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The role of vegetation in the retardation of rill erosion
¢ATENA ELSEVIER
Catena 29 (1997) 145-159
The role of vegetation in the retardation of rill erosion Ming-ko Woo a, Guoxiang Fang b, Peter D. diCenzo c a Department of Geography, McMaster University, Hamilton, Ont. L8S 4K1, Canada b Guangzhou Institute of Geography, Yellow Flower Hill, Guangzhou 510070, People's Republic of China ¢ Department of Geography, Erindale College, University of Toronto, Mississauga, Ont. L5L 1C6, Canada
Abstract Large rills with catchment areas measuring tens to hundreds of square metres pose serious soil erosion problems to South China, after deforestation has exposed the deeply weathered granitic slopes to the impacts of tropical rainstorms. Runoff and sediment discharges of rills respond rapidly to rainfall, but compared with bare fills, vegetated rills show considerably lower amounts of flow and sediment yield from unit rainfall input. Rainfall interception is one main reason for reduced runoff, and enhanced infiltration also lowers surface flow from vegetated areas. Potential sediment yield from unit runoff is greatly reduced when the rills are grown with fern, and this provides a far more effective mechanism for soil erosion control than the more temporary measures of installing sediment traps along the rills. Keywords: Vegetation; Rill erosion; Sediment; South China
1. Introduction Misuse of the land in South China has caused severe slope erosion. Sheet erosion is very intense where the vegetation cover is stripped, and active gullying removes sizeable portions of the hills to create scars tens of metres in relief. Intermediate between sheet erosion and gullying is the formation of rills which measure up to several metres deep and across (Fig. 1). Some rills have developed drainage networks with catchment areas in the order of tens or hundreds of square metres. It is these large rills (which approach the dimension of gullies in other erosional environments) that are the concern of the present study. Rill incision occurs when hydraulic tractive forces exceed resistance of the topsoil (Rauws and Govers, 1988; Torri et al., 1987), and is frequently associated with a sharp increase in sediment concentration (Bryan, 1990; Rauws, 1987). Once rills are estab0341-8162/97/$17.00 © 1997 Elsevier Science All rights reserved. PH S 0 3 4 1 - 8 1 6 2 ( 9 6 ) 0 0 0 5 2 - 5
146
M.-k. Woo et al. / Catena 29 (1997) 145-159
Fig. 1, Runoff and sediment collectors at the outlet of fills ER2 (left) and ER3 (fight). The channel of ER2 is shaded by fern and seven lines of sediment traps are emplacedacross ER3. Lower end of the fill catchments are lined by cemented bricks which direct the sediment and flow to hoses that lead to collector tanks. Each upper tank is a flow-dividerand only 1/10 of the overflowis collected by the lower tanks. lished, soil loss from slopes is greatly increased. Morgan (1977) found that the presence of rills in a runoff plot increases sediment yield by about 40 times, indicating that filling is a very important agent of soil erosion. Enlargement of fills may develop into gullies (Govers, 1987; Planchon et al., 1987), or they may be integrated with the permanent drainage system as the upper part of the drainage network (Bryan, 1987). Both rill and interfill erosion have resulted from deforestation in subtropical China. Given judicious re-vegetation of the slopes, soil losses may be retarded (Hudson, 1981; Morgan, 1986; Thomas, 1988). For the weathered granitic hills of the region, sheet wash can be reduced significantly once a sparse fern cover is re-established (Woo and Luk, 1990). Vegetation may play a useful role in reducing rill erosion also. The purpose of this study is to determine the effectiveness of vegetation in controlling the erosion of large rills in South China.
2. Study area and methods Granitic rocks outcrop in many parts of South China, usually producing a hilly landscape with rounded hilltops of moderate gradient (20°-30°). Weathering under a subtropical climate has produced a deep weathering mantle that may reach 50 m. The natural forest on these granitic hills have long been destroyed, and periodic deforestation
147
M.-k. Woo et al. / Catena 29 (1997) 145-159
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Fig. 2. Contour maps showing topography of several study rills.
and stripping of the secondary woodlands, the fern and the grass cover, have exposed many slopes to erosion by heavy rainfall. Rills soon developed, many of them carving deeply into the easily erodible weathered granite, and integrating into ephemeral drainage networks. To study the runoff and sediment discharges from these rills, seven large rills and one incipient rill located in Shenchong basin, Deqing County, western G u a n g d o n g P r o v i n c e (23o10 ' N, 11 l ° 5 0 ' E), w e r e m o n i t o r e d (Fig. 2). T h e s e rills were selected to include the typical range o f c a t c h m e n t sizes and varying degrees o f v e g e t a t i o n c o v e r a g e (Table 1). A V - s h a p e d brick and c e m e n t f l o w collector was constructed at the outlet o f each rill to direct water and s e d i m e n t into a f l o w divider, f r o m which o n e - e i g h t h or one-fifth o f Table 1 Characteristics of experimental rills Rill
Catchment Orientation Channel Cover conditions area (m 2) (°) Slope (°) Catchment
RI
97.8
233
29
R2
106.9
230
23
R5 66.9 R6 84.2 R7 21.1 ER 1 45.1 ER2 89.1 ER3 102.6
201 159 5 229 240 249
28 26 38 26 26 25
Several plantation trees on bare slope Several plantation trees on bare slope Bare slope Sparse pine cover Fern-covered slope Moderate pine cover Moderate pine cover Sparse pine cover on bare slope
Rill Bare Thick fern growth Bare
Heavy fern growth Bare Pine needle covered Fern growth adjacent to bare rill channel Bare, with sediment traps
148
M.-k. Woo et al. / Catena 29 (1997) 145-159
the flow and the overflowed sediments were discharged into a container. At the end of each storm, the water in the container and the sediments in both the flow divider and the container were measured, and these collector devices were then cleaned out for the subsequent storm. Total water and sediment discharges were determined on a storm by storm basis, except for two events in 1987 when detailed sampling was carried out during the storms at two experimental rills. During the storms, water and sediment were also sampled at short time intervals at a nearby bare slope plot (No. SB1) which measured 9.4 m 2 (see Woo and Luk, 1990 for a description of this plot and the sampling method). Several rain gauges were set up in the basin, including a recording gauge, and manual gauges placed underneath a woodlot of predominantly South China pine (Pinus massoniana). A tensiometer was installed on the floor of rill R5, and readings were taken daily whenever possible. The shear strength of slope and rill materials was measured using a Pilcon torvane during a dry period (24 July 1988), a period with moderate rainfall (10 August 1988), and after heavy rainfall (27 May 1989). Measurements were taken during periods of different soil moisture conditions because Luk (1985) demonstrated that the shear strength decreases with increasing soil moisture content. Infiltration experiments were performed at several sites using a double-ring infiltrometer. Erosion pins were set up in the rills and the interrill areas, but the result from only one summer was of limited use and will not be reported here. In 1989, a series of sediment trap-lines was set up perpendicular to the channel of rill ER3 and extending from the interrill area, across the rill floor. Each line consisted of twigs and grass mats up to 0.3-0.5 m high. By mid-summer, the considerable amount of sediments deposited behind these lines rendered these sediment traps ineffective for the remaining study duration.
3. Water and sediment production 3.1. Rain and soil conditions
The rainy season in South China is between late April and early September. Frontal rain dominates the earlier parts of the wet season while typhoon-related rainfall prevails after July. Most rain events are of low to moderate intensities, but there are occasional high intensity events that exceed 20 mm h -1 . The near surface soil moisture responds readily to rainfall. A tensiometer installed at a depth of 30 cm on the floor of rill R1 showed that the soil became saturated after moderate rainfall (Fig. 3), though it is to be noted that the rain-generated runoff in the rill channel further contributed to soil saturation. The soil dried out quickly also, as high evaporation rates were common between rain events. The shear strength at the soil surfaces is responsive to moisture changes. After a dry period, the shear strength of bare soil measured 3 9 . 0 5 _ 9.09 kPa while it was 31.50 ___12.33 kPa under heavily forested soils (24 July 1988). After moderate rain, the corresponding values were 29.00 + 6.06 and 2 7 . 7 8 _ 7.69 kPa (10 August 1988). It appeared that the shear strength decreased with higher moisture content, and the decrease was larger for bare soils.
M.-k. Woo et al./ Catena 29 (1997) 145-159
149
o
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-i 1
40
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80-
60-
o ¢= 0
40
i
ul
v
20
0
,/
I
i
10
20
1
July
10 20 August 1989
Fig. 3. Rainfall and tensiometer readings obtained at 0.3 m depth below the floor of a bare rill RI.
A comparison of the soil strength for the interrill and rill zones during a dry period and after heavy rain is given in Table 2. When dry, the interrill and rill soil strengths were similar, and was close to the value for the bare slope. After heavy rain (125 mm in 5 days), the shear strength dropped considerably, particularly for the rill floors, possibly because of added saturation by the flow in the channels. Our finding agrees with Luk's (1985) experiment which showed that the shear strength decreases as soil moisture increases. There was a hint of higher shear values in the vegetated rills, possibly due to the presence of fern roots, but the difference in strength with bare rill floors is not significant statistically (t-value = 1.0 at 19 degrees of freedom). Shear strength measurements of freshly deposited sediments trapped in rill ER3 yielded a mean value of 3.0 kPa which was significantly lower than the mean of 14.14 kPa for the floor (Table 2). These represent materials that may have been deposited at the tail end of a runoff event.
Table 2 Shear strength of surface soil at rill and interrill zones Location
lnterrill
Rill
Remarks
Dry period (24 July 1988) RI
34.35 ± 9.1 n=ll
32.00 + 11.65 n=ll
Bare rill
Wet period (27 May 1989) R1/R5/ER3
24.60 ± 8.18 n = 35
14.14 + 4.29 n = 14
Bare rills
R2/R6
24.36 5= 11.07 n=14
19.17 + 5.81 n=6
Fern in rill floors
Values are in kPa, measured by Pilcon Torvane; n is number of samples.
M.-k. Woo et al. / Catena 29 (1997) 145-159
150
Their extremely low shear strength (cobesionless) allows them to be readily flushed out of the rill when the next flow event occurs.
3.2. Storm runoff and sediment yield Runoff and sediment production were measured at three locations during a storm on 11 August 1987. One site was a bare slope surface plot (SB1) with a surface area of 9.4 m 2. The others were rill catchments, with R5 being a bare rill (catchment over 66.9 m 2) and R6 having a fern-covered fill floor (catchment area 84.2 m2). Fig. 4 summarizes the rainfall at 10 min intervals, and instantaneous runoff and sediment load for the three sites plotted on logarithmic scale. Although the time of hydrograph rise was not observed, the peak runoff and sediment yield response to peak rainfall could not have been delayed for more than 15 min. The increases in both water and sediment discharges were extremely rapid (in the order of minutes) as the rainfall intensity increased; and as rainfall intensity decreased, both water and sediment deliveries dropped just as abruptly at all three sites. For the bare surface plot, peak sediment concentration preceded peak runoff, but due to coarse sampling intervals (about 10 rain), this phenomenon of sediment flushing was not observed at the rill sites. Given the presence of loose materials in the rill floor and their extreme low shear strength, it is likely that their rapid entrainment and transport occurred as the hydrograph rose.
10
1.0
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. 0.01
0.1
~ 0.01
r
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| E
J
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,
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i
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,
1500
,
=
1530
,
J
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1600
i
i
i
1630 11A~. 19~7
"t430
1500
1600
1630 11 Aug. 1907
Fig. 4. Rainfall at 10 rain intervals, runoff and sediment load from a bare surface plot, a bare rill and a vegetated fib during a rainstorm, 11 August 1987.
M.-k. Woo et aL / Catena 29 (1997) 145-159
151
A comparison of the bare surface plot with the bare rill data shows that their unit runoff responses were similar but the sediment concentration was much higher in the rill (Fig. 4). Since both water and sediment discharges changed quickly during the storm, a slight difference in the time of sampling may give rise to discrepancies in the peak concentration values. Such immediate responses were recorded in rainfall simulation experiments carded out on the slope plots (measuring 10 m 2) with varying percentages of fern cover (Woo and Luk, 1990). Even under a 100% fern coverage, runoff started in less than 5 min after rain began. Compared with the rill with bare floor (R5), the rill where the floor was overgrown with fern (R6) showed a reduction in runoff. Sediment concentration in the runoff from R6 was one order of magnitude lower than R5, and dropped to zero about 30 min after the flow began, possibly indicating that the vegetation was able to trap and retain the transported sediments when the rain declined. For all three sets of data, the response of water and sediment yield to rainfall was flashy and short-lived. Similarity between the bare slope and the bare rill, but a divergence between the bare and the vegetated rills suggests reduction of water and sediment yields due to the presence of vegetation growth in the rill floor. Data from this storm event and results from Slattery and Bryan (1992) indicate that with fluctuating rainfall intensities and intermittent storm durations, it is unlikely that equilibrium runoff and sediment discharge conditions are attained. In the discussion to follow, only total sediment yield on a per storm basis will be reported. 3.3. Vegetation and throughfall
Rainfall interception by vegetation reduces the water input to the slopes. Average throughfall obtained by rain gauges placed under a pine and fern canopy was compared with rainfall in the open area. Based on six storms with rainfall in the open ranging from under 20 nun to over 80 mm, a linear relationship was obtained between rainfall ( r ) and throughfall (rt): rt=bo +blr
(1)
The value of b o and b 1 are - 6.0 and 0.867, with a standard error of 5.5 mm and a correlation coefficient of 0.97. As most storms being studied tended to be of moderate to high intensities, Eq. (1) was simplified by taking a regression intercept of zero, so that: r t = br
(2)
and the empirical value of b for the study area was 0.75. In the following discussion, throughfall will be approximated by Eq. (2), as a fraction of rainfall. 3.4. Runoff from rainfall
Runoff from a single storm is related to storm rainfall through the water balance: q = r - e -f
(3)
where q is runoff, r is rainfall, e is evaporation and f if infiltration, all of which are expressed in nun per unit area. Due to the brevity of most storm runoff periods, the
M.-k. Woo et al. / Catena 29 (1997) 145-159
152
Table 3 Empirical relationships between rainfall (r) and runoff (q), through the equation q = cr; and between runoff and sediment discharge (qs), through the equation qs = k(s.q )n~ with s being channel slope Rill
c
k
m
R1 R2 R5
0.53 (0.91) 0.05 (0.84) 0.55 (0.77)
1.7 1.2 1.7
2.5 1.0 2.5
R6 R7 ER1 ER2 ER3 ER3 (May to 2 July) ER3 (after mid-July)
0.05 (0.64) 0,43 (0~74) 0.32 (0.86) 0.13 (0,77) 0.42 (0.89)
0.3 6.0 1.0 2.5
1.0 1.4 1.0 1.7
1.2 1.5
0.9 1.3
r 2 values are given in brackets.
evaporative loss term is mainly attributable to rainwater interception by the vegetation. For a rill catchment without vegetation, Eq. (3) may be simplified to: q = r -f
(ha)
while for a vegetated rill catchment: q = r t -f'
= br -f'
(4b)
where f ' is infiltration into vegetated soils and f is infiltration into bare soils. It is also postulated that runoff is proportional to rainfall, through an empirical relationship expressed by the rational formula (Gray, 1970). q = cr
(5)
This approach is readily amenable to quantification through regression of rainfall and runoff from various rill catchments. Table 3 lists the empirically obtained c-values for different experimental rill catchments. Several groups of rills are identified. The bare catchments of R1 and R5 have high values of c = 0.54. Catchments with pine-covered slopes are bare rill (ER3) or pine-needle covered channels (ER1), have lower c-values. The lowest c values of about 0.05 are obtained from vegetation catchments and with thick fern growth in the rills (R2 and R6). These rills experience rainfall losses through interception by the pine trees and then by the fern in the channels. Higher c-values for the bare rill catchments than for the vegetated rill catchments can be attributed to the rainfall interception effect. Take the relationship for bare catchment of q = 0.54 r, and compare it with that for a bare rill in a pine-clad catchment (ER3) where q = 0.42 r. Throughfall received at the ground surface of ER3 is r t = 0.75 r. If the runoff response of a vegetated catchment to throughfall is similar to the runoff response of a bare catchment to rainfall, one expects q t = 0.54 r t = 0.54 (0.75 r) = c r r Here, the subscript t denotes vegetated catchment values. The computed value of c t for ER3 is 0.41 which is comparable with the field result of 0.42 (Table 3). Thus, interception losses account for the difference in c-coefficients between the vegetated and the bare catchments. The case of fern growth in the rill channels is discussed below.
M.-k. Woo et al./ Catena 29 (1997) 145-159
153
0.25
0,20 Rill floor with fern 0.15 are, interrill area C o
~
~
Seddimentstrapped on
~0.10
IIIfloor
_e 0.05
0
I
[
1000
2000
3000
Time (s)
Fig. 5. Infiltrationat severalplots in the fill and interrillzones. 3.5. Infiltration in fern-covered rills The previous example considers the infiltration rates to be similar between bare and vegetated interrill areas. Where the rill channels are thickly grown with fern, the root system will significantly alter the rate of infiltration, as is demonstrated by double-ring infiltrometer experiments made in a fern-clad rill floor and in sediments trapped in the rill bed (Fig. 5). The values thus obtained can only be regarded as potential infiltration rates under saturated conditions, but it is clear that infiltration in the fern plot was about 1.8 times that of the bare sediment plot. The enhancement of infiltration by fern-growth may be treated theoretically as follows. For a rill catchment of Area A, with a fern-clad segment occupying Area a, the bare area will be (A - a). For a given storm, the unit area runoff produced by the bare zone will be given by Eq. (4a) (i.e. q = r - f ) . From Eqs. (4) and (5), infiltration into the bare soil is: [=r-qo=qo/co-qo=qo(1/Co
- l)
(6)
The subscript o is introduced to denote the bare plot. From the fern-covered rill-strip, unit area runoff (q') is: q'= br - f '
(7a)
Here, interception is involved so that throughfall (as given by Eq. (2)) is incorporated; and f ' is infiltration into the fern zone. From Eq. (5), r = q,,/co, and Eq. (7) may be written as q ' = b(qo/Co) - f '
(7b)
M.-k. Woo et al. / Catena 29 (1997) 145-159
154
Taking the catchment as a whole, unit area runoff (qB) may be obtained as the areaUy weighted runoff from the bare and the fern-covered segments: qB = [aq' + ( A - a ) q o ] / A or
(8)
q'= [ AqB- ( A-a)qo]/a
Equating Eqs. (7b) and (8): q' = bqo/C o - f ' = [ aqB - ( a - a ) q o ] / a
From this: (9)
f ' = qo[ b / c o + ( a - a) / a ] - ( a / a ) q 8
Dividing Eq. (9) by Eq. (6), we obtain the ratio: f ' / f = [ b / c o + ( A - a) / a - A q B / a q o ] / ( 1 / C o
- 1)
(10)
This expresses the rate of infiltration enhancement resulting from the growth of fern in the catchment. To obtain numerical estimates of this ratio, consider the situation where the entire catchment is overgrown with fern. Then: lim(f'/f)
a--*A
= (b/co)/(1/c
o - 1)
(11)
because qB/qo is negligibly small, as is indicated by the empirical evidence for where there is significant fern growth in the rills (e.g. the c-values for R2 and R6 in Table 3). Taking b = 0.75 for Eq. (2), and a mean c o value of 0.54 for bare catchments such as R1 and R5 (Table 3), Eq. (11) yields f ' / f = 1.6. This ratio is close to the value of 1.8 obtained by the infiltrometer measurements. Thus, both theoretical and experimental results suggest that for the study area, the growth of fern will significantly increase the infiltration rate. This, together with interception, accounts for the low runoff responses to rainfall where the rills are overgrown with fern.
4. Vegetation and sediment yield 4.1. Theory
Sediment production in rills depends on the tractive force exerted by the fluid (Rauws and Govers, 1988; Toni et al., 1987), and the shear strength of the soil material (Rose et al., 1990). The tractive force, as indicated by the shear stress exerted by runoff on the sides and the floor of the rill, comprises a drag and a lift component (Savat and DePloey, 1982). The magnitude of runoff is its main controlling variable. The shear strength of the materials depends on the degree of saturation, and more importantly, on the types of material. Field measurements at the study site showed that the shear strength of materials deposited on the rill channel is an order of magnitude lower than that of the weathered granite which forms the sides and the bed of the rills. Materials deposited at
M.-k. Woo et al. / Catena 29 (1997) 145-159
155
the end of previous flow events are therefore much easier to be entrained. Rose et al. (1990) noted that sediment concentration in rill flow typically fluctuates between an upper (transport) limit and a lower (source) limit determined by the strength characteristics of the materials entrained. The upper limit is set by the tractive force of the flow, and its ability to entrain the previously dislodged materials. The lower limit is set by the capability of the flow to detach particles from the original soil matrix which forms the walls and the bed of the rill. Under laboratory experiment conditions, rill erosion rate is dependent on shear stress r (e.g. Torri et al., 1987): (12)
r = pgRs
where p is density of water, g is acceleration due to gravity, s is the channel slope, and R is the hydraulic radius. For rills where flow depth (d) is shallow relative to width (w), R may be approximated by d which is related to discharge (Q) through the following: R = d = O/wu
(13)
with u being the flow velocity, u can be determined if the Darcy-Weisbach friction factor ( i f ) is known: (14)
u = ( 8 g R s / f f ) 1/2
Combining Eqs. (13) and (14) or: d = (Qeff/8gsw2)l/3
or: r = p( gsQ/w)Z/3(ff/8)'/3
=/3(sQ/w)Z/3
(16)
with /3 being a constant. For cohesionless sediments such as those often used in laboratory experiments, the relationship between sediment discharge (qs) and shear stress may take the form of DuBoys' equation: q,=x(z-
z,.) y
(17)
where x and y are empirical coefficients (e.g. the data from Fig. 1 in T o m et al., 1987, may be expressed in this form) and for cohesionless materials r,, << z where ~', is the critical shear stress. Substituting (16) into (17) yields: q, = x ( / 3 s Q / w ) (y+ 2/3)
(18)
In the following sections, the coefficients will be estimated from the field data after converting discharge into runoff per unit catchment area (q = Q / A where A is the catchment area). If channel width is taken to be invariant, the empirical relationship will be: qs = k ( s q ) m
where k and m are to be estimated empirically.
(19)
156
M.-k. Woo et aL / Catena 29 (1997) 145-159
Under field conditions, the materials entrained during a storm is far from homogeneous, and often include loose particles and aggregates deposited by the afterflow of the previous storm, materials delivered from overland flow from the interrill areas, and cohesive weathered granite detached from the rill walls and the rill bed. Hence, in contrast to laboratory experiments where only incohesive materials are involved, one expects the field data to show considerable scatter in the sediment-runoff relationship. It is currently not possible to estimate sediment detachment and transport rates for rills at the field site. However, it is reasonable to consider Eq. (19) as an expression of the potential rate if all the materials entrained are of low shear strength. This then allows an estimation of the upper enveloping curve for maximum sediment yield under a given discharge. 4.2. Vegetation effects
Vegetation growth offers protection against rill erosion. This is achieved by (1) reducing runoff through enhanced infiltration as previously discussed; (2) the binding
10,000 BareRIIIs, R1 1,000
~ < ~
+ R5 s Slope
~,k~-/_/ ~'/
+
100 / lO 'm Cm
.o >"
+
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100
1
E
.,.~ .o
o
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0.01 ~
S Slope
O,~
t ' / ,
*
0.001 . . . . . . . . . . . . . . 0.01 0.1 1 Rill Runoff (q in mm)
10
Fig. 6. Sedimentyield and runofffrom various storms, and the estimatedpotentialsedimentyields (straight lines on the logarithmicplots) for bare rills (R1, R5) and vegetatedrills (R2, R6).
M.-k. Woo et al. / Catena 29 (1997) 145-159
157
effects of roots and of organic matter supplied by microbial decomposition (Smith and Elliott, 1990), thus increasing soil resistance to sediment entrainment (or ~'c in Eq. (17) is increased); and (3) reducing flow velocity (therefore sediment detachment and transport capabilities) as vegetation offers greater resistance to flow (Gilley et al., 1986). Then, for a given discharge rate, sediment yield from a vegetated rill is expected to be less than that for a bare rill. Using field data from Shenchong, the k and m coefficients were estimated as straight lines that envelop the data plotted on logarithmic scales (Fig. 6). 5. Management implications
This study lends theoretical support and empirical evidence to the usefulness of vegetation as an agent for rill erosion control. Vegetation growth in the rill catchment and along the channel helps to reduce runoff and decrease sediment yield through several processes. (1) Rainfall interception: Much of the rain intercepted by vegetation is lost to evaporation, or is at least retained in the canopy so that the ground is buffered from
E
"~ 1,000
._~
w
,~
800
._~ ~"
E
E ~ E ~
6O0
400 200
o 10
E E .E
ao
~
50
n,"
40
70 80 90 100 0
I0
20
30
40
50
60
Rill Runoff (q in mm) Fig. 7. Nomogram showing the relationship between rainfall, runoff and potential sediment yield for rills of 25 ° channel slopes, under different cover conditions.
158
M.-k. Woo et aL / Catena 29 (1997) 145-159
intense raindrop impacts. Where there are layers of vegetation cover, multiple interception will further extend the timing of rainfall input while the amount of evaporative loss also increases at the expense of runoff. (2) Infiltration: The abundance of roots and cracks in the soil created by plant growth enhances infiltration. Pine needles covering the rill and the interrill zone also encourage infiltration (Woo and Luk, 1990) as is the case for ER1. The net effect is a reduction in surface runoff. (3) Resistance to flow: Vegetation growth on the rill floor increases resistance to flow so that sediment entrainment rates are reduced, and some twigs and roots may act as sediment traps. It is important to note that vegetation shading the rill rather than growing within the rill itself has limited effect on erosion control. Such a case is shown by ER2. Field results of this study can be summarized by a nomogram showing the relationship between rainfall, runoff and potential (maximum) sediment yield for rills with channel gradient of 25 ° which is common for the study area (Fig. 7). The nomogram is not meant for prediction usage, but was presented merely to demonstrate that the growth of vegetation in the rill catchment and along the rill channels reduces runoff significantly. The efficacy of vegetation growth in the rill channel as a natural retardant of erosion can be compared with the performance of sediment traps emplaced across the rill catchment. During the early stage after the traps were installed in catchment ER3 (Fig. 1), the traps were effective in reducing sediment discharge so the m-coefficient was 0.9, a value which was close to that of the fern-grown rills such as R2 and R6. After mid-July, the traps were gradually filled and became much less effective in retaining additional sediments produced during subsequent storms. The m-value rose to 1.3. It is surmised that under normal conditions, the traps become full after one or two rain seasons. They will then cease to function and fresh traps have to be installed. In contrast, vegetation growth in rills continues to retard sediment loss for long periods, and this argues in favour of using re-vegetation as the primary means of rill erosion control in South China.
Acknowledgements Research was supported by the International Development Research Centre of Canada, and the Canadian Natural Sciences and Engineering Research Council. We wish to thank Kathy Young and Dan Clark for field assistance, and X.X. Li, Director of Forestry Research Station at Shenchong, for facilitating the field work. The support of S.H. Luk and Q.Y. Yao is gratefully acknowledged. We are deeply appreciative of the referees for their useful comments.
References Bryan, R.B., 1987. Processes and significance of rill development. Catena Suppl., 8: 1-15. Bryan, R.B., 1990. Knick'point evolution in rillwash. Catena Suppl., 17: 111-132.
M.-k, Woo et aL / Catena 29 (1997) 145-159
159
Gilley, J.E., Finkner, S.C., Soomer, R.G. and Mielke, L.N., 1986. Runoff and erosion as affected by corn residue: Part IL Rill and interrill components. Trans. ASAE, 29: 161-164. Govers, G., 1987. Rill development processes. Catena Suppl., 8: 17-34. Gray, D.M., 1970. Handbook on the Principles of Hydrology. Canadian National Committee for I.H.D. National Research Council, Ottawa. Hudson, N.H., 1981. Soil Conservation. Cornell University Press, NY, 324 pp. Luk, S.H., 1985. Effect of antecedent soil moisture content on rainwash erosion. Catena, 12: 129-139. Morgan, R.P.C., 1977. Soil erosion in the United Kingdom: field studies in the Silsoe area, 1973-1975. Nat. Coll. Agr. Eng. Silsoe, Occasional Paper 4. Morgan, R.P.C., 1986. Soil Erosion and Conservation. Longman, Harlow, 198 pp. Planchon, O., Fritsch, E. and Valentin, C., 1987. Rill development in a wet savannah environment. Catena Suppl., 8: 55-70. Rauws, G., 1987. The initiation of fills on plane beds of non-cohesive sediments. Catena Suppl., 8:107-118. Rauws, G. and Govers, G., 1988. Hydraulics and soil mechanical aspects of rill generation on agricultural soils. J. Soil Sci., 39:111-124. Rose, C.W., Hairsine, P.B., Proffltt, A.P.B. and Misra, R.K., 1990. Interpreting the role of soil strength in erosion processes. Catena Suppl., 17: 153-165. Savat, J. and DePloey, J., 1982. Sbeetwash and rill development by surface flow. In: R.B. Bryand and A. Yair (Editors). Badland Geomorphology and Piping. GeoBooks, Norwich, pp. 113-126. Slattery, M.C. and Bryan, R.B., 1992. Hydraulic conditions for fill incision under simulated rainfall: a laboratory experiment. Earth Surface Processes and Landforms, 17: 127-146. Smith, J.L. and Elliott, L.F., 1990. Tillage and residue management effects on soil organic matter dynamics in semiarid regions. In: R.P. Singh, J.F. Parr and B,A. Stewart (Editors). Dryland Agriculture: Strategies for Sustainability, Adv. Soil Sci., 13: 69-88. Thomas, D.B., 1988. Conservation of cropland on steep slopes in eastern Africa. In: W.C. Moldenhauer and N.W. Hudson (Editors). Conservation Farming on Steep Slopes. Soil and Water Conservation Society. World Association of Soil and Water Conservation, Ankeny, IO, pp. 140-149. Torri, D., Sfalanga, M. and Chisci, G., 1987. Threshold conditions for incipient rilling. Catena Suppl., 8: 97-105. Woo, M.K. and Luk, S.H., 1990. Vegetation effects on soil and water losses on weathered granitic hillslopes, South China. Phys. Geogr., 11: 1-16.
Catena 29 (1997) 145-159
The role of vegetation in the retardation of rill erosion Ming-ko Woo a, Guoxiang Fang b, Peter D. diCenzo c a Department of Geography, McMaster University, Hamilton, Ont. L8S 4K1, Canada b Guangzhou Institute of Geography, Yellow Flower Hill, Guangzhou 510070, People's Republic of China ¢ Department of Geography, Erindale College, University of Toronto, Mississauga, Ont. L5L 1C6, Canada
Abstract Large rills with catchment areas measuring tens to hundreds of square metres pose serious soil erosion problems to South China, after deforestation has exposed the deeply weathered granitic slopes to the impacts of tropical rainstorms. Runoff and sediment discharges of rills respond rapidly to rainfall, but compared with bare fills, vegetated rills show considerably lower amounts of flow and sediment yield from unit rainfall input. Rainfall interception is one main reason for reduced runoff, and enhanced infiltration also lowers surface flow from vegetated areas. Potential sediment yield from unit runoff is greatly reduced when the rills are grown with fern, and this provides a far more effective mechanism for soil erosion control than the more temporary measures of installing sediment traps along the rills. Keywords: Vegetation; Rill erosion; Sediment; South China
1. Introduction Misuse of the land in South China has caused severe slope erosion. Sheet erosion is very intense where the vegetation cover is stripped, and active gullying removes sizeable portions of the hills to create scars tens of metres in relief. Intermediate between sheet erosion and gullying is the formation of rills which measure up to several metres deep and across (Fig. 1). Some rills have developed drainage networks with catchment areas in the order of tens or hundreds of square metres. It is these large rills (which approach the dimension of gullies in other erosional environments) that are the concern of the present study. Rill incision occurs when hydraulic tractive forces exceed resistance of the topsoil (Rauws and Govers, 1988; Torri et al., 1987), and is frequently associated with a sharp increase in sediment concentration (Bryan, 1990; Rauws, 1987). Once rills are estab0341-8162/97/$17.00 © 1997 Elsevier Science All rights reserved. PH S 0 3 4 1 - 8 1 6 2 ( 9 6 ) 0 0 0 5 2 - 5
146
M.-k. Woo et al. / Catena 29 (1997) 145-159
Fig. 1, Runoff and sediment collectors at the outlet of fills ER2 (left) and ER3 (fight). The channel of ER2 is shaded by fern and seven lines of sediment traps are emplacedacross ER3. Lower end of the fill catchments are lined by cemented bricks which direct the sediment and flow to hoses that lead to collector tanks. Each upper tank is a flow-dividerand only 1/10 of the overflowis collected by the lower tanks. lished, soil loss from slopes is greatly increased. Morgan (1977) found that the presence of rills in a runoff plot increases sediment yield by about 40 times, indicating that filling is a very important agent of soil erosion. Enlargement of fills may develop into gullies (Govers, 1987; Planchon et al., 1987), or they may be integrated with the permanent drainage system as the upper part of the drainage network (Bryan, 1987). Both rill and interfill erosion have resulted from deforestation in subtropical China. Given judicious re-vegetation of the slopes, soil losses may be retarded (Hudson, 1981; Morgan, 1986; Thomas, 1988). For the weathered granitic hills of the region, sheet wash can be reduced significantly once a sparse fern cover is re-established (Woo and Luk, 1990). Vegetation may play a useful role in reducing rill erosion also. The purpose of this study is to determine the effectiveness of vegetation in controlling the erosion of large rills in South China.
2. Study area and methods Granitic rocks outcrop in many parts of South China, usually producing a hilly landscape with rounded hilltops of moderate gradient (20°-30°). Weathering under a subtropical climate has produced a deep weathering mantle that may reach 50 m. The natural forest on these granitic hills have long been destroyed, and periodic deforestation
147
M.-k. Woo et al. / Catena 29 (1997) 145-159
./
•. . . . . . . .
~.
/~,~
"
../~-_.'~-~." \ \ \ \ . X --~'-
--_~-~ L L ' ~ , "~"
"-
"
/
i
~ - - ~ ~. i
l ~'a:8",' ~
\ f - "
"
~ , ' -
#
i I
.
\
i
..~
. - - - - ~ ,#~~
"-, i "~
~,'
~'~,"i,
'
....
R2 ~
,
1
1
contourl in m ~
#
....
~,,,
R6 -'~
. . . . . . Rill
Irbltnlry datum
0
\
Tensiometer
Fig. 2. Contour maps showing topography of several study rills.
and stripping of the secondary woodlands, the fern and the grass cover, have exposed many slopes to erosion by heavy rainfall. Rills soon developed, many of them carving deeply into the easily erodible weathered granite, and integrating into ephemeral drainage networks. To study the runoff and sediment discharges from these rills, seven large rills and one incipient rill located in Shenchong basin, Deqing County, western G u a n g d o n g P r o v i n c e (23o10 ' N, 11 l ° 5 0 ' E), w e r e m o n i t o r e d (Fig. 2). T h e s e rills were selected to include the typical range o f c a t c h m e n t sizes and varying degrees o f v e g e t a t i o n c o v e r a g e (Table 1). A V - s h a p e d brick and c e m e n t f l o w collector was constructed at the outlet o f each rill to direct water and s e d i m e n t into a f l o w divider, f r o m which o n e - e i g h t h or one-fifth o f Table 1 Characteristics of experimental rills Rill
Catchment Orientation Channel Cover conditions area (m 2) (°) Slope (°) Catchment
RI
97.8
233
29
R2
106.9
230
23
R5 66.9 R6 84.2 R7 21.1 ER 1 45.1 ER2 89.1 ER3 102.6
201 159 5 229 240 249
28 26 38 26 26 25
Several plantation trees on bare slope Several plantation trees on bare slope Bare slope Sparse pine cover Fern-covered slope Moderate pine cover Moderate pine cover Sparse pine cover on bare slope
Rill Bare Thick fern growth Bare
Heavy fern growth Bare Pine needle covered Fern growth adjacent to bare rill channel Bare, with sediment traps
148
M.-k. Woo et al. / Catena 29 (1997) 145-159
the flow and the overflowed sediments were discharged into a container. At the end of each storm, the water in the container and the sediments in both the flow divider and the container were measured, and these collector devices were then cleaned out for the subsequent storm. Total water and sediment discharges were determined on a storm by storm basis, except for two events in 1987 when detailed sampling was carried out during the storms at two experimental rills. During the storms, water and sediment were also sampled at short time intervals at a nearby bare slope plot (No. SB1) which measured 9.4 m 2 (see Woo and Luk, 1990 for a description of this plot and the sampling method). Several rain gauges were set up in the basin, including a recording gauge, and manual gauges placed underneath a woodlot of predominantly South China pine (Pinus massoniana). A tensiometer was installed on the floor of rill R5, and readings were taken daily whenever possible. The shear strength of slope and rill materials was measured using a Pilcon torvane during a dry period (24 July 1988), a period with moderate rainfall (10 August 1988), and after heavy rainfall (27 May 1989). Measurements were taken during periods of different soil moisture conditions because Luk (1985) demonstrated that the shear strength decreases with increasing soil moisture content. Infiltration experiments were performed at several sites using a double-ring infiltrometer. Erosion pins were set up in the rills and the interrill areas, but the result from only one summer was of limited use and will not be reported here. In 1989, a series of sediment trap-lines was set up perpendicular to the channel of rill ER3 and extending from the interrill area, across the rill floor. Each line consisted of twigs and grass mats up to 0.3-0.5 m high. By mid-summer, the considerable amount of sediments deposited behind these lines rendered these sediment traps ineffective for the remaining study duration.
3. Water and sediment production 3.1. Rain and soil conditions
The rainy season in South China is between late April and early September. Frontal rain dominates the earlier parts of the wet season while typhoon-related rainfall prevails after July. Most rain events are of low to moderate intensities, but there are occasional high intensity events that exceed 20 mm h -1 . The near surface soil moisture responds readily to rainfall. A tensiometer installed at a depth of 30 cm on the floor of rill R1 showed that the soil became saturated after moderate rainfall (Fig. 3), though it is to be noted that the rain-generated runoff in the rill channel further contributed to soil saturation. The soil dried out quickly also, as high evaporation rates were common between rain events. The shear strength at the soil surfaces is responsive to moisture changes. After a dry period, the shear strength of bare soil measured 3 9 . 0 5 _ 9.09 kPa while it was 31.50 ___12.33 kPa under heavily forested soils (24 July 1988). After moderate rain, the corresponding values were 29.00 + 6.06 and 2 7 . 7 8 _ 7.69 kPa (10 August 1988). It appeared that the shear strength decreased with higher moisture content, and the decrease was larger for bare soils.
M.-k. Woo et al./ Catena 29 (1997) 145-159
149
o
A
E
g
20' l r - I
-i 1
40
60
A ¢g ,,Q m i-
80-
60-
o ¢= 0
40
i
ul
v
20
0
,/
I
i
10
20
1
July
10 20 August 1989
Fig. 3. Rainfall and tensiometer readings obtained at 0.3 m depth below the floor of a bare rill RI.
A comparison of the soil strength for the interrill and rill zones during a dry period and after heavy rain is given in Table 2. When dry, the interrill and rill soil strengths were similar, and was close to the value for the bare slope. After heavy rain (125 mm in 5 days), the shear strength dropped considerably, particularly for the rill floors, possibly because of added saturation by the flow in the channels. Our finding agrees with Luk's (1985) experiment which showed that the shear strength decreases as soil moisture increases. There was a hint of higher shear values in the vegetated rills, possibly due to the presence of fern roots, but the difference in strength with bare rill floors is not significant statistically (t-value = 1.0 at 19 degrees of freedom). Shear strength measurements of freshly deposited sediments trapped in rill ER3 yielded a mean value of 3.0 kPa which was significantly lower than the mean of 14.14 kPa for the floor (Table 2). These represent materials that may have been deposited at the tail end of a runoff event.
Table 2 Shear strength of surface soil at rill and interrill zones Location
lnterrill
Rill
Remarks
Dry period (24 July 1988) RI
34.35 ± 9.1 n=ll
32.00 + 11.65 n=ll
Bare rill
Wet period (27 May 1989) R1/R5/ER3
24.60 ± 8.18 n = 35
14.14 + 4.29 n = 14
Bare rills
R2/R6
24.36 5= 11.07 n=14
19.17 + 5.81 n=6
Fern in rill floors
Values are in kPa, measured by Pilcon Torvane; n is number of samples.
M.-k. Woo et al. / Catena 29 (1997) 145-159
150
Their extremely low shear strength (cobesionless) allows them to be readily flushed out of the rill when the next flow event occurs.
3.2. Storm runoff and sediment yield Runoff and sediment production were measured at three locations during a storm on 11 August 1987. One site was a bare slope surface plot (SB1) with a surface area of 9.4 m 2. The others were rill catchments, with R5 being a bare rill (catchment over 66.9 m 2) and R6 having a fern-covered fill floor (catchment area 84.2 m2). Fig. 4 summarizes the rainfall at 10 min intervals, and instantaneous runoff and sediment load for the three sites plotted on logarithmic scale. Although the time of hydrograph rise was not observed, the peak runoff and sediment yield response to peak rainfall could not have been delayed for more than 15 min. The increases in both water and sediment discharges were extremely rapid (in the order of minutes) as the rainfall intensity increased; and as rainfall intensity decreased, both water and sediment deliveries dropped just as abruptly at all three sites. For the bare surface plot, peak sediment concentration preceded peak runoff, but due to coarse sampling intervals (about 10 rain), this phenomenon of sediment flushing was not observed at the rill sites. Given the presence of loose materials in the rill floor and their extreme low shear strength, it is likely that their rapid entrainment and transport occurred as the hydrograph rose.
10
1.0
- {7--:~ I
. 0.01
0.1
~ 0.01
r
E
| E
J
J
,
1430
i
J
,
1500
,
=
1530
,
J
I
1600
i
i
i
1630 11A~. 19~7
"t430
1500
1600
1630 11 Aug. 1907
Fig. 4. Rainfall at 10 rain intervals, runoff and sediment load from a bare surface plot, a bare rill and a vegetated fib during a rainstorm, 11 August 1987.
M.-k. Woo et aL / Catena 29 (1997) 145-159
151
A comparison of the bare surface plot with the bare rill data shows that their unit runoff responses were similar but the sediment concentration was much higher in the rill (Fig. 4). Since both water and sediment discharges changed quickly during the storm, a slight difference in the time of sampling may give rise to discrepancies in the peak concentration values. Such immediate responses were recorded in rainfall simulation experiments carded out on the slope plots (measuring 10 m 2) with varying percentages of fern cover (Woo and Luk, 1990). Even under a 100% fern coverage, runoff started in less than 5 min after rain began. Compared with the rill with bare floor (R5), the rill where the floor was overgrown with fern (R6) showed a reduction in runoff. Sediment concentration in the runoff from R6 was one order of magnitude lower than R5, and dropped to zero about 30 min after the flow began, possibly indicating that the vegetation was able to trap and retain the transported sediments when the rain declined. For all three sets of data, the response of water and sediment yield to rainfall was flashy and short-lived. Similarity between the bare slope and the bare rill, but a divergence between the bare and the vegetated rills suggests reduction of water and sediment yields due to the presence of vegetation growth in the rill floor. Data from this storm event and results from Slattery and Bryan (1992) indicate that with fluctuating rainfall intensities and intermittent storm durations, it is unlikely that equilibrium runoff and sediment discharge conditions are attained. In the discussion to follow, only total sediment yield on a per storm basis will be reported. 3.3. Vegetation and throughfall
Rainfall interception by vegetation reduces the water input to the slopes. Average throughfall obtained by rain gauges placed under a pine and fern canopy was compared with rainfall in the open area. Based on six storms with rainfall in the open ranging from under 20 nun to over 80 mm, a linear relationship was obtained between rainfall ( r ) and throughfall (rt): rt=bo +blr
(1)
The value of b o and b 1 are - 6.0 and 0.867, with a standard error of 5.5 mm and a correlation coefficient of 0.97. As most storms being studied tended to be of moderate to high intensities, Eq. (1) was simplified by taking a regression intercept of zero, so that: r t = br
(2)
and the empirical value of b for the study area was 0.75. In the following discussion, throughfall will be approximated by Eq. (2), as a fraction of rainfall. 3.4. Runoff from rainfall
Runoff from a single storm is related to storm rainfall through the water balance: q = r - e -f
(3)
where q is runoff, r is rainfall, e is evaporation and f if infiltration, all of which are expressed in nun per unit area. Due to the brevity of most storm runoff periods, the
M.-k. Woo et al. / Catena 29 (1997) 145-159
152
Table 3 Empirical relationships between rainfall (r) and runoff (q), through the equation q = cr; and between runoff and sediment discharge (qs), through the equation qs = k(s.q )n~ with s being channel slope Rill
c
k
m
R1 R2 R5
0.53 (0.91) 0.05 (0.84) 0.55 (0.77)
1.7 1.2 1.7
2.5 1.0 2.5
R6 R7 ER1 ER2 ER3 ER3 (May to 2 July) ER3 (after mid-July)
0.05 (0.64) 0,43 (0~74) 0.32 (0.86) 0.13 (0,77) 0.42 (0.89)
0.3 6.0 1.0 2.5
1.0 1.4 1.0 1.7
1.2 1.5
0.9 1.3
r 2 values are given in brackets.
evaporative loss term is mainly attributable to rainwater interception by the vegetation. For a rill catchment without vegetation, Eq. (3) may be simplified to: q = r -f
(ha)
while for a vegetated rill catchment: q = r t -f'
= br -f'
(4b)
where f ' is infiltration into vegetated soils and f is infiltration into bare soils. It is also postulated that runoff is proportional to rainfall, through an empirical relationship expressed by the rational formula (Gray, 1970). q = cr
(5)
This approach is readily amenable to quantification through regression of rainfall and runoff from various rill catchments. Table 3 lists the empirically obtained c-values for different experimental rill catchments. Several groups of rills are identified. The bare catchments of R1 and R5 have high values of c = 0.54. Catchments with pine-covered slopes are bare rill (ER3) or pine-needle covered channels (ER1), have lower c-values. The lowest c values of about 0.05 are obtained from vegetation catchments and with thick fern growth in the rills (R2 and R6). These rills experience rainfall losses through interception by the pine trees and then by the fern in the channels. Higher c-values for the bare rill catchments than for the vegetated rill catchments can be attributed to the rainfall interception effect. Take the relationship for bare catchment of q = 0.54 r, and compare it with that for a bare rill in a pine-clad catchment (ER3) where q = 0.42 r. Throughfall received at the ground surface of ER3 is r t = 0.75 r. If the runoff response of a vegetated catchment to throughfall is similar to the runoff response of a bare catchment to rainfall, one expects q t = 0.54 r t = 0.54 (0.75 r) = c r r Here, the subscript t denotes vegetated catchment values. The computed value of c t for ER3 is 0.41 which is comparable with the field result of 0.42 (Table 3). Thus, interception losses account for the difference in c-coefficients between the vegetated and the bare catchments. The case of fern growth in the rill channels is discussed below.
M.-k. Woo et al./ Catena 29 (1997) 145-159
153
0.25
0,20 Rill floor with fern 0.15 are, interrill area C o
~
~
Seddimentstrapped on
~0.10
IIIfloor
_e 0.05
0
I
[
1000
2000
3000
Time (s)
Fig. 5. Infiltrationat severalplots in the fill and interrillzones. 3.5. Infiltration in fern-covered rills The previous example considers the infiltration rates to be similar between bare and vegetated interrill areas. Where the rill channels are thickly grown with fern, the root system will significantly alter the rate of infiltration, as is demonstrated by double-ring infiltrometer experiments made in a fern-clad rill floor and in sediments trapped in the rill bed (Fig. 5). The values thus obtained can only be regarded as potential infiltration rates under saturated conditions, but it is clear that infiltration in the fern plot was about 1.8 times that of the bare sediment plot. The enhancement of infiltration by fern-growth may be treated theoretically as follows. For a rill catchment of Area A, with a fern-clad segment occupying Area a, the bare area will be (A - a). For a given storm, the unit area runoff produced by the bare zone will be given by Eq. (4a) (i.e. q = r - f ) . From Eqs. (4) and (5), infiltration into the bare soil is: [=r-qo=qo/co-qo=qo(1/Co
- l)
(6)
The subscript o is introduced to denote the bare plot. From the fern-covered rill-strip, unit area runoff (q') is: q'= br - f '
(7a)
Here, interception is involved so that throughfall (as given by Eq. (2)) is incorporated; and f ' is infiltration into the fern zone. From Eq. (5), r = q,,/co, and Eq. (7) may be written as q ' = b(qo/Co) - f '
(7b)
M.-k. Woo et al. / Catena 29 (1997) 145-159
154
Taking the catchment as a whole, unit area runoff (qB) may be obtained as the areaUy weighted runoff from the bare and the fern-covered segments: qB = [aq' + ( A - a ) q o ] / A or
(8)
q'= [ AqB- ( A-a)qo]/a
Equating Eqs. (7b) and (8): q' = bqo/C o - f ' = [ aqB - ( a - a ) q o ] / a
From this: (9)
f ' = qo[ b / c o + ( a - a) / a ] - ( a / a ) q 8
Dividing Eq. (9) by Eq. (6), we obtain the ratio: f ' / f = [ b / c o + ( A - a) / a - A q B / a q o ] / ( 1 / C o
- 1)
(10)
This expresses the rate of infiltration enhancement resulting from the growth of fern in the catchment. To obtain numerical estimates of this ratio, consider the situation where the entire catchment is overgrown with fern. Then: lim(f'/f)
a--*A
= (b/co)/(1/c
o - 1)
(11)
because qB/qo is negligibly small, as is indicated by the empirical evidence for where there is significant fern growth in the rills (e.g. the c-values for R2 and R6 in Table 3). Taking b = 0.75 for Eq. (2), and a mean c o value of 0.54 for bare catchments such as R1 and R5 (Table 3), Eq. (11) yields f ' / f = 1.6. This ratio is close to the value of 1.8 obtained by the infiltrometer measurements. Thus, both theoretical and experimental results suggest that for the study area, the growth of fern will significantly increase the infiltration rate. This, together with interception, accounts for the low runoff responses to rainfall where the rills are overgrown with fern.
4. Vegetation and sediment yield 4.1. Theory
Sediment production in rills depends on the tractive force exerted by the fluid (Rauws and Govers, 1988; Toni et al., 1987), and the shear strength of the soil material (Rose et al., 1990). The tractive force, as indicated by the shear stress exerted by runoff on the sides and the floor of the rill, comprises a drag and a lift component (Savat and DePloey, 1982). The magnitude of runoff is its main controlling variable. The shear strength of the materials depends on the degree of saturation, and more importantly, on the types of material. Field measurements at the study site showed that the shear strength of materials deposited on the rill channel is an order of magnitude lower than that of the weathered granite which forms the sides and the bed of the rills. Materials deposited at
M.-k. Woo et al. / Catena 29 (1997) 145-159
155
the end of previous flow events are therefore much easier to be entrained. Rose et al. (1990) noted that sediment concentration in rill flow typically fluctuates between an upper (transport) limit and a lower (source) limit determined by the strength characteristics of the materials entrained. The upper limit is set by the tractive force of the flow, and its ability to entrain the previously dislodged materials. The lower limit is set by the capability of the flow to detach particles from the original soil matrix which forms the walls and the bed of the rill. Under laboratory experiment conditions, rill erosion rate is dependent on shear stress r (e.g. Torri et al., 1987): (12)
r = pgRs
where p is density of water, g is acceleration due to gravity, s is the channel slope, and R is the hydraulic radius. For rills where flow depth (d) is shallow relative to width (w), R may be approximated by d which is related to discharge (Q) through the following: R = d = O/wu
(13)
with u being the flow velocity, u can be determined if the Darcy-Weisbach friction factor ( i f ) is known: (14)
u = ( 8 g R s / f f ) 1/2
Combining Eqs. (13) and (14) or: d = (Qeff/8gsw2)l/3
or: r = p( gsQ/w)Z/3(ff/8)'/3
=/3(sQ/w)Z/3
(16)
with /3 being a constant. For cohesionless sediments such as those often used in laboratory experiments, the relationship between sediment discharge (qs) and shear stress may take the form of DuBoys' equation: q,=x(z-
z,.) y
(17)
where x and y are empirical coefficients (e.g. the data from Fig. 1 in T o m et al., 1987, may be expressed in this form) and for cohesionless materials r,, << z where ~', is the critical shear stress. Substituting (16) into (17) yields: q, = x ( / 3 s Q / w ) (y+ 2/3)
(18)
In the following sections, the coefficients will be estimated from the field data after converting discharge into runoff per unit catchment area (q = Q / A where A is the catchment area). If channel width is taken to be invariant, the empirical relationship will be: qs = k ( s q ) m
where k and m are to be estimated empirically.
(19)
156
M.-k. Woo et aL / Catena 29 (1997) 145-159
Under field conditions, the materials entrained during a storm is far from homogeneous, and often include loose particles and aggregates deposited by the afterflow of the previous storm, materials delivered from overland flow from the interrill areas, and cohesive weathered granite detached from the rill walls and the rill bed. Hence, in contrast to laboratory experiments where only incohesive materials are involved, one expects the field data to show considerable scatter in the sediment-runoff relationship. It is currently not possible to estimate sediment detachment and transport rates for rills at the field site. However, it is reasonable to consider Eq. (19) as an expression of the potential rate if all the materials entrained are of low shear strength. This then allows an estimation of the upper enveloping curve for maximum sediment yield under a given discharge. 4.2. Vegetation effects
Vegetation growth offers protection against rill erosion. This is achieved by (1) reducing runoff through enhanced infiltration as previously discussed; (2) the binding
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M.-k. Woo et al. / Catena 29 (1997) 145-159
157
effects of roots and of organic matter supplied by microbial decomposition (Smith and Elliott, 1990), thus increasing soil resistance to sediment entrainment (or ~'c in Eq. (17) is increased); and (3) reducing flow velocity (therefore sediment detachment and transport capabilities) as vegetation offers greater resistance to flow (Gilley et al., 1986). Then, for a given discharge rate, sediment yield from a vegetated rill is expected to be less than that for a bare rill. Using field data from Shenchong, the k and m coefficients were estimated as straight lines that envelop the data plotted on logarithmic scales (Fig. 6). 5. Management implications
This study lends theoretical support and empirical evidence to the usefulness of vegetation as an agent for rill erosion control. Vegetation growth in the rill catchment and along the channel helps to reduce runoff and decrease sediment yield through several processes. (1) Rainfall interception: Much of the rain intercepted by vegetation is lost to evaporation, or is at least retained in the canopy so that the ground is buffered from
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158
M.-k. Woo et aL / Catena 29 (1997) 145-159
intense raindrop impacts. Where there are layers of vegetation cover, multiple interception will further extend the timing of rainfall input while the amount of evaporative loss also increases at the expense of runoff. (2) Infiltration: The abundance of roots and cracks in the soil created by plant growth enhances infiltration. Pine needles covering the rill and the interrill zone also encourage infiltration (Woo and Luk, 1990) as is the case for ER1. The net effect is a reduction in surface runoff. (3) Resistance to flow: Vegetation growth on the rill floor increases resistance to flow so that sediment entrainment rates are reduced, and some twigs and roots may act as sediment traps. It is important to note that vegetation shading the rill rather than growing within the rill itself has limited effect on erosion control. Such a case is shown by ER2. Field results of this study can be summarized by a nomogram showing the relationship between rainfall, runoff and potential (maximum) sediment yield for rills with channel gradient of 25 ° which is common for the study area (Fig. 7). The nomogram is not meant for prediction usage, but was presented merely to demonstrate that the growth of vegetation in the rill catchment and along the rill channels reduces runoff significantly. The efficacy of vegetation growth in the rill channel as a natural retardant of erosion can be compared with the performance of sediment traps emplaced across the rill catchment. During the early stage after the traps were installed in catchment ER3 (Fig. 1), the traps were effective in reducing sediment discharge so the m-coefficient was 0.9, a value which was close to that of the fern-grown rills such as R2 and R6. After mid-July, the traps were gradually filled and became much less effective in retaining additional sediments produced during subsequent storms. The m-value rose to 1.3. It is surmised that under normal conditions, the traps become full after one or two rain seasons. They will then cease to function and fresh traps have to be installed. In contrast, vegetation growth in rills continues to retard sediment loss for long periods, and this argues in favour of using re-vegetation as the primary means of rill erosion control in South China.
Acknowledgements Research was supported by the International Development Research Centre of Canada, and the Canadian Natural Sciences and Engineering Research Council. We wish to thank Kathy Young and Dan Clark for field assistance, and X.X. Li, Director of Forestry Research Station at Shenchong, for facilitating the field work. The support of S.H. Luk and Q.Y. Yao is gratefully acknowledged. We are deeply appreciative of the referees for their useful comments.
References Bryan, R.B., 1987. Processes and significance of rill development. Catena Suppl., 8: 1-15. Bryan, R.B., 1990. Knick'point evolution in rillwash. Catena Suppl., 17: 111-132.
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Gilley, J.E., Finkner, S.C., Soomer, R.G. and Mielke, L.N., 1986. Runoff and erosion as affected by corn residue: Part IL Rill and interrill components. Trans. ASAE, 29: 161-164. Govers, G., 1987. Rill development processes. Catena Suppl., 8: 17-34. Gray, D.M., 1970. Handbook on the Principles of Hydrology. Canadian National Committee for I.H.D. National Research Council, Ottawa. Hudson, N.H., 1981. Soil Conservation. Cornell University Press, NY, 324 pp. Luk, S.H., 1985. Effect of antecedent soil moisture content on rainwash erosion. Catena, 12: 129-139. Morgan, R.P.C., 1977. Soil erosion in the United Kingdom: field studies in the Silsoe area, 1973-1975. Nat. Coll. Agr. Eng. Silsoe, Occasional Paper 4. Morgan, R.P.C., 1986. Soil Erosion and Conservation. Longman, Harlow, 198 pp. Planchon, O., Fritsch, E. and Valentin, C., 1987. Rill development in a wet savannah environment. Catena Suppl., 8: 55-70. Rauws, G., 1987. The initiation of fills on plane beds of non-cohesive sediments. Catena Suppl., 8:107-118. Rauws, G. and Govers, G., 1988. Hydraulics and soil mechanical aspects of rill generation on agricultural soils. J. Soil Sci., 39:111-124. Rose, C.W., Hairsine, P.B., Proffltt, A.P.B. and Misra, R.K., 1990. Interpreting the role of soil strength in erosion processes. Catena Suppl., 17: 153-165. Savat, J. and DePloey, J., 1982. Sbeetwash and rill development by surface flow. In: R.B. Bryand and A. Yair (Editors). Badland Geomorphology and Piping. GeoBooks, Norwich, pp. 113-126. Slattery, M.C. and Bryan, R.B., 1992. Hydraulic conditions for fill incision under simulated rainfall: a laboratory experiment. Earth Surface Processes and Landforms, 17: 127-146. Smith, J.L. and Elliott, L.F., 1990. Tillage and residue management effects on soil organic matter dynamics in semiarid regions. In: R.P. Singh, J.F. Parr and B,A. Stewart (Editors). Dryland Agriculture: Strategies for Sustainability, Adv. Soil Sci., 13: 69-88. Thomas, D.B., 1988. Conservation of cropland on steep slopes in eastern Africa. In: W.C. Moldenhauer and N.W. Hudson (Editors). Conservation Farming on Steep Slopes. Soil and Water Conservation Society. World Association of Soil and Water Conservation, Ankeny, IO, pp. 140-149. Torri, D., Sfalanga, M. and Chisci, G., 1987. Threshold conditions for incipient rilling. Catena Suppl., 8: 97-105. Woo, M.K. and Luk, S.H., 1990. Vegetation effects on soil and water losses on weathered granitic hillslopes, South China. Phys. Geogr., 11: 1-16.