Rainwater Catchment

The Turks and Caicos islands are a group in the lower Bahamian range of islands.  Grand Turk is the island where the seat of government resides.  Other large islands are Salt Cay, Parrot Cay, North Caicos, South Caicos, Middle Caicos, and Providenciales.  Providenciales (called “Provo” by the locals) is the island that we have chosen for our operation.   It is rated in the top 5 resort destinations in the world by TripAdvisor.  Grace Bay beach is the most beautiful beach that I have ever seen in my life, offset by the turquoise ocean protected by the reef.  When we went there 2 years ago for the first time, Pam and I fell in love both with the island and with its inhabitants.  Great folks!

The problem that atolls like Provo have is no good groundwater.  What you pump out of the ground is brine water that is equal in salinity to ocean water.  What is common on atolls like Provo is the capture of rainwater and retention in cisterns for each building.  The island has gone one step further by establishing a Reverse Osmosis plant for water production and supply to all.  Unfortunately, the cost associated with such a commercial process makes the price per gallon prohibitive for a hydroponic operation.   
Now some math: The rule of thumb for rainwater catchment systems is that for every square foot of roof 0.6 gallons of water can be captured for every inch of rain.  Our greenhouse in Arkansas has 3000 square feet of roof, thus every 1” of rain supplies us with 1800 gallons.  Each plant will use 0.25 gallons per day in fertilized water.  750 plants per greenhouse equals 187.5 gallons per day consumed. 
The annual rainfall for Provo is a shade less than 50 inches with 20 inches coming in October and November.   January’s rain fall is about 2 inches, gaining 3600 gallons or 19 days, February comes in at 1.5 inches or 14 days, and March at 1 inch or 9 days of water.  So the peak tourist season of January through March runs a monthly deficit of anywhere from 11 days to 22 days of water requirement. 
 So the question yet unanswered is how much water storage is needed. 
In Arkansas, we have a 2000 gallon cistern that pumps into a 10,000 gallon above ground tank.  I believe that a 10,000 gallon tank that is full by the start of peak season can, with the addition of the extra rainfall, suffice. And we plan on using some city water to enhance the local flavor of the tomatoes; however, the daily amount of municipal water will not exceed 10% of the total; cost and salinity being limiting factors. 
The costs involved in this methodology are mainly upfront costs, ie. tanks, pipes, pumps. 
Saddling up to an existing building with a gutter system would enhance our chances of having adequate water.  The trick would be to find such a building which had a gutter system in place but no cistern OR find a building that has both a catchment system and a cistern, but never fully utilizes the amount of rainfall on its roof, like a church or a department store. 
However, in the long run reality will be the final arbitrator of this issue. 
The beauty of this venture comes from meeting very informative people along the way.  Having joined the American Rainwater Catchment Association and gone to their workshop, I met people in this field who were knowledgeable and helpful.  What with the droughts in Texas and California, they were very busy, but the systems that they designed fit the bill perfectly.  The people at Acer Tanks even sent Jeremy over to Proctor to show us how to erect the tank (he damn near erected it himself). 
Now, our rainwater is caught on two Stainless Steel gutters that traverse the greenhouse, then goes through a Vortex filter before entering into the underground tank only to be pumped up into our 10,000 Pioneer tank before going through 2 filters and a UV light on its way to the fertilizer injector and plants. 
The rainfall is free and Sustainable!  Cool!