Hydrology and Erosion
Thomas L. Thurow
Properties of Water
Concepts of the Hydrologic
- Transit Through the Terrestrial System
Effects of Grazing on Hydrologic Processes
- Transit Through the Terrestrial System
Concepts of the Erosion Process
- Water Erosion
- Wind Erosion
Effects of Grazing on Erosion
List of Figures and Tables
"Water is the driving force of nature" This fundamental observation
by Leonardo da Vinci underscores the fact that water is the essential medium
of biogeochemical cycles and of life itself (Smith 1974). Thus, the development
of ecologically and economically sound grazing tactics requires a clear
understanding of the interaction of grazing on hydrologic processes. The
objective of this chapter is to review how grazing effects the hydrologic
Properties of Water
The importance of water to ecosystem function is largely a result of the water molecule's unique bonding structure. The strong dipolar nature of a water molecule results in an asymmetrical charge which strongly attracts one molecule to another. Van der Waals' forces (the attraction of a positively charged nucleus to the negatively charged electrons of neighboring molecules) and hydrogen bonds (the attraction of the two hydrogen atoms of one water molecule to the oxygen atoms of adjacent molecules) causes much stronger intermolecular bonding than is the case for most chemical bonds, such as covalent bonds which also bind oxygen and hydrogen atoms in water.
The combined effect of the water molecule's bond structure is that water
is a substance possessing a unique set of physical and chemical properties.
These properties are:
In addition to water's vital role within an organism, water is also a major force influencing the topographic and functional attributes of landscapes. This is because:
The hydrologic cycle is the process of water movement through the environment. On a worldwide basis, the amount of water is essentially constant, but the amount of water present at any given locale at any given instant varies depending upon how much water enters the system, the mode and rate of transit within the system, and how water exits the system. The fundamental avenues of input transit, and output of water movement are outlined in Figure 6.1.
Precipitation. Air heated at the earth's surface rises into the atmosphere and carries water vapor with it. As the air cools the water vapor begins to condense on small particles of matter suspended in the atmosphere. As the condensation process progresses clouds are formed. Coalescence of these droplets continues until the mass of the suspended droplets is sufficient for gravity to overcome the upward force of rising air. At this point, the droplets fall as rain, or if cold enough, snow or hail.
Underground and Overland Flow. Water originating from a source other than onsite precipitation may be an important component of an ecosystem's water balance. Water flowing in surface channels or in shallow groundwater reserves may be accessed by deep-rooted trees and shrubs. When roots reach a water table the plants are referred to as phreatophytes. Phreatophytic communities (e.g., oases and riparian communities) are an important component of and ecosystems.
Dew. Water vapor condensation forming dew on vegetation or soil is generally a negligible component of an ecosystem's water balance. However, dew may constitute a significant water source in localized coastal and montane regions such as in northern Kenya (Ingraham and Matthews 1988) and along the Pacific Ocean in North and South America (Azevedo and Morgan 1974).
Transit Through the Terrestrial System
Interception. As precipitation reaches the earth's surface it either strikes objects such as vegetation, litter, or rocks (i.e., interception) or falls unimpeded to the soil. Some of the intercepted water adheres to the objects and returns to the atmosphere via evaporation without ever reaching the soil. Some of the intercepted water runs down plants (stemflow) or drips off intercepting objects, resulting in a redistribution of water reaching the soil. All of the intercepted raindrops dissipate at least some of the kinetic energy associated with their fall.
The amount of water retained by an intercepting object is governed primarily by its water-holding capacity which varies primarily as a function of area (i.e., cover), morphological characteristics, and storm intensity. Interception loss, the amount of water intercepted and returned to the atmosphere without ever reaching the soil, is proportionately greater if the precipitation occurs as many small events since interception capacity may account for a large proportion of each such storm's total volume of precipitation. Interception loss is a relatively smaller proportion of the total volume of large storms since a large portion of the rain event occurs after the interception capacity of an object has been exceeded. The intensity with which raindrops strike an object is also important since a greater proportion of gentle rainfall is able to adhere to the cover surface compared to an intense driving storm.
The precipitation that reaches the soil may either run off, be stored in surface depressions, or enter the soil, Water stored in surface depressions or soil eventually exits the system via deep drainage, evapotranspiration, or seepage into surface or underground runoff streams. A small portion of water adheres to the soil particles (hydroscopic water) and is not available for plant use.
Surface Detention. Surface detention is a function of micro-relief, slope, soil texture, soil structure, and soil depth. Micro-relief formed by topography, vegetation growth, and accumulated litter influences the size, shape, and number of depressions and the amount of water that can be detained in them. Runoff generally increases as slope increases because of the associated decrease in size of detention storage sites. Water ponded on the surface is ultimately either lost via evaporation or enters the soil.
Infiltration. Infiltration is the process by which water moves into the soil. Infiltration rate is the quantity of water passing through the soil surface per unit of time. When the soil surface is saturated, infiltration stabilizes at a rate reflecting the interrelated hydrologic effects of the soil and vegetation characteristics of the site. This steady state rate is termed the terminal infiltration rate.
A low terminal infiltration rate is an indication of poor soil structure. Soil structure is the arrangement of soil particles and the intervening pore spaces. The structural characteristic of the soil is determined by the degree to which soil particles are held together in individual clusters which are termed aggregates (Fig. 6.2). Aggregation occurs when soil particles are mechanically bound by roots, fungal hyphae, and/or adhesive byproducts of organic matter decay and microbial syntheses. These mechanically bound particles are then cemented together by resistent humus components which form chemical bonds (Brady 1974). The porosity (pore volume) of the soil is a function of soil texture and the degree to which the soil is aggregated. Porosity and pore size determines the rate of movement of water into soil. Large macropores which aid high infiltration rates increase with improved aggregation (Allison 1973). The formation of soil aggregates is aided by any action that mixes the soil thus promoting contact between decomposing organic matter and inorganic soil particles. This action can be accomplished by wetting and drying, freezing and thawing, the physical activity of roots and burrowing animals, and soil churning by hooves or farm implements.
Aggregation alone is not a guarantee of high infiltration rate. The other key factor that must be considered is the stability of the aggregates. Aggregate stability is the collective measure of the degree to which soil particles are bound together and the stability of those bonds when wetted. Aggregate stability is used as an index of soil structure and as an empirical definition of aggregation (Kemper and Rosenau 1986; Boyle et al. 1989). The aggregates creating the soil pore structure must maintain their structural integrity when wet if infiltration through those pores is to occur. If the aggregate bonds are upstable when wetted, the clay particles disperse so the aggregate cluster begins to break into smaller pieces (slaking). These particles are then carried by the water and lodge in the remaining pores, making them smaller or sealing them completely (Lynch and Bragg 1985). This is one way in which soil crusts are formed. A "washed in" layer where clay particles have clogged soil pores to form a crust may reduce infiltration rate by as much as 90% (Boyle et al. 1989).
Aggregate bonds may be broken by the kinetic energy of raindrops striking the soil. Cover intercepts and dissipates raindrop energy before it strikes the soil, thereby protecting aggregate structure. Vegetation type affects the amount and structure of associated cover, therefore the infiltration rate differs among vegetation types (Figure 6.3 and Figure 6.4). The amount of cover, and hence the rate of infiltration, is usually greatest under trees and shrubs, followed in decreasing order by bunchgrass, shortgrass, and bare ground (Blackburn 1975; Thurow et al. 1986). Moreover, infiltration rates vary seasonally because of variation in growth dynamics (Thurow et al. 1988a). For example, cover on sites dominated by annual species is generally highly variable over time because the amount of cover rapidly increases during warm, moist periods that favor growth but rapidly declines during dormant seasons. The result is that the soil surface is poorly covered during some portions of the year and well covered at others. This is in contrast to the cover dynamics of perennial shrub's and grasses in that the amount of cover provided by these species fluctuates much less between seasons.
Another important difference between vegetation types relative to their affect on infiltration rate is related to the amount of litter. For example, bunchgrasses and shrubs tend to produce greater amounts of foliage than annuals and short grasses. The fallen foliage accumulates as litter which in turn leads to an increase in soil organic matter. Litter also creates a more consistent temperature and moisture microenvironment that favors microorganism activity. These factors enhance formation of stable soil aggregates which aids infiltration.
The hydrologic characteristics of various vegetation types can be expected to confer some competitive advantages to vegetation types with the greatest infiltration rates. Much of the water that flows overland does not reach a stream and leave the site. Rather, it flows for a short distance until it reaches an area of higher infiltration capacity that can accommodate both the failing precipitation and the overland flow. The net result of this process is that some areas receive more water than others. Since the infiltration capacity of some vegetation types is greater than others, the result is that in a vegetation mosaic the mineral soil near some species may receive more water from a storm producing a runoff event than soil near other species.
Runoff. Overland flow begins when the amount of water at the soil surface exceeds the amount of water entering the soil and when the storage capacity of surface depressions are filled. Runoff is the portion of precipitation that exits a watershed via overland flow. Water that exits the watershed without entering the soil is called surface runoff. Water that enters the soil before returning to a surface stream is called interflow or seepage flow.
Storm characteristics influence the amount of runoff. Storm intensity varies between different regions of the world and typically varies in different seasons within a region. For example, in the southwestern U.S. summer rainfall occurs predominately as thunderstorms, characterized as brief, intense rainfall events, whereas winter rainfall occurs predominately as frontal storms, characterized as prolonged, low-intensitv rainfall events. The low-intensity characteristic of frontal storms results in precipitation reaching the soil at a slower rate than the rate at which it can enter the soil. Consequently, the precipitation from these storms is able to soak into the ground soon after it strikes the soil. Conversely, during the high-intensity rain associated with summer thunderstorms, the rainfall rate exceeds the rate at which water can enter the soil. Therefore, flash-flooding is most likely to occur in the summer. The direction of a storm, especially in mountainous regions, also affects the amount of runoff. For example, a storm moving down a drainage produces greater runoff and peak flow than a storm moving up a drainage since the rain input moves with the accumulating overland flow.
Evapotranspiration. Transpiration is the process whereby water vapor is released to the atmosphere by passing through permeable membranes or pores of living organisms. Evaporation is the process whereby water vapor enters the atmosphere from soil or from surface water. The term evapotranspiration encompasses the combined effect of both these processes.
The amount of water which returns to the atmosphere via evapotranspiration is affected primarily by soil and plant characteristics. Amount of transpiration is dependent upon climatic conditions (such as relative humidity, wind, temperature, etc.), soil moisture, morphological vegetation characteristics, stage of phenological development, and the amount of transpiring tissue (see Chapter 4). Generally, transpiration is lower from sites dominated by herbaceous vegetation than from sites dominated by trees or shrubs because of inherent physiological differences relative to water use efficiency and growth dynamics. For example, graminoides often enter a dormant state during periods of water- or temperature-induced stress so transpiration losses are minimal during such periods as compared to trees and shrubs which usually maintain a substantial number of green leaves. Also, trees and shrubs tend to have more extensive root systems than grasses thereby increasing access to soil water. Trees and shrubs intercept more water than grasses, therefore grass sites contain less intercepted water to lose via evaporation than trees or shrubs. For these reasons, conversion of shrub or forest cover to grass cover may increase the amount of runoff and deep drainage on a watershed (Hibbert 1983).
While evaporation from a moist soil surface proceeds at a rapid rate, a thin layer of dry soil at the surface can dramatically reduce the rate (Penman 1948; Veihmeyer 1964; Ripple et al. 1972). In the absence of soil cracks, soil water can be depleted by evaporation to a depth of about 10 cm in clay-textured soils and about 20 cm in sandy soils (Sosebee 1976). Other soil characteristics such as color (dark soils absorb more heat and lose more water through evaporation than light soils) may also have a significant effect on potential evaporation rates. But in general, evaporational water loss from unsaturated soils is a relatively small portion of the total outflow.
Deep Drainage. Water movement through a soil column after it has entered the soil is called percolation. Water that percolates beyond the reach of plant roots is termed deep drainage. The volume of deep drainage depends on the amount of infiltration, the evapotranspirational demand, and the substrate transmission characteristics which are primarily a function of regional geological features.
Amount and type of cover influence deep drainage to the degree that they effect infiltration rates and evapotranspiration loss. For example, from a 1-year study in south Texas, Weltz (1987) estimated that deep drainage accounted for about 10%. of total precipitation on bare soil, 2% under herbaceous cover, and 0% under shrubs. The lower evapotranspiration loss on the bare plots (80% compared to 95% and 98% loss on herbaceous and shrub plots, respectively) offset the increased runoff on the bare plots (10% compared to 3% and 2% on herbaceous and shrub plots, respectively). Thus, the net effect was that more water was available in the soil column for drainage beneath the bare ground sites. On sites that have hard pans or caliche barriers, deep drainage is reduced to a small portion of precipitation regardless of cover type. In such situations the bare ground sites store more water because there is no transpiration loss via vegetation. The resultant higher antecedent soil moisture on the bare plots results in greater runoff (Carlson et al. 1990). These data from vegetated Texas shrublands bear a close similarity to the water balance of the environmentally similar Burkea savanna in South Africa (Whitmore 1971).
Effects of Grazing on Hydrologic Processes
The degree to which livestock grazing can influence water distribution over a landscape can be conceptually evaluated by examining grazing effects on the hydrologic cycle. The vegetation and soil characteristics of a site are the prime determinants influencing how water is partitioned to each of -the potential pathways of water movement Understanding how vegetation and soil factors affect the quantity and -quality of water, and how grazing alters these factors, is essential to the development of grazing strategies aimed at sustained production by conserving the water and soil resource.
Precipitation. Amount, intensity, duration, form, and temporal and spatial distribution of precipitation are intrinsic to climatic conditions, so they are viewed as being beyond the influence of grazing management. An exception may occur in some regions where prolonged, intensive grazing has caused serious land degradation. The extensive removal of vegetation cover and litter may cause an increase in local albedo (surface reflectivity) of the land surface (Otterman 1977), which reduces the amount of heat absorbed by the land surface and which in turn diminishes the convective activity in the lower atmosphere required for cumulus cloud formation. Furthermore, bare ground tends to cool more at night than vegetated soil surfaces which tends to reduce the opportunity for convective precipitation events (Charney et al. 1975). Extensive litter removal over a large region also reduces the population of bacteria such as Pseudomonas syringae, which are important sources of nuclei for the formation of raindrops or ice crystals in clouds. A reduction in the quantity of these raindrop nuclei may lead to reduced precipitation (Vah et al. 1976).
Interception. The amount and intensity of precipitation reaching a soil surface may be influenced by grazing to the degree that grazing alters the amount and type of vegetation cover on a site. The extent of rainfall interception differences among plant communities was documented for rangeland on the Edward's Plateau of Texas. The estimated annual interception loss for a site dominated by stoloniferous grass was 10.8% of annual precipitation. A site dominated by bunchgrass had an estimated annual interception loss of 18.1%, while oak trees and the litter beneath the trees intercepted about 46% of annual precipitation. However, due to the concentrating effects of stemflow, soil near the bases of trees received about 222% of annual precipitation whereas areas more than 100 mm from the trunk received only about 50.6% of annual rainfall (Thurow et al. 1987). Studies of eastern U.S. forests have shown that conifers intercept 3-8 times as much precipitation as deciduous trees (Douglass 1983). These data indicate that shifts in the kind or amount of vegetation associated with grazing and brush management affect interception, which in turn affects the amount and kinetic energy of the precipitation actually reaching the soil.
Surface Detention. The effect of large herbivores on surface detention of water are mixed and dependent on grazing intensity. For example, the hoofprints left by livestock increase micro-relief if the stocking intensity is moderate. If stocking intensity is heavy, micro-relief is reduced due to disaggregation of soil structure resulting in the soil surface becoming either loose dust if soil moisture is low, or flattened and compacted if soil moisture is great. Heavy grazing also often decreases micro-relief by reducing litter accumulation and bunchgrass growth forms.
Infiltration. The terminal infiltration rate is sensitive
to the type of grazing management used on the site (Fig.
6.5). The key components affecting infiltration impacted by grazing
are listed in Table 6.1. The extent to which
livestock grazing effects these variables is largely dependent upon grazing
intensity. The impacts of grazing and the magnitude of hydrologic response
differ from region to region depending upon the interrelationships associated
with the particular mix of these parameters. Livestock effects on the parameters
that influence infiltration fall into two broad categories: vegetation
impacts and trampling impacts.
Vegetation impacts. The herbivorous nature of grazing animals clearly results in the removal of a portion of the vegetation. Removal of vegetation a&cts aggregate stability in several ways:
The degree to which grazing reduces cover, reduces litter, and changes the species composition of cover is dependent upon the frequency and intensity of grazing. This physical removal of vegetation by herbivores is superimposed on fluctuations of vegetation cover resulting from seasonal variation in growth dynamics as influenced by climatic factors. The combined effects of climate and livestock grazing intensity
strongly influences seasonal fluctuations of cover that in turn contributes to seasonal fluctuations of infiltration rate (Thurow et al. 1988a).
The principle objectives of most livestock grazing systems (see Chapter 7) are to maintain or improve forage production and/or to improve forage harvesting efficiency. Maintenance or improvement of forage production is directly related to water infiltration rates. Indeed, drought stress caused by poor infiltration has been documented to be a major problem limiting production in the western U.S. (Boyle et al. 1989). Therefore, the long-term success of a grazing system depends on how well increased livestock harvest efficiency (which reduces cover and biomass) is balanced with the need to maintain aggregate stability (which is improved by increased cover and soil organic matter).
The extent to which grazing causes a change in species composition is a prime factor determining hydrologic condition of a site (Wood and Blackburn 1981; Gamougoun et al. 1984; Thurow et al. 1988a) (Figure 6.3 and Figure 6.4). Density of herbaceous perennials is an especially important indicator of hydrologic condition in many regions because their decline is usually associated with increased runoff. This insight provides an understanding of the basis for the range condition classification system outlined in Chapter 5, since infiltration rate in many respects is a synthesis of the measures used to evaluate range condition. For example, as grazing intensity in a mixed grassland increases, the vegetation composition shifts from midgrass to short- grass dominance (Rhodes et al. 1964; Sharp et al. 1964; Thurow et al. 1988b), with the most severe changes being associated with heavy stocking (Ellison 1960). Heavy stocking tends to result in intense defoliation of palatable species resulting in their decline (Dyksterhuis 1949). Many bunchgrass species are palatable and nutritious, but if closely grazed the above-ground apical meristems are damaged (Sims et al. 1982). Therefore, heavy grazing intensity, regardless of grazing strategy, does not appear suited for long-term maintenance of hydrologically desirable bunchgrass species (Thurow et al. 1988b). Moderate or light grazing, regardless of grazing strategy, generally has little effect on bunchgrass cover (Ellison 1960; Rich and Reynolds 1963) and thus has little affect on infiltration rate (Blackburn 1984).
Trampling impacts. Grazing animals also reduce infiltration by breaking the soil aggregate structure due to the force applied by hooves. A commonly asserted hypothesis is that intense trampling activity associated with high stock densities enhances infiltration (OTA 1982; Walter 1984). Intense trampling results from concentrating livestock on small areas for short periods of time, creating a "herd effect" (Savory 1978; 1979). The result of this "hoof action" is hypothesized by these authors to enhance infiltration rate and reduce erosion, even when conventional stocking rates are doubled or tripled (Goodloe 1969; Savory and Parsons 1980; Savory 1983).
Research conducted to date does not support the hypothesis that a hydrologic benefit accrues by increasing the magnitude of trampling. Length of rest, rather than intensity of livestock activity, appears to be the key to soil hydrologic stability (Warren et al. 1986a).
It is difficult to separate the disaggregating effects of the force associated with hoof impact from the force associated with raindrop impact. Studies aimed at determining the impacts of livestock trampling in the absence of concomitant removal of vegetation have generally shown that trampling increases soil compaction (i.e., bulk density) (Alderfer and Robinson 1947; Kako and Toyoda 1981; Willat and Pullar 1983); mechanically disrupts soil aggregates (Beckmann and Smith 1974); reduces aggregate stability (Knoll and Hopkins 1959); and destroys cryptogamic cover (i.e., cover provided by algae, moss, and lichens) (Loope and Gifford 1972; Brotherson and Rushforth 1983). Moreover, these studies have shown the magnitude of negative impacts from trampling increases as stocking intensity increases (Willatt and Pullar 1983). The degree of damage associated with trampling at a particular site depends on soil type (Van Haveren 1983), soil water content (Robinson and Alderfer 1952), seasonal climatic conditions (Warren et al. 1986a), and vegetation type (Wood and Blackburn 1984). For example, Warren et al. (1986b) show that repeated high- intensity trampling decreases aggregate stability and increases bulk density which in turn reduces infiltration rates and increases surface runoff and interrill erosion. Trampling dry soil did indeed chum the soil surface. However, this "hoof action" reduced the size of naturally occurring soil aggregates and increased the bulk density of the surface soil layer. Trampling moist soils destroyed existing soil aggregates by compacting them into a comparatively impermeable surface layer composed of dense, unstable clods (Figure 6.2). Both of these outcomes were detrimental to infiltration rate and interrill erosion (Warren et al. 1986c) (Table 6.2).
Compacted trails may increase as the number of pastures is increased within an intensive rotational grazing system (Walker and Heitschmidt 1986; Andrew 1988). The low porosity of trails leads to a low infiltration rate resulting in concentrated runoff which eventually creates gullies. To prevent trailing on fenced or unfenced land, sufficient water points must be provided so that grazing pressure is evenly distributed. Livestock must regularly return to the same sites if water points are spaced too far apart, resulting in the creation of a series of radial paths leading to water. On sites of low productivity it is unlikely to be economical for a rancher to install the fences and water supplies necessary to avoid damage from livestock concentration. Yet it is these low-productivity sites which are often most fragile and susceptible to accelerated erosion. Therefore it may be in management's long-term interest to expend the capital necessary to disperse grazing if grazing use of these sites is to continue.
As previously discussed, one way in which crusts are formed is the clogging of the surface pores by disaggregated soil particles. Crusting is commonly associated with sifty soils having a low organic matter content and low aggregate stability (Blackburn 1975). A stop-gap approach often used in an attempt to manage crusted soils is to concentrate grazing so that the soil surface is disrupted by hoof action (OTA 1982). livestock trampling does indeed break the crust, incorporate mulch and seeds into the soil, and aid seedling emergence. However, this result is short-lived because the subsequent impact of falling raindrops re-seals the soil surface (i.e., the unstable soil pores will become plugged) after the first several minutes of an intense rainstorm. To effectively address a soil crusting problem, livestock grazing systems must concentrate on addressing poor aggregate stability which is the cause of the crusting. Livestock grazing systems that promote an increase in plant and litter cover and an increase in organic matter produces the only lasting effect in reducing soil crusts (Blackburn 1983). Not all types of crusts are bad. Some types of crusts formed by lichens, moss, and algae (i.e., cryptogamic crusts) play an important role in and environments by stabilizing soils otherwise susceptible to wind erosion. The reduction of water infiltration associated with cryptogamic crusts may be offset by the benefits they provide in slowing runoff and evaporation, leading to a net soil water benefit (Johansen 1986). Cryptogamic crusts are prone to deterioration resulting from trampling or air pollution (Hawksworth 1971).
Runoff. Management of grazing lands must be designed to cope with the storm characteristics of the region. For example, regions unlikely to experience runoff can afford to allow more protective cover to be grazed than regions where flood danger is extreme due to thunderstorms, slope, soil texture characteristics, etc. Areas of grazing lands susceptible to runoff and erosion may be best managed by minimizing grazing disturbance prior to and during the period when runoff and erosion danger is greatest. Protection of riparian areas is particularly important in regions where large seasonal runoff events are likely because these areas serve as buffer strips to filter sediment and slow the rate of overland flow. Riparian vegetation growth can also stabilize banks that would otherwise be susceptible to erosion by peak flows.
Evapotranspiration. Vegetation cover tends to reduce evaporation rates by shading the soil and reducing wind velocity. However, the greater interception and transpiration loss associated with the greater vegetative cover usually more than offsets the benefits of reduced evaporation (Lull 1964). Frequent, heavy grazing results in less transpiration loss per unit area due to removal of transpiring tissue. However, the water use efficiency of the remaining plant tissue is likely to be lower (Caldwell et al. 1983).
Water evaporates more rapidly from compacted soils than from well aggregated, friable soils. Consequently, soil compaction by grazing animals can increase evaporation from the soil (Sosebee 1976).
Deep Drainage. A decrease in percolation rate associated with increased bulk density may slow and reduce downward movement of water. Trampling normally does not alter bulk density much beyond a depth of 25 cm (Alderfer and Robinson 1947), although such effects have been recorded to a depth of 150 cm on wet soils (Lull 1959).
Concepts of the erosion Process
Natural or geologic erosion results from climatic and topographic conditions and is independent of human activities. About 80% of the world's land surface considered susceptible to geologic erosion (Fig. 6.6) are classified as rangeland. Accelerated erosion is defined as an increase in soil erosion associated with human activities relative to changes in vegetation cover and/or the physical properties of the soil. The rate of erosion must be equal to or less than the rate of soil formation if sustained, long-term productivity is to be maintained.
The erosion process develops in three basic phases: detachment, transportation, and deposition. Different soils exhibit different responses to each of these phases; sand, for example, is more easily detached than clay, but clay particles are more easily transported. Erosion is a function of the erosivity (Le., energy of the water or wind acting on the soil) of the detachment factor and the erodibility (i.e., a function of the soil's physical characteristics, topography, type of land use, and type of vegetation) of the soil (Smith and Wischmeir 1957).
Soil erosion by water is largely dictated by mean annual rainfall. The amount of rain in regions with annual rainfall under 350 mm is usually insufficient to cause serious erosion. At the other extreme, annual rainfall of over 1000 mm leads to such a complete vegetative cover that the soil is effectively protected from raindrop impact. Regions with annual precipitation between these two extremes are most susceptible to severe water erosion. Erosion is of course increased whenever the protective cover is removed from sites with higher precipitation (Fig. 6.7).
Sediment production is closely related to runoff (Blackburn et al. 1986) since runoff is the principle agent of soil detachment and transport. Water erosion is typically characterized in stages corresponding to a progressive concentration of runoff. Evidence of erosion can be quickly determined by a variety of indicators (Fig. 6.8). The first stage, interrill erosion, combines the detachment of soil resulting from raindrop splash and its transport by a thin flow of water across the surface. This thin flow of water is highly turbulent as a result of raindrop impact and has a high erosive capacity. Extreme interrill erosion is evident when, for example, soil pedestals are formed by erosion around an area covered by a resistant material such as rock. The fact that the surrounding soil is eroded without undercutting the soil under the resistant cap illustrates that raindrop splash is the major transport mechanism, rather than surface flow. Clearly, therefore, although no running water is observed, water erosion may still be taking place. Rill erosion begins as the diffuse water movement causing interrill erosion concentrates into discrete flow paths. Gully erosion is generally defined as the point when rills increase in size to the point they can no longer be driven across by a truck. Streambank erosion is defined as soil displaced from the banks of rivers or streams.
The amount of interrill erosion varies depending upon graminoid growth form in that interrill erosion is less when equal cover is provided in bunchgrass vegetation types than sodgrass types. This is because the bunch growth form and the accumu- lated litter at the base of the bunch provide an effective obstruction to overland flow. By slowing or diverting the course of overland flow the kinetic energy of runoff is reduced, resulting in decreased sediment transport capacity. Bunchgrass clumps which are mounded above the level of surrounding soil indicate erosion and deposi-
tion of soil transported by splash or suspension from the exposed interspaces. Due to their diffuse basal characteristics, stoloniferous grasses or annuals generally do not have the capacity to catch and hold sediment Consequently, while runoff is typically related to the amount of cover, interrill erosion is more strongly related to vegetation type.
The major pollutant from grazed watersheds is sediment. Livestock
grazing generally does not significantly increase bacteria contamination
of runoff as long as grazing intensity is light to moderate and animals
are kept away from riparian zones (Doran and Linn 1979). If livestock grazing
is heavy and not restricted from riparian zones, fecal coliform indicator
bacteria counts mav increase 10-fold above back- ground counts (Tiedemann
et al. 1987). Fecal coliform bacteria is not pathologically harmful itself,
but it is a useful indicator of the likely presence and concentration of
associated harmful bacteria which do constitute a threat to public health.
Because of the close association of runoff with water erosion, any practice that reduces runoff reduces erosion. Therefore practices that increase infiltration rate and surface detention reduce sediment loss. Changes in species composition associated with grazing therefore effect the amount of interrill erosion. Differences in interrill erosion within a vegetation type are related to the amount of cover and litter accumulation associated with contrasting grazing systems (Figure 6.9). Increased trampling intensity reduces soil structure, making soil particles more susceptible to detachment (Table 6.2).
Reduction of cover and standing crop also exposes the soil more directly to the erosive force of wind. If the grazing intensity causes a reduction in cover, or if trampling disaggregates soil particles, wind erosion will increase.
Satterlund (1972) developed the critical point deterioration concept to explain what happens if erosion is not controlled (Figure 6.10). Beyond the critical point, erosion continues at an accelerated rate which cannot be reversed by the natural processes of revegetation and soil stabilization, even if the initial cause of disturbance (e.g., intensive grazing) is corrected. As runoff and soil erosion increase, less water and fewer nutrients are retained to support the level of plant growth needed for surface soil protection. The plants that do grow are scarcer and hence receive greater focus of continued grazing pressure. The microclimate deteriorates leading to less microorganism activity needed for soil aggregate formation and a harsher environment for germination. These factors contribute to even more soil exposed to raindrop impact, further accelerating surface runoff and erosion. This spiraling pattern of deterioration (Figure 6.11) eventually results in decertification. Desertification is the diminution or destruction of the biological potential of land (Dregne 1987). A converse pattern occurs during recovery if the critical point has not been passed. Management intervention on most grazing lands is usually not economically viable once the critical point has been passed. Therefore, it is vital that management of the resource be sensitive to the hydrologic relationships of the site so that the decertification process s never initiated.
Land use practices and the concern for soil and water conservation vary
greatly depending upon whether the management time horizon is focused on
short-term economic gain or long-term sustained yield. Traditional economic
theory asserts management and conservation of the soil resource will occur
when the discount rate for future profit potential exceeds current returns.
In overpopulated regions of the world the need for immediate survival outweighs
consideration of future productivity. Consequently, people in these areas
may indeed understand that the resource is Deteriorating but are not in
a position to change their use pattern since their immediate concern is
to produce enough to stay alive. In developed countries the factors affecting
discount rate decisions are usually not so dire; however, land use practices
which cause accelerated erosion may be prompted by the need to generate
high yield to remain financially solvent (see Chapter
9). Loss of soil fertility and water-holding capacity associated with
erosion also reduces the magnitude of future benefits that could be gained
through introduction of improved livestock breeds or better husbandry techniques.
The loss of the soil resource is potentially more serious than consumptive use of other commodities because once accelerated erosion is allowed to develop beyond he critical point, resource deterioration continues regardless of whether the destructive land use continues. Soil loss cannot be effectively restored through management since topsoil formation occurs at the rate of 1 in. formed every 300-1000 years. Soil accumulated in the past because conditions for its presence were in equilibrium with its environment. If grazing disturbance is not too great, stability will return and Accelerated erosion gradually cease. However, if disturbance has been too disruptive he equilibrium is lost and will not return on its own, resulting in a "death spiral" towards decertification. Once this equilibrium point has been exceeded, the cost of restoring equilibrium is far greater than the initial energy which caused the original Destabilization. For this reason, the first priority of grazing land management should )e to maintain the soil resource and hydrologic condition of the site. These efforts form the foundation of sustained long-term productivity.
The hydrologic condition of rangelands is the result of complex interrelationships
of soil, vegetation, topography, and climate. Maintenance or improvement
of Hydrologic condition and soil retention are critical determinants of
long-term sustained production. It is possible to interpret and anticipate
livestock affects on Hydrology by understanding how livestock use of grazing
lands impacts the soil and Vegetation parameters.
The rangeland water balance is determined by the characteristics of incoming Precipitation and by the outflow associated with runoff, evapotranspiration, and deep drainage. The amount of water available for forage production is dependent on the Characteristics of the components affecting the water balance. The greatest potential impact that grazing has on rangeland hydrology is the effects on parameters which Determine infiltration rate (broadly: soil structure, amount of cover, and type of cover). Runoff not only represents a loss of water, it is also an erosive force that transports topsoil and nutrients from the site. Water and soil leaving the site are resources unavailable for future forage production, which translates into reduced livestock and wildlife production. It is therefore evident that the long-term success of any grazing management strategy is dependent upon that strategy's ability to maintain or improve the hydrologic condition and soils of the site.
Figure 6.1 The water cycle showing major processes and pathways of water movement through a watershed.
Figure 6.2 Conceptual architecture of a soil aggregate and the changes in soil aggregate structure caused by trampling under wet and dry conditions.
Figure 6.3 Mean infiltration rate for three vegetation types, Edwards Plateau, Texas.
Figure 6.4 Water budgets and amount of interrill erosion, runoff, and interception from oak, bunchgrass, sodgrass, and bare ground dominated areas, Edwards Plateau, Texas.
Figure 6.5 Mean infiltration rates for four grazing treatments six years after they were initiated in the Edwards Plateau, Texas.
Figure 6.6 Regions of the world most susceptible to geologic wind and water erosion.
Figure 6.7 Relationship between water erosion and wind erosion with mean annual presipitation.
Figure 6.8 Visual evidence indicating accelerated erosion.
Figure 6.9 Mean interrill erosion for different grazing treatments on the Edwards Plateau, Texas
Figure 6.10 Relationship of deterioration of a site by erosion to the rate of revegetation.
Figure 6.11 Conceptual pathway
of changes in the hydrologic condition of a site.
Table 6.1 Broad relationship between infiltration rate, grazing intensity, and various soil and vegetation attributes.
Table 6.2 Mean infiltration rate and interrill erosion in relation to trampling intensity and water content at the time of trampling on the Edwards Plateau, Texas.