Help me save lots of winter rainwater and use it in the summer!

• Water engineering question: help me save lots of winter rainwater and use it in the summer! I live in Seattle, where the winters are very wet and the summers are very dry. I have a large yard and grow a lot of vegetables, many of which (like tomatoes) need lots of water to produce fruit. So my water bills in July / August are upwards of $150 per month. The engineer in me thinks, 'what a waste of resources; I should collect the water in the winter and reuse it in the summer!' The cheapskate in me heartily agrees. So, I've got my downspout emptying into two rain barrels and one large rubber tub (obtained for free), connected in serial. This gives me 250 gallons of water capacity. A pump sucks the water out of the tub and into an underground watering system where it waters my whole yard (largely though soaker hoses). So far, so good. However, 250 gallons isn't nearly enough. This summer, I'll probably go through my whole supply in two to three weeks. I'd like to store a minimum of 1,000 gallons. So I've been brainstorming ideas for keeping more water around. Here are some plausible ideas: 1. Buy a bigger tub. Lots of places sell industrial water tanks. But they're expensive, even on eBay / craiglist (used 325-gallon tubs run around $60 -> $120). New, they're about 50 cents / gallon. 2. Build a cistern. Basically, this would be a small underground swimming pool, with a concrete floor and walls, and some kind of lid. Rainwater would flow into the cistern and my pump would pull it out. Even a modest size (10' x 10' x 3') cistern would store 2,200 gallons. However, my construction skills are limited: I can dig and pour a concrete floor, but I'm not sure how to pour walls, or build a sealing lid. 3. Dig a series of holes and line them with pool liner. Use the dirt walls as structural support and the liner to make it impermeable. Basically the same as #2, but simpler to build. Still not sure how to fashion a sealing lid, though. 4. Dig a well, and use a well pump. This is less implausible than it sounds because I happen to live at a geographic low point (near MLK and Union, if you know Seattle). When I dug post holes this spring, we hit the water table less than three feet down. But I don't know how deep it will be in the summer, and I don't know how environmentally dangerous that is. 5. Forget about it, because the county is already solving this problem for me, far better than I ever could, by tapping watersheds and rivers. Any suggestions? I don't mind doing lots of semi-skilled labor, but I'd like to keep the cost down. I don't care if the water is potable or not; it just has to not kill plants.

• Answer:

  If you hit the local water table at a depth of 3 feet, and can legally drill an irrigation well (you usually need to pull permits with local authorities and register it for monitoring purposes, particularly if it is over a certian flow rate or cassion size), that's your best bet. A water table that high pretty much precludes effective use of a normal underground cistern, since when you empty a cistern sitting in a site with a water table like that, it is going to badly want to float up out of the ground. If you didn't have that issue, you'd probably talk to septic tank manufacturers, and simply have one of their cistern type tanks set, which is generally just a variant of a septic tank, set up with blocked drainage outlets, and equipped with a pump access hole in its lid. Most places that sell pre-cast septic tanks sell cisterns, too. The dig the hole, deliver the tank on a truck with a crane, and plop it in, and cover the hole. Takes maybe 3 or 4 hours, at a cost of a couple thousand dollars, if no major excavation problems are encountered.