High-density rainwater harvesting

High-density rainwater harvesting

Nick Kelley

AGEC 606

Dr. Ron Griffin

May 07, 2013

High-density rainwater harvesting 2

Introduction

Rainwater harvesting is a practice that has existed for thousands of years 15. While this

practice has wax and waned through the centuries and societies that have used it, the current state

of climate change and increasing presence of severe droughts in many areas of the world has

prompted a new interest in rainwater harvesting (RWH) over the past couple of decades.

There are numerous sources available that discuss the various facets of RWH, from the

different systems and components to the numerous (and almost exclusively) positive

consequences of this practice. This paper intends to highlight a specific RWH scenario, referred

to as high-density rainwater harvesting (HDRWH), and the potential negative effects of such a

scenario. In this report, HDRWH is defined and given context in relation to other RWH

scenarios. Some regulations regarding RWH, specifically in the United States, are summarized

along with related incentive programs and tax rebates. The potential negative downstream effects

of HDRWH are discussed and finally, an outlook on the potential future of a HDRWH scenario

is provided.

High-Density Rainwater Harvesting

Firstly, to allow for the analysis and discussion of high-density rainwater harvesting, the

term must first be defined. Here lies the first obstacle. The exact term “high-density rainwater

harvesting” has never been defined (at the time of this writing). In the realm of rainwater

harvesting, it seems, terms are often only loosely defined, if ever defined at all. In performing a

cursory web search for the string “high density rainwater harvesting”, only two results were

found, both referencing the same document: a case study about the integrated water management

of Cabbage Tree Creek in Brisbane, Australia 10. In this document, “high-density rainwater


harvesting” is listed twice and is never defined or even compared to anything else to give some

sort of context. Performing extra searches for the string using other search engines and databases

yielded no further results of any significance.

In order to define HDRWH, a similar term must first be evaluated. Large-scale rainwater

harvesting (LSRWH) is a term that can be commonly found in the literature and resources

regarding rainwater harvesting, but unsurprisingly, is never strictly defined. Typically, LSRWH

is used to refer to two different scenarios, one of which appears synonymous with HDRWH. The

first scenario, unrelated to HDRWH, is that of a single rainwater harvesting (RWH) system that

is considered “large”. Any criteria for what makes a RWH system large is never given. From the

way the term is used for this scenario, “large” is usually assumed to be any system that is bigger

than a residential single-home RWH systemA. Whether this is considered by the catchment area

of the system, the storage capacity, or both, is unknown. What the threshold between a standard

residential system and a “large-scale” system is, again, never defined. Usually, LSRWH is used

to refer to systems installed on commercial property, given that the area and building sizes are

often larger than the average house.

The second scenario that LSRWH can refer to is that of many individual systems

installed in a specified area, usually in the context of an area with higher population density such

as a city or suburban development. In this case, the size of the individual systems is not as

important as the number of systems in the area and this is where HDRWH comes in. For this

paper, HDRWH refers to a scenario in which a high percentage of rooftops in a specified area

have RWH systems. To illustrate, compare two identical suburban neighborhoods both having

exactly 100 homes, all of which are more or less the same size and shape. Say neighborhood A

has only 10 homes with RWH systems while neighborhood B has 90 homes with RWH systems.

A But this can bring up the question, “What if you have a big house?”


The percentage, or density, of RWH systems in these two neighborhoods would be 10% and

90%, respectively. Neighborhood B would be considered a HDRWH scenario while

neighborhood A would not.

Now HDRWH must be further defined so that one can differentiate between low-density

and high-density scenarios. Ideally, more than two categories would be used, such as very low-

density, low-density, medium-density, high-density, and very high-density, but to keep things

simple we will consider only two categories. For this paper, high-density will be considered any

area having at least 70% of its rooftops utilizing RWH systems. This value is completely

arbitrary but it provides the ability to distinguish between potential high-density and low-density

scenarios. Additionally, one further stipulation must be made. In this paper, HDRWH implies

any urban or suburban area that is also near a watercourse (or drains into one) and typically

having downstream users present on that watercourse. This last criterion is important for

assessing possible negative effects on downstream users, which will be discussed later in this

paper.

Currently, there are no cities or suburbs that fit the criteria of HDRWH set forth in this

paper. There are communities that utilize a high percentage of rooftops for RWH, but have no

downstream users or even really a ‘downstream’ at all. These communities are typically found

on islands where water is usually discharged directly into the sea. These places also do not have

to contend with legal or political obstructions of RWH since rain water may be their only local

source of fresh water. The island of Bermuda, for example, relies on RWH as it has no

freshwater streams or aquifers 7. In fact, RWH is not only encouraged in these locales, but is

often mandatory by law. These types of communities are the exception to the scenario of

HDRWH defined here.


Searching for known densities or percentages of buildings with RWH systems in a

particular city has given no useful results. Either these numbers are not known or they are not

well documented or publicized. Various searches of scientific literature and internet sources have

yielded no known communities with a HDRWH scenario. Such a scenario does not currently

appear to exist. However, with many states in the US offering incentives and rebates for

installing RWH systems and some cities even requiring their installation by law, coupled with

the increasing effects of climate change on regional precipitation patterns and water

consumption, it is not implausible that HDRWH will someday become a reality. Until then,

potential consequences of HDRWH discussed in this paper, and many LSRWH projects that

currently exist, are purely hypothetical but are considered possible in the event of an actual

HDRWH scenario.

Regulations

While HDRWH scenarios do not currently seem to exist, there are cities with regulations

in place that require the installation of RWH systems in new buildings. If a HDRWH scenario

ever does become a reality, it will most likely be through the enacting of regulations and laws

requiring the installation of RWH systems. These regulations will most likely be in response to

water scarcity issues, though in some areas water quality may be the driving factor.

Remaining within the US where water rights and laws vary from state to state, there are

several cities of significance that have put into place regulations requiring that new buildings

have RWH systems installed. The specifics of these regulations, similar to state-level laws, vary

from city to city. This section will summarize the regulations of Tucson, AZ, and Santa Fe, NM,

to illustrate the current trend and possible future of RWH in the US.


The city of Tucson, Arizona, adopted an ordinance B in 2008 (and enacted in 2010)

stating that new commercial projects must receive at least 50% of their irrigation water from

harvested rainwater. The first such regulation of its kind in the US, the ordinance goes on to

stipulate that eligible properties must also “[…] prepare a site water harvesting plan and water

budget, meter outdoor water use and use irrigation controls that respond to soil moisture

conditions at the site” 3. However, the requirement of 50% is waived during periods of drought.

New Mexico is considered to have passed some of the most progressive RWH policies and

regulations in the US 6. Among them is the water harvesting ordinance C enacted in Santa Fe

County. This ordinance not only requires RWH systems be installed in new commercial projects

like the Tucson ordinance, but also includes residential projects and further specifies RWH

systems criteria. The following list, taken from the Water Conservation Projects and Programs

page of the Santa Fe County website 13, summarizes the criteria for new residential and

commercial projects:

Residences 2,500 ft2 of heated area or less must utilize rain barrels, cisterns, or other

catchment basins.

Residences 2,500 ft2 of heated area and greater must install an active rainwater catchment

system comprised of cisterns

o Cisterns must be buried or partially buried.

o Cisterns must hold 1.15 gallons per ft2 of residential heated area; this figure can

be adjusted based on landscaping, but not eliminated.

o Landscaping must be watered by a pump and drip irrigation system connected to

cisterns.

B Commercial Rainwater Harvesting Ordinance No. 10597C Santa Fe County Ordinance No. 2003-6


For commercial properties, the criteria are similar to residences of 2,500 ft2 of heated and greater,

requiring that an active rainwater harvesting system be installed.

Cisterns must be buried, partially buried, or enclosed within an insulated

building/structure.

Cisterns must hold 1.5 gallons per ft2 of roofed area; this figure can be adjusted based on

landscaping but not eliminated.

Both Arizona and New Mexico use the doctrine of prior appropriations when it comes to

surface water 6 and both clearly allow RWH, even enforcing its practice in certain municipalities.

Many other states also allow the practice of RWH though currently none appear to include the

more aggressive regulations found in Tucson and Santa Fe. Such states include Illinois, North

Carolina, Ohio, Oklahoma, Oregon, Rhode Island, Texas, Utah, Virginia, and Washington,

among others 8. These states are noted for having established some form of legislation, policy, or

program specifically regarding rainwater harvesting. Rainwater harvesting may be practiced

legally in other states but they may have no official laws or regulations defining or concerning

such practices. Typically, wetter states do not have significant water scarcity issues and so the

subject of rainwater harvesting may not be an important one. However, these states may have

significant water quality issues, in which rainwater harvesting can provide decreases in

downstream pollution due to reduction in stormwater runoff 9.

One exception to the list of prior appropriations states friendly to RWH is Colorado, in which

rainwater harvesting was previously considered illegal but has recently adopted new legislation

allowing RWH under very specific circumstances. In 2009, Colorado passed two laws loosening

the restrictions on RWH. In particular, Senate Bill 09-080 allows for a residential RWH system


only if the residence is not connected to a domestic water system that serves more than three

single-family dwellings 6. Additionally, this water may only be used for 1) ordinary household

purposes, 2) fire protection, 3) the watering of livestock or domestic animals, and 4) the

irrigation of up to one acre of gardens and lawns. Clearly this law is only useful for those being

supplied by well water in rural areas and is not meant for suburban or urban populations. It also

stipulates that rainwater may only be harvested from the rooftop of the residence; no other

catchment areas may be built or used.

Colorado’s restrictiveness regarding RWH is due to the state’s water laws, which essentially

considers precipitation to be already appropriated to senior rights holders as this water will

eventually contribute to stream flow in some form 6. Maybe Colorado is being overbearing and

paranoid when it comes to the actual downstream effects of RWH, or maybe they are protecting

their water rights holders from the possible consequences of a future HDRWH scenario.

Incentives

Among the states friendly to RWH, most of them also include some form of incentive to

encourage the use this practice. These incentives are typically in the form of credits or rebates

that offer taxpayers a percentage of the cost of installing a new RWH system up to a certain

dollar amount.

Starting in 2007, the state of Arizona provides a one-time credit of 25% of the cost of a

system up to $1000 6. There is a state budget limit of $250,000 annually for the tax credit

program and as of 2010 the state had yet to reach this limit, indicating there is still room for

growth in the RWH market in Arizona.

In Texas, rainwater harvesting equipment and supplies are exempt from sales tax 14. At

the municipal level, the City of Austin may provide a rebate of up to $500 for residential


customers installing a RWH system, while commercial users may receive a rebate of up to

$40,000. In San Antonio, the San Antonio Water System will rebate up to 50% of the cost of

installing water-saving technology, including RWH systems to commercial, industrial, and

institutional users. However, these projects must remain in service for at least 10 years and water

use data from before and after installation must be provided to determine if conservation goals

are met.

The city of Portland, Oregon provides a discount of up to 100% off of on-site stormwater

management charges if the property owner manages stormwater runoff in a way that helps to

“[…] protect rivers, streams and groundwater from the damaging effects of stormwater runoff.”

1. This discount applies to both residential and commercial properties in which residential

properties must manage runoff from rooftops only while commercial properties must include

paved areas as well. Stormwater management techniques include disconnecting downspouts and

directing roof drainage to landscaped areas, installing soakage trenches, rain gardens, or rain

barrels 2.

Rainwater harvesting tax credits shorten the return on investment (ROI) period, however,

this period can vary greatly depending on the size and cost of the RWH system installed, local

precipitation patterns, cost of municipal water, the efficiency of the RWH system (leaks,

evaporation rates), and the uses of the system (outdoor irrigation, toilet flushing, laundry).

In a study performed in the Metropolitan Area of Barcelona, Spain 4, it was found that

harvested rainwater was frequently underused in the sample systems. Initial capital costs were

usually perceived as high but were lower per household in multi-family buildings. The cost per

household for a 20 m3 system used for landscape irrigation in a multi-family building was

estimated at about 633€ ($827 US) per household, compared to 8,864€ ($11,581 US) for a


single-family building. The low cost of water in the area also increased the theoretical payback

periods. At a price of 1.01€/m3 of municipal water (approx. $5/1000 gal), payback periods in a

single-family household were essentially unattainable, but a higher water rate of 3.22€/m3 (about

$16/1000 gal) saw payback periods from 33 to 43 years, depending on system size and assuming

a 0% discount rate D. It was also found that payback periods decreased if the rainwater was used

for outdoor irrigation instead of indoor uses in single-family households. As for multi-family

dwellings, shorter payback periods were possible with a higher water rate, indoor water use only,

and a discount rate of 0%, with payback periods in the range of 21 to 26 years.

Other sources, mainly RWH system suppliers, often cite shorter payback periods.

According to Ecozi, a UK provider of rainwater harvesting and greywater recycling systems, the

payback period on a residential system is about 15 years depending on initial capital cost, and

commercial systems can see returns in as little as 3 years 5, though how these numbers were

calculated is never mentioned. They also state that RWH offers shorter payback periods than

most other sustainable technologies.

Downstream Water Quantity

One of the areas with the least amount of knowledge or research regarding the up-scaling

of RWH is that of possible negative downstream effects. Much of the publicized material

supporting RWH states only positive downstream effects such as reduced flooding and pollution

from stormwater runoff 9. However, this material is often published by companies or

organizations that either sell RWH systems or are trying to obtain funding for RWH projects. But

in their defense, the RWH scenarios they often refer to are not large enough to cause observable

D Domenech and Sauri mention that since RWH provides positive environmental and social benefits, choosing a discount rate that matches the interest rate would undervalue its benefits. In their analysis, a social discount rate of 0% was chosen as well as a more conventional rate of 4% for comparison.


negative downstream effects with regards to water quantity. Even hypothetical scenarios often

disregard this aspect and only focus on water savings and stormwater reduction at the harvesting

site. At the scales and densities of current RWH implementation, local watersheds do not see any

harm downstream of the where RWH occurs, at least not anything significant enough to warrant

more research on the matter. But what if a city located along a watercourse were to install RWH

systems on 75% of its rooftops? This question is central to HDRWH scenarios.

The National Resources Defense Council (NRDC) conducted analyses of the potential

water savings of several hypothetical RWH densities in eight US cities: Atlanta, GA; Austin,

TX; Chicago, IL; Denver, CO; Fort Myers, FL; Kansas City, MO; Madison, WI; and

Washington, DC 9. They modeled two scenarios looking at the volume of rainwater that could be

captured if different percentages of rooftops were utilized for RWH. The first scenario involved

capturing and using all of the first inch of rainfall from each storm event throughout a year and is

the least conservative of the two scenarios studied (the second scenario involved capturing and

using only some of the first inch of rainfall). In their model, RWH densities of 25%, 50%, and

75% were used (though the term “high-density rainwater harvesting” is never mentioned). In

their findings, a HDRWH scenario of 75% and capturing only the first inch of rainfall would

divert roughly anywhere from 31 to 51% of all rooftop runoff annually. Diverting one-third to

one-half of rooftop runoff would surely have an impact on downstream water quantity, and if

more than the first inch of rainfall from each storm event was captured, these percentages of

diverted rainwater would increase. And like all the other literature discussing RWH, nowhere

does the NRDC mention any potential negative impacts on downstream water quantity E.

E Admittedly, some of the cities in the study do not really have any downstream users as their water drains directly into the sea or other large water body (Lake Superior in the case of Chicago). One could argue that anyone pumping water out of these coastal discharge zones could be considered a downstream user, but that would be with regards to water quality, not water quantity.


Not only are downstream users a consideration when it comes to HDRWH, but so is the

downstream itself. Instream flow is defined generally as the amount of water flowing through a

channel over specified amount of time. Many states have adopted legislation or programs to

consider the different instream flows required to support the various uses and users of water

along a watercourse. These include drinking water supply, recreation, navigation, hydropower,

wetlands conservation, water quality, and aquatic habitat support 16. Depending on a number of

factors and conditions, each of these uses and users (if present) require different minimum

instream flows. Typically, human water uses are balanced with ecological considerations when

determining instream flow requirements. A water user such as a city must leave enough water in

the watercourse to support instream considerations further downstream such as aquatic habitat,

recreation, navigation, water quality F, etc.

While Colorado may be considered restrictive when it comes to RWH, they may be

considered progressive when it comes to instream flow protection. The Colorado Water

Conservation Board was established in 1973 and is “the sole entity that may hold instream flow

rights” 6, providing significant protection to instream flows, something that many other states

have only recently begun to consider. With all the importance that Colorado puts on instream

flows and ensuring that every drop of water reaches its intended users, Colorado may never see a

HDRWH scenario. That does not discount other states, however, as determining instream flow

requirements for multiple uses along a single watercourse can prove difficult, including how

those uses are prioritized (irrigation, municipal drinking water, hydropower, etc). Every

watercourse is different as are the users along that watercourse. Everyone has different priorities

and they all claim theirs to be the most important. Perhaps significant establishment and

F Water quality is not discussed in this paper as the main focus is on water quantity. However, there are many sources that discuss the positive effects of RWH on water quality, and the negative effects of decreased instream flows.


enforcement of instream flow requirements may inhibit future RWH projects, limiting the

theoretical RWH densities that a city may be able to obtain.

Outlook

Currently there are no regulations in place that require the retrofitting of RWH systems

into preexisting buildings, even in cities as progressive as Tucson and Santa Fe. However, there

are municipalities that do require the retrofitting of other conservation technologies such as low-

flow toilets, faucets, and showerheads. This is a possible next step for cities to take on the path to

a HDRWH scenario. Santa Fe County has a Commercial Retrofit Program, established in 2005,

that requires that all businesses upgrade to low-flow plumbing fixtures 13. The San Antonio

Water System has a similar program for commercial, industrial, and institutional users to help

them upgrade equipment and practices in order to save water 11. The program goes beyond

plumbing fixtures to include replacing water-cooled devices with air-cooled equipment,

capturing and reusing air conditioner condensate, cooling tower modifications, industrial laundry

equipment upgrades, and eliminating water-intensive phases of industrial processes. Similarly,

the city of San Francisco also has a program that provides aid to all non-residential water users

for retrofit projects 12. Some specific upgrades they list include cooling tower pH controllers,

water efficient ice machines, medical equipment steam sterilizers, and dry vacuum pumps. While

these latter two programs are merely incentives that provide rebates for upgrades, Santa Fe

appears to be the closest to maybe someday requiring the retrofitting of older buildings with

RWH systems.

And while some cities are currently requiring the installation of RWH systems in new

buildings, those regulations have not been in place for very long. These cities were already of a

significant size by the time these regulations were enacted and further development may be slow.


However, since state water plans usually use planning horizons of several decades, older

buildings will eventually be destroyed and replaced with new buildings having RWH systems.

The NRDC takes an optimistic outlook on the growth of RWH in the US. They claim that RWH

densities of 50% are achievable in most cities and that by the year 2030, about half of the

buildings in the US will have been built after the year 2000, implying that about half of the

buildings in the US will have RWH systems 9.

Summary and Conclusions

High-density rainwater harvesting is currently a hypothetical scenario, though it that may

become a reality in the decades to come. For now, the possible negative impacts of HDRWH are

purely speculative.

Many states in the US currently allow and even encourage the practice of RWH. Some

cities, such as Tucson and Santa Fe, go so far as to require RWH systems be installed in new

buildings. These kinds of regulations, often enacted in the face of growing water scarcity, are

most likely the beginnings to a future HDRWH scenario.

As water scarcity issues (and water quality in some cases) continue to grow, states, cities,

and individual water users will continue to adapt and shift in order to meet freshwater demands.

These adaptations will include infrastructural changes as well as political and social changes.

Among these changes is the perception of rainwater harvesting and its potential benefits in water

conservation. Rainwater harvesting continues to struggle for acceptance in some areas while it is

required in others. The potential future for increased popularity and massive up-scaling of

rainwater harvesting seems possible in some areas, though such scenarios may have unforeseen

consequences. Currently, there is no thorough research or modeling regarding potential negative


consequences of high-density rainwater harvesting, especially on downstream users. This report

concludes that the possibility of such a scenario should be considered more likely than it

currently is, and that more research should be performed in effort to curtail possible future

complications.


References

1. City of Portland [Internet]. Portland (OR); c2013. Stormwater discount program; 2013 [cited

2013 May 02]; [about 2 screens]. Available from:

https://www.portlandoregon.gov/bes/41976

2. City of Portland [Internet]. Portland (OR); c2013. Technical assistance; 2013 [cited 2013

May 02]; [about 3 screens]. Available from:

https://www.portlandoregon.gov/bes/article/390681

3. City of Tucson [Internet]. Tucson (AZ); c2012. Sustainability in government - water

resources; 2013 [cited 2013 May 04]; [about 2 screens]. Available from:

http://cms3.tucsonaz.gov/ocsd/sustainability-rainwater

4. Domènech L, Sauri D. A comparative appraisal of the use of rainwater harvesting in single

and multi-family buildings of the Metropolitan Area of Barcelona (Spain): social experience,

drinking water savings and economic costs. J Clean Prod. 2011;19:598-608.

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04]; [about 7 screens]. Available from:

http://www.ecozi.com/advice/rainwaterharvesting.html

6. Gaston TL. Rainwater harvesting in the southwestern United States [research paper].

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8. National Conference of State Legislatures [Internet]. National Conference of State

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9. National Resources Defense Council. Capturing rainwater from rooftops: an efficient water

resource management strategy that increases supply and reduces pollution. New York City

(NY): National Resources Defense Council; 2011 Nov.

10. National Water Commission (AU). Integrated resource planning for urban water - Cabbage

Tree Creek. Case study. Brisbane (AU): Marsden Jacob Associates (AU); 2011 Mar.

Waterlines Report Series No 41, Section 6.

11. San Antonio Water System [Internet]. San Antonio (TX); c2013. Large-scale retrofit rebate

program; [cited 2013 May 04]. Available from:

http://www.saws.org/Conservation/commercial/retrofit.cfm

12. San Francisco Public Utilities Commission [Internet]. San Francisco (CA); c2013. Grant

assistance for water efficient equipment retrofits; [cited 2013 May 04]; [about 2 screens] .

Available from: http://www.sfwater.org/index.aspx?page=512

13. Santa Fe County [Internet]. Santa Fe (NM); c2013. Projects and programs; [cited 2013 May

04]. Available from:

http://www.santafecountynm.gov/public_works/water_conservation/projects_and_programs

14. Texas Water Development Board (US). The Texas manual on rainwater harvesting. Third

edition. Austin (TX): Texas Water Development Board (US); 2005.

15. Texas Water Resources Institute. Rainwater harvesting: a new water source. Newsletter.

College Station (TX); 1997. Volume 3, Number 2. Available from:

http://twri.tamu.edu/publications/archive/


16. University of North Carolina [Internet]. Chapel Hill (NC): UNC-Chapel Hill; c2010.

Instream flows; [cited 2013 May 04]. Available from:

http://sogweb.sog.unc.edu/Water/index.php/Instream_flow

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