Preferential Flow Control: Key To Sustainable Land Management
July 25, 2000
Hawaii ASAE Preferential Flow Symposium
Robert M. Dixon, Ph.D and Ann B. Carr
This paper traces the course of research conducted by the senior author and several coworkers beginning in 1960 and continuing to the present time. A review of water infiltration literature and some early infiltrometer trials led to the formulation of the Air-Earth Interface (AEI) Concept for water infiltration into initially dry soils which states that the microroughness and macroporosity of the AEI regulate the exchange of surface water and displaced soil air across the AEI with the rough-open interface having very high exchange rates and with the smooth-closed interface having very low rates . This concept was supported by a series of sprinkling infiltrometer studies, wherein manipulation of surface microroughness and macroporosity (roughness and openness) provided an order of magnitude control over water infiltration. In other words, management of these two surface conditions controls preferential flow in macropores, thereby providing a means for controlling infiltration. Controlling infiltration at high levels is the key to sustainable land management.
The Problem: Land Desertification
Land desertification began some 10 millennia ago with the domestication and herding of livestock and the selection and cultivation of wild plants (Lowdermilk, 1935; Dregne, 1983). These two human activities produce bare soil which quickly becomes smoothed and sealed under the impact of falling raindrops, thereby greatly reducing infiltration and accelerating runoff and erosion (Dixon, 1966). Desertification of the natural land resource base is the world's No. 1 problem directly or indirectly causing social unrest, malnutrition, hunger, starvation, political instability, armed conflicts and mass human migrations.
The Solution: Preferential Flow Control
This paper traces the course of field research conducted by the senior author and several coworkers beginning in 1960 and continuing to the present time. The vast amount of literature on infiltration reveals that field studies often stress the importance of surface conditions, whereas laboratory studies using soil columns indicate that profile properties largely determine infiltration rates (Dixon, 1966; Philip, 1957). The senior author's field research concluded that two physical properties of the soil surface--microroughness and macroporosity--interact to control the infiltration rates of an initially dry soil (Dixon and Peterson, 1971). This finding led to the formulation of the Air-Earth Interface(AEI) Concept of infiltration which states that the microroughness and macroporosity (roughness & openness) of the AEI regulate the exchange of surface water and displaced soil air across the AEI with the rough-open interface having very high fluid exchange rates and with the smooth-closed interface having very low rates. That is, these two surface conditions control two-phase preferential flow in the soil during an infiltration event.
A series of studies conducted under widely ranging climatic, edaphic, and vegetal conditions showed that manipulation of AEI roughness and openness could easily provide an order-of-magnitude control of water infiltration into dry soils (Dixon, 1977). A number of new infiltrometers were invented--a border irrigation infiltrometer and several types of closed-top infiltrometers-to elucidate the effects of soil-air pressure and soil-air counterflow on infiltration. Data from these infiltrometers showed that just a few millibars of soil air pressure could greatly reduce infiltration and that macropores connected to the crests in a microrough surface served as preferential flow routes to relieve this pressure (Dixon, 1975; Dixon and Linden, 1972). Thus, these results were consistent with the AEI Concept of infiltration.
To realize the many potential benefits of wide-range infiltration control through field application of the AEI Concept, a new land treatment method called land imprinting was conceived and devices called land imprinters were invented in 1976. The rolling rangeland imprinter, with a front-mounted drop seeder, was compared with a no-till rangeland drill for establishing forage species at several overgrazed sites in the Sonoran Desert near Tucson, Arizona. A series of studies found that the imprinting practice was greatly superior because of much better surface control of rainwater along with other resources important in seed germination and seedling establishment (Dixon, 1990). The V-shaped imprinted pockets or indentations, formed by angle-iron imprinter teeth, efficiently funneled rainwater, seed, plant litter and splash-eroded topsoil together at the bottom of the vee where these resources could work in concert to germinate seeds and establish seedlings. In contrast, the continuous furrows created by the no-till rangeland drill, tended to bleed resources downslope until encountering a flow obstruction, at which place infiltration, sedimentation and deposition would occur. The end result was a sparse and spatially spotty stand as compared with uniform stands of seedlings in the geometrically closed, V-shaped imprints.
Early rolling imprinters, designed from 1976 to 1985, were massive machines having large-diameter rollers with complex patterns of imprinting teeth welded to their circumferences (Dixon and Simanton, 1977). Imprinters designed after 1985 were smaller in diameter, lighter in weight, cheaper to fabricate, and easier to transport. They also had simpler and more efficient imprinting patterns. A drop seeder was mounted above the imprinting roller so that the seeder agitator could be directly driven by the tips of the imprinting teeth. The seeder is designed to dispense the morphologically diverse seed mixes commonly used in ecological restoration (Dixon and Carr, 2000). It can also inoculate the soil with beneficial organisms such as cryptogams and mycorrhizae.
The AEI Concept for wide-range control of infiltration gradually evolved into the AEI Model for reversing global land desertification through imprintation and revegetation (Dixon, 1977). This model verbally and diagrammatically describes the four interrelated and interacting processes: Desertification, Infiltration, Imprintation and Revegetation. Desertification smooths and seals the normally rough-open interface, thereby greatly decreasing infiltration with resultant increases in water runoff, erosion, flash flooding, and sedimentation. Imprintation efficiently converts the smooth sealed desertified AEI back into the rough open condition, thereby restoring infiltration to high levels. Imprints greatly accelerate the revegetation process by funneling and concentrating resources to, in turn, germinate seeds and establish seedlings. Seedling establishment is enhanced by the favorable microclimate created by imprints. The AEI model can help develop and guide the cultural practices for sustainable agriculture, agroforestry, and ecological restoration. Imprinter seeding has already restored perennial grasses on 20,000 hectares of degraded land in the Sonoran Desert of southern Arizona and is currently being used to restore native vegetation at several degraded land sites in the Mojave Desert of southern California where annual precipitation is only about one-half that of the Sonoran Desert (Dixon and Carr, 1994).
Summary and Conclusions
The series of studies described briefly herein were directed ultimately to making severely desertified land productive again through greatly improved rainwater infiltration and erosion control at the soil surface. Spatiotemperal scales were realistic. Sprinkling and closed-top infiltrometer frames were one-meter square and runs were one to two hours in duration. Before elimination by grazing cattle, the stands of perennial grasses in the Sonoran and Chihuahuan Deserts probably ranged from about one to 10 plants per square meter depending on annual precipitation. Microwatersheds created by the improved rolling imprinters are about 30-cm square. Thus nine of these tiny watersheds can fit inside a one-meter-square infiltrometer or plant sampling frame.
Geometrically, imprinted watersheds are V-shaped, right angle troughs with ends closed and with a small wing at the top. Each of these closed-basin microwatersheds can hold from 3 to 5 liters of rainwater, thereby extending the infiltration period following intense rainfall. These microcatchments are stabilized by the soil firming action of imprinting. While shaping the microwatersheds, imprinting increases the area of a flat desertified surface by about 30%.
Watershedding and infiltrating are at the microscale level to meet the needs of individual plants. Conventional seeding implements (drills and planters) leave continuous furrows which tend to bleed resources downslope instead of holding them in place to meet the needs of each and every plant beginning with seed germination and seedling establishment. Thus, the long-held dream of holding soil and water resources in place to control erosion, grow crops, restore natural ecosystems, and rebuild topsoil has to a great extent been realized by these 30-cm square imprinted watersheds. Although the initial applications of imprinting have been in the drylands, this technology may also be appropriate for more humid regions, especially on sloping lands and hillsides where better infiltration and erosion control is especially critical.
The AEI Model represents a problem-solution strategy for restoring endangered natural ecosystems and degraded agricultural cropping systems. It identifies the general problem to be desertification and the resultant low rainwater infiltration rates. It pinpoints the cause of low infiltration to be loss of roughness and openness (microroughness and macroporosity) of the soil surface. The first step in solving the desertification problem then, is to restore roughness and openness, thereby accelerating infiltration and preferential flow. A new no-till process and several new machines--imprintation and imprinters--were developed to restore these two physical surface properties in the most cost-effective way possible. Then, the solution to restoring desertified ecosystems logically becomes imprinter seeding--a new no-till practice that greatly accelerates the revegetation process.
Preferential flow in soils is a natural process contributing greatly to infiltration in relatively undisturbed woodlands and prairies. However, the higher the level of land disturbance or desertification, the lower will be the contribution of preferential flow to infiltration (Dreccer, 1993). Undisturbed soil contains a well-developed network of surface-connected macropores, a microrough surface, and a covering of plant litter-conditions favoring rapid preferential fluid flow and rapid rainwater infiltration.
Imprinting greatly increases infiltration by providing surface conditions favoring accelerated preferential flow of surface water downward and displaced soil air upward. These surface conditions include (1) plant litter partially imbedded in the soil surface to absorb the energy of raindrop impact, thereby preventing the surface clogging of macropores, (2) plant litter on the soil surface to feed burrowing soil invertebrates and (3) a network of closed surface basins (imprints), the troughs of which preferentially infiltrate rainwater while the crests preferentially exhaust displaced soil air.
The preferential flow process is driven by just a few millibars of surface water head and displaced air pressure. The infiltration process is accelerated because of the relatively high fluid conductivity of the soil's network of macropores, some of which are soil-surface connected. Potential preferential flow rate under the conditions listed previously, exceeds the rainfall rate of most high intensity storms, thus surface runoff and erosion are prevented.
In conclusion, preferential flow control is indeed the key to sustainable land management. And land imprinting integrated with a multitude of new technologies can provide a wide-range of control over preferential flow. New advances in technology will complement the preferential flow effect of imprinting including (1) plastic coating of seeds for controlled dormancy permitting fall no-till planting of most crops, (2) pest resistant crop varieties and (3) crop varieties bred for high populations and solid planting. Row crops will be relegated to history as agricultural sustainability becomes reality.
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