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Rainwater Collection

The basics of why and how, and is it helpful or harmful?

Water is arguably the most important natural resource we need. Without energy sources such as fuels and electricity, our daily lives are disrupted. Without water, we die in a matter of days. Water is a requirement for growing food. It is also used heavily by individuals, businesses, and industries for all kinds of purposes. Water is the one necessity for life to exist.

Where Fresh Water Comes From

The hydrological cycle provides inland areas with fresh water. With the help of the energy from the sun, water molecules evaporate from the surfaces of lakes, rivers, and oceans. Water molecules are also released into the air through transpiration in plants. Those water vapor molecules rise up in the atmosphere, cool, and condense into liquid droplets, forming clouds. When enough of these droplets combine, they become heavy enough to precipitate out of the atmosphere and fall in the form of rain, snow, sleet, or hail. Some of this water seeps into the ground in a process called infiltration. Water that doesn't infiltrate the ground flows downhill over the land – called runoff. Eventually, the runoff collects in rivers, lakes, and oceans, where the cycle begins again.

Evaporation and Transpiration >

Condensation >

Precipitation >

Infiltration and Runoff >

Collection >

repeat ...

Humans generally gather fresh water for use from the collection stage of the hydrological cycle. We pull it from rivers, lakes, and artificial lakes called reservoirs. We also drill wells to get water that is stored underground in porous rock in structures called aquifers. Similar to how fossil fuel deposits are non-renewable pockets of hydrocarbons, many aquifers contain non-renewable pockets of fossil water. These "confined aquifers" contain water that was sealed off from their source millennia ago and are either no longer being refilled or are being refilled at a rate much slower than the rate at which we extract water from them. Water extracted from these aquifers is not a sustainable source of water.

In order for water use to be sustainable, we can't extract water from aquifers faster than they are refilled by infiltration, and we can't extract water from rivers and lakes faster than they are refilled by precipitation. If we do either of these things, eventually, we will run out of fresh water. Hypothetically, we could desalinate seawater and pump it inland uphill to the places that need it. However, the infrastructure and energy required to do this at scale is unreasonable.

Rainwater Collection

A rainwater collection system (aka rainwater harvesting) effectively captures the precipitation that falls on a given area and stores it for later use on site. It pulls water from the infiltration and runoff stage of the hydrological cycle.

Doesn't this prevent rainwater from recharging aquifers by not letting the water seep back into the ground? Kinda, but only to a very small extent, and the benefits of collecting rainwater onsite have a significant beneficial effect overall.

Much of the rainwater runoff evaporates before it has a chance to recharge any aquifers. Only a small fraction of the water that falls on a house's roof will ever make it to an aquifer. And what water does make it to an aquifer will be required to be extracted from it for use by the people, including those whose house it initially fell on. But now energy is required to pump and purify that water for domestic use. Overall, capturing rainwater for use onsite is a much better use of water than allowing it to run off and then pumping it back to the house for use via the municipal water system. This is especially true in dry desert environments where water is scarce.

The built environment in cities and suburban areas significantly reduces the permeability of the ground and, therefore, the water infiltration rate. In areas covered by concrete, asphalt, and compacted dirt, less of the precipitation infiltrates the ground to recharge groundwater levels, and more of it runs off. This large amount of runoff makes cities and suburbs more prone to flooding from runoff. By reducing runoff, rainwater collection systems can reduce flooding and urban pollutants being washed into waterways. Often, the water that runs off a roof ends up in the yard, where it tends to flow into the most porous dirt up against the house's foundation, loosened from being dug up during construction, where it can find its way into basements and crawlspaces. Collecting this rainwater can help reduce the risk of basement flooding.

How To Build a Rainwater Collection System

A rainwater collection system consists of a catchment area (such as a roof), gutters and downspouts, a first flush diverter and screens, a storage tank, optional filtration, and a distribution system. Essentially, when it rains, water falls on the roof, gets strained, and flows into a tank for later use on the property.

A rainwater collection system can be used to water a few plants, wash cars, provide all outdoor irrigation, or supply all of your domestic water needs. How you build your system will depend on what you want to do with it.

The first flush diverter is a device that ensures that the first flush of rainwater, which may contain debris and contaminants that have been sitting on the roof, is diverted away from the main storage tank. It consists of a vertical pipe with a lightweight ball in it that floats upward when the pipe gets filled with water. The first water that flows through the gutters, downspouts, and a screen to remove debris fills up this pipe with the most contaminated water. When it's full, the ball floats up and seals off the flow into the diverter, so the rest of the rainwater flows past into the water storage tank. There is a tiny hole in the bottom of the diverter that lets the water slowly drip out over the next few days.

Depending on intended use, storage tanks can range from small 50-gallon rain barrels to large cisterns. Less frequent rainfall will require a larger storage tank. The storage tank should be opaque and not let light in, which would allow algae to grow. The tank also needs an overflow vent for the water to drain out if a large storm fills the tank. The overflow vent should be large enough to handle the same volume of water that is filling the tank, and it should have a fine mesh screen over it to prevent mosquitos and other insects from getting in the tank and laying eggs.

Depending on the intended use, the collected rainwater may need to be filtered and purified to remove sediments, bacteria, and other contaminants. This can include coarse filters, carbon filters, UV purifiers, and other treatment systems.

Finally, a distribution system consisting of a pump and piping is used to get the collected rainwater to the intended point of use. If you want the water pressure to be equivalent to standard residential water pressure of 45-80 psi, you'll need to ensure your pump has that capacity. Flow rates at different "head" heights must be considered for your application. Some sort of pressure tank should be used to prevent the pump from cycling on and off constantly and shortening its life. While technically, you can build a water tower for this, in order to achieve a standard 50 psi of pressure, your water tower would need to be 115 feet tall. Using a well pressure tank that has a diaphragm separating an air chamber from a water chamber is a much better way of reducing pump cycling. You'll also need to determine how the pump should be controlled for your application. This might be a high/low pressure switch. It's also a good idea to have a low water level cut-off switch to disable the pump when the tank is out of water. This can be accomplished with a float switch.

All rainwater storage tanks, pumps, and pipes must be prevented from freezing in the winter. This might mean keeping everything in a heated and insulated space, or winterizing the system in the fall.


Before installing a rainwater collection system, it's important to check state regulations around collecting rainwater. Many states like Arizona and Michigan encourage and offer incentives for rainwater collection. Others place a limit on the amount you can collect. Some states, like Colorado, have even banned rainwater collection altogether.

Your type of roofing material should be considered since it affects the water quality. Metal roofs are ideal for rainwater collection. Clay and concrete roofs work as well, but they absorb some water in their pores. Asphalt and composite roofs can work, but there is a chance water quality can be affected by some of the adhesives used. Solar panels make a great collection surface, as does slate. Roof materials you don't want to collect rainwater off of include:

  • Cedar Shake: Wood shingles are usually treated with fire retardants, which you don't want in your water.

  • Copper: Copper is an herbicide that is naturally resistant to algae and moss growth, and it will leach into rainwater, so it isn't ideal for garden irrigation.

  • Lead: Lead flashing is still available in some parts of the USA, so test for lead if you're harvesting it for drinking water.

  • Biocides: Some rooftops are treated with biocides. Zinc or copper-treated shingles are common in the Pacific Northwest to curb mold, moss, and algae growth on the roof.

Freeze protection is important for the whole rainwater collection system. You should have a strategy for preventing freezing in the winter before you begin, or you'll have a headache later.

My Rainwater Collection System

Since I live in the desert, where we receive only 8.5 inches of precipitation per year (the average for the United States is 30.2 inches), it's very important to use water as effectively as possible. So, I decided to install a rainwater collection system myself to learn about how it works and explore whether it can be implemented at scale to make a significant difference in water use.

For simplicity, I decided to use my rainwater collection system for only outdoor water use, which includes irrigation and washing dirt bikes and mountain bikes. I put my collection tanks in the backyard, and I had to install gutters. I opted for oversized 6" gutters and extra gutter hangers to handle larger amounts of water when it rains hard since I only wanted a downspout where water could be collected. I have 2 downspouts from which I collect water, which are fed by 3288 square feet of garage and house roof, which means it can collect up to 2050 gallons of water for every inch of rain.

Each downspout flows into a debris screen and a first flush diverter to filter out most debris and sediment before flowing into 500-gallon tanks. Each tank has an outlet and a shutoff valve on the bottom that flows into an insulated box that contains a 1/2 horsepower jet pump, a pressure tank, and a flow meter to see how much water the system has pumped. From there, the water flows through underground pipes that I connected to my irrigation system. The pump provides constant pressure to the irrigation system, which is controlled by valves on programmable timers. The pump turns on when the system pressure drops below 40 psi, and turns off when it exceeds 60 psi. Since I put a check valve at the outlet of the pump, when I run out of water in the tank, I can switch to city water by simply opening the main valve and connecting the irrigation system to the city water. I have a second pump setup that pressurizes a hose bib that goes into my garage.

I installed the system in February 2023. The system saves over 10,000 gallons of water per year. I also tore out the grass that the house came with and planted about a hundred fruit trees instead. Not only is grass a lot of maintenance, it takes a ridiculous amount of water. Now I use the collected rainwater to grow food on my little 1/3 acre plot with less water than having grass.

Is this going to save the world? No. However, rooftop rainwater collection and onsite usage work well and make a meaningful difference in water use. Knowing what I now know, I would install a rainwater collection system again.

Questions for you:
  • Do you have a rainwater collection system? If so, how does it work for you?

  • Do you plan on installing a rainwater collection system?

  • What arguments are there for or against rainwater collection that are not covered in this article?

Resource for more information on building your system:


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