We learned in elementary school how the water cycle goes. The amount of water we have on and above earth is all there is. The clouds gather moisture, rain falls to the ground and runs off into creeks, low water crossings, and eventually rivers swell. Oh yes, some of it gets into the ground to migrate into the aquifer. And the cycle starts again, but somewhere in about the middle humans, flora and fauna use what is available to them. Nothing on earth can live without water. So the questions are: How much water can we expect to collect in a year? How much do we use? How to cut down on water consumption?
There are a lot of variables in how much water you can collect and store. What is the average annual rainfall for Blanco County? What is the size of your roof? What is the affordable tank size you can install? And the list goes on.
Let’s make a few assumptions. 1) We are going to store all of it, no loss from type of roof or first flush operation. 2) Our roof size is a modest 2,800 sq ft. Roof square footage (sq. ft.) is calculated by the flat size of a roof (birds’ eye view), the slope or pitch of the roof does not matter in this case. 3) We expect 1" of rainfall at a time. 4) We are after the total number of gallons we can collect per 1" of rainfall. 5) Total number of gallons available annually. Now for the hard part; oh no, Math!
I will make this a bit easier: One square foot of surface with 1" of rain equals 0.623 gallons of water. Our 2,800 sq. ft. roof would yield 1,744.4 gallons of water. Let’s say 1745 gallons for the sake of easier math. Blanco County annual rainfall is 25.5 inches, according to a study conducted at Bamberger Ranch in cooperation with Texas A & M Rangeland Management, completed in this decade. From information available on the Internet, some reports indicate as much as 32" of rainfall. We will use the average of 26 inches per year from the study done in our county. The total amount we can capture is 45,370 gallons in a year. Fortunately it does not all come at the same time. Our storage capacity could be considerably less to accommodate our usage and frequency of rainfall.
Household use: The average person uses approximately 75 gallons per day. A household of 4 would use 300 a day or 109,200 per year. As you can see, our rainfall total is less than half. Because we are a wasteful society, it is prudent we all begin water conservation. With not much effort, that daily amount can be greatly reduced. The most important thing you can do whether you harvest rainwater or not is: Do not let water run needlessly. The second thing you can do is if you have to let it run, catch it in a bucket or tub for other uses such as watering potted plants, pets water bowls, hand washing, birdbaths, etc. It truly does not take much effort to reduce water usage, only persistence. I could go on and on about household conservation, but I will not.
Outdoor use: An irrigation system on your property could use the harvested rainwater. The average, all things being equal and ideal, amount of water needed is 1" per week for gardens and lawns, etc. That would take 52" of rain per year and, as you well know, that does not happen from rainfall or from irrigation. Yet plants still live and thrive as individual needs are different than the whole. Nature is a wondrous thing. Blanco County gets about half of that amount on a good rainfall year. Water restrictions need to be observed no matter the source of supply, especially in drought conditions.
Recap: Water is precious and nothing lives without it. Rain clouds give us free water. There is no guarantee when the next rain will be. We each choose how we wish to use it. We are all in this together. Be water-wise and caring. A Rainwater Harvesting Workshop will be held at Meadows of Flat Creek Lavender Farm and Ranch in the afternoon of October 3 at 852 Flat Creek Road, Johnson City, during the 2nd Annual Texas Hill Country Lavender Festival. For more information about the workshop and Rainwater Harvesting, call the Blanco County AgriLife extension office at 830-868-7167. We will be glad to help.