Xeriscape Garden Design

and Rainwater Harvesting


We are so used to turning on the faucet and getting good, clean water to run out that we take it for granted. The infrastructure was built over the last century and has supplied us cheap, plentiful water for decades. However, access to cheap, plentiful water is becoming a thing of the past, not only in the Austin area, but around the country and the world.

City Of Austin water utility plans to make more than $1 billion in capital improvements, including the proposed Water Treatment Plant #4, through 2015. Debt service for those projects is set to grow from about $165 million in 2010 to about $219.5 million in 2015, a 32 percent increase. Operations and maintenance costs will grow from $177.3 million to $217.9 million, a 22 percent increase. Water rates will rise. Austin Water officials said the proposed rate increases are necessary to make the debt payments and are appropriate, citing the new water plant’s long-term economic benefits.

Just how much water rates will go up is always a matter of contention, but even the official numbers state that from 2011 to 2015 the increase will be approximately 6% per annum. More accurately the 2011 number was predicted at 5.70%. In reality the number was like 10.10%. Following that trend we can only assume future numbers will be skewed upward as well.

This is a great time to think about harvesting rainwater.

The process does have an initial investment attached to it. You will need a water barrel (or barrels) and if you don’t already have guttering on your house, you will need to that also. But, once the system is set up, the water is FREE! Some people want to harvest rainwater for household use and this is possible, but it includes treatments such as UV or Ozone to make the water potable and we’re not going to talk about that in this article. We are concentrating on water for the landscape.

Most barrels hold between 50-100 gallons of water and should be mounted a few feet above the ground. That will give you a static water pressure (SWP) capable of powering a drip line or soaker hose into nearby flowerbeds or vegetable gardens. Remember that every foot you raise your storage tank increases the SWP by about .433 psi. It may be less due to pressure loss through friction (1 psi ~ 3.21 feet of fresh water head).So, if you need the harvested rainwater to irrigate a lawn you will need to add a pump to your system.

The formula for figuring out how much rain water you can collect off your roof is roof square footage x .623 gallons per square inch of rainfall x annual rainfall. Square footage of the roof of the house x amount of rain and the last variable is .623. A cubic foot of water being equal to 7.48 gallons, which when divided by 12 (i.e. inches in a foot) equals .623 gallons per inch of rain. So, for instance, if you have a 2000 sq ft roof area that has been guttered, you can expect to harvest over 1,200 gallons of water from a single inch of rain! Also, consider this: even in the severe drought of 2011 the Austin area still received 16.90 inches of rain, converting to a yearly rainwater harvesting total of over 21,000 gallons for the same sized roof!

Remember that different roof types yield different amounts of water. A 95% efficiency can be expected from most roofs (metal, tile, concrete or asphalt) but if you have a gravel roof the rate will drop to around 70%.