Rainfall catchments improve transplant survival
Fred Edwards, David Bainbridge, and Tom A. Zink
Deserts of southwestern United States experience low, infrequent and unfavorable rainfall distribution. This often limits soil moisture, with favorable conditions for plant establishment occurring perhaps only one out of ten years. Lack of water is usually a critical problem for revegetation and restoration efforts in these areas and irrigation costs can easily double project costs. Ancient desert civilizations did not often have skills or resources to transport water over long distances, but they did develop systems for utilizing local water supplies even in the harshest deserts (Shanan and Tadmor, 1979). Over 2,000 years ago the Nabateans, used water harvesting ditches and fields and flash floods with spreader dams to irrigate crops in the Negev Desert, creating the first large scale "runoff" farms (Evenari et al., 1991). Similar techniques were also historically used in America, North Africa, Mexico and South Arabia. The use of rainfall catchments is on of the most promising techniques developed by these ancient farmers. Rainfall catchment systems provide many advantages over other irrigation techniques. They are simple and inexpensive to construct and can be built rapidly using local materials and manpower. Runoff water has a low salt content and, because it does not have to be transported or pumped, is relatively inexpensive. The purpose of this experiment was to determine if rainfall catchments could be adapted to a restoration setting; if they would improve survival of native species container out plantings; which native species are most suitable for catchment plantings; and what is the most effective catchment size.
The experiment was established on a restoration site located at Fort Irwin, approximately 30 miles northeast of Barstow, California in the Mojave Desert. The site had recently been used as a temporary encampment during training activities in early 1997. Disturbance was mainly the destruction of the Larrea tridentata (cresote) and Ambrosia dumosa (bur sage) vegetation along with moderate soils compaction. Compacted soils at Fort Irwin have typically been sandy loam soils with very little organic matter, (less than one percent), low bicarbonate extractable phosphorous (<10 ppm) and low soil nitrogen (<5 ppm N03). The site was approximately 4.7 hectares in size on a gentle (less than 3%) slope with a southern aspect at 950 meters elevation.
Since bare ground and soil compaction accelerate surface runoff and erosion, a primary concern and objective for the project was to reestablish vegetation while reducing topsoil loss and preventing the formation of erosion gullies that could interfere with training activities. To accomplish this, 128 V-shaped catchments were installed perpendicular to the slope and drainage patterns on the site in February 1998. To determine to most effective catchment size for shrub establishment, four different size catchments, 4 m^2, 9 m^2, 16 m^2, and 25 m^2 were constructed. The species planted were native to the site and included: Ambrosia dumosa, Atriplex polycarpa, Encelia farinosa, Ephedra nevadensis, Hymenoclea, Isomeris arborea, Larrea tridentata and Prosopis glandulosa. A total of 512 plants were planted in the catchments with four shrubs spaced one meter apart in the deepest corner of each catchment. For comparison, 630 plants were planted in groupings of 5 (with at least one meter spacing between plants) outside the catchments. These clusters were randomly interspersed with the catchments but not immediately down slope from any catchments. All plants received supplemental water during planting and at approximately one month intervals during the summer months of June, July, and August. Survival inside and outside the catchments and the effect of catchment size were compared using a Chi square test of independence. Survival was monitored twice, at four months and one year after planting. To determine how each species performed in the catchments, Chi-square contingency tables were also calculated for each species.
Survival, both four months and one year after planting, was significantly higher inside the catchments (p-value <= 0.0001) than outside. Overall survival inside the catchments after one year was 83% versus 64% outside. Catchment size did not significantly effect survival at either the four month or the one year sampling. One year after planting, the highest survival (87%) was found in the 4 m^2 meter size catchments. The next highest was in the 25 m^2 meter size catchments (86%), with the lowest survival (80%) on the 9 m^2 and 16 m^2 catchments. Out of the eight species planted, three species, (Larrea, Encelia and Isomeris) had only slight increases or the same survival as outside catchments. The other five species (Ephedra, Ambrosia, Hymenoclea, Atriplex and Prosopis) had significantly higher survival rates inside the catchments (Figure 3.1-1).
Figure 1: Percent survival by species inside and outside catchments one year after
Using rainfall catchments in arid environments is not a new concept but has been rarely used in landscaping and restoration work (Ehrler et al, 1978). Archeological ruins in the Middle East and America show that a productive and extensive agricultural system can be build using rainfall catchment technology to habitat restoration in arid and semi arid environments is a low cost, low technology method for improving transplant survival as we suggested in the CalTrans report in 1995. Site selection, however, is important. Gradient on the site should be between 1-7%. Since square or rectangular catchments, like the ones used in this experiment, are the easiest shapes to stake out they are the most common; but, basin shapes can be tailored to suit the geography of the site with less disturbance while maintaining a more natural appearance. Catchment basins are susceptible to siltation and erosion if undesired runoff is allowed to enter the system from up slope so protective diversion ditches should be constructed above areas subject to extensive ground flow.
Traditionally, the tree or shrub is planted in the basin near the lowest point of the catchment, where the water would be deepest and the dike highest; but for many desert species this is not desirable (Orev, 1988). This study suggests that for some species the benefits of added water collected in the catchment may be off set by problems caused from short periods of flooding and submersion. The reduced survival that for some species the benefits of added water collected in the catchment may be off set by problems cause from short periods of flooding and submersion. The reduced survival in the large catchments may be related to this problem. After an intense rainstorm, catchments fill with water and may remain flooded for several days. During this time, the roots and lower stem suffer from lack of aeration and may be vulnerable to funal diseases. In this study, seedlings were planted one meter apart in the deepest corner of the catchment and on several occasions, water was observed sitting in the catchments. Larrea on our site may have suffered in this manner. On several occasions Larrea transplants inside the catchments did not green up as rapidly as ones planted outside. Knowing the infiltration rate of the soil will help determine the best planting spot. Fast draining soils would be better candidates for planting at the bottom of the basin. Slow draining soils should be planted on the dike or basin border. In this way the water collected in the basin will moisten the soil above the water line by capillary action.
The results of this experiment were inconclusive regarding the best catchment size. Based on survival, it appears that catchments larger than 4 m^2 or 2 x 2 meters on this site did not improve survival; however, the few supplemental waterings during the summer may have served to equalize survival and mask differences between catchment size. Smaller catchments in the Colorado desert were inadequate during low rainfall years, but studies have shown that small individual catchments have higher relative water yield per unit surface area than larger catchments. The sizes used here may not be appropriate to other sites and potential water yields should be estimated before decisions regarding catchment design are made (Shanan and Tadmor, 1979). The appropriate size for catchments will vary dependent on four main physiographic factors: (1) the runoff producing potential of the local microclimate (annual rainfall, peak rainfall intensity, and the minimum expected annual precipitation); (2) the soil surface condition (cover, vegetation, crust, stoniness); (3) the gradient and evenness of slope; and (4) the water retaining capacity of the soil in the root zone profile. These all contribute to the runoff threshold coefficient which is a key factor in determining the optimum size for a catchment (Howell, 1989). The catchment surface itself can be left unaltered, altered (defoliated and compacted) or treated with living soil crusts or water resistant chemicals to enhance runoff.
Catchment construction is low tech and adaptable to local labor and materials. To better control catchment size in this experiment most of the catchments were built by hand. Subsequently, on a nearby restoration site, we found that scraping the ground in four passes using a small tractor with a bucket (approx. one meter in length) followed by a final shaping using a shovel was a better construction technique that produced catchments roughly two meters in length. For us, this combined construction technique made the best use of labor and equipment, but where either labor or equipment are in the short supply, adjustments in the construction of the catchments can be made to fit. With the ease of construction and improved survival, the use of catchments is highly recommended for restoration projects undertaken in the deserts of California and the southwestern United States.
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