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 municipal 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 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 direct it to either an underground cistern or to a wet well for pumping into above-ground storage.

This report details such a system for 0.15 ha (0.37 acre) of orchard and garden area on a 0.4 ha (1 acre) residential lot in San Antonio, Texas.

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. 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.

Locating the main channels near the lot edges will help capture runoff and shallow subsurface flow from adjacent land, making and effective catchment area of 0.4 ha.

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 negligible without it. At the bottom of the channel and covered with a layer of coarse gravel are one or more drainage pipes. Above this, and forming the dual-media filter, is about 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 either side 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.

The planted areas around the channel is raised so they do not become waterlogged when the ground is saturated.

This lot slopes gradually, about 0.4%, too gradually for water to flow reliably, along its length from North to South. The orchard and garden area extends about 100m from the south edge of the lot and spans its 30 m width. To drain the entire length to the back, the drainpipes at the bottom of the main channels need to slope at least 2%, a drop of 2 meters front to back. Each main channel is in four sections, each discharging to a channel ½ m lower. To avoid excavating caliche, the bottoms of the main channels start near ground level at their northern ends.

Since this area has hard caliche underlying it at a relatively shallow depth, the bulk of the cistern is above ground and will require pumps to capture as much rainwater as quickly as possible while it is raining and for a time after it stops.

The wet well is also the destination for filter backwash before it is removed and composted. This report does not detail the construction of the wet well nor specify the pumps needed, though it must be noted the pump capacity is assumed to be sufficient to keep up with the runoff conveyed to the wet well long enough to fill the tank.

The size of the cistern is limited by practical considerations of size and expense, since storing enough irrigation water to get through the worst expected droughts¹⁾ is impractical. However, an analysis of three years of daily rainfall data from 1999-2002²⁾ shows how even a small tank can substantially reduce the amount of irrigation water needed from other sources. (The data for this graph is based on the simple model detailed below.)

³⁾

It seems surprising how much irrigation water even a relatively small rainfall collection tank can save. Although a 460 m³ cistern would have been required to store all the irrigation water needed for the time period analyzed, a tank one-tenth the size would have reduced the use of water from other sources by more than half. A detailed cost/benefit analysis is needed to choose the best tank dimensions.

Although water can simply be pumped from the cistern through ordinary garden hose, aqueducts are a more permanent method to irrigate the area quickly. Though it would not be needed every day, an average daily requirement of 1/10” (¼ cm) of irrigation water is assumed for the garden and orchard areas.⁴⁾ This report does not detail the construction of these aqueducts.

This rainfall collection system is based on a simple model. Captured runoff is collected in small cross-channels and conveyed to two main serpentine north-south filtering channels.⁵⁾

1. Captured rainfall volume Q_runoff (m³/day) is calculated using the rational method of runoff estimation: Q_runoff = CIA_catch = Q_in.
   • The runoff coefficient C = 0.5.⁶⁾
   • The daily rainfall intensity I is from a TCEQ monitoring station nearby.
   • A_catch = 0.4 ha.
2. The average daily irrigation need is Q_irr = A_irr x I_irr cm/day = Q_out; I_irr=0.25 cm/day.
   • A_irr = 0.15 ha.
3. The amount of irrigation water needed from other sources is Q_ext.
4. With a time step of one day, V_n (m³) = Q_n (m³/day) x 1 day.
5. The volume of the storage tank V_tank (m³) = S.
   • The tank is drawn down before water is used from other sources.

Appendix B: Tank Size Analysis details these calculations for about six months of daily operation using a 100 m³ storage tank and the other assumptions listed above.

The filtering channels act as detention basins and remove contaminants. When the runoff flow is slow it filters through the dual-media (scoria and sand) filter. When the flow is higher, water fills the channels and overflows from one to another, north to south, allowing larger particles to settle out of the water before the water flows into the next channel. At the southern boundary, the channels empty into a final cross-channel to convey water to a wet well so it can be pumped into a storage tank.

When the filter media needs cleaning, water from the tank can be pumped backward through the system to backflush the filters. This will suspend contaminants long enough for them to be washed downstream to the wet well where they can be collected for composting.

Rainwater collection and storage can significantly offset the use of irrigation water from other sources needed for intensively cultivated gardens and orchards. Even a fairly small storage tank can reduce dependancy on less sustainable water sources than the precipitation already falling on the area. In addition, by reducing runoff, this practice is particularly suitable to urban and suburban areas (where community gardens are often located) and will help decrease the flooding common in cities.

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