It never rains in southern California, but when it does…...

       We are in a drought, my town of Ramona has had only 5" of rain this yr.  instead of  about 15" that as our norm.  Private water Wells are dropping down so low they are sucking air!  Then there are the people on "City water" they are being told to reduce water waste. some are told not to water the yard.  The rest are told not to water the sidewalk or un landscaped areas.  Even that a $500 fine could be imposed if they waste.
   I tell folks get a rainwater catchment system, it's great for plants.  People reply it doesn't rain enough.   I say,  It's a sin to use tap water in landscape pots because the salts/minerals in tap water are extreemly high and the build up from time where the water evaporates  and the minerals stay makes for poor plant performance in many plant varieties.      Not only is rain water free, it is water that otherwise is a burden to the storm drains and municipal flood control system.              
    So, let's look at what your assets are.  The roof is larger than your house footprint due to the eaves.  One might say oh come on you can't get that much water from a roof….How about 623 gallons per thousand sq. ft. per inch of rain!   so, if you have 2,000 sq. ft. and live where you get severe drought rainfall of only (5"),  you can collect over 6,000 gallons.
    So, you ask what can 6,000 do? Well, flush toilets for two people over a year!  Then it rains again.  Now why would we want to use fully treated tap water to just flush down the drain?   Why would we use tap water to water a landscape?   I will tell you what people use tap water to flush toilets and spray all over outdoors,  because they can!,  water is cheaper than dirt! But wait, will be be allowed to continue to waste water like this?  No,  we can not continue this waste, it is not sustainable!.  That is to say,  it is not "green" or even politically correct.  Some areas wasting water in criminal.   As I said earlier, it is a sin…. in any case.
   So, why are we short of water?   we have oceans and desalination has been done for decades…  The answer is we use so much water that when every home in San Diego uses 130 gallons a day and there are 2 million buildings the amount needed exceeds what we have!  Population is growing, as are applications of life that use water.  Did you that every product we consume requires water to make?  so we as people consume the hundreds of gallons daily just in our diet.  This is called the water footprint.   Google that term and read up,  I will let you go so you can.. thanks..