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.
Site Description
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.
Methods
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.
Results
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
Discussion
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.
References
Ehler, W.L., D.H. Fink abd S.T. Mitchell. 1978. Growth and yield of jojoba plants
in native stands using runoff-collecting microcatchments. Agronomy Journal 70:1005-1009.
Evenari, M., L. Shanan and N.H> Tadmor. 1991. The Negev: Challenge of a Desert.
Boston: Harvard University Press.
Howell, D. 1989. How to harvest water with microcatchments. Permaculture Drylands
Journal 5.
Orev, Y. 1998. Some considerations in the planning of runoff farming. In Desertification
Control Bulletin 16:13-16.
Shanan, L. and N.H. Tadmor. 1979. Microcatchment System for Arid Zone Development.
Hebrew University, Jerusalem. 99 p.