The average rainfall in four major U.S. cities studied, if collected by households, would be enough to offset the biggest use of water in the home: flushing toilets. Drexel University researchers say that using rainwater instead of treated municipal water for toilet flushing could reduce consumption of nearly one-quarter of potable water, dramatically lowering stresses on watersheds, drinking water treatment and distribution systems and urban drainage infrastructure.

Researchers studied four of the largest metropolitan areas in the country—Philadelphia, New York, Seattle and Chicago—to determine whether rainfall is sufficient to allow for such a change and, if so, what effect it would have on domestic water demand and storm water runoff generation in those cities.

: A residence with a 1,000-gallon rainwater-harvesting system could reduce runoff by over 40%. Image credit: Drexel University.“When the natural landscape is replaced by a building, rain can no longer infiltrate into the ground,” says Franco Montalto, associate professor in Drexel’s College of Engineering. “It runs off and is captured in drains, where it can cause downstream flooding, carry pollutants that settle out of the air into local water bodies or—in the case of a city like Philadelphia or New York—cause the sewer to overflow, which leads to a discharge of untreated wastewater into local streams and rivers."

Capturing rainwater, Montalto says, can help to reduce the demands on the water treatment system and ensure that it will still function properly during heavy rainfall events.

The process of collecting and using roof runoff has been working its way into vogue among urban planners and water managers over the last couple decades and has been implemented widely in California in the wake of its water crisis. The Drexel study is one of the first to crunch the numbers and sort out the feasibility and benefit of offsetting potable water use for non-potable purposes while simultaneously reducing generation of undesirable urban storm water runoff.

Taking into consideration the cities’ annual rainfall patterns, residential population and roof areas, the team calculated that, with enough water storage capacity—a little more than a standard 1,000-gallon home storage tank—a three-person family in a home with an average-sized roof would have enough water to offset over 80% of its flushes throughout the year simply by diverting their downspouts to collect stormwater. This would reduce overall household potable water demand by approximately 25%.

In general, greater potable water savings are estimated in cities with either larger roof areas or lower population density, according to the researchers. However, such savings would be accompanied by smaller reductions in runoff, they add. Philadelphia and Seattle are the two cities where the percentage of water savings would be greatest if residential neighborhoods were all equipped with rainwater harvesting systems.

From a stormwater management perspective, an average residence with a 1,000-gallon rainwater-harvesting system could reduce runoff by over 40%, say the researchers. This would vary by residence—according to the size of the water storage container and the water demand for toilet flushing—but as a whole cities could see a significant reduction in the amount of stormwater their infrastructure would have to handle during each storm.