Restoration in the Colorado Desert: Management Notes
Prepared for the California Department of Transportation District 11m 2829 Juan Street San Diego, CA 92138 July 1994
Matthew W. Fidelibus and David A. Bainbridge
The desert regions of southern California experience low andsporadic annual rainfall. Low annual rainfall and unfavorabledistribution of rainfall throughout the year limit soil moisturein this and other arid and semi arid zones (Boers, 1986) andplant production is often limited by water availability (Shanan,1979). To establish and maintain plants, techniques for improvingcapture and utilization of local water supplies must be developed(Shanan, 1979). Although irrigation can overcome water shortagesif surface or ground water is available, a drainage system may beneeded to maintain a favorable salt balance (Boers, 1986). Acomplete irrigation and drainage system is not always practicalor desirable if the immediate region lacks water, or if plantsare being established in a low or no maintenance area. Anirrigation system may also be undesirable from an environmentalpoint of view when preserving or establishing native plant stands(Ehrler, 1978). Water harvesting, a method of exploitingavailable precipitation, is practiced in many arid and semi- aridregions for both economic and environmental reasons.Microcatchments could be much more widely used in California toreestablish native vegetation.
Ancient desert civilizations rarely had the skills orresources to transport water over long distances, but they diddevelop very effective systems for utilizing water (Shanan,1979). Middle eastern civilizations, for example, used flashfloods to irrigate the flood plains of streams, and in somecases, check dams and flood bypass structures were constructed tocontrol water flow. These were the first runoff farms. Similartechniques were used in America, North Africa, Mexico, and SouthArabia. It is unclear why the extensive systems in the mid Eastwere abandoned, but it appears likely that it was for political,rather than environmental reasons.
Michael Evenari, Leslie Shanan, and Naphtali Tadmore havestudied the uniquely adapted farming methods employed severalthousand years ago on the Negev Desert of Israel for many years.During the reign of King Solomon and his successors between thetenth and sixth centuries B.C., the Nabtaens appeared as tradersin the Negev (Hall, 1979) The Nabateans eventually developed thetechnology to collect and direct runoff form the hills to crops.During the Byzantine empire from 250 B.C. to A.D. 630m Nabtaeanagriculture reached its peak with improved water harvestingmethods that were practiced on a large scale (as much as 300,000ha). Crop production from these extensive systems is impressivein a desert with annual rainfall of 3-4 inches (80-100mm) peryear; similar to the normal annual rainfall recorded in theColorado desert. Israeli researchers were intrigued by theancient farms, and one of the farms, Wadi Mashash was restored inthe early 1960's in an attempt to "relearn" the ancienttechniques of water harvesting. The positive results of the WadiMashash restoration inspired Israel to invest in water harvestingresearch as a means of supporting many types of crops (Shanan,1979). Microcatchments have also been used to establishvegetation for parks and roadside rest areas. These studiesformed a solid foundation for modern microcatchment development.
Modern microcatchments systems harvest runoff water in alaminar flow , water depth less than 1/8 inch (1-2 mm), and flowvelocity less than 2.75 inches/sec (7 cm/sec) (Shanan, 1979). Thecatchment area can be left unaltered, altered (defoliated andcompacted) or treated with living soil crusts or water resistantcompounds to enhance runoff. The collected water us theneffectively "stored" below the soil surface where it isavailable for plants but protected from evaporation higherrelative water yield per unit surface area than the larger runofffarm catchments. The size of catchment can be tailored to providean optimal runoff volume for individual plants. Microcatchmentsenhance leading and often reduce soil salinity (Shanan, 1979).The use of microcatchments techniques in Arizona has continuedthe productive use of land retired from ground water irrigatedagriculture (Karpiscak, 1988)
Microcatchment systems provide many advantages overalternative irrigation schemes. They are simple and inexpensiveto construct and can be built rapidly using local materials andmanpower. The runoff water has a low salt content and because itdoes not have to be transported or pumped it is relativelyinexpensive. The hydrological data needed for an efficient designcan be collected through observations over two to five years evenin areas with limited rainfall. Finally, the systems are easy tooperate and maintain and relatively safe from failure.
Many types of domesticated crops have been successfullyestablished using microcatchments, but water harvesting has alsobeen used to supplement rainfall for water stressed nativevegetation. Ehrler (1978) compared the growth and seed productionof jojoba (Simmondrisia chinensis) plants that were given one ofthree treatments: T0)no water harvesting catchments; T1) cleared,smoothed and rolled 20 meter square catchments; and T2) like T1plus a water repellent coating. The plants receiving supplementalwater form micro catchments , especially the treated catchments,were larger in volume and produced flowers and seeds than theuntreated plants. figure 1. Even native plants that are welladapted to hot and dry conditions will usually benefit fromsupplemental water provided by microcatchments (as illustrated bythe increased height of creosote bush, Larrea divarivata,alongside desert highways).
Rainfall and Climate of the California Deserts
Although rainfall patterns vary in the California deserts,rainfall can generally be described as bimodal with a peakprecipitation occurring in January, and declining gradually to alow in June when there is essentially no rainfall. A second peakoccurs in many parts of the Colorado desert in August as a resultof tropical storms. In all areas, the potential evaporation farexceeds the average precipitation. In 1975, at the United StatesDate Gardens at Indio, for example, total precipitation amountedto only three inches (76mm), while free evaporation (from an openpan) was 112.96 inches (2869 mm), more than forty times the yearsof precipitation (Lenz, 1981).
Most of the Colorado desert (Coachella Valley and relatedarea) normally receives less than five inches (127 mm) of rainper year. This aridity is the result of several environmentalfactors: the stable Pacific high pressure cell which divertsstorms to the north; the cold California current whichprecipitates rain before it reaches land, and series of mountainsalong the western periphery of the Colorado desert which create arain shadow effect (Bainbridge & Virginia, 1986.) Stormsgenerated from tropical air masses moving north from the Gulf ofCalifornia can drop rain equal to the yearly average in a matterof minutes (Bainbridge & Virginia, 1986). Storms such asthese recharge soil moisture and are important in the spring andfall for establishing seedlings. In 1991, late spring rainsresulted in considerable creosote bush germination andestablishment at a site in Imperial county and at Red Rock CanyonState Park in the Mojave Desert.
The high temperature and limited annual rainfall ofCalifornia's deserts result in plant water stress. Astemperatures rise, the rate of evaporation increases and with itthe water demands of the vegetation (Nir, 1974). Peaktemperatures in the 100's ェ (38 イ) occur from June throughSeptember when less than 1.5" (37.5mm) of precipitationnormally occurs.
Temperature and precipitation values can be combined tographically portray the aridity of a region. A graph ofprecipitation values in millimeters plotted against temperaturevalues in イ with a scale of 1イ :2 mm is a common climaticyardstick. When the curve of precipitation values is below thetemperature curve, then the amount of precipitation is smallerthan twice the temperature length and intensity of the dryseason. Graphs of temperature and precipitation values forseveral weather stations in the Colorado desert illustrate theyear round water deficit, figures 2, 3.
Wide temperature variation, daily and annual, ischaracteristic of both the Mojave and Colorado deserts ofsouthern California. In the summer months. the difference betweenmean high and low temperatures can be greater than 30ェ (17イ), 100ェ to 70ェ (38-21 イ) (NOAA).
Ambient air temperature and soil temperature have a pronouncedeffect on plants. The high air temperatures of the summer,increase the plants water demand for evapo-transpiration. InJanuary, mean low temperatures are between 35-40 ェ (1.5-4.5 イ), with highs between 60-70 ェ (16-21 イ ) a span of 25 ェ.
Freezing temperatures are not uncommon on clear winter nights,but hard freezes only occur about every decade. Several hours ofbelow 26 ェ temperatures can damage sensitive plants (Bainbridge& Virginia, 1986). In August 1989 a rainfall initiated growthof many palo verde seedlings in the Colorado desert. The majorcauses of death in the initial months were herbivore, frost,tramplings, and flood damage. Thus freezing can affect theseplants as significantly as high temperatures ( Bainbridge &Virginia, 1990).
Bare soil temperature in the sun may reach 57-64 ェ (14-18イ) or greater above ambient air temperature under windlessconditions. When the wind is blowing, exposed soil temperaturesmay be only 4 イ above ambient air temperature (Wallace &Romey, 1972). Optimum root growth for mesquite and many othersummer active desert plants occurs at soil temperatures in thehigh 80's ェ (27 イ ), usually not achieved in until earlysummer.
There are four types of microcatchment systems:micro-watersheds, runoff strips, contour bench terraces, andcatchment basins (Shanan, 1979). Of these, runoff strips andcontour bench terraces are best suited for agriculture as theyrequire extensive mechanical re-shaping of the surroundingterrain and create regular patterns which are inconsistent with anatural appearance. In runoff strips, a series of"saw-teeth" ridges are built by moving soil laterallyand longitudinally with a grader to increase the natural slopeand form a contour strip to collect runoff on a parallelcultivated strip.
In contour bench terrace systems, the runoff strips of thecontributing area are left unaltered, but the cultivated stripsare leveled longitudinally to distribute the runoff evenly(Shanan, 1979). Mico-watersheds and catchment basins can be builtusing hand labor and are this less expensive and more adaptableto revegetation projects.
Micro-watershed systems include mound and strip collectors.Strips can be built with mechanical equipment of by hand. Thesteps are bordered on each side by ridges 8-20 inches (20-50 cm)high and 6.5-16.5 feet (2-5 meters) apart. The result is a seriesof linear strips well suited for crops such as corn. The wafflegarden of the Zuni in New Mexico are a form of crossed stripcatchments.]
In mound systems the soil surface is shaped by hand into 4-20inch (10-50 cm) tall mounds spaced 2-5 meters apart. Whenorganized into a regular pattern, this system is suitable formany types of farm crops, including melons and squash ( Shanan,1979). The mounds are also effective when arranged in a morerandom manner for revegetation and restoration efforts.
Since catchment basins involve a minimum amount of labor andless manipulation of the surrounding terrain, they are probablybest suited for revegetation and restoration. A gently slopingplain (ideally with slopes less than 5%), is divided into plotsby small earth ridges 10-20 cm high and 20-30 cm wide (Shanan,1979). The ridges are constructed with the soil excavated from aplanting basin about 40 cm deep or from the uphill side of theridge. These can be constructed by hand, or with a small plow.Catchment basins are susceptible to siltation and erosion ifundesired runoff is allowed to enter the system, so protectivediversion ditches are often constructed above areas subject toextensive ground flow.
The gradients of the microcatchments should be between 1-7%.Square or rectangular plots are easy to stake out so they arecommonly used, however basin shapes for revegetation sites shouldbe tailored to suit the geography of the site to reducedisturbance and retain a more natural appearance.
Potential water yields must be estimated before otherdecisions regarding microcatchment design can be made (Shanan,1979). To determine expected yields from microcatchments threerainfall characteristics must be evaluated: 1)the average annualrainfall, 2) peak rainfall intensity, and 3) the minimum expectedannual precipitation. The average annual rainfall must be knownto predict the long term water availability.
Consideration of a site for microcatchment construction mustalso include four main physiographic factors: the runoffproducing potential; the soil surface condition (cover,vegetation, crust, stoniness); the gradient and evenness ofslope; and the water retaining capacity of the soil in the rootzone profile (Shanan, 1979). These all contribute to the runoffthreshold coefficient which is a key factor in determining theoptimum size for catachment (Howell, 1989). Other factorsaffecting the infiltration capacity of a particular area include:the moisture content of the soil; macropores in the soil as aresult of decaying roots or burrowing animals; and the compactionof the soil (Shanan, 1979).
The optimal size of the microcatchment for each speciesdepends upon many factors including normal precipitation, thesoil quality, and the slope (Evanari, 1971). The size and depthof the planting basin in relation to the size of the catchmentarea is also important. These factors determine the size of thesurface area wetted by runoff and the volume and depth of thewater column in the soil. If the infiltration rate of the soiland the water demands of the plant are known, the size of acatchment basin can be estimated. If a particular species ofshrub requiring 30 inches of rain per year is being grown in aregion of 15 inch average annual precipitation, and then anadditional 15 inches of rain is needed. If the catchment soil hasa runoff of 10% (a typical runoff volume for untreated desertsoils), then a 100 square foot catchment should yield enoughwater to meet the water requirement for the shrub (Howell, 1989).
Effective precipitation is defined as that which producesrunoff (Toy, 1987). Surface infiltration is often proportional tovegetation cover, so as cover decreases, infiltration decreases.This results in greater volume and velocity of runoff for a givenrainfall event. Catchments areas are often defoliated ( acondition faced in most revegetation- restoration efforts), andgiven additional treatments to increase runoff.
To illustrate the potential advantage of treatedmicrocatchment areas, rainfall can be reported as a volume ratherthan as a depth ( Myers, 1967). One millimeter of rain equals oneliter of water per square meter assuming 100% runoff. Severalrunoff enhancing treatments have been evaluated on microcatchmentbasins. In 1972, Fink et al. compared runoff yields untreatedbasins with basins treated with paraffin wax basins covered withbutyl sheets. Paraffin (candle wax) is applied to the soil byhand, in the form of granules, at a rate of one to two pounds persquare yard. Wax with a low melting point, 120-150 ェ (49-65イ), will melt within a few days in hot desert environments toform a solid coating on the soil surface (Fink, 1973). The waxtreated soils yielded 90% runoff compared to 30% runoff onuntreated soils, and 100% runoff from a butyl covered plot. Waxtreatments are best for sandy soils, and some plots have remainedeffective after five years, sufficient time for plantestablishment.
In soils with a clay content between 5-30%, sodium salts(including sodium chloride) effectively reduce permeability bydispersing clay particles (Cluff and Forbel, 1978). When used ina suitable soil, the sodium is absorbed by clay, and the qualityof the runoff water is tolerable. The quality generally improvesafter the initial one of two runoff events. Theses treatmentsonly work on the clay fraction of a soil, however, and will notbe effective on sandy or course grained soils. Salt treatmentswill often be inappropriate for revegetation sites.
Many types of synthetic membrane materials have also been usedto increase runoff. Plastic membranes , such as polyethylene andvinyl, have been used on extensive revegetation projects inChina, They are very effective but generally last less than oneyear. Butyl rubber and chlorinated polyethylene sheeting lastsmuch longer, but these materials are expensive and must be wellsecured to protect them from wind damage.
Asphalt, concrete and other hard surfaces in urban andsuburban areas can also be used to channel water to catchmentbasin plantings. Landscaping on streets in Tucson is increasinglywatered this way and the potential for for highway plantings ishigh, despite water quality concerns from road surface runoff(Howell & Howell,1991;Virginia & Bainbridge, 1986).
Enhancing the development of natural surface crusts hasapplications for increasing runoff. In many soils, crusts formafter rains; either by mechanical action due to impactingraindrops or by chemical action such as hydration and dispersion(Freebair et al,. 1991). In areas with segregated soil types athin layer of well oriented clay forms after wetting, enhancingrunoff (Tackett &pearson, 1965). Microphytes such as bacteriaand lichen living on the soil surface also contribute tocrusting. Researcher in Israel have observed that colonies ofblue-green algae (cyanobacteria) secrete threads that entrap sandparticles producing a thin, grayish soil crust (Gillis, 1992).This crust my decrease infiltration and increase runoff.Brotherson and Rushforth (1983) found that microphytic crusts innorthern Arizona allow deeper penetration of applied water, andreasoned that the crusts seal the soil surface, reducingevaporation (West, 1990). Harper and Marble, (1988) however foundthat sites heavily crusted with dark colored cyanobacteria lostsignificantly more water from the upper 7-5 cm of soil thanintermixed, scalped plots. They concluded that the dark crustsabsorbed more solar radiation than the light colored, uncrusted,soil leading to higher evaporation. More research on the effectsof microphytic crusts on soiled-d water relationships is needed,but their potential use as biological "coatings" formicrocatchments appears promising.
Lack of water is often a critical problem for revegetaionefforts of arid lands. Successful transplanting and directseeding may require temporary supplemental water. After aninitial establishment phase, the demand for water is much reducedfor survival but remain important for growth. Spalding (1990)found that Larrea transpiration rates increased 8-9 times whenirrigated. While most of these species respond positively tosupplemental water, they generally need well drained soils due tolow resistance to fungal infections and oxygen deficiency (Fisheret al., 1988). Unlike the shallow diffuse roots of typicaldomesticated plant species, many Colorado desert plants developdeep tap roots that eventually reach the water table. In Arizona,live mesquite, Prosopis glandulosa, roots have been found morethan 150 feet below the soil surface. However, phreatophytes suchas mesquitem and more drought tolerant species such as fourwinged salt bush (Atriplex canescens), white bur sage (Ambrosiadumosa) and cresote bush (Larrea divaricata) all need relativelyhigh surface soil moisture to germinate and become established.Conditions suitable for plant establishment may only occur onceor twice a decade when favorable moisture patterns develop (Bainbridge & Virginia, 1986).
Ambrosia dumosa like Larrea, is very drought tolerant andprefers well drained soils, but it is more tolerant than creosotebush to low soil oxygen levels 9Lunt, 1973) that might beencountered in catchment basins. White bur sage isdrought-deciduous and loses its leaves during drought stress(Bamberg, 1975). Under favorable conditions it exhibits highphotosynthetic and transpiration rates. Ambrosia dumosa, a goodcandidate for revegetation in the Colorado desert, should respondfavorably to micro catchments.
Atriplex canscens also appears to be a good candidate formicro catchments. A study showed that Atriplex seed emergence wasalways greater in shade and correlated with rainfall (Hennessy,1984). Scholl (1985) observed that stands of saltbush in NewMexico established themselves in basins and furrows on a minesite where soil moisture content was higher. Adequate soilmoisture is also very important for transplanting this species.
Mesquite like fourwing saltbush, white bur sage, and creosotebush requires supplemental water during the first few months ofestablishment. Over-irrigation may decrease growth and floodingmay kill mesquite trees, so the trees should be planted on amound in the basin or on top of the dike where they will be safefrom prolonged flooding.
Construction and Maintenance
The first step in constructing microcatchment basins isclearing the catchment area of weeds. The catchment and basinarea should then have it contour lines staked so that thecatchment area can be shaped into evenly sloped basins tomaximize runoff and minimize erosion (Shanan, 1979).Shaping canthen be carried out with hand tools, plows or graders. Shapingshould be undertaken when soil moisture is near field capacity ifpossible (Dutt, 1981). The catchment area is smoothed with handrakes following rough shaping. Soil treatments, if used areapplied after smoothing. The soil is compacted following thefirst rain storm. Compaction can be done by foot or tools onsmall small catchments or with rollers.
Basin size depends upon requirements. Small basins (less than9m^2), can be constructed by hand labor, but larger catchmentsshould be built with equipment (Shanan, 1979). Basin should beshaped to form inverted truncated pyramids. The soil removed fromthe basin area is deposited and spread on the border ridges. Fourworkers can excavate a basin 3.5m^2x20 cm deep and raise borderchecks 20 cm's high around the 250 m^2 catchment area in aboutone hour (Evenari, 1975).
Microcatchments are traditionally planted with shrub or treeseedlings. To take advantage of peak precipitation and favorabletemperature, plantings of native shrubs in the Colorado desertshould be planned before precipitation peaks. The soil around theplating spot should be loosened before planting as compactedsoils retard tap root growth which can be essential forsuccessful establishment (Bainbridge & Virginia, 1990).
Traditionally, the tree shrub is planted in the basin near thelowest point of the catchment, where the water would be deepestand the ridge highest. However, for many desert species this isnot desirable (Orev, 1988). After an intense rainstorm, thecatchment will fill with water, and may remain flooded forseveral days. During this time, the roots and lower stem sufferform lack of aeration and may succumb to fungal diseases. Theinfiltration rate of the soil will help determine the bestplanting spot. Fast draining soils would be better candidates forplanting at the bottom of the basin. Slow draining soils shouldbe planted on the dike or basin border. The water collected inthe basin will moisten the soil above the water line by capillaryaction.
In many cases, the best place to plant desert shrubs incatchments is immediately above the mean high water line, or nearthe top ridge where the topsoil from the basin is placed, figure4. Observations of conventional plantings reveal both weeds andshrubs appear to thrive on this site and are absent from theplaces where water stays longer (Orev, 1988).
Microcatchments work well and should be more widely used.Microcatchment basins have been utilized for crops for thousandsof years. new technology has improved soil runoff potential andincreased the positive effects of microcatchment basins.Microcatchments are relatively inexpensive and require littlemaintenance. They are now being extensively used in Israel forplanting crops and have excellent potential for revegetatingsites in the California Deserts and other arid and semiaridregions.
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