Simple Water Harvesting in the Landscape
Consider using storm water runoff to supplement your regular irrigation method. Runoff is a valuable resource often lost to the street, which also contributes to downstream flooding and erosion problems that are often acute in urban areas. Although a regular irrigation method, such as drip irrigation, is needed for proper plant establishment and adequate early growth, using rain water can stretch irrigation periods, lower water bills, leach growth-deterring salts from the soil, as well as furnish plants with natural nutrients and better quality water. Many desert plants (especially those native to the area), once established, can be weaned from irrigation entirely and still perform well if they capture adequate runoff.
Water harvesting includes “active” systems for runoff collection, storage, and forced distribution, but anyone can easily practice “passive” water harvesting with only an investment of a little effort. Passive water harvesting is the practice of directing and capturing or slowing runoff to water plants usually by simple soil grading. Done properly, one can capture and deliver a substantial amount of runoff to plants, even during fairly light rains. Basic techniques include swales (shallow channels), berms, (raised soil on the downslope side of the tree basin), and depressions or basins, often using rock protection where gradients are steep or a thick bed of organic mulch (straw, aged manure, fallen leaves, and/or bark chips) as a moisture-conserving and soil-building mulch. Some combination of these can be applied in almost all landscapes, but best if designed at the planning stage. Using a builder’s tripod level, a water tube level, or string level will ensure accuracy, but even a basic familiarity with runoff patterns is often enough to do the job.
Figure 1 shows a row planting of trees along a conventional city street. In most cases adjacent property slopes toward the street, which is ideal for street tree plantings. In this example, the trees are planted in an 8 – 12 inch depression shaped like a funnel with a flat bottom to capture a wide area of sheet flow from the sidewalk and yard. The deeper the depression the wiser it is to make the slopes of the basin more gradual, and to use a thick, moisture-conserving mulch to lessen the likelihood of someone stumbling into the basin. The wider the basin the better – ideally the diameter of the basin is 1.5 to 3 times wider than the diameter of the mature canopy of the tree planted within it. This is because the roots uptake the majority of the harvested moisture beyond the drip line of their canopy. If you don’t have enough room to make such a wide basin, do what you can – a small basin or a number of small basins spread out around the plant is better than no basin. A building roof (as shown) is a great potential source of additional runoff you can harvest within the basin.
Figure 2 shows an informal street tree planting, appropriate in many settings, on a street without curbs. Since most streets are crowned to drain from the center, their impermeable surface is a great source of runoff. This example shows trees planted in a basin below street grade, connected by a shallow swale. Interconnecting the basins will help, in many cases, to equalize the flow to streetside trees where sheet flows from the street are not uniform.
There are other ways to direct and capture runoff for various planting arrangements through creative grading, but attention should be paid also to the landform character in the process. Grading for water harvesting should be considered an integral part of the landscape. It will be most satisfying if the grades contribute to the landscape form, as opposed to a system of ditches, furrows and rigid circular tree wells.
For more information on water-harvesting strategies see the books
Rainwater Harvesting for Drylands and Beyond Volume 1 and 2 by Brad Lancaster and www.HarvestingRainwater.com.
Images of Water Harvesting Along Streets
Images of Water Harvesting Parking Lots
Images of Water Harvesting Traffic Calming