Subsurface Drip Irrigation

By Larry Keesen

In 1989 we followed testing of several different SDI systems installed in test plots at California State University’s Center for Irrigation Technology. We were intrigued by how well two of the systems worked and the lack of root intrusion after four years had gone by. At that point, we started by testing a three foot wide by forty foot strip of turf between a walk and a shrub bed. That was over twenty years ago and it’s still working well.

In 2004 we designed a SDI system for an 11.5 acre site of existing turfgrass with the dripper lines installed 4-inches below the surface in clay soil. The drip flow rate was 0.6 gallons per hour (gph) with emitters installed every 12-inches inline and 18-inches between rows. (Because lower flow emitters provide better lateral movement of water in the soil, we now use 0.26 gph or 0.4 gph emitters routinely.) Before the system was installed the site was using 38-inces per sq. ft. or 23.5 gallons per sq. ft. annually. The 6 year average water use after installation was 18.7-inches per sq. ft. 11.7 gallons per sq. ft. annually which is a 50 percent water savings.

We designed a 1.7 acre streetscape with medians using SDI on this newly constructed highway. The contactor brought in a special soil (loamy-clay-sand mix) with organic matter, fertilizer and most importantly soil with a pH of 6 which is slightly acidic. This pH is considered optimum for lateral water movement in the soil. The soil mix was dumped onto landscape areas and thoroughly rototilled into the subsoil to a depth of 6 inches. This creates a gradual soil texture transition.

The drip flow rate was 0.26 gph with emitters installed every 12-inches inline and 18-inches between rows. The dripper lines were pinned to the topsoil surface which was 4-inches below finish grade and then 4-inches of topsoil mix was applied. It is important that the applied topsoil is sufficiently compacted to match the density of the soil below the dripper line to aid in even water absorption. In July turfgrass seed was applied with hydro mulch and germinated without any supplemental overhead watering. A fertilizer injection system was installed to reduce maintenance costs.

Each dripper had a check valve that held water back for up to 4½ feet of elevation change. This streetscape had serious elevation changes, so we had a surveyor mark every 4 foot of elevation change and a header and footer were installed 6-inches apart (H pattern) with one line connecting each with a check valve in the connecting line.

Sports fields with turf are using SDI at a rapidly increasing rate. Watering is done even when the turf is being played on. Injuries from irrigations heads are nil making play safer and reducing liability. SDI also softens the soil and prevents compaction and which reduces injuries. Use slice aeration when reseeding and top-dressing.

CAUTION: We don’t recommend the use of SDI in areas with large existing trees which can suck up water leaving little water for the existing turf (especially on the south side). For example, we did a site with SDI and new sod with a new 4” tree planted in the area. When everything was done, the area of sod on the south side of the tree was drying up and additional SDI lines were added to keep the sod alive. Even though we keep the SDI at a 4-inch depth, we know tree roots have pinched poly pipes in the past and may do the same to SDI.

WATERING SUMMER SCHEDULE (June 10 September 15).

1. Using .26 gph drippers, spacing 12-inches by 15-inches between rows and watering 10 to 11 cycles (8 to 10 minute cycles) would require a total runtime of 90 minutes to apply a ½ - inch of water. Using a .46 gph dripper with the same spacing and 7 to 10 cycles (6 to 8 minute cycles) the total runtime would be 58 minutes to apply ½ - inch of water. Watering for longer periods of time would result in water draining past the root zone.
2. SDI watering is best operated every other day or 3 days a week depending on the climate. Sandy soils may need watering every day.
3. Subsurface drip irrigation

COMPARATIVE ANALYSIS