SOME BASIC PRINCIPLES

Numerous plot studies carried out in the field under a wide range of agricultural conditions show that vegetation cover can control soil erosion by water on sloping land. Typical data are shown in Table 5.1 from research carried out at Séfa, Senegal (Roose, 1967). Erosion and runoff are lowest under protected forest cover and highest on bare ground with about three orders

Table 5.1 Relationship between mean annual soil erosion on slopes and vegetation cover (after Roose, 1967) Annual runoff (% of rainfall) Annual soil loss (t/ha)

Vegetation cover Replications Mean Range Mean Range

Protected forest 7 0.67 0.10–1.18 0.08 0.02–0.22

Burned forest 9 0.90 0.30–1.52 0.18 0.02–0.51

Groundnut 24 22.8 8.1–42.5 6.89 2.91–16.30

Cotton 3 28.0 0.9–42.7 7.75 0.47–18.52

Sorghum 9 20.6 11.2–35.0 7.82 1.19–22.71

Maize 1 30.9 10.34

Millet 2 34.7 26.4–39.7 10.34 8.10–12.57

Bare soil 7 40.1 22.3–53.1 25.13 6.93–54.48

of magnitude difference between the erosion rates for the two conditions. Crops, such as groundnuts, cotton, sorghum, maize and millet, result in about 100 times more erosion than the protected forest but only

Slope Stabilization and Erosion Control: A Bioengineering Approach. Edited by R.P.C.Morgan and R.J.Rickson. Published in 1995 by E & FN Spon, 2–6 Boundary Row, London, SE1 8HN. ISBN 0 419 15630 5.

about one-third to one-half of the erosion recorded from bare ground. One reason for these differences in erosion is that the vegetation is interacting with the erosion processes in two ways: one relates to the effect of the overall vegetation cover and the other to its spatial configuration or layout. ‘Vegetation cover’ is used here as a broad term to encompass the combined effects of canopy, plant stems and roots.

5.2.1

ROLE OF VEGETATION COVER

The data in Table 5.1 show a very strong relationship between the mean annual rate of erosion and the mean annual runoff. This implies that the major role played by vegetation in controlling water erosion is hydrological. As indicated in Chapter 2, this role is exerted largely through the infiltration process. The infiltration rates of vegetated soils are higher than those of bare soils because the growth of the root network and the presence of soil fauna open up the pore system; also the return of organic material to the soil contributes to the stability of the soil aggregates and therefore to the stability of the pores which are less likely to close as the soil wets up. In addition, vegetation leads to lower antecedent moisture contents because of the removal of water from the soil through evapotranspiration, and to changes in the effective rainfall intensity at the soil surface because of interception of rainfall by the canopy. The combination of the ability to take in water at a higher rate and in greater quantity means that vegetated soils are less likely to generate runoff.

A simple way of evaluating the effect of vegetation on runoff volume is to use the runoff coefficients from Cook’s method (United States Soil Conservation Service, 1953) and Hudson’s (1981) method of estimating runoff for small catchments, relate them to a percentage vegetation cover and then express the coefficients relative to the coefficient value for bare soil to obtain a set of soil loss ratios (Table 5.2). These Figure 5.1 Hydrological, erosion and nutrient systems related to the engineering role of vegetation.

ratios represent the soil loss under a given vegetation cover as a proportion of that under bare soil. If, for simplicity, it is assumed that soil loss varies directly with the volume of runoff— in practice it varies with runoff raised by a power of between 0.67 and 1.8 (section 2.3.2)— these ratios show an exponential decrease with increasing percentage cover (Figure 5.2; Rickson and Morgan, 1988). In reality, there will be some degree of variability from this relationship because of vegetation effects other than cover; for example, stem density, and differences in the

Table 5.2 Derivation of soil loss ratios expressing the effect of vegetation cover on the volume of runoff Cover type Estimated % cover Catchment characteristic^a Soil loss ratio (CCbare/

CCveg)^b

Bare soil 0 25 1.0

Cultivated land with poor

cover 10 20 0.8

Cultivated land with fair cover 25 15 0.5

Cultivated land with good

cover, scrub or grass 50 10 0.4

Forest, grass or scrub 90 5 0.2

aCatchment characteristic (CC) values based on Cook’s Method (United States Soil Conservation Service, 1953) and Hudson’s (1981) method.

bCC bare=catchment characteristic value for bare soil; CCveg=catchment characteristic value for vegetated soil.

types and species of the plants making up the vegetation community.

A further way in which vegetation cover controls erosion was demonstrated by Hudson (1981) in a mosquito gauze experiment. Two experimental plots, 1.5 m wide and 27.5 m long, were set up in Zimbabwe on a clay loam soil with a 5% slope. They were kept bare of vegetation by hand-weeding. Both plots were exposed to the same natural rainfall but one plot was covered by a double layer of fine-mesh wire gauze, simulating a dense (90–100%) vegetation cover. As can be seen from Table 5.3, the soil loss from the plot Figure 5.2 Relationship between the soil loss ratio and percentage vegetation cover taking account of runoff volume (after Rickson and Morgan, 1988).

covered by the gauze is less than 1/100th of that from the uncovered plot. This experiment indicates the protective effect of a cover close to the soil surface in breaking up the raindrops so that they reach the soil from

Table 5.3 Soil losses recorded in the mosquito-gauze experiment (after Hudson, 1981) Soil loss (t/ha) Zanchi (1983). Here, the difference in erosion between the two plots was much less with the soil loss from the covered plot being about one-tenth of that from the uncovered plot (Table 5.4). Runoff on the covered plot was about one-third of that on the uncovered plot, showing that, even without a rooting system, a cover close to the soil surface can reduce runoff.

These experiments simulate the protective effect of a low-growing vegetation cover described in Chapter 2. The cover reduces the energy of the rainfall at the soil surface which, in turn, reduces the rate of soil particle detachment by raindrop impact. This means that there is less material available for transport by any runoff that is generated and, also, less infilling of the soil pore spaces by detached fine particles. The soil is thus protected against surface crusting and sealing so that high infiltration rates are maintained.

Whilst the data and experiments cited above demonstrate the effect of vegetation cover, how that effect is brought about is better understood by considering the individual roles played by the plant canopy, stems and roots.

Plant canopy

As shown in Chapter 2, it is important that the vegetation canopy is uniform and either close to or in contact with the soil surface to obtain the maximum protection. In this instance, research with grasses (Lang and McCaffrey, 1984), crop residues (Laflen and Colvin, 1981) and stones (van Asch, 1980) indicates that the soil loss ratio, defined here as relating to soil detachment by raindrop impact, decreases exponentially with increasing percentage canopy cover (Figure 5.3; Rickson and Morgan, 1988). As the height of the cover above the ground increases, the relationship becomes linear and the cover is less effective because of the greater fall height of the leaf drainage. Where leaf drainage occurs with large diameter drops because of the

coalescence of raindrops on the leaves, and the canopy is 1 m or more above the ground, the vegetation no longer affords any protection and soil loss ratios increase with increasing percentage cover. This effect has been modelled theoretically by Styczen and Høgh-Schmidt (1986) from a consideration of the physics of raindrop impact as an inelastic collision between the water drop and the soil surface. It is supported Table 5.4 Soil losses and runoffs recorded in a net-covering experiment (after Zanchi, 1983)

Soil loss (t/ha) Runoff (mm)

Year Covered plot Bare plot Covered plot Bare plot

1978 0.5 6.3 45 98

1979 1.3 66.3 88 392

1980 1.2 48.0 67 302

1981 0.8 19.1 82 509

1982 16.7 83.2 292 615

1983 2.7 35.2 111 216

Six-year totals 23.2 258.1 685 2132

Figure 5.3 Relationship between the soil loss ratio and percentage vegetation cover taking account of the effect of soil detachment by raindrop impact. Asterisks denote relationships allowing for the effect of leaf drainage (after Rickson and Morgan, 1988).

by field measurements of soil detachment rates under maize (Morgan, 1985) and laboratory studies under Brussels sprouts (Noble and Morgan, 1983). Figure 5.4 shows a typical relationship of the soil loss ratios for detachment by raindrop impact as a plant grows. Initially the soil loss ratio is high because little of the ground surface is covered. The ratio then reduces exponentially because the cover is close to the soil surface in the early stages of vegetative growth. As the height of the canopy rises, however, with further growth, the ratio increases in value (Morgan et al., 1986).

Plant stems

In addition to the hydrological and protective roles, vegetation can influence water erosion through its hydraulic effect arising from the roughness that vegetation imparts to flowing water. For typical shallow flows on slopes, it has been shown (Morgan, 1980) that the sediment transport capacity of the runoff varies with Manning’s n raised by the power of −0.15. If it is assumed that n=0.01 for bare soil, the n^−0.15 value for this condition is 1.99. If it is further assumed that the bare soil condition is represented by a soil loss ratio of 1, ratios taking account of the effect of vegetation on flow velocity can be obtained by calculating n^−0.15 values for different values of Manning’s n and expressing them as a ratio of 1.99. The results, plotted in Figure 5.5 (Rickson and Morgan, 1988), show that the ratio decreases rapidly as Manning’s n increases from 0.01 to 0.05 but that further increases in n have little additional effect.

Plant roots

In addition to the effects on infiltration described above, plant roots have a mechanical effect on the soil. By penetrating the soil mass, they reinforce it, bringing about an increase in cohesion and, hence, in soil shear Figure 5.4 Application of the relationship shown in Figure 5.3 to selected crops showing how the soil loss ratio changes with as the vegetation cover increases with plant growth. ●, Brussels sprouts; ▲, potatoes; ■, maize.

strength. Also, a fine root mat close to the soil surface may act like a mulch or low-growing vegetation cover and protect the soil from erosion. Dissmeyer and Foster (1985) propose soil loss ratios to take account of the root effect (Figure 5.6). An increase in the percentage area occupied by fine roots produces an exponential decay in the soil loss ratio. As expected, the effect is much greater for rooting systems which spread laterally close to the soil surface than for systems with a strong vertical development characterized by tap roots.

Figure 5.5 Relationship between the soil loss ratio and Manning’s n (after Rickson and Morgan, 1988).

Figure 5.6 Relationship between the soil loss ratio and the percentage area occupied by fine roots (after Dissmeyer and Foster, 1985).

Effects of vegetation growth

A comparison of the slopes of the lines on Figures 5.2, 5.3, 5.5 and 5.6 reveals how the separate effects of vegetation on erosion change as the vegetation develops. A summary of this comparison is given in Table 5.5 which shows the changes in the soil loss ratios that result from given changes in the percentage vegetation cover (Morgan, 1987). In the early stages of vegetation growth (0–20% cover), the most important effect is through the reduction in soil detachment by raindrop impact. This means that it is extremely important to aim for a good ground cover of vegetation to obtain the maximum effect at this stage. If this is not achieved, increasing the vegetation cover from 0 to 20% will enhance soil particle detachment although at a rate which would be offset by reductions in the soil loss ratio due to lower runoff volumes and greater soil cohesion.

As the vegetation cover increases from 20 to 60%, the effects on soil detachment, runoff volume and soil cohesion are roughly equal, assuming that the vegetation is at ground level and provides a uniform cover which means that tussocky and tufted species must be avoided. With increases in vegetation cover above 60%, the most important effect is through increases in soil cohesion.

Although this analysis supports the view, first expressed above, that vegetation plays an important hydrological role, it also implies that this is not the most important mechanism by which erosion is reduced.

More important is its protective role in reducing soil detachment, as illustrated by the mosquito-gauze experiments, and, for long-term effectiveness, the reinforcement of soil strength by the root network. The hydraulic effect of vegetation in reducing runoff velocity is always subsumed by these other effects.

5.2.2

VEGETATION LAYOUTS

Vegetation can be organized in a layout that will reduce erosion risk on slopes. A vegetation layout aligned across slope, ideally on or close to the contour, will reduce effective slope length and impede or obstruct overland flow due to increased surface roughness. These effects will reduce the accumulation of runoff volume downslope and reduce the flow velocity which in turn will reduce the kinetic energy and, therefore, the capacity of the flow to detach and transport soil particles. Indeed, the reduction in velocity may be sufficient to prevent potentially erosive velocities from being attained. Any small reduction in the erosive power of the flow will have a dramatic impact on reducing the transporting capacity of that flow, as this varies with the fifth power of the velocity (see Chapter 2). Reduced flow velocity results in localized Table 5.5 Changes in the soil loss ratio as a function of changes in percentage vegetation cover (after Morgan, 1987)

Change in soil loss ratio

Change in % cover Change in Manning’s n Detachment Runoff volume Roughness Fine roots

0–20 0.01–0.03 0.43 (0.17) 0.36 0.16 0.22

20–40 0.03–0.05 0.19 (0.18) 0.19 0.06 0.19

40–60 0.05–0.07 0.14 (0.17) 0.12 0.03 0.15

60–80 0.07–0.09 0.11 (0.17) 0.11 0.03 0.14

80–100 0.09–0.11 0.09 (0.18) 0.04 0.02 0.08

Figures in parentheses denote increases in the soil loss ratio as a result of leaf drainage. All other values denote decreases.

advantageous in situations where water is the limiting factor to vegetation growth. In some circumstances, however, water ponded behind vegetation aligned across slope may break through as a concentration of flow, thus increasing the risk of erosion by rilling.

5.3