Subirrigation with perforated drainage pipe
Fred Edwards and David Bainbridge
Use of subirrigation with a perforated pipe delivery system was tested in the upper Mojave Desert at Fort Irwin on a native plant windbreak that is 0.5 miles long. Topography on the site is flat with a gentle slope to the southeast. Soil on the site from a highly compacted sandy loam on the western end with large amounts of fine dust, the result of heavy vehicle traffic, to a coarse loose sand on the eastern end.
The windbreak was installed in three phases. These phases included site grading, perforated pipe installation, and planting. A motor grader was used to cut a trench for the perforated pipe and to erect a berm to discourage vehicle traffic from cutting through and damaging the plantings. This berm also catches and directs rainfall runoff from the street and the roadside to the plantings. The trench was cut to a depth between 30 and 40 centimeters and the berm raised to a height of 60 to 80 centimeters. A schematic of the grading and plant positions is showin in figure 3.2-1.
After the trench was cut and berm erected, 800 meter of 3 inch diameter flexible ABS slotted drainage pipe was installed in 33 m (100 ft) sections. The ends were ben upwards and staked in place with rebar to create watering standpipes. After the standpipes were secured, short segments of the perforated pipe were buried by hand to keep the pipe in position while it was buried with a motor grader.
Figure 3.2-1: Windbreak Design. The finished berm height was between 60 and 80 cm and the depth for the perforated pipe was between 20 and 30 cm. Distance from edge of the road was between 5 and 10 meters. Plantings were made on the leeward side of the berm to benefit from roadside runoff and next to and over the perforated pipe to minimize hand watering.
The windbreak was planted with six different native desert plant species: Honey Mesquite (Prosopis glandulosa), Screwbean Mesquite (Prosopis pubescens), Bladderpod (Isomeris arborea), Desert Willow (Chilopsis linearis), Palo Verde (Cercidium floridum) and Cat Claw Acacia (Acacia gregii), see Table 3.2-1. The seedlings were placed 1.6 meters apart in a single row on the leeward (north) side of the berm to facilitate initial hand watering. For the smaller plants, a two foot high Supertube treeshelter secured with rebard was used for protection. To increase visibility and discourage vehicle traffic from crossing over the berm, additional treeshelters were staked in placed alongside some of the larger plants.
Table 3.2-1. Windbreak Species Composition
|Scientific Name||Common Name||Number||Percent|
|Prosopis glandulosa||honey mesquite||137||27|
|Cercidium floridum||palo verde||128||25|
|Chilopsis linearis||desert willow||85||17|
|Prosopis pubescens||screwbean mesquite||78||15|
|Acacia gregii||cat claw acacia||13||3|
Initial watering was done by hand, but irrigation was then shifted to a water truck that would pump water into the vertical standpipes. This proved to be fast and efficient. The watering was done once a month during the summer, starting in May, 1998.
By June 1998, overall survival was 87 percent. Both Prosopis glandulosa and P. pubescens (honey and screwbean mesquite) had the highest survival at 98 and 97 percent respectively. At 23 percent, Acacia gregii had the lowest survival of all the species planted in the windbreak. The number of plants installed and survival eight months after planting are summarized in Table 3.2-.2.
Table 3.2-2. Survival eight months after planting
|Scientific Name||Number Planted||Alive 10/27/98||Percent|
By October 1998, one year after planting, overall survival dropped slightly to 84 percent. Prosopis glandulosa at 97 percent and P. pubescens at 96 percenta again had the highest survival while Acacia gregii had the lowest. Survival at the one year mark is summarized in Table 3.2-3.
Table 3.2-3: Survival one year after planting
|Scientific Name||Number Planted||Alive 10/27/98||Percent|
Subirrigation with buried slotted drainage pipe was effective and economical. One year after planting survival remained very high at 84%. After planting in October 1997, plants had approximately one month of relatively mild conditions to overcome transplant shock and acclimatize to the site before the onset of cooler winter temperatures. The El Nino winter of 1997-98 provided above average winter precipitation. Together, the fall planting and above average percipitation contributed to excellent survival. Survival was poorest for Acacia which had been outplanted from tall pipe containers which were difficult to transplant from at this site and root damage probably occurred. The soil type, root damage during planting for moist soil with limited drainage may have contributed the low survival. The use of smaller container sizes in conjunction with an early fall or spring planting when temperatures are warmer may improve success with this species.
The loose alluvial soils and windbreak positioning relative to drainage patterns has been problematic. Flooding and some erosion of the berm, with burial of some seedlings in the eastern section of the windbreak resulted from storms. In March a motor grader to placed a second berm along the eastern section of the existing windbreak to protect the seedlings from further flood damage. This effectively protected the seedlings through the vulnerable first year seedling stage. Once seedlings have outgrown the treeshelters, we recommend that the shelters be removed and placed alongside the plant as a visual deterrent to traffic. Future maintenence should include grading to restore the berm where it has been buried with sediment or damage by vehicle traffic.
This is a very effective, relatively low cost method for establishing plants in remote areas without conventional irrigation. Planting in fall with initial hand watering into tree shelters and then subirrigation during the following summer is recommended. In many areas thay may be sufficient for establishment. In very dry areas where more growth is desire continued summer irrigation may be required.