Rainwater collection as primary water source in dry area

Hello everyone,

I've been lurking for a while and learning a lot. Now I have some questions.

I'm interested in buying some property in a remote high-desert area. It doesn't get a lot of rain, about 11.5 inches a year, plus about a foot of snow. Based on my current utility bills, I use between 500 - 600 gallons of water a month. I found information on the UofAz website that says the formula for how much rainwater you collect is (roof area) * (inches of rainfall) * 0.623 = water in gallons. Based on that information I believe I need at least an 1100 ft² collection area.

So, my idea was basically to build a water house kind of thing that houses the storage cisterns a few feet off the ground. Then there would be a smaller cistern that the storage cisterns filter into as the water is used. I made a very, very rough model in Google SketchUp (attached). The current model is a square building, but in reality it would probably be round and made from cob. It has three 2500 gallon cisterns. I chose those because they seemed to offer the most value. I can get plastic 2500 gallon cisterns for around $950 a piece. The smaller cistern that it filters into is 50 gallons. The filter would be made from sand and carbon. The walls are pretty high, almost 16', which is what I am most concerned about. In the current model the walls are 2 ft thick. It also has an 8 pitch square hip roof with a total area of around 1550 ft².

The majority of the rain falls between July and October, inclusive. The other eight months of the year rainfall is less than an inch a month and sometimes less than half an inch. Based on the UofAz site's formula, I believe during the driest month it should collect around 400 gallons. (The total for the year would be around 11,000 gallons.)

At this point, I know a sum total of zero about building anything. I was wondering if this seems feasible, or am I totally thinking wrong on this? Being able to get sufficient water from rain is a major factor in whether or not I will buy this property.

I would truck water in to fill it up the 1st time that way you always have an extra 4 months of emergency water or maybe even a well.
How did you bring you water usage down to such a low number, 20gallons per day.
Do you use a composting toilet? You can also use the grey water a second time in a courtyard "greenhouse".
It would lower the temp and increase the humidity in the summer while producing fruits for you.

Do you have a plan in place for when you water usage goes up, parties/holiday, growing family/kids, getting older using more water esp.

I really haven't done anything special to reduce my water usage. I have a desert landscape that doesn't require watering and a front-loading low-water washing machine. That's basically it. I do live alone, which probably makes a huge difference.

I want to have an aquaponic greenhouse, so that is additional collection area. The main house would also provide more collection area. In fact, I could build the main house directly next to the rainwater house, or even as an extension of it, and basically double the collection area.

I think a well would be too expensive, at least in the beginning. Well digging in this area is very expensive.

I would closely look at building laws for the county in question.
They can vary widely from county to county.

For example, I am looking at a property, and the county says that I can build an 'out building' without needing a permit if it is at least 20' from any permitted building. If it is closer, or adjoining, not only would it need a permit, but the county assessor could appraise it at a higher rate, since it now technically becomes part of the living space.

A little research now could save some grief (and $$$) over the long haul.

It's outside the city limits in AZ, so no need to worry about code.

Code is one thing. The County Assessor is something else.
He's the guy who figures out how much you need to pay them each year.

Most counties now are having money problems.
He needs to figure out how they are going to raise more. Ouch!

For what do you build a house over the tanks? And why do you have that many tanks? The price per liter (or per gallon for you) is far higher if you have two tanks rather than one.
We are not living in the dessert and our tank is like this: One tank on rubble evened out and raked a bit in. Because we're fancy there's even some timber around the rubble but it keeps the rubble in place. There are some leaf eaters on the gutters. And the water is pumped into the house.
We're getting our gutters rehung because the slope is not right and much of the water is being lost. I believe that it is important having enough downpipes, because the water wll be lost if the gutter is too long.
Your rainfall is dam low.