Here in northern New Mexico, we average about 14 inches of rainfall a year. That's fairly standard in most desert gardens. The rainfall may vary a couple of inches, depending on elevation, proximity to the coast or mountains. Now, 14 inches a year isn't very much rain.
But let's look at how much water that is.
We can calculate runoff from rain, taking into account evaporation, at 1 gallon per inch of water. So a typical 2,000-square-foot house, receiving 14 inches of rain a year will generate a whopping 28,000 gallons of water. Now, since deserts receive a lot of sun, and humidity is almost non-existent, we need to calculate for evaporation. Experts agree on .08 as a factor in calculating evaporation.
Multiply 28,000 gallons X .08 = 2,248 gallons lost to evaporation, which leaves us with 25,752 gallons of water a year from rainfall...most of which goes unharvested. That comes to over 70 gallons a day in wasted water.
There are lots of ways to effectively harvest and retain water. One way is using swales and berms as we described in our last article.
Another method is the use of dry streambeds.
Here's how you create one.
At the point where the rainwater hits the ground (directly under a canale or gutter, for instance), you'll want to create some sort of catch basin. The object here is to slow the water down, and harvest some of it. A great way to do this is with a rain barrel (which you can find at nurseries), a steel drum, or any other container that you like. This must be raised off the ground to create gravity flow. It must also have an overflow mechanism to allow excess water to slough off into your streambed.
From the catch basin, dig a shallow trench to whichever plants you want to water. Meander it a little to make it look more natural. I would run some water through the trench to make sure that it flows properly to the plants.
Next, line the trench with a heavy plastic liner. It can be clear plastic or black plastic. The color doesn't matter.
Next, cover the plastic with small rounded river stones. To create a more natural look, you may want to also include different sized gravel, even some pea gravel, mixed in.
Plant native grasses and wildflowers along the edge of the streambed, and place some larger river stones here and there to enhance the naturalized look.
Two books you might want to read are Rainwater Catchment Systems for Domestic Supply by John Gould and Erik Nissen-Petersen, and Branched Drain Greywater Systems by Art Ludwig
