by Penny Pawl, U. C. Master Gardener

Swales are ditches or trenches that catch rainwater for the purpose of conserving it in the soil. And if you top the swale with compost, you will gradually improve the water-holding capacity of the soil underneath.

Most swales are created on hillsides to keep rainwater from washing downhill, but mine are on the Napa Valley floor. They are not straight but rather bend around the plants already in the area. They are about two feet wide and 12 to 15 inches deep. Compost was spread on top of the soil including inside the swale.

Last winter, rains filled the swales up three or four times. Each time, it took about three days for the water to be absorbed and stored under the swale. I have been planting a dry habitat garden in this area. Eventually, these drought-tolerant Mediterranean plants, which have been in the ground only one to two years, will be on their own, with no additional water.

Because my plants were young, I watered every few weeks this past summer. A slow-growing Hollywood juniper that had been in the ground for a long time, with no water, doubled in size this past  year. I can only presume it is because of the additional moisture provided by the swale.

As the roots of the plants go deeper and I continue to add compost on the surface, the water-holding capacity of this area should increase.

Last winter I placed a couple of garbage cans under downspouts, covering the cans between rains. I used this water on potted plants throughout the winter. Since then I have acquired two 300-gallon plastic containers. They rest on a raised platform under rain gutters, so after they fill, I can attach a hose and get some water pressure.

Because of the West’s growing water shortage, people are devising more ways to save rainwater. Some methods are elaborate, while others, like mine, are simple and inexpensive.

You can buy adapters to cut into gutters to direct the water and flat-sided tanks that butt up against the side of your house. These tanks are lidded and usually made of dark plastic so that light does not get inside. If kept covered, the water stays clean and free of mosquito larvae.

So how much rainwater can you glean from your roof? Most roofs are pitched, so a 1500-square-foot house may have a roof with double that surface area. In that example, an inch of rain will produce 1000 gallons.

To see how much rainwater you can save, check the chart at www.rainwater-green.com/RainfallCharts.pdf.  Driveways, parking lots and streets also displace rainwater. In our area, any water not retained likely ends up in the river and bay

Some states do not allow collection of rainwater. Others allow limited collection to protect the interest of those downstream. California does not restrict rainwater collection, so you may harvest what you can.

Of course you can also save on water use by selecting plants that tolerate our dry-summer climate. When choosing plants, check their water requirements. You can also make plans to start eliminating lawn as that patch of grass is the biggest water user you have.