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Passive versus Active Rainwater Harvesting

by Doug Pushard

There is wide spread interest in water conservation and specifically in capturing and reusing rainwater in both residential and commercial buildings to reduce costs, reduce the environmental impact of the building and lessen the load on the municipal sewer and stormwater systems in the arid southwest where droughts are a way of life.

Harvesting rainwater from rooftops is one solutions to conserving our precious water, where it can be used instead of municipal drinking water for many non-drinking water (i.e. non-potable) applications (e.g. landscape, toilet flushing) as well as drinking water. There are two general types of rainwater catchment systems - "active" or "passive". Most professionally installed systems incorporate aspects of both to maximize the water conserved.

Active rainwater catchment refers to systems that actively collect, filter, store and reuse water. The storage is usually the most visual aspect of an active system (i.e. large tanks), but they also generally incorporate pumps, and sometimes filters that require electricity (e.g. ultraviolet lights). These are active components that require regular ongoing maintenance to run efficiently and effectively.

In comparison, passive harvesting systems incorporate no mechanical methods of collecting, cleaning and storing rainwater. The intent with passive rainwater management is to create areas to contain waters until they can naturally be absorbed into the land. Vegetative swales, dry creek beds, and pervious concrete or pavers are types of passive collections systems. Passive systems can be relatively inexpensive and are generally simple to design and build.

Which is best? Just as in the building trade where there is a debate on the merits of active and passive homes, the same split sometimes occurs in the rainwater harvesting community. Some will advocate active over passive and vica versa. In most cases, one or the other can be used and be highly effective; however, there are cases where one technique is better than the other, so both should be considered when looking at catchment systems.

While active systems are more complex and generally more expensive than passive systems, they have the ability to store large quantities of both winter precipitation and summer monsoon rains and move it to specific locations where and when it is needed. Additionally, they are generally tied to irrigation systems but also can be used for indoor water use. Both types of catchment systems greatly reduce overall water consumption, but one major distinction is that active systems can also help reduce indoor water consumption.

Passive systems require little maintenance and can be incorporated in most new or existing landscapes without major surgery or costs. Building berms and swales to channel rainwater and slow it down to allow it to infiltrate are great places to start. Brad Lancaster's book Rainwater Harvesting for Drylands and Beyond is a great resource for those interested in fully understanding how these systems work and how they can be used on both large and small projects.

In my experience it is generally the background and training of the individual that leads to an inclination to either active or passive systems. Both are valid methods of harvesting rainwater and neither should be neglected as an option. Sometimes the site or the demand of the water will lead to a 'solution' that is better for a site than another. For example, commercial crop growing will require active catchment because they systems have very specific demands on water - timing and placement - that can not be fulfilled with passive systems. Residential potable water systems will require active. (Potable Rainwater - Filtration and Purification) Other situations where active may be the 'best' solution include: apartment complexes, high-rise buildings, commercial buildings with large parking lots, and large parks or school grounds. On the other hand, most residential systems should incorporate passive systems at the very least and a lot of yards can just use passive only.

In Santa Fe and surrounding areas where get nearly 1/3 of our precipitation in the winter months below ground active systems can capture and use this water during our typical dry springs, before the monsoons start. But this extra capture requires below ground cisterns which will generally drive the cost of the system up substantially (i.e. 2-3 more expensive) due to the costs of burying the tank and the tank is specifically manufactured to be buried and consequently made with heavier materials.

Harvesting rainwater is a great option for conserving our precious water(Before You Harvest: Conserve) . Either passive or active catchment systems will save water and should be incorporated in any harvesting project you are considering. Before you start, it is always best to start with water conservation. Whether indoor or outdoor, cutting down your water use will always reduce the size and cost of any system you will install.

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