I have been dreaming of a sustainable water collection system since long before we moved onto our little urban farmlet.  I dreamed and schemed, but could not get far – the project was too big for my brain.  An all-or-nothing girl, my much more reasonable boyfriend has at last convinced me that incremental & adaptive will ultimately get this project finished.

So I am starting with a few cisterns.  But what what size? How many? Where?  Our house is tiny (800 sq.ft), with lots of little roofs.  If your house is large you can harvest rainwater from just one downspout into one large cistern, plumb it to your garden and you are finished.  Our many tiny roofs required more planning.

Collecting runoff for later use is a volumetric balance between water quantity and timing.  Although ‘it rains a lot in Seattle’, our rain is seasonal (dry summer).  In climates with regular rainfall throughout the year, you don’t need to store much water because your cistern will fill again soon.  The longer your dry spells, the larger the cistern you will need to tide you over until it rains again.

Here is what I did:

I calculated the aerial surface area of each roof section, and then entered my native habitat: Spreadsheetlandia.  Using formulas found online, I calculated the volume of rain that I could expect to collect from each roof section each month.

inches rain x 0.623 = gallons/SQFT x roof SQFT x 0.9 efficiency = volume that month

OK, that’s how much water I can collect.  How much water do I need?

Our bimonthly water bill tells me we use approximately 20,200 gallons of water per year.  That is a lot of water.  Our wee house does not generate that much runoff water in an entire year.  But we are starting small – just water for the garden for now.  The difference in water use between summer and winter tells me how much water I use for the garden; about 5200 gallons, mostly between June and October.  I calculated that I don’t have enough roof area to get away with just one cistern – I will need several.

Rain collection is from the NE, SE, and SW corners of the house, assuming 90% efficiency.

Our tiny house is squished into the NW corner of our 6000 SQFT property.  Ideally we’d put cisterns on the N side of the house where we can’t grow food and the cisterns would not be exposed to sunlight.  But ideal spaces were next to none.  Most spaces either would not fit a cistern, would take over valuable garden or patio space, or would be very unsightly.  Finally, my yardens are mostly uphill from the house.  I was hoping to gravity feed my crops, but clearly something was going to have to give.

What I finally decided on was three 500-gallon cisterns; two collector tanks placed at downspouts by the house, and one as a ‘head’ storage tank located at the top of the property by the main garden.  The plan is to fill the head tank from the collector tanks, transferring water as-needed by portable solar/electric sump pump or pedal power (suggestions welcome).

The cisterns will be full all winter, then dip down past zero by August.  It looks like I will ultimately need more head tanks or to route more gutters to the collection tanks to tide us through the summer, but it is a start. By building more ollas for irrigation I hope for some water savings that will make this stored water go even further!

Clearly 1500 gallons won't meet all of our watering needs, but it is a start.

Next Post: Installing the cisterns!

UPDATE: 2012 Cistern Report