Rainwater Harvesting and Re-Use:
A Tool for Stormwater Runoff Reduction

By Greg Kowalsky and Kathryn Thomason, P.E.

Learning Objectives

After reading this article you should understand:

  • Engineering considerations when runoff reduction is the goal
  • Introduction to Building Codes and Water Laws
  • Components of a complete Rainwater Harvesting System

Online quiz for this article is no longer available.


Water Conservation- Until recently, the primary goal of Rainwater Harvesting in the United States was conservation. Portions of the country with limited municipal supplies or groundwater resources implemented Rainwater Harvesting systems for primary or supplementary water supplies, typically on a small residential scale. With the green movement, the use of Rainwater Harvesting became more wide-spread to reduce the environmental impact of development and population growth, especially in municipalities with finite water resources.

With water conservation as the goal, designers seek to maximize the supply by expanding the catchment area and reducing consumption, ensuring the cistern is full and water is available. The dry season presents the most challenge in this scenario as supply dwindles and demand remains constant or increases.

When used strictly for conservation, Rainwater Harvesting can be difficult to defend economically in the United States because the price of water is very low — less than a penny a gallon in many places. The current pricing structure of municipal water does not account for the "external" environmental toll of waterways running dry, the cost of increased energy demand, and the deferred maintenance of aging infrastructure. Until these costs are factored in, Rainwater Harvesting will be difficult to justify solely as a cost-savings measure.

Runoff Reduction- Today's approach to stormwater management, developed during the last few years and often referred to as Low Impact Development (LID), strives to eliminate post-construction runoff by reducing impervious area and infiltrating wherever practical. There are many sites where these two options do not provide enough runoff reduction to meet regulations. Engineers must overcome high groundwater, soils with low permeability, bedrock, and dense sites with limited space for surface infiltration, and Rainwater Harvesting offers them another solution.

When Rainwater Harvesting is employed as a stormwater Best Management Practice (BMP), engineers are faced with the opposite challenges of conventional design for water conservation. For runoff reduction, engineers must have storage capacity to catch the next storm event and demand in the water budget to empty the cistern. The wet season can be challenging because irrigation demand is low, supply is high, and engineers must find additional applications to use the harvested runoff.

Because of the new regulations, if runoff cannot be infiltrated onsite, building permits may be in jeopardy in many jurisdictions. While water prices won't provide the financial justification for an investment in a Rainwater Harvesting system, plan approval and project viability will. LID regulations are creating a large financial incentive for the Rainwater Harvesting systems.

Rainwater Harvesting and LEED

Rainwater Harvesting can provide a significant reduction in municipal water usage, especially when combined with water conservation strategies, and can be a powerful tool to meet the water reduction goals the U.S. Green Building Council (USGBC) has set forth in Leadership in Energy and Environmental Design (LEED) 2009. There are eight points available for Water Reduction, two points available for Innovative Wastewater Technologies, and two points available for Stormwater Design. The total of 12 possible points in these categories can provide a big head start toward LEED certification.

Building Codes

For years, rainwater harvesting has been excluded from building codes, creating barriers for its adaptation. Rules are changing and the International Plumbing Code and local codes are steadily being updated to allow Rainwater Harvesting. While engineers must always check the local codes, common requirements include:

  • Catchment — rooftop runoff is generally preferred; surface runoff may be subject to more treatment or not allowed.
  • First Flush Diversion and Pretreatment — codes may require runoff from the first flush to be bypassed and pretreated using gutter guards and screening before storage.
  • Storage — vent and screen to prevent insects, rodents, or other pests from entering the cistern. Aboveground cisterns may be subject to seismic and wind loading and applicable permitting. Provide access for inspection and maintenance.
  • Re-Use Water Line — plumb a separate water line. In many codes, this water line is purple so it is readily identified and not accidentally connected to potable fixtures.
  • Separate from Municipal Supplies — backflow preventers or air-gaps in make-up tanks are required to prevent cross contamination.
  • Disinfection — depends on the location and application. While rules vary, Ultraviolet (UV) disinfection for irrigation is commonly required and chlorination may be required for toilet flushing.  
Table 1: Factors to consider in conservation and stormwater runoff-reduction objectives for rainwater harvesting
  Conservation Focus Stormwater Focus
Primary Goal Reduced municipal demand Eliminate runoff (pollution prevention)
Secondary Benefits Reduce SW Runoff, Energy, CO2 Conservation, Energy, CO2
Catchment Area Maximize to Increase Supply Minimize to Reduce Supply
Water Usage Minimize and Conserve Find Re-use Applications
Seasonal Challenge Dry Season — not enough rain Wet Season — to much rain
Cistern Goal Keep it full Empty it quickly
Economic ROI Negative — "external costs" not included in market price of water Positive — best LID solution in many cases


Water Law and Prior Appropriation

Water law in the arid west is a complicated topic that impacts Rainwater Harvesting. As the U.S. expanded westward, water resources were claimed by the first entity to use the water for a beneficial purpose such as ranching, farming, mining, or industrial use.

This first-in-use concept allocates a set annual volume or set flow rate to the owner of the water right and prevents an upstream user from withdrawing water without claim. Today, all of the water resources have been allocated and these water rights are valuable assets that are bought, sold, and exchanged. Water law impacts Rainwater Harvesting because runoff from a property feeds a waterway that has likely been fully appropriated. In short, a property owner does not own the runoff from their property. Each state interprets water laws differently and most arid states are beginning to clarify the legality of harvesting runoff. For example, Colorado tends to be stricter and only allows small rain barrels for private residences with wells, while Washington State has declared capturing rooftop runoff for re-use legal.

Water law, which limits upstream runoff reduction to protect the owners of water claims downstream, is in conflict with stormwater regulations that require significant runoff reduction. If Rainwater Harvesting continues to grow as a BMP, the overlap between these laws and regulations will need to be clarified.

Re-Use Applications for Harvested Water

Irrigation is the most common application for harvested water. Treatment requirements are lower, which makes the harvesting system simpler, and the demand can quickly drain the cistern to make room for the next storm event. However, relying solely on irrigation for net-annual runoff reduction is rarely enough. In most locations, there is significant rainfall and irrigation is unnecessary. The Pacific Northwest and Southern California, for example, have the majority of rainfall in the winter when there is no irrigation demand. To get meaningful runoff reduction on a net annual basis, engineers will need to find additional applications beyond irrigation. Other possible applications may include:

  • Toilet Flushing — creates year round water demand. Multi-story buildings increase the number of toilets under the rooftop and create more demand than irrigation for denser developments.
  • Washing Machines — After toilet flushing, washing machines create the most non-potable demand in residential applications. Harvested water, if treated appropriately, can be used as the source of cold water for washing.
  • Hose Bibs and Outdoor Washing — Many commercial, industrial, and government buildings perform outdoor washing operations that are well suited for harvested water. Vehicle washing, window washing, and cleaning of photovoltaic solar cells are common examples.
  • Process Water — Commercial or industrial projects may have water-intensive processes. Cooling water is required for many industrial operations and can represent a large, steady demand for harvested water.
  • Potable Applications — are not practical for most projects where runoff reduction is the goal. Demand is a small portion of the water budget; treatment costs are higher and monitoring is required to meet drinking water standards.

UrbanGreen SRPE Rainwater Harvesting CisternsStormwater Management stormfilter for Rainwater Harvesting treatment

A four-phase rainwater harvesting system from CONTECH Construction Products Inc. is installed at Alta Vista Park in the city of Redondo Beach, Calif. The system includes a CDS pretreatment system, two UrbanGreen SRPE cisterns, and a Stormwater Management StormFilter.

Components of a Complete Rainwater Harvesting System

Rainwater Harvesting systems are created from common building blocks. The size and complexity of the building blocks depends on the project and application. All systems will incorporate the following items in some form:


Rooftops are the most common catchment and are major contributors to total runoff in dense developments. Although relatively clean, dirt, dust, and leaves will contaminate runoff, and animals can be a source of pathogens. Also, roofing material could leach additional pollutants.

Hardscape Surfaces tend to be more contaminated than rooftops but contribute a large area in lower-density developments. Patios, walkways, parking lots, and roadways may lend themselves to harvesting if appropriate treatment is paired with suitable water usage applications.

First Flush Diversion and Pretreatment

First flush diversion is required to meet some building codes and is based on the assumption that runoff from the beginning of a rainfall carries more pollutants. Many of these diversion structures are built into the downspout system and are sized to divert a sitespecific volume. The code in Portland, Ore., for example, requires the first 100 gallons per 1,000 square feet of collection area to be diverted.

Pretreatment cleans the water prior to storage and provides several benefits. It protects downstream pumps, filters, and fixtures from damage or clogging, and lowers cleaning and maintenance costs by keeping pollutants out of the cistern and filters. It also reduces the amount of organic matter and biological oxygen demand (BOD) in the cistern, decreasing the likelihood of creating anaerobic conditions and associated odors. Common pretreatment tools include gutter screens, downspout and in-ground screening devices, passive filters, and bioretention. When Rainwater Harvesting is used as a runoff-reduction BMP, pretreatment should be fully evaluated and sized for the large flow rates many sites will produce. Small systems that are common for residential applications may not be adequate.


Above ground cisterns are common for smaller systems, can range from hundreds of gallons to 20,000 gallons or more, and are often manufactured in metal, concrete, and plastic. Freezing, wind loads, and seismic loads should all be considered when designing an above ground cistern. Soil types must be able to withstand the weight, and engineered foundations may be required. For smaller sites, above ground cisterns can be small enough to provide economical storage, but for larger sites, the engineering, foundation, and installation cost can prove prohibitive.

Below ground cisterns save surface space and may be more economical for larger sites. Freezing is not a concern if depth is available and there are no seismic or wind loads. Groundwater can be a concern, but most systems can include anti-buoyancy features. Metal, plastic, concrete, and fiberglass cisterns are common for underground applications.

Additional storage features should be evaluated. Calming inlet devices create quiescent flows that disturb pollutants and sediments as little as possible. Floating outlets ensure the cleanest water just below the surface is used first. All openings should be screened, the cistern should be vented, and all cisterns should incorporate an overflow.

Day Tanks, Pressure Tanks and Make-up Water

Day-tanks provide a convenient location to provide an air gap between potable and re-use water. Pressure tanks can also be used with back-flow preventers, allowing pumps to cycle less frequently. Most systems will incorporate municipal make-up water to ensure the end-use application — such as toilet flushing or irrigation — is not interrupted during dry periods.


Pumps are sized to meet the maximum instantaneous demand for all combined applications. They also provide standard city water pressure to meet code and operational standards. Duplex or even triplex pumps are common to ensure water service is not interrupted.


Filtration is the most common type of treatment after storage. Staged filtration is common, which targets larger particles first and smaller particles with a second filter. Many systems can be back-flushed if pressure loss increases across the filter. Ultra filtration is an option if an additional level of treatment is required.


Harvested water can harbor a variety of pathogens. Even though the majority of water will be used for non-potable applications, people could come in contact with re-use water that potentially poses health risks. Current rules and building codes vary. For example, Los Angeles has published guidance requiring disinfection for almost all applications including irrigation while Oregon does not require disinfection for irrigation. Disinfection technology has become affordable — especially when compared with the overall cost of a harvesting system — and should be considered for all systems.

UV radiation is common, affordable, and provides a high level of effectiveness (99.9 percent or better kill rate of pathogens). Typically, it is applied just prior to delivery as the last treatment step. One drawback is it does not provide any residual disinfection capability. Over time, the few remaining pathogens can reproduce and contaminate the water downstream. UV is best used for applications where the water will be used immediately, such as irrigation, and avoided where water will be stored, such as toilet flushing.

Chlorination is commonly used for applications that require residual disinfection capability and is common for non-irrigation applications. There are both manual and automatic systems that can be economically scaled to meet the demand.

Ozone can also be used as a method of disinfection but is often avoided because of its potential corrosive effects on downstream plumbing.

Reverse Osmosis can be used but is more energy intensive and has a low yield, sending a large percentage of the harvested water directly to the sewer system during back-flushing.


Harvesting requires pumps and power, allowing automated controls. Systems can automatically switch to municipal backup supplies if the cistern is dry, perform back-flushing of filters when pressure loss increases, and manage disinfection dosing. Controls can also perform ongoing monitoring and communication. It is common to measure cistern levels, water usage, pump hours, UV bulb life, and other important system information.

Communication over the Internet can provide remote monitoring and alert capabilities as well.

Typical components of an underground rainwater harvesting cistern.

Modeling Runoff Reduction

To meet runoff-reduction requirements, engineers must show the amount of water that the Rainwater Harvesting system will collect and use. So, in addition to calculating the amount of runoff from a site, they must develop a demand model and water budget for the targeted re-use applications. By overlaying the runoff model, storage capacity, and demand model, engineers can calculate the net annual runoff reduction.

Daily or Continuous Models

To accurately estimate runoff reduction that aRainwater Harvesting system will provide, engineers should employ a daily model of the system. Although monthly models are useful for rough average rainfall and demand estimates, they are not detailed enough for final design.

Demand models should account for all water use applications including weekly and seasonal fluctuations. For example, commercial offices will have higher demand on weekdays and irrigation will occur only during the summer. By accounting for day-to-day fluctuations in weather and water usage, a continuous model can accurately demonstrate the net annual runoff reduction that the Rainwater Harvesting system will provide.


Rainwater Harvesting is an effective stormwater BMP for runoff reduction. Secondary environmental benefits include reduced municipal water consumption, net energy savings, and reduction of associated greenhouse gases. Building codes are changing and many include provisions for Rainwater Harvesting, and it's important to understand the local requirements before moving forward with a Rainwater Harvesting project. System components are scalable, allowing engineers to design a system that is optimal for their project.

There are many applications for harvested water beyond irrigation and engineers should look for additional water demand to increase the effectiveness of their system. By incorporating Rainwater Harvesting, engineers can meet stormwater regulations, earn points toward LEED, and reduce demand on municipal water supplies. Turning polluted runoff into a valuable resource will be a major step forward for sustainable development.

Greg Kowalsky, BSME, is the Low Impact Development product manager for CONTECH Construction Products Inc. He is an active member of the American Rainwater Catchment Systems Association (ARCSA) and has six years of experience in stormwater design and 15 years of engineering experience.

Kathryn Thomason, P.E., is a senior design engineer with CONTECH and currently specializes in stormwater treatment, detention, and rainwater harvesting design.


  • Georgia Rainwater Harvesting Guidelines, in accordance with Appendix I Rainwater Recycling Systems of the 2009 Georgia Amendments to the 2006 International Plumbing Code.
  • Roof-Reliant Landscaping, Rainwater Harvesting with Cistern Systems in New Mexico, 2009, Nate Downey, principal author, Randall D. Schultz, editor, Ken Wilson, designer, New Mexico Office of the State Engineer, www.ose.state.nm.us.
  • 2008 Oregon Plumbing Specialty Code, Chapter 16, Part I Gray Water Systems and Chapter 16, Part II Rainwater Harvesting Systems.
  • Code Guide, Office of Planning and Development, Portland Ore., Rainwater Harvesting – ICC – RES/34/#1 and UPC/6/#2.
  • Oregon Smart Guide, Rainwater Harvesting, Department of Consumer & Business Services, Building Codes Division, Salem, Ore.
  • The Texas Manual on Rainwater Harvesting, Texas Water Development Board, Third Edition.
  • Focus on Rainwater Interpretive Policy, Water Resources Program, October 2009, Publication Number: 09-11-026.
  • Water Resource Program Policy Regarding Collection of Rainwater for Beneficial Use, Pol 1017, Washington Department of Ecology.
  • LEED 2009 for New Construction and Major Renovations, U.S. Green Building Council.
  • Approval Request for Rainwater Harvesting Matrix, From: City of Los Angeles Department of Sanitation, To: Los Angeles County Department of Public Health, April 14, 2010.
  • State of Colorado Senate Bill 09-080, Concerning limited exemptions for water collected from certain residential rooftops, 2009 Legislative Session.