The content for this article was originally featured in the Daily Journal of Commerce Landscape Special Section on June 27, 2024.

Landscape architects must take on the role of advocate for harvesting rainwater onsite early in the design process.

Advocating for progressive water strategies, inside and outside a building, must be a focus for every landscape architect. Rain is ever present in Seattle, and we are lucky to draw from pristine, reliable rivers. To that end, despite having the highest water bills amongst major cities in the U.S. (as much as $275/month for a typical household), the cost isn’t enough to spur change, and most Seattleites take our clean, seemingly abundant water for granted.

But water isn’t a never-ending resource, and the environmental benefits for creating more resilient and restorative urban infrastructure are clear. Landscape architects are perfectly positioned to maximize the impact our projects have on our region’s water usage. We need to use that power!

When designing landscapes for the urban environment, as the scale of a building increases, the financial justification for lowering water usage becomes more apparent. Exploring and evaluating ways to harvest rainwater onsite is necessary for every building type.

Early in the design process, landscape architects should assess the viability of capturing and re-using stormwater or greywater to determine the highest and best use of water on any given site. That starts with the question: What is this water fit to do?

Potable vs. non-potable

Reducing demand for resources is often the first and most essential step when designing deep green buildings. However, when thinking about water resources, designers must also begin assuming that municipally cleaned potable water is only routed to uses that need it (aka drinking, sinks, laundry, etc.). For all non-potable uses, such as toilet flushing and irrigation, teams should be designing separate water reclamation systems.

The rainfall from the roof at Watershed is conveyed to the building’s cistern via a stepping downspout on the north side of the building. (Photo by Built Work Photography / Meghan Montgomery)

As often happens in commercial real estate, mechanical engineers, who would ultimately design a water reclamation system, are not fully engaged until well into the building design process. Landscape architects must take on the role of advocate for early project water goals.

Optimal cistern sizing

Using rainwater for irrigation is intuitive – rain falls, and the landscape comes alive each spring. However, when the goal is to reduce demand for municipal water, using rainwater for irrigation is often not the highest and best use for this resource. In a typical urban, mixed-use project, often ten times the volume of water is needed to flush toilets than is needed for irrigation. In addition, given Seattle’s extended dry season each summer, it is necessary for irrigation water to be collected and stored many months before it is to be used.

Allocating space for rainwater capture is always going to be a challenge, which is why it is important to optimize the volume of water captured by targeting a year-round use that continually empties the cistern and allows for more rain to be collected. That best use in the Pacific Northwest is for flushing toilets.

System design

Water reclamation systems for toilet flushing closely resemble the design of commercial irrigation systems. Once a location for the cistern is identified, the remaining equipment is often co-located with the irrigation equipment. Ultimately the debris/sediment filters as well as smaller micron filters are identical between the two uses. A UV filter is commonly added to ensure viral and bacterial growth is suppressed. We have had projects that shared this equipment for both systems, but it is more common to have some redundancy.

An automatic loop that regularly filters the water stored in the cistern will also reduce the chances that anything grows in the tank.

It is also imperative to include a domestic water back-up for periods of drought, but that the point of input is downstream of the storage tank. Do not fill up the cistern if the water level dips below a certain level – this will prevent the next storm from being collected, and the system will lose efficiency.

Exterior downspouts are more common with Seattle’s stormwater code. This makes it easier to route water to bioretention planters.

Projects have elected to forgo the domestic back-up connection altogether, saving the cost of the municipal connection. Deep green certifications stipulate that an owner is allowed to fill a cistern at the beginning of a performance period to account for regional variation in rainfall. As climate change impacts the size and frequency of storms, a municipal back-up could create a more resilient system. For critical building functions, the back-up is essential.

Doing better

Onsite stormwater mitigation (slowing and cleaning of stormwater) is now required by many jurisdictions, and as such, developers that have been active over the last eight years to understand the cost and space needed for green infrastructure. For many urban developments, this has increased landscape construction budgets by 30-50%. Most water reclamation systems will not provide stormwater detention because a cistern is not designed to release captured water slowly. Surely toilet flushing would mimic the slow release that a detention tank provides, but jurisdictions are not convinced to allow these systems to be combined.

Likewise, it should be best practice to route roof water through planters to provide the function of an irrigation system, but this has only emerged in the design of bioretention planters. Researchers have shown that sending stormwater through soil removes toxins that have been shown to be responsible for the decline of salmon populations, however even in areas where water quality measures are not required, shouldn’t we still be implementing this strategy everywhere?

As the line blurs between the definitions of stormwater and greywater and the reality that the toxins killing salmon are found settling on all surfaces, not just where cars and roads leave behind tire dust, we must accept that we can do better to protect water as a resource. And that by innovating, optimizing, and combining all water systems inside and outside of a building, we will begin to see significant improvements in the environment around us. If we all work towards this goal, we will be even more surrounded by demonstrations of the value of integrating nature with the built environment.