Adapting RWH to rainfall intensity changes due to climate change

Climate change is affecting rainfall intensity.
Do rainwater harvesting systems need to adapt as the intensity increases and possibly the gap between rainfalls?


A call to assess the need to adapt our RWH designs to the new rainfall patterns. Patterns with increased intensity that we are experiencing and that the scientists are predicting.

The following press release got wide coverage:

Climate Change: When It Rains It (Really) Pours

Study in ‘Science’ says warmer climates lead to more extreme rainstorms

Virginia Key, Fla. (August 8, 2008) —

Climate models have long predicted that global warming will increase the intensity of extreme precipitation events. A new study conducted at the University of Miami and the University of Reading (U.K.) provides the first observational evidence to confirm the link between a warmer climate and more powerful rainstorms.

One of the most serious challenges humanity will face in response to global warming is adapting to changes in extreme weather events. Of utmost concern is that heavy rainstorms will become more common and more intense in a warmer climate due to the increased moisture available for condensation. More intense rain events increase the risk of flooding and can have substantial societal and economic impacts.

To understand how precipitation responds to a warmer climate, researchers in this study used naturally-driven changes associated with El Niño as a laboratory for testing their hypotheses. Based on 20 years of satellite observations, they found a distinct link between tropical rainfall extremes and temperature, with heavy rain events increasing during warm periods and decreasing during cold periods.

“A warmer atmosphere contains larger amounts of moisture which boosts the intensity of heavy downpours,” said Dr. Brian J. Soden, associate professor at the University of Miami Rosenstiel School of Marine & Atmospheric Science.

The report, “Atmospheric Warming and the Amplification of Precipitation Extremes,” previewed in Science Express this Thursday, August 7, and published in an upcoming issue of Science, found that both observations and models indicated an increase in heavy rainstorms in response to a warmer climate. However, the observed amplification of rainfall extremes was found to be substantially larger in the observations than what is predicted by current models.

“Comparing observations with results from computer models improves understanding of how rainfall responds to a warming world” said Dr. Richard P. Allan, NERC advance fellow at the University of Reading’s Environmental Systems Science Centre. “Differences can relate to deficiencies in the measurements, or the models used to predict future climatic change”

So what does it mean for those of us who are creating RWH systems?

I believe we have to assess its impact and adapt. There are two impacts of increased rainfall intensity.

The first impact is our ability to capture all the rain when it is intense. Some of the systems in the field i have seen, would not capture all the water during intense downpours. My home system captures some water from a sloping roof and some from a flat roof. The flat roof can ‘flood’ and while most of the water will be channeled into the tank, the upslope end also has a drain that just shoots water out onto the street. The sloping roof sends a lot of water into the collecting gutter, which can overflow. The sand filter so far seems to be able to cope, but if cannot absorb all, it overflows into the driveway. So i lose some ‘capacity’ when rainfall is intense – how much i don’t know.

The second impact is when the rain becomes more sporadic. With sufficiently large gaps between downpours, each rain becomes a ‘first’ rain. Since we normally discard the first rain, we would capture very little rain!
To ensure maximum RWH, we have to create a way to discard the first part of the rain.
This requires some manual effort and/or ingenuity, which i am sure that the community is capable of, once it is considered a necessity.

My thoughts are that we first need to assess whether our systems in general will cope with intense downpours. If there are gaps of a week or more, do these rains count as first rains? Of course, depending on dust, pollution, bird population, etc. the answers will vary.

Depending on the answers, do we need to design catchments and filters to handle a sudden burst?

A discussion is requested and any experiences are welcome.

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