A Tale of Two Cities
- December 8, 2010
Growing up in Birmingham, England, it was common place for every home gardener to have a 25- to 50-gallon rainwater barrel attached to a convenient downspout in the front and back of the house, to the garage if located at the rear of the garden, and/or to the greenhouse. Gardeners would then supplement summer rainfall on vegetable and flowerbeds and/or water the interior of the greenhouse using a watering can filled from the rain barrel. I remember my father extolling the virtues of rainwater over tap water for growing good tomatoes and lettuces and decrying the waste of letting all that good rain go down the drain. It wasn’t that this saved us any money—back then our water bill was a fixed flat rate, not a variable metered volume charge—rather it was a common-sense approach that fitted into a general ethos of frugality and efficiency that characterized his, and earlier, generations.
Later on, as a junior consultant in soil and water conservation, I found myself assessing rainwater harvesting activities in rural development projects across Africa, documenting, for UNICEF and others, the various applications of rainwater harvesting technologies for the purpose of providing onsite sources of potable water in areas where public systems were absent, and where indigenous, communal water sources such as shallow groundwater were inadequate as safe drinking supplies. On one assignment, I lived for several months in a thatched-roof traditional dwelling, supplied solely with runoff captured from an adjacent rock outcrop and stored in a reinforced concrete tank hand-built next to the house by local craftsmen trained by the Danish international development agency I was working for. Previously, as a graduate student, I had researched surface runoff harvesting systems in the Negev Desert of Israel, where ancient civilizations depended for their year-round supply on runoff water stored in excavated cisterns that drained from rocky hillsides and developed terraced agricultural systems supported by flood irrigation captured from distant hillsides using a network of carefully planned diversion ditches. Thousands of people were supported this way in an area where annual rainfall typically totaled around 90–100 millimeters (4 inches), often with six to eight months of the year with no rainfall whatsoever.
A major impetus for this paper was that I recently began a private garden remodeling project in Oakland, CA, offering a return to my frugal roots and to the footsteps of my father because I have the opportunity to design and install a rainwater catchment system to make use of a conveniently located, 900-square-foot section of roof area with downspouts that can be configured to supply a storage tank located next to the house. The tank would be at an elevation that would permit simple gravity flow of water to the terraced ornamental and vegetable garden areas below through drip or soaker hose. (Local regulations do not currently permit residential irrigation systems to irrigate untreated stored rainwater or graywater through sprinkler application.)
My decision to consider this project clearly goes back to my English roots, but more directly dates back to a month I spent working with the Department of Sustainability and Environment of the State of Victoria, Australia, assisting its staff in their implementation of a 50-year planning framework for water management. Faced with prospects of intense droughts and potential long-term supply reductions from existing catchment and reservoir systems and a growing population, Victoria—in line with the Australian government—has adopted an aggressive, innovative, and holistic approach to water planning and management.
Household rainwater tank installation in suburban Australia
Laid out in the 2004 blueprint Our Water, Our Future, the government of Victoria is promoting a wide variety of water efficiency and other measures that take the notion of integrated water resources planning to a level not yet found across the United States (Government of Victoria 2004). One of the technologies being promoted in Victoria, both for existing and new construction, is domestic rainwater harvesting. Since the days of the early settlers, it has been common across Australia for rural households not connected to a municipal water distribution system to use rooftops and rainwater storage tanks to provide domestic supplies. However, it is becoming increasingly common to find rainwater tanks being employed as part of water conservation measures in urban centers, for both homes and businesses, substituting for existing supplies where retrofitted to buildings, and reducing incremental system demands where installed on new construction.
Rainwater is being promoted not only for landscape irrigation, but also through integration to the internal plumbing of the building for flushing toilets and washing clothes. Less complicated and problematic than graywater systems (i.e., the reuse of wastewater from sinks, showers, and clothes washers), rainwater harvesting is promoted as a means of creating a hybrid potable/non-potable supply system for domestic water uses without having to develop a municipal dual-plumbing system for a complete community or treat domestic graywater to deal with human pathogens or chemicals introduced to it prior to its collection. Rooftops provide the source of water, onsite tanks provide the storage, and a limited modification of internal plumbing provides the distribution. For existing buildings, this usually requires the strategic location of a rainwater tank on an exterior wall adjacent to a bathroom or laundry room and a direct connection through the wall to the toilet tank or the washing machine located inside. A simple manual control valve can allow users to switch between the rainwater supply and the regular potable supply for each device, or alternately, the rainwater tank could be filled during a rainless period from the potable supply to save the need to switch sources.
According to the Government of Victoria Web site, some 38,000 rebates have already been issued for the installation of rainwater harvesting systems as part of the Water Smart Gardens and Homes program (Government of Victoria 2010a). As of June 2010, a rebate of AUS $150 (approximately US $125) was being offered to homeowners for installation of a tank of 600–1,999 liters (approximately 160–540 US gallons), along with an additional AUS $150 for a toilet connection. For larger tanks connected to a toilet and/or laundry, rebates are higher, ranging from AUS $500 (approximately US $425) for a 2,000- to 3,999-liter (approximately 540–1,080 US gallons) tank to AUS $1,000 (approximately US $850) for a 4,000-liter (approximately 1,080 gallons)-or-greater-sized tank (Government of Victoria 2010b). Tank design and installation must comply with Australian standards and guidelines. This program is an enhanced version of the rebate scheme offered as part of the Australian government’s $12.9-billion Water for the Future plan, which has a National Rainwater and Greywater Initiative to help people use water more efficiently and wisely (Government of Australia 2010a). All Australians meeting the requirements have had the opportunity since January 30, 2009 and through March 31, 2014 to receive a grant of up to AUS $500 (approximately US $425) towards the purchase and installation of a rainwater tank of 2,000-liter (approximately 540 US gallons) capacity or greater for existing buildings. This program does not cover new construction since, in many Australian jurisdictions, integrated rainwater harvesting systems are now mandated by building codes as part of design requirements (Government of Australia 2010a).
System Assessment
In order to assess a rainwater harvesting system, three basic pieces of information are required. The sophistication with which this information is used depends on the data quality available and the assumptions and mathematical procedures incorporated into the analytical instrument (e.g., monthly or daily accounting of runoff and demand). They are size and type of catchment area, rainfall quantity (at least monthly, ideally daily), and volumetric demand (at least monthly, ideally daily). Volumetric demand can be estimated (e.g., by using evapotranspiration data and square footage of different irrigated vegetation, estimates of toilet flushing frequency, etc.) or based on actual historical consumption.
There are a number of performance calculators available in the public domain for assessing water harvesting performance. Two of the more impressive include that developed by the San Francisco Public Utilities Commission (SFPUC), specifically to size rainwater tanks for irrigation and non-potable use in the City of San Francisco (SFPUC 2010b), and that developed by the University of Warwick, UK, to size rainwater tanks for domestic use (potable and/or non-potable) anywhere in the world, for which rainfall data is available (University of Warwick 2010). Both use monthly rainfall and demand totals.
For this paper, however, I have used my own spreadsheet tool for water tank performance assessment, the prototype of which I developed 25 years ago when I was still a graduate student, and which I recently dusted off and modified for use with my garden project. Like the others, the tool has some complicated algorithms, but is basically a simple mass-balance calculation that takes a set of monthly precipitation data, inputs a rooftop catchment area and an assumed runoff coefficient (to allow for evaporation, retention, first-flush, and so forth—it seems pretty standard to assume 0.85 for rooftops), and produces a monthly runoff total. (Note that, in doing so, as with most monthly calculators, it necessarily assumes that the rainfall is distributed evenly across the month. However, we know that this is not the case, and a more sophisticated model would make the calculation at a higher resolution of a daily scale, either using actual daily rainfall records or some stochastic procedure that used magnitude and frequency data to simulate the spacing of rainy and rain-free days that typically result in a given month’s average total. It is expected that a daily model would predict lower yields and greater overflows for smaller tanks compared to larger tanks).
It also uses actual or potential evapotranspiration data (or pan evaporation data as a less accurate means to determine water needs), along with a coefficient for a particular vegetation type if needed and the total irrigated area to estimate the monthly irrigation demand to be supplied, if available, with stored water in the tank over the course of each month. (Note that, as with rainfall, it is not necessarily the case that irrigation takes place equally on every day of a given month, and a more sophisticated model might incorporate a more disaggregated approach based on an assumed or actual watering regimen.) In addition, the tool provides the option of adding an average daily non-potable water demand for toilet flushing, clothes washing, and so forth as desired, calculated as a monthly total, if it is planned to also use the rainwater harvesting system to meet interior or exterior non-irrigation needs currently met from the potable water supply. Note that non-potable use is assumed, for the purpose of this analysis, to be a constant 77 cubic feet per month. This is calculated assuming that one ultra-low flow toilet (1.6 gallons per flush) is connected to the rainwater tank and it is flushed daily four times each by three residents, a total of 12 flushes and 19.2 gallons per day, 576 gallons per month.
In the absence of a more detailed data set for Melbourne, Australia, mean monthly pan evaporation data was used for both Oakland and Melbourne to calculate the irrigation demand for the landscape area. For a more accurate assessment of the performance for a rainwater harvesting system that calculates monthly demand, actual evapotranspiration, for example, that’s supplied by the California Irrigation Management Information System (CIMIS), and an appropriate vegetation coefficient would need to be used. CIMIS data is available for the Oakland Foothills region where my rainwater harvesting project would be located, calibrated for residential (turfgrass) vegetation, and thus this could be used in an assessment of irrigation demand. However, similar evapotranspiration data was not available for the Melbourne urban area. Pan evaporation data will typically overestimate plant water demands, the degree to which it will do so varying from month to month depending on the time of year, and thus the amount of photosynthesis and evaporation taking place. However, any overestimate in irrigation demand in this analysis will compensate for the fact that irrigation efficiency has not been factored in to the calculation; the ability of the irrigation system to supply water that can actually be used by the vegetation can be anything from 0.75 to 0.95 for drip and soaker irrigation, meaning that actual water needs will be greater than calculated water needs using evapotranspiration data. The calculations used in this paper can thus be assumed to provide a general order of magnitude of the irrigation demand for a 1,000-square-feet garden area to be supplied in part by rainwater storage.
City West Water is one of the three major water utilities providing water to the City of Melbourne, Australia, the principal population center in the State of Victoria. It serves around 800,000 of the city’s more than three million residents. To promote water use efficiency, among other initiatives City West Water offers rebates on rainwater tanks (up to US $850), graywater systems (up to US $425), and dual-flush low-flow toilets (US $85) (City West Water 2010a). In Oakland, CA, water is provided by the East Bay Utility District (EBMUD) to around 1.2 million consumers. EBMUD does not currently offer rebates for domestic rainwater harvesting or graywater systems, but it does offer customer rebates on high-efficiency toilets (US $50) and clothes washers (US $100), on self-adjusting irrigation controllers (US $200), and for turfgrass replacement (up to US $500) (EBMUD 2010a). Both City West Water and EBMUD offer assistance with education, audits, showerheads, and water-saving garden products such asmulch, etc.
In addition to their contrasting programs with respect to rebates, it is informative to look at their residential water fee structures. Both utilities charge a combination of fixed service charges and variable commodity rates to customers. Of importance in determining the cost-effectiveness of a given water conservation and efficiency measure, of course, are the variable rates. Table 3 shows the relative pricing for water for City West Water and EBMUD residential customers and the effect of the inclining rate conservation-based tariff structure that both have adopted. At the highest tier, those high-volume water users who would benefit financially the most from an on-site source of water that could substitute for municipal supplies, a 100-cubic-foot (748 gallon) billing unit of water (ccf) costs $3.27 for EBMUD customers and $10.23 for City West Water customers (given that City West charges an additional $3.80 per billing unit as a volume-based fee for wastewater treatment). Note that these are the values I have used in my analysis—a lower-volume water user with smaller household consumption would need to use the lower tiers in a similar calculation. Clearly, water is a much more expensive commodity and represents a larger share of household expenditure for the residents of Melbourne, than it does for us in Oakland. It, thus, affects greatly the relative cost-effectiveness and, hence, attractiveness of water conservation and efficiency measures, all things being equal.
Dollars and Tanks
Water tank prices will vary based on design and so-forth. A perusal of various Web sites indicated that round polyethylene water tanks certified to Australian national standards for potable water storage cost around AUS $500 (US $425) for a 1,000-liter (264-US-gallon) tank, AUS $750 (US $637) for a 2,500-liter (660-US-gallon) tank, and AUS $1,000 (US $850) for a 5,000-liter (1,320-US-gallon) tank for the Melbourne area including taxes and delivery. This is broadly similar to costs in the US, where an assessment of tank prices at various internet suppliers indicate that tanks made from food grade safe Food and Drug Administration-approved, California-compliant polyethylene plastic, and suitable for potable water storage cost around US $100 for a 100-gallon tank, US $250 for a 250-gallon tank, US $350 for a 500-gallon tank, and US $800 for a 1,500-gallon tank exclusive of local sales taxes and delivery. What is not factored in is the cost of installation, which if a system is to be installed in an existing construction to include non-potable uses (requiring an element of dual-plumbing) to comply with official code regulations and through a permitting process will usually require a qualified plumber in addition to necessary materials such as valves, joints, piping, and so forth. For more straightforward irrigation-only installations, modifications will be more straightforward and could be completed by a homeowner. Thus, no installation costs have been added to this assessment, but likewise, no subsidy would be available without a non-potable connection given the terms of the Victoria program. Given that plumbers cost between $50–100 per hour in the Bay Area (it seems to be about half as much in Melbourne), it seems reasonable to assume that installation, which should take a day or so to attach the necessary pipes to toilets and so forth, might be around an added US $500 for labor and materials in Oakland and US $250 in Melbourne. However, these are probably underestimates.
The relative costs of water, water tanks, and assumed installation costs can be used to estimate the cost-effectiveness of the same rainwater installation tank for the two localities with and without the rebates provided. Using a simple net-present value calculator developed by the Shatz Energy Research Center (2002), the equivalent net present value of the installation and its simple payback period, among other indicators, can be calculated by inputting the installation cost both with and without rebates and the value of the water that would otherwise have needed to be purchased from the water provider at current tariffs (annual savings). A discount rate must also be specified to determine the future value of money, for example, the interest rate offered on municipal bonds (e.g., 3%) could be selected or perhaps a homeowner might use their mortgage interest rate as a suitable yardstick (e.g., 5%). For the purpose of this study, 5% was used as a reasonable measure of the discounting of money.
Conclusions
What is also clear from this assessment is that rainwater harvesting, all things being equal, is much more effective when practiced in a climate that has a more even distribution of rainfall. Even though Melbourne’s annual average rainfall is only 40% higher than Oakland’s, a 100-gallon tank there can produce between 107 and 170% more supply compared to in our Mediterranean climate and a 2,000-gallon tank can yield 73 and 96% more supply (with and without non-potable use respectively). Along with relatively low water tariffs, another critical limitation for rainwater harvesting effectiveness in California is thus our long summer dry season and our imbalance between rooftop supply and irrigation demand.
A final observation concerns the benefits gained from using a rainwater harvesting tank not only for irrigation, but also for interior non-potable uses like toilet flushing. What is clear from this analysis is that financially, a rainwater tank that is connected for non-potable water use in a location like Oakland would still probably not make economic sense due to the low-cost of water and the high cost of labor required to make the connection to the water-using fixtures inside. In Melbourne, with its higher water costs and lower labor costs, this is not the case, both with and without subsidies. Connecting tanks for interior water use is cost-effective and has a positive net-present value at all sizes with or without subsidies. From an efficiency perspective, being able to use rainwater for non-irrigation uses, which are more constant and less seasonal, means that for a given roof size and catchment tank combination and investment, more of the available water can be put to use with less of that water being lost to tank overflow in months when rainfall and, hence, roof runoff is high and irrigation demand is low. In Melbourne, adding non-potable use to irrigation use increased the performance of the rainwater system (volume of water substituted for potable) by 54% for a 100-gallon tank and 40% for a 2,000-gallon tank. In the Mediterranean climate of California, the benefits of adding non-potable use were even higher, with the performance of a 100-gallon tank increasing by 101% and a 2,000-gallon tank by 57%. With California’s more skewed monthly rainfall and evaporation distributions and higher maximum monthly rainfall totals, more overflow will occur for virtually all the roof, tank, and irrigated area size combinations.
In thinking about rainwater harvesting as an option for urban water conservation, I clearly would prefer my project to be cost-effective, i.e., pay for itself within a reasonable period of time. However, as an environmentally conscious individual and a professor of geography and environmental studies teaching classes on sustainability and on water management, I recognize that reducing my utility bills through water savings is only part of the broader equation I might consider. For philosophical and ethical reasons, I might still do what many, who wish to become more self-sufficient and environmentally sustainable, choose to do—implement a project that is not in of itself financially viable.
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