In Tucson, as in many communities in Baja Arizona, in an average year, more rain falls on the surface area of the community than all its citizens consume of utility water in that same year.
This rainwater is the best water for our plants and soil. It is salt-free, unlike much of our groundwater and imported surface waters, such as those from the Colorado River, that carry salts which can build up in irrigated soil and impede plants’ ability to photosynthesize and utilize water. Rainwater is a natural fertilizer, containing sulfur, beneficial microorganisms, mineral nutrients, and nitrogen. And it’s free.
Nonetheless, because of mound-like landscapes, soil scraped and raked bare, and excessive paving, we drain the vast majority of that high-quality rainwater out of our communities almost as quickly as it arrives.
The unnaturally exposed soil and pavement of our streets and backyards, along with the sun-baked exterior walls of our homes, schools, and other buildings, drains still more water by absorbing the heat of the sun during the day and reradiating that heat back out at night, increasing temperatures up to 10˚. This leads to another drylands statistic, one that may shock you—but can also empower you:
Our average annual rainwater income/gain is about 11 inches of rain a year.
But our potential water loss to evaporation is about 100 inches per year.
1. Improve your water gain by planting the rain before you plant any vegetation (or plant the rain beside vegetation if the plants are already in the ground). Capture rain in bowl-like—as opposed to mound-like—shapes in your landscape to capture and infiltrate, rather than drain, the rain.
2. Emphasize the placement of these basin-shaped rain gardens next to and below impervious surfaces like roofs, roads, and patios from which water runs off. That way you can double or even triple the available rainfall in the basins by capturing both rainfall and runoff, which becomes runon, and that’s right on!
3. Decrease potential water loss by planting shading vegetation, like low-water-use, native, food-producing trees that will then grow to shade and cool roads, patios, and the east-, west-, and even north-facing walls of adjoining buildings. This will reduce unwanted sun exposure on our buildings’ walls and windows in the morning and afternoon of the hot months. (Leave the south-facing wall, beneath an appropriately-sized roof overhang or awning, open to the winter sun, which hangs low in the southern sky, so you can get free heat, light, and solar power when you need it most.) The runoff from the buildings and paved surfaces then freely runs into the rain gardens to irrigate the trees, while the trees passively shade and cool the pavement—reducing water loss both to runoff and evaporation. The City of Tucson just passed an ordinance that all new city streets must be designed and built to harvest at least a half-inch rainstorm’s worth of water to freely irrigate street-side vegetation shading the street and walkways.
4. Mulch the surface of the soil to make it more porous to speed up the rate at which water infiltrates, which also reduces the loss of soil moisture to evaporation. Compost and woody organic matter are the best mulch as they increase the fertility of the soil and plant growth. Furthermore, this mulch feeds beneficial soil microorganisms, such as mycorrhizal fungi, which tap into and expand the surface area of associated plants’ roots. The plants can then more efficiently uptake harvested water, as the fungi give the plants water and minerals, while the plants give the fungi carbohydrates and sugars. At the very least, don’t rake up and throw away your fallen leaves. (They’re called “leaves” because you are supposed to leave them as mulch beneath your plantings.) Fallen seedpods of mesquite, ironwood, palo verde and other trees also make great mulch.
5. If you want any higher-water-use plantings such as fruit trees, be sure to plant your graywater before you plant your fruit tree(s). (Or if your fruit tree is already planted, then plant the greywater next to the fruit tree.) Graywater is the drainwater from household bathroom sinks, showers, bathtubs, and washing machines. The volume of graywater running down the drain of the average Arizona family household is enough to meet about half of the average family’s landscape irrigation demand. If you use non-toxic, salt-free soaps and detergents, your greywater can be directed to the same mulched basins that capture your rainwater. No tanks, pumps, or filters are needed. In times of rain, the basins act as rain gardens. In times of no rain, they act as graywater gardens. As long as you are home, that graywater flow to your plants can be perennial—even in the driest of times.
See the “Graywater Harvesting” page for information on what ingredients and products are good or bad to use.
6. If you have an air conditioner, direct its salt-free condensate water to the rain gardens instead of the sewer. You’ll only get about a ¼ gallon per day of condensate from a home air conditioner in the dry season, but it can be as much as 18 gallons a day in the humid season. Condensate from commercial air conditioners can be as much as hundreds of gallons of water a day.
Taking these steps which harvest, rather than drain, free, local waters transform dehydrating landscapes into rehydrating landscapes that provide myriad additional benefits such as more local food, enhanced flood control, diverse wildlife habitat, beauty, and more life which can also lead to more rain.
This is because clouds are more likely to form from cooled atmospheric moisture evapotranspired through plant leaves than the warmer moisture evaporated from bare soil. In addition, raindrops are more likely to condense around tiny, richly-textured, air-borne particles of organic matter generated by the vegetation (such as pollen) than the less textured and less cool particles of dust from exposed dirt. We can choose to work with these natural systems or against them. I think you’ll find going with the flow is always the easiest and most abundant path. ✜
Brad Lancaster is the author of the award-winning books, Rainwater Harvesting for Drylands and Beyond, Volumes 1 and 2, and manages HarvestingRainwater.com, which show you how to make the most of the water in your landscape—even if you are renting your home and don’t have a yard.