— David Wagner 2007/04/14 11:31

The productivity and reliability of gardens and orchards in San Antonio, Texas are limited by rainfall characterized as sporadic at best, with occasional deluges punctuating an otherwise arid climate. Although the average annual rainfall is sufficient for most crops, it is usually not distributed evenly enough for reliable production. The municiple water available is hard enough to damage the soil by the accumulation of mineral salts if it is used regularly as irrigation water. For all but the most severe droughts, storing the stormwater runoff from a plot of land should provide enough irrigation water to grow on it most crops suitable for this climate.

An engineered garden path collects and cleans localized stormwater runoff for storage in a cistern and for later use as irrigation water. The path is actually the top of a dual-media (scoria and sand) filter designed to capture, detain, and clean initial stormwater runoff. The path also acts as a channel to capture the remaining stormwater runoff and to direct it to either an underground cistern or to a wet well for pumping into above-ground storage.

Smaller (1 m) path-channel-filters extend from near the E-W lot centerline, diagonally ESE and WSW, and feed the main (2 m) channels near the east and west lot boundaries. Locating the main channels near the lot edges will help capture runoff and shallow subsurface flow from adjacent land. These slightly serpentine filtering channels run South for 100 meters to discharge into a cross channel near the easement at the southern edge. The cross channel conveys water to a wet well and pump station used to fill the cistern.

A typical section of this engineered path is a channel 1-2m wide with a rounded bottom. The bottom may be a concrete half-pipe if the soil is permeable, or simply lined with plastic if water loss would be negligable without it. At the bottom of the channel and covered with a layer of coarse gravel are one or more drainage pipes. Above this, forming the dual-media filter, is approximately 0.25m of sand and 0.25m of coarser (and lighter) scoria.

To capture shallow subsurface flow, the channel walls are drystone masonry. In fact, this channel is not much more than a pair of retaining walls and a good use for urbanite. On both sides of the channel is a raised strip of permanent plantings of bushes and trees designed to be irrigated by the filter backwash water flowing over the channel walls. A stone sill marks the edge of this backwash overland flow area. The other side of this stone sill is suitable for regular food crops, raised so the plants do not become waterlogged when the ground is saturated.

This report will detail such a system for 0.3 ha (0.75 acre) of orchard and garden area on a residential lot in San Antonio, Texas.

This lot slopes gradually, about 0.004 m/m, too gradually to flow reliably along its length from North to South. The orchard and garden area extends about 100 m from the South edge of the lot and spans its 30 m width. To drain the entire length to the back, the drain pipes at the bottoms of the channels need to slope at least 0.02 m/m, a total drop of 2 m. To avoid excavating caliche, the bottoms of the main channels start near ground level or above.