Jan 18, 2010

It's Raining! Now What?

It's been hard to miss the message over the past year that we're in a drought. And Californians have responded remarkably well: residential water use was reduced by 9.4% in Santa Clara County, 12% in Los Angeles County, and up to 25% in San Diego County from previous periods. I still see far too many gratuitous lawns, thirsty "exotic" plants and wasteful irrigation systems. But on the whole, we're at least trying to handle our water.

So the next place to look is, naturally, increasing our supply. And on the cusp of a major El NiƱo week like the one currently forecast for the Bay Area, the opportunities seem abundant. So why isn't everyone harvesting rainwater? To understand why it's not quite that simple, let's do a little math:

    First, let's assume we're in for nine inches of rain — that's 3/4 foot — over the next week or so.

    Second, let's assume we can harvest that rain from the roof of our detached 2-car garage, which has a roof area (equals footprint) of about 20 feet by 20 feet.

    So if we catch every drop that falls on our roof, we would harvest 20' * 20' * 3/4' = 300 cubic feet.

    Using a handy-dandy volume converter, we find that 300 cubic feet equals 2,244.156 gallons.

Holy crap! Seriously?! More than two thousand gallons? That can't be right.

    Well, actually, it isn't right. Because we won't catch every drop. Some will splash away, some will get trapped in the gutters, some will leak out. So a capture rate of 60% is usually considered reasonable, and our potential volume actually is 2,244.156 * 60% = 1,346.494 gallons.

Holy crap! Seriously?! More than thirteen hundred gallons?

Yep. And unfortunately, that huge number is less than 5% of the annual water needs of a 1,000 square foot lawn. Never mind that if you're serious about water conservation, you don't have a 1,000 square foot lawn; are you getting a sense of the quantity of water we're talking about? Say you did have a 20'x50' patch of grass: 1,346 gallons of rainwater would irrigate it for all of about two weeks. For the year you'd need twenty times that, or around 27,000 gallons.

Which raises another issue, specific to Mediterranean climates like ours where most of the rain comes in one season, as opposed to throughout the year: storage.

We get most of our rain during the winter, when plants are dormant and evapotranspiration rates are low. We don't need the captured rainwater now. We need it six to nine months from now, when skies are sunny and the ground is parched. Even if you have no lawn and your xeric garden only needs 1,300 gallons, where do you store what you've saved?

Rain BarrelRainwater PillowRain BoxRainwater HogThere are in-ground cisterns, which may be large enough but are pricey and complicated to install. There are classic, above-ground rain barrels, which are bulky and difficult to link together for additional capacity. There are interesting systems like the Rainwater Pillow which can efficiently store 1,000 gallons or more, but may not be ideal for exposed outdoor locations. And there are modern above-ground tanks such as the Rain Box and Rainwater Hog, which link together with slim rectilinear profiles that use space efficiently but can become pricey.

How pricey? The Rainwater Hog sells in the neighborhood of $500 per 50-gallon tank. To catch 1,346 gallons requires 27 tanks, or $13,500. The Rain Box is more economical, at about $250 per 75-gallon box. But that's still 18 boxes, or $4,500.

Even if cost isn't a consideration, space may be. The Rainwater Hog has such a slim profile — just 20" wide by 10" deep — that it can be mounted not only vertically against walls, but also horizontally, e.g. beneath a deck. But no matter how you set them up, 27 tanks would take a lot of room: far more than the 20' wide wall of our two-car garage. The Rain Box is bigger, about 24" wide by 20" deep, and not designed to mount horizontally; so 18 boxes would need at least 36', or almost two full walls of the garage.

I don't mean to discourage anyone from catching and reusing every drop possible. Even if your "rain barrel" is a garbage can, that's 20 or 30 gallons you don't need to draw from a reservoir. But it won't be your only solution, and in fact might raise more questions, e.g. what do you do with the overflow? We all can install green roofs, detention basins and porous paving, which will help the rain get into the groundwater where it actually can do some good. But these solutions aren't the same as storage; and they're not cheap, either.

I guess my point is that it ain't easy to save the world. It's probably not economical, and you probably won't get your money back. Serious rainwater harvesting requires some serious commitment, and we're not all there just yet. But even if you're not ready to shell out thousands of dollars to store thousands of gallons, you have plenty of other options. Maybe you can swap out your lawn for a delightful garden of unthirsty plants. Maybe you can mulch those plants with 3" of compost instead of leaving the soil bare. Maybe you can redo your driveway with pervious pavers instead of asphalt. Maybe you can take shorter showers or make other changes that reduce your water footprint.

Maybe you can't do much; but you can do something. And what better time to start than now — while there's a break in the weather?


Daffodil Planter said...

Excellent post, John, laying out the facts so clearly!

John said...

Thanks, Daff! I just wish I had more answers than questions…

Deviant Deziner, aka Michelle said...

Hi John,
I have two rain barrels ( recycled merlot oak vineyard casks ).
To tell you the truth, it's more about feeling good about doing something rather than really making a dent in the real problem.
Both barrels are full and I'll use the water this summer but it is just a proverbial drop in the bucket.

I have room for more oak rain barrels but when you figure the cost of the barrel ( and I'm getting a good deal ) of 25.00 each vs. how much that same amount of water costs via my North Marin Water Company, I'm certainly not saving any money, ... just a bit of conscience savings and perhaps a few drops of good karma
On the average my water bill is about $ 18 to 20 dollars a month in the height of the dry season.

Thanks for a thought provoking article.
michelle derviss

John said...

Michelle, you're right on the money (ha): much like installing solar panels or driving hybrid cars, the eco-friendly solution often does our conscience more good than our climate. I shudder to think of how much petroleum etc. goes into those plastic rain tanks. But feeling good has value, too! And when you figure that your water bill (like the cost of gas) probably is kept artificially low by any number of factors, perhaps the financial calculus becomes irrelevant, and it's just about the number of gallons you don't pull from a reservoir.

Personally, even if they never held a drop I might keep oak casks just around for the scent. Clean conscience and a merlot state of mind? Priceless.

Chookie said...

The tiny tanks you mention are practically worthless, and poor value for money. It's very difficult to store a reasonable amount of water because of the amount of space it takes up. There are companies here selling narrow tanks to put down the side of your house or use as a 'wall', and updated forms of our traditional round corrugated iron tanks. I've also seen water bladders, which are meant to go under decks or floors. 10,000 litres would be a reasonable amount in my climate to tide me over dry spells... but that's 10 cubic metres (353 cubic feet) needed for water storage! That's a pretty big footprint on your block of land!

Greywater reuse, now...

ScottHokunson said...

Very interesting post, John. It certainly gives us a lot to consider. Here in the east we do not suffer the water shortages as you do in California, but we still try to get clients to catch and use rain water, use pervious materials, and filter run off. Thanks for giving us something to think about! Scott

Cecie Harrington said...

The Original Rainwater Pillow is ideal for outdoor locations. The material can tolerate temps down to -30 degrees. We use a three part durable material with a tear strength of 550 lbs per inch and has a proven life of 20 + years ( military grade) Our material is NSF 61 approved and FDA compliant which means no VOC or off gassing. Our pillows are custom built to your space requirements which allows for max water volume collection and fewer fittings ( as compared to all those fittings necessary to link together smaller modular tanks to achieve the same collection volume).The Original Rainwater Pillow kits start at 1,000 gallon and go up to 40,000 gallon. The 1,000 gallon kit is $2,500 and includes filters, pillow pump and all fittings for a complete self installation. As the pillow sizes increase, the cost per gallon drastically decreases. The pillows are light weight and economical to ship ( 1,000 gallon pillow rolls up and weight 49 lbs) thus reducing the overall carbon foot print. Thanks for the mention on your blog. For more info www.rainwaterpillow.com

John said...

Scott, I can only wish for midsummer thunderstorms here! That's the thing about a Mediterranean climate, even more than a desert: it's feast or famine (which in turn affects things like wildfire season). If we'd all been a lot smarter a lot sooner, we might not be in some of the trouble we're in.

Chookie, you're talking about roughly double the volume I use in my example, and yes, that is a huge footprint, no matter whether it's vertical or horizontal. This is my problem with the Rainwater Pillow (see below), the smallest of which has a 10' x 8' footprint.

Cecie, I'm so glad you weighed in with more detail! Now, if some of us like the product and love the price but don't have 80 square feet of land free, could the Pillow be mounted vertically, in some sort of enclosure? I could easily see mounting one against the wall of my garage, for 3000 gal. of storage in far less space than any tank would require.

KatePresents said...

Excellent post! I get asked about rainwater harvesting from time to time. I've done the math, too, (I had to do something in grad school) and in San Diego you'd need a cistern the size of a small school bus to water a typical residential lot here just for the summer months. I am glad that I have your article to send people to to get more information.

John said...

Kate, thank you! San Diego is my second home, and I appreciate you keeping it beautiful!

You remind me to mention that the critical variable in this equation — the amount of rain expected — can be found by plugging "annual rainfall" and the name of your city into a search engine such as Google. For Palo Alto it's close to 16 inches annually; for you in San Diego it's probably more like 10 inches. (And don't forget to divide those inches by 12 to convert them to feet for the cubic-feet calculation.)

Dirty Girl Gardening said...

I use the water pillow on my farm... it works ok. It was one of those impulse purchases I thought would change my life. But it more looks cool than anything else... pretty difficult to use and a complete pain to store. The guys that work for me like to push each other over on it and flop around, so I guess it had a couple uses. :)

John said...

DG, it's good to hear your real-world experience with the Pillow. If nothing else, we know it's durable, right?!

rebecca sweet said...

Excellent, excellent post...I currently have 2 clients who are interested in installing some sort of method to catch the rain and I'm so glad to find such detailed information to pass along to them...I'm just sad to hear there isn't a perfect solution out there...ah well...soon, I'm sure. Hoping to see more innovative methods at the SF Garden show...

cecie said...

Hey Dirty Girl Gardening.. You do not have The Original Rainwater Pillow system that we have perfected over years of experimentation, Give me a call and I will be happy to assist you with the correct setup or solve any problems you maybe having.
John, we build custom size pillows to fit the unobstructed space ( any dimensions) . The custom size pillows utilize all the available space and maximizes the collection volume in one unit.

EastBound said...

Has anyone tried this kind of system? http://www.rainxchange.com/

I seems to provide storage without taking up space, be aesthetically pleasing, and allow the water to be used when its really needed. I wonder if you could do a home-made version?

Gardenerd said...

We have 2 official rain barrels and we use our green waste bin for a third. Last year, here in Los Angeles, we had enough periodic rain to fill our barrels over and over through the fall and winter. We were able to shut off our irrigation system from November through April and hand-water our vegetable garden and front yard of drought tolerant and native plants. We have room for 2 more rain barrels and will certainly take advantage of that when we can afford more.

John said...

EastBound, the RainXchange™ System is essentially a cistern with a pump to recirculate the rainwater through a water feature such as a pond or waterfall, where it drains back into the basin. For someone more mechanically inclined than I, this should be possible to replicate, although the folks at AquaScape have made it so turnkey I don't know why you'd want to. I don't see a real advantage of the RainXchange system being "modular" (other than buying just the number of modules you need): unless you'll particularly enjoy excavating and expanding it, you would still want to size it to your maximum runoff volume from the start.

Gardenerd, you embody the reason for a modular above-ground system such as the Rainwater Hog or the RainBox. Even though these units can be more expensive per gallon, they're so easy to expand that your total system cost can be amortized over years when budget becomes available.

Karen said...

We use garbage cans and place them underneath areas that get a lot of runoff. Is there anything wrong with using this inexpensive system?

John said...

Saving water and saving money?! Karen, there is nothing at all wrong with that.