Does Rainwater Harvesting Make Sense in Dry Climates?

Does Rainwater Harvesting Make Sense in Dry Climates?
By Jesse Froehlich, ARCSA A.P.
Founder, BlueBarrel
The answer to the question above is simple:


If you’re satisfied with that, you can stop reading here. And maybe visit for some great rainwater harvesting options!

If you need a little more convincing, read on.


Being in the rainwater harvesting business in arid California, I often hear comments like this:

“In our climate it rains in winter when we’re not irrigating. Then when we need the water in summer, there’s no rain. I just don’t see how rainwater harvesting makes sense with such a long dry season.”

Let’s dig into this predicament. We have to ask ourselves where our water comes from during this long, hot summer.

Whether you’re a municipal ratepayer or on a private well, there’s a good chance your water is pumped from underground aquifers. Or perhaps it comes from surface waters, or a combination. In any case, can you imagine what happens to these shared water sources as the dry summer drags on?

That’s right: they draw DOWN, DOWN, DOWN!

And unfortunately there’s another compounding factor: when it finally does rain again, this abundant fresh water resource sheets off our hardscapes, and hurries AWAY, AWAY, AWAY!

See below for an illustration of the effect of our hardscapes on nature’s hydrologic cycle. The water that is intended to infiltrate and recharge groundwater is largely prevented from doing so in our urban and semi-urban environments. The effect over time is overdraw, and in dry climates we are especially vulnerable.
Aquifer Depletion in California

That big downward-sloping blue line shows us what the California central valley aquifers have been up to (or “down” to) over a recent 40-year period: a steep decline. And with a rapidly growing population in this region, the problem is only getting worse.

So let’s consider this: What if you could do your share to counteract this trend by meeting even some of your summer irrigation demand with stored rainwater?

But this begs another question:


It is amazing just how much water you can catch off of a relatively small rooftop surface, even in a dry climate. Every square foot of catchment surface will yield about 0.6 gallons for every inch of rain. Check out the sizing tools on BlueBarrel’s website to refine the calculation for your own rooftop.

Rainwater Harvesting System

Now that you see the numbers in action, don’t forget about garages, workshops and sheds! You really don’t need much surface area to fill a rainbarrel, or even a series of them.

With these numbers in mind, the question isn’t about whether there’s enough water to catch, it’s about how much space you want to dedicate to water storage.

So let’s dig into THAT.

We all dream of a rainwater catchment system that can meet our year-round demand. (I sure do!) And as shown by the numbers above, that system may be possible even in arid California.

It is likely, however, that your budget and/or spatial constraints limit your ability to store that much water. So catch what you can and enjoy the benefits in proportion!

There are three ways to manage a rainwater catchment system if you can’t store enough water to meet your entire dry-season demand:

  1. Store what you can, use it while it lasts (which may be longer than you think if you use gravity-fed drip irrigation), and switch back to your existing water source when your rainwater runs dry.
  2. Store what you can, use it while it lasts, and then re-fill your rainwater tanks with your backup water source to continue using your gravity-fed drip system throughout the dry season.
  3. Dedicate your rainwater catchment system to a particular garden zone, like a perennial border or raised vegetable beds, and size the system to meet that zone’s irrigation need all summer long (BlueBarrel can help you strike this balance with a professional Site Assessment).

[NOTE: The BlueBarrel System includes specialty vent pieces with screened hose-swivel ports. This allows for safe and easy dry-season refilling if you’re using Strategy 2. We also offer a streamlined drip irrigation connection to ensure you are using your water efficiently.]

See below for a real-life case study from a recent BlueBarrel project at a Santa Rosa, CA residence.


Climate change projections for the North Bay predict a decrease in average annual rainfall. But there’s more to the story: this diminished rainfall is expected to come in the form of stronger storms that occur less frequently. And in fact this is exactly what we have experienced in recent years.

Ironically, this presents an additional opportunity for Rainwater Harvesters: multiple irrigation and recharge cycles over the course of a year. (This means much less storage required to meet the entire year’s irrigation need, and much higher efficacy for small rainwater catchment systems.)

Last year we experienced heavy rains in autumn followed by an exceptionally dry winter. Then we had significant rainfall again as late as June! With the first autumn rains of this year falling in late September, North Bay Rainwater Harvesters got two full irrigation cycles (one in winter and one in summer), spanning a mere 12 weeks during the hot summer season! We’re on track for the same pattern in the current cycle.

[In this article I’m not focusing on the stormwater mitigation side of things, but it’s worth interjecting that with heavier rain events (as predicted), stormwater impacts will be more severe. Rainwater harvesting takes a chunk out of your hardscape impact in addition to your draw on scarce resources–a double win. Some jurisdictions are beginning to charge stormwater impact fees based on the amount of hardscape on a site. Homeowners can usually gain off-set credits by installing rainwater harvesting systems.]


If climate change is on your mind, it’s worth noting the huge nexus between energy and water. 20% of per capita energy use in the state of California is dedicated to pumping, treating, heating, and transporting water. And in turn, there is a large water demand associated with cleaning and cooling energy generating facilities. When you reduce your draw on pumped and treated water sources, you shrink your carbon footprint as well. And with this energy savings, you contribute to a collective reduction in water needed for power delivery. Like many things in life, this is an interconnected cycle.

The conversation on climate change also brings up the awareness of a heightened need to be prepared for emergencies. Rainwater Harvesters experience tremendous peace of mind knowing they have many gallons of water stored onsite for the case of an emergency. It is important to note that stored rainwater is not potable without treatment, but it’s a wonderful emergency drinking water supply as it can easily be treated for human consumption with the same treatment methods that campers and backpackers use.

[NOTE: The BlueBarrel System includes customizable isolation valves so you can hold water in some barrels for emergency supply while using others to feed your irrigation line.]


Is it a concern that water may sit in tanks for months before being used? Not with a well designed rainwater catchment system. Depending on your system’s overflow handling, you may actually be pushing older water out as newer water enters.

But even if rainwater is held in storage for extended periods, there are a couple of best practices that help prevent problems in your tanks:

  • Use dark-colored, opaque tanks. Sun exposure will quickly lead to algal growth, but this is completely prevented when sunlight is excluded.
  • Keep your tanks in the shade. If tanks are kept out of direct sunlight the temperature will remain cool enough to prevent bacterial growth in the tanks.
  • Keep up with routine seasonal gutter clean-outs. The most important time to sweep out your gutter is in the fall before the first rains.
  • Rough-filter the water on the way into the tanks. This serves to keep out any major particulates that can cause in-tank water quality to decline.
  • If storing for emergency purposes (i.e. holding the water and not drawing it down), drain and recharge your system at least once per year.

An optimized rainwater harvesting system (in a dry climate!) will drain by the end of the dry season so you’ll get a fresh recharge at least every year. I say “at least” because this wonderful water source is also good for washing cars, defrosting windshields, rinsing boots, gardenwares and lawn furniture, and watering pets, so you may find you draw down your tanks a bit (and in turn fill them up again!) even in the thick of the rainy season.

[NOTE: The BlueBarrel System includes the code-specified pre-filtration and of course uses opaque food-grade drums. We can help you work out your system siting strategy here.]


At this point my suspicion is that here in California we’re just not dry enough! It’s the very dry southwestern states (AZ, NM, TX) with annual rainfall averages as low as 8” – 12” that are spearheading the re-birth of rainwater harvesting in the USA with favorable policies, incentives, and active licensing programs. Many California jurisdictions are beginning to follow suit.

When it’s really dry, rainwater harvesting is a no-brainer. Of course when it’s really wet, it’s a no-brainer, too. Let’s not fall victim to the Goldilocks Complex. Let’s harness this fresh abundant resource to improve our gardens, be prepared for emergencies, and restore the broken link in our urban hydrologic cycle before it’s too late!

Call your city or county to see about rebates and incentives that may be available to you for rainwater harvesting, and check out the resources at to customize your very own rainwater catchment system!

Runoff in Developed Landscapes

Our Broken Hydrologic Cycle

In our developed environments, groundwater doesn’t recharge the way it would in nature.

In a natural landscape, about 50% of precipitation hits the ground and infiltrates to hydrate soils and recharge groundwater. About 40% evaporates (or evapotranspirates through the metabolic activity of plants and animals); and only about 10% becomes runoff.

In a developed landscape however (including our towns and cities), with 75% impervious groundcover (think roads, parking lots, buildings, driveways, and even lawns), only 15% of water is allowed to infiltrate, while a whopping 55% becomes runoff. In an urban environment, this runoff sheets off the hardscape picking up automotive, industrial, and other pollutants along the way, rushing them to our sensitive waterways.

Rainwater Harvesting gives us the opportunity to hold this hardscape runoff onsite and allow it to infiltrate at a more natural rate if we use it to irrigate our gardens when it’s not raining.

Santa Rosa Rainwater Harvesting Analysis

Santa Rosa, CA Garden

The image above is the rainwater catchment system sizing analysis for a BlueBarrel project I completed in Santa Rosa, CA in October, 2013.

This homeowner wanted to irrigate a 250-square foot vegetable garden only with rainwater, so we sized her system to meet this irrigation need all summer long. We used a conservative safety factor to account for the especially dry conditions we’ve experienced in Santa Rosa over the last couple of years.

Our analysis showed us that even with dry years under consideration, 2,300 gallons of storage will serve this garden throughout the summer!

The lime green line shows the projected tank volume over the course of three years. We optimized the system size, so it nearly runs dry in a very dry year, but still holds some water by the time it starts refilling again in autumn. The purple line indicates supplemental water need. It’s hard to see this line in the diagram because it’s completely flat-lined at zero.

And best of all, we were able to catch all the water we need off her 480-square-foot workshop roof. Even with a small catchment surface, the system will fill with around 8” of rain. This means she gets additional bang for her buck with multiple recharges each year, depending on her water usage pattern.

We have reserved the option to tie in more rooftop surface for a quicker fill if we continue to experience extreme drought.

What can you do in your garden? Contact BlueBarrel for a professional Site Assessment.

To Boot, Rainwater is Just Too Good!

As if all this weren’t enough, serious gardeners know that rainwater is a good thing to have on hand, no matter how much rain (or how little!) you get.

Water Quality: Rainwater is the highest quality water source available to plants for three reasons:

1. Rainwater is free of the salts, minerals, and heavy metals that leach into groundwater and surface waters–a 100% soft water source.

2. If caught from a rooftop and stored in barrels, rainwater will accumulate a small amount of organic matter which can be very beneficial to your garden—a light application of fertilizer every time you water.

3. Rainwater is slightly acidic. Most organically grown plants prefer a soil pH of 5.5 – 6.5: on the acidic side of the neutral pH 7. Rainwater can help you maintain that perfect soil pH balance!

You will notice the difference, especially with young plants (they are particularly sensitive to salts and chemicals), and also with potted plants, whose containers tend to accumulate salts and minerals over time. In an open garden, you’ll also get much less soil salinization and mineral buildup with rainwater.

Gravity Feed: And for the resource-conscious gardener there’s even more! Stored rainwater is an ideal match for drip irrigation. In fact, drip irrigation will work by gravity feed, even on a flat site. Drip irrigation does not require pressure, it just requires water in the line. So as long as your water level is above the high-point on your drip line, you will get water to your garden with no added energy inputs!