the parallel sides of the trapezoid).

Area of triangular roof =  (where b = base and h = height of the triangle), among others

Digitalized Areas Corrections

It is important to confirm the accuracy of the digitalized maps and the subsequent areas using ArcGIS. To do this, actual areas of a few buildings in Melbourne and Adelaide will be measured and compared with the corresponding digitalized areas calculated using geometry calculation tool of ArcGIS. This will be done by selecting five buildings in each of the two cities and physically measuring the dimensions of their rooftops and using them to calculate.

the rooftop surface area. The buildings may be private homes, educational institutions, health facilities, hotels, offices, etc. The actual calculated rooftop areas will then be compared with the values obtained from ArcGIS geometry calculation tool. The comparison will then be used to determine correction factor for each of the selected buildings. The correction factor is simply obtained by dividing the actual measured area of the building rooftop with the digitalized area.

The average correction factor will then be calculated from the individual correction factors of the buildings. This correction factor will be used to modify the digitalized areas. The modified or corrected areas will be obtained by multiplying the digitalized areas with the average correction factor.

 Estimation of Potential Volumes of Harvested Rainwater

The areas computed using ArcGIS and later corrected by the average correction factor will be used to estimate the rainwater volumes that can be harvested from the rooftops of buildings in Melbourne and Adelaide annually using a rational method. The estimation is done using equation 1 provided in introduction section:

Rainfall harvested per year = roof surface area (m2) x average annual rainfall (m) x collection efficiency

Where values of average annual rainfall in the two cities and collection efficiency are also needed. The average annual rainfall for the cities will be obtained from meteorological stations. The calculation of harvested rainwater volumes will be done in Excel

Estimation of Water Demand

It is also important to determine the gap between water demand in Melbourne and Adelaide and the volume of water that can be supplied with rainwater harvesting systems. To achieve this, water demand in the two cities will be estimated. To start with, the digitalized maps will be used to identify domestic or residential buildings. It is obvious that water demand for these two categories is different. Estimating domestic water demand is not that easy considering that every household has unique water needs and uses the water differently. For example, some households have water saving plans while others do not have such plans. Also, households use more water during summer season than winter season. There is also the issue of different number of persons per household. The water demand in this study will only be domestic water demand.

The domestic water demand will be estimated by considering the number of households in Melbourne and Adelaide, the number of persons per household, and water demand per person per day. Alternatively, domestic water demand can be obtained by multiplying the average water demand per person per day with the total number of people in the city (Biswas & Mandal, 2014). Since the population of each of these two cities is known, it will be easier to determine domestic water demand using this alternative. Random sampling will be used to determine the average water demand per person per day. A total of 50 households will be selected in each of the two cities then interviews or questionnaires will be used to collect the required data.

The main data to be collected is the amount of water that each household member consumes every day. The participants will be requested to provide this data by reading water meters directly or obtaining this information from monthly water bills. In either way, the average water demand per person shall be obtained by dividing the total monthly volume consumed by the household per month with the number of household members. The value obtained will then be divided by the number of days of the month so as to get average daily water demand per person.

To begin with, the number of persons in the two cities (population) will be obtained from relevant government departments and agencies in the City of Melbourne and the city of Adelaide. The daily water demand will then be determined from the average values obtained from 50 households in each city. Total annual domestic water demand will then be determined as follows:

Total annual domestic water demand = average daily water demand per person x no. of persons in the city x 365   

Comparing Water Demand and Supply Gap

After determining the water demand, it will be compared with the volume of potential rainwater harvested in the two regions. This data will then be used to simulate a yearly water balance model. The model will be created in Excel. One of the limitations of this methodology is that yearly water balance models are less precise than monthly and daily water balance models (Wang, et al., 2011).

Conclusion

Water scarcity is a problem that is affect many cities across the world. Current issues such as increasing urban population and climate change are worsening the water crisis problem in cities. Australia is the driest continent on earth and the fact that most of the Australian cities are affected by water crisis is not a surprise. One of the approaches that individuals, companies and governments are using to boost water supply especially in urban areas is use of rainwater harvesting systems.

This paper focuses on the potential of rainwater harvesting systems in Melbourne and Adelaide. These are two Australian cities with looming water crisis. Rainwater harvesting systems are an alternative source of water for many cities. The rainwater harvesting systems can be used in Melbourne and Adelaide to supplement existing water sources, especially the mains supplies. The rapidly increasing urban population in Melbourne and Adelaide has greatly stressed mains water supplies hence the need for alternative water sources is inevitable.

The main aim of this study is to investigate the potential of rainwater harvesting systems in Melbourne and Adelaide. This is so as to establish if these systems can be reliable in alleviating water scarcity in the two cities by supplementing mains supply. This will be achieved by creating individual water balance models for Melbourne and Adelaide. The specific objectives of the study are to evaluate and predict the potential of rainwater harvesting in Melbourne and Adelaide; to determine the reliability of rainwater harvesting systems in .

Melbourne and Adelaide; to assess the effects of Australia’s climatic condition variation on the efficiency and reliability of rainwater harvesting systems in Melbourne and Adelaide; to assess the amount of water used in Melbourne and Adelaide and determine if rainwater harvesting systems are a feasible alternative for water scarcity; and to create a water balance model for the cities of Melbourne and Adelaide.

The literature review of this paper has discussed various aspects of rainwater harvesting system including: components of rainwater harvesting systems, functions of rainwater harvesting systems, advantages and disadvantages of rainwater harvesting systems, types of rainwater harvesting systems, types of rainwater storage tanks, procedure for estimating harvested rainwater volume, and the water balance model.

The research will be carried out using a comprehensive methodology that includes the following aspects: describing the study area, obtaining the maps of study area from Google Earth, calculating the total roof surface area of residential buildings in the study area using ArcGIS’s georeferencing and digitalization tools, determining correction factors for the roof area and using them to correct the digitalized areas, estimating potential volumes of harvested rainwater, estimating water demand,  and determining the gap between water demand and water supply. All the data and information collected will be used to create a water balance model for Melbourne and Adelaide.

The average annual rainfall received by Melbourne and Adelaide is 666 mm and 536 mm respectively. This amount of rainfall is relatively low but it can still be used to harvest some volumes of rainwater to supplement mains water supplies in the two cities. The water balance model will determine the percentage of water consumption that is accounted for by the rainwater harvesting systems. It is expected that rainwater harvesting systems in Melbourne and Adelaide account for small percentages of water consumption or demand in the two cities. However, the water balance models created can be used to improve this. Information from water balance models is useful in establishing ways of improving the potential, efficiency and reliability of rainwater harvesting systems.

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