- Email: [email protected]
Effect of antecedent soil moisture content on rainwash erosion
CATENA
Vol.
12,
129-139
Braunschwcig 1985
EFFECT OF ANTECEDENT SOIL MOISTURE CONTENT ON RAINWASH EROSION Shiu-hung Luk, Toronto SUMMARY The effect of antecedent soil moisture on rainwash erosion was evaluated in a series of field experiments by using 0.84 m" erosion plots and, the modified Toronto simulator which delivered artificial rain at an intensity of 50 mm hr -~ (0.38 J m-~s-~kinetic energy). It was observed that antecedent moisture content is a significant factor contributing to variations in measured rainwash eroson. Extrapolation from the collected data suggests that rainwash differs by 4 to 5 times if the full range ofantecedent moisture content is considered. For the soils tested which are cohesive in nature, the effect of antecedent moisture is not confined to enhanced runoff capacity, but also shear strength reduction.
1.
INTRODUCTION
Results from mini-plot experiments under simulated rainfall indicate that rainwash erosion varies with different samples of the same soil in the laboratory (BRYAN & LUK 1981) and with different plots at the same site (LUK & MORGAN 1981, LUK 1982). Since the experimental results were obtained under the most practicably 'constant' conditions, the observed differences in erosional response were referred to as ~inherent soil loss variability'. It was suggested that this variability can be attributed to soil surface characteristics such as microreliefand microtopography, antecedent soil moisture content, soil shear strength and aggregation characteristics (LUK 1982, 1983). In addition, variabilities which are unexplained may be of a random nature and they are considered to be scale-dependent (BURROUGH 1983). The present study is primarily concerned with the effect of antecedent soil moisture content on rainwash erosion. The relationship between soil moisture content and runoff generation capacities as well as soil strength will also be discussed. The significance of soil moisture in relation to soil erosion has been considered for a long time (e.g. BAVER 1937) but much of the emphasis of the soil moisture l~actorhas been on its effects on infiltration and runoff generation rather than on shear strength and aggregation characteristics (e.g. NEAL 1937, ELLISON 1934, HORTON 1945). Recent work beginning with CHORLEY (1959) has revealed the potential influence of soil strength on erosion. Both cone penetration resistance (PAUL & DE VRIES 1979) and torsional shear resistance (ARMAN et al. 1975) have been considered suitable measurements. Other methods have been tested and results compared (O'SULLIVAN & BALL 1982). The cone penetration resistance was found to be linearly dependent on soil water tension within a limited range of moisture tensions close to saturation (PAUL & DE VRIES 1979, MULLINS & FRASER 1980). Measurements using other methods have produced similar results (UTOMO & DEXTER 1981b, BALL & O'SULLIVAN 1982). Another aspect is the relationship between shear strength and measured erosion. By using single drops, CRUSE & LARSON (1977) and subsequently AL-DURRAH & BRADFORD (1981, 1982) have ISSN 0341 - 8162 Coourioht IQRfi bu G A T F N A V F R I A ~
n - R R R 9 ('~remllnnen. Re~feAI W ~ = r m = n l l
130
LUK
observed significant relationships between soil shear strength and splash detachment in the laboratory. Relationships involving rainwash erosion have yet to be established. For aggregation characteristics, the influence of soil moisture on aggregate formation and aggregate stability has been evaluated (GABRIELS & DE BOODT 1974, HARTMANN & DE BOOT 1975). Recently, UTOMO & DEXTER (198 l a, 1982) have investigated the complex effects of wetting and drying on soil aggregation characteristics. The precise effect of wetting and drying was shown to relate to many factors, including tillage.
o~
E o
-m-
1
O.
Drop diameter
(mm)
Fig. 1: Drop size frequency for simulated rainfall compared with natural rainfall of the same intensity (50 mm hr-l).
2.
PROCEDURES
The modified Toronto pattern rainfall simulator (LUK 1982)was used in this study. The simulated rainfall was sprayed from nozzles (1.19 m m aperture) located on six radial arms which were rotated atop two sections of T.V. antenna towering. Each radial arm is 2.54 m in length; the speed of rotation is 4 rpm which produces an average wetting frequency of 2.5 s; and there are 27 nozzles on each arm. The 'rainfall' was sprayed at a water pressure of 80 kPa on an annular 18 m 2 in area, permitting 4 to 6 erosion plots e~ich 3 × 3 ft (0.84 m 2) to be wet-
SOIL MOISTURE AND RAINWASH EROSION
131
Tab. 1: BASIC SOIL PROPERTIES Soil Properties Sand % (0.063 - 2 mm) Silt % (0.002 - 0.063 ram) Clay % ( < 0.002 mm) Organic matter (%) Water stable aggregates % > 2 mm >0.5 mm Water-stable aggregates % (with sand correction)
-~ 37 51 12
Guelph s ---
5.21 40.3 73.8 > 2 mm 32.7 > 0.5 mm 72.3
0.41
'g 35 49 16
Font s
3.48
--0.21
10.5 7.5
48.1 71.0
13.0 11.2
--
28.2 48.2
-
2: sample mean; s: sample standard deviation
ted. With an apical flail height of 7.16 m above plot surfaces, drop sizes ranging from 0.4 to 5.8 mm, and a rainfall intensity of 50.8 m m hr -~, the simulator achieved a kinetic energy delivery rate of 0.38 J m -2 s -l (Fig. 1), which is practically 100% of the average amount of energy found in natural storms with the same intensity (LAWS & PARSONS 1943). Two soils in southern Ontario: the Font loam and the Guelph silt loam which are both Groy-Brown Luvisols (GILLESP1E et al. 1972, HOFFMANN et al. 1963) were selected for rainfall simulation studies. Selected properties of these soils are listed in Table 1. At each site, the surface which was mostly covered by grass was unilbrmly scalped to a depth of 35-40 mm by using a manual 'sod-cutter'. This helps to eliminate the influence of surface vegetation and most root systems. Four to six 0.84 m 2 plots were then established in the wetted annular by inserting 15 cm high steel boards to a depth ofapproximately 15 m m on the upslope and the two sides of the plot. At the downslope edge, a Gerlach-type trough was installed and securely attached to 15-1itre capacity plastic buckets. In total, 12 sets of data were collected for each of the Font and the Guelph soils. For the Font soil, three adjacent sites each with four plots were tested while for the Guelph, only two sites each with six plots were used. The rainfall simulator was calibrated at the Erindale Campus of the University of Toronto by using 20 7.5-cm gauges. It was found that the areal variation in rainfall amounts produced by the machine has a coefficient of variation of less than 10% of the target rainfall under calm conditions ( < 8 km hr-l). However, when winds > 8 km hr -I were present, it was observed that the wetted annular was disturbed, the finest drops were removed from the area subjected to the simulated rainfall, and the areal uniformity ofrainfall intensity was also distorted. To minimize experimental errors induced by wind interference, the following procedure was followed. Days with high winds were avoided. When light to moderate winds occur ( < 15 km hr-l), wind-screens made of open-weave polyethylene with dimensions of 7.5 x 4 m were erected on the up-wind side of the site shortly before the beginning of the experiments. At the end of the experiments, rainfall data obtained from the four 7.5-cm gauges located at the corners of each plot were examined. Plots which had not received an average amount of rainflall within + 10% of the target (25.4 m m in 30 minutes) were discarded. In addition, a correction factor ( (target rainfall) / (actual rainfiall) ) was applied to the accepted plot data. In this study, various soil characteristics were determined as follows. Gravimetric soil
132
LUK
moisture samples were collected at the two upslope corners of each plot immediately before the beginning of the experiment by coring to a depth of 1.5 cm. Microtopography was determined by using a specially designed microreliefgauge which is described as follows. Seven cross pieces in an aluminium frame were perforated at a regular spacing of 10 cm so that 49 aluminium rods of I cm in diameter and 69 cm in length can be lowered through the perforations onto the soil surface. When properly levelled, the length of the rods extending above the top edge of the frame, as measured by a vernier depth gauge, indicates the relative heights of the 49 sample points on the ground surface. A surface roughness index (R t ) similar to the one used in LUK (1982) was then computed by the following formula:
Rt =
n-
where h a is measured height for each ofthe 49 sample points, h is mean measured height and n is the number of sample points. Resistance of surface soil to torsional shear stress (or torsional shear strength) was determined by using a hand-held Pitcon torvane with a vane diameter of 19 mm and height of 29 ram. Bearing capacity, which is closely related to unconfined compressive strength, was measured by a Vicksburg-type proving ring penetrometer. Soil bulk density was determined by extracting 54-ram cores l - 2.5 cm below the soil surl:ace. in the laboratory, aggregate size distribution was measured by using the standard wet-sieving technique (YODER 1936). Air-dried samples were passed through an 8-ram sieve before wetted by direct immersion and wet-sieved for 30 minutes (amplitude of 15 mm and stroke of 40 per minute). Torsional shear strength was tested in a mechanised torvane with a vane diameter and height of 12.5 mm.
3.
RESULTS
Ten sets of collected data on antecedent moisture, rainwash and runoffwere accepted and they are presented in Figures 2 and 3. The positive relationship observed between rainwash and runoff (Fig. 2) is consistent with previous findings (e.g. LUK & MORGAN 1981 ). There are also clear evidences of a positive relationship between rainwash and antecedent moisture but the scatter of points suggest possible interference by variations of other soil properties. A case in point is plot 2-4 for the Guelph soil where a relatively high rainwash was observed at an antecedent moisture of only 7.2% (Fig. 3). Inspection of the data on surPace soil properties show that this plot has a particularly high bulk density of 1.37 compared to the mean of 1.21 g cm -3. The soil surface properties which were determined tbr each plot are presented in Table 2. As indicated by the C.V. levels, there are considerable variations ofthese properties which probably have affected the measured rainwash at the selected sites. Therelbre, a suitable statistical technique is required to isolate the effects of these properties and to reveal the inlluence of antecedent moisture. Multiple regression techniques were used Ibr this purpose. In the regression analysis, measured rainwash erosion constitutes the dependent wlriable and antecedent moisture along with surface bulk density, surt~ace aggregate size,
SOIL MOISTUREAND RAINWASHEROSION
133
Rainwash. Wr (g) 800
400
!
D
200
Wr = --98 2 8 + 34 60 Fin
10
15
20
150
100
50
0
.0
i
i
I
.5
1_0
1.5
Runoll. Fin (dm 3 )
Fig. 2: Relationship between rainwash and runoff, Guelph silt loam (A) and Font loam (I-q).The regression coefficients are significant at the 0.05 level.
slope steepness and surface roughness are the independent wlriables. Both untranslbrmed and lgl0-transformed rainwash were used in separate computations. The best regression equations for the Guelph and the Font soils are presented in Table 3 (Equations 3.1,3.2, 3.4 and 3.5). it can be seen that significant results were obtained using either the untranstbrmed and the Igl0-transtbrmed rainwash as the dependent wiriable. The R2 values achieved w~ry between 88 and 96% and the coefficients of determination are significant at the 0.05 level. These wdues compare very favourably with results obtained in previous studies (e.g. CAMPBELL 1970, SOONS 1971, LUK 1975) where minwash was measured at the same plot scale but under natural rainfall. For all the equations computed, antecedent moisture was selected as one of the significant independent wlriables (at the 0.05 level), thus confirming its important influence on rainwash erosion. Moreover, other significant independent wlriablcs which pertain to the soil surlhce characteristics were also selected, reflecting that the local variations
134
LUK
Rain~ash (g) 6O0
400
(3
200
O
O D
0
100 2-4
&
50 ak dk
0
I
I
10
20
30
Anlecedent rnoisture (%)
Fig. 3: Relationship between rainwash and antecedent moisture, Guelph silt loam (1) and Font loam (r--I).Plot 2-4 is labelled. The arrow refers to the liquid limit of the Font soil as determined by the vane shear test.
in soil properties are also signilicant. To demonstrate the precise influence of the soil moisture filctor, Equations 3.2 and 3.5 with lgl0-transformed rainwash as the dependent wiriable (Table 3) were selected and residual equations (Eq. 3.3 and 3.6) were computed by substituting the mean of bulk density and water-stable aggregates > 2 mm into Equations 3.2 and 3.5 respectively. The residual equations were then used to estimate rainwash erosion rates under dilTerent antecedent moisture conditions (Table 4). These conditions include the saturation state which occurs during the spring-melt period, and the "wilting point' (or approximately 15 bars moisture tension) which may exist during the summer season. The percent soil moisture used for these estimations is based on field moisture samples collected at different times of the year. For comparison purposes, the ratio of estimated rainwash relating to the saturated state
SOIL MOISTURE AND RAINWASH EROSION Tab. 2:
135
SOIL SURFACE CHARACTERISTICS Guelph
Soil chanlcteristics Antecedent moisture (%)
~ 14.9
s 5.8
Water-stable aggregates (%) > 2 mm >0.5ram
40.3 73.8
10.5 7.5
.
Font
c'v.* 2 0.39 21.6 0.26 0.10
s 3.3
48.1 71.0
c.v. 0.15
13.0 0.27 11.2 0.16
Bulk density (gcm -3)
1.21
0.07
0.06
1.30
0.05 0.04
Surlhce roughness (ram)
2.63
0.66
0.25
3.64
0.51 0.14
Slope angle (°)
4.56
1. I 1
0.24
8.09
1.56 0.19
3,14
0.09
Torsional shear strength (kPa)
33.1
23.7
1.70 0.07
Bearing capacity(kPa) 533.2 100,0 0.19 331.6 17.4 0.05 * Coefficient of variation = ~/s where-ff is sample mean and s is sample standard deviation. Tab. 3:
Equation Number
REGRESSION ANALYSIS RESULTS
Dependent Constant Variable Term
GUELPH SILT LOAM 3. I Wr -- 392.37. 3.2 Ig~oWr -1.636 3.3 Igl0Wr 1.368 FONT LOAM 3.4 Wr 3.5 Igl0Wr 3.6 Igl0Wr
286.979 2.394 1.946
Partial 0d
2.5210 0.0179 0.0179
Regression Ys
Coefficient Gt
353.94 2.485
-- 0.6522
R2
Remarks
0.963 0.888 ys =
13.9576 0.02339 0.02339
-- 5.8742 -- 0.00931
1.209 g c m -3
0.882 0.903 G t = 48.102%
0d: antecedent soil moisture content (%); Ys : soilbulk density (g cm-3); G t : water-stable aggregates > 2 mm (%); Wr: rainwash (g). For equation 3.1,3.2, 3.4 and 3.5, all the partial regression coefficients and coefficients ofdetermination are signilicant at the 0.05 level. Tab. 4:
EXTRAPOLATED RAINWASH EROSION FOR SELECTED MOISTURE CONSTANTS
Soil
Wr at 0dw (g)
Wr at 0ds (g)
Wr(0ds ) Wr(O dw )
Guelph Silt Loam Font Loam
35.29 151.43
149.47 761.90
4.2 5.0
Extrapolation based on Equation 3.3 and 3.6; 0dw, 0ds : soil moisture content near the wilting point and saturation; for Guelph silt loam, 0dw = 10%, 0ds = 45%; for Font loam, 0dw = 10%, 0ds = 40%.
136
LUK
to that relating to the wilting point was computed for each of the two soils tested. This ratio was found to vary between 4 and 5 (Table 4). This result demonstrates that fora selected field site where the soil properties can be assumed to be relatively 'homogeneous', measured rainwash erosion according to the specific experimental conditions described above may differ by as much as five times if the full range ofantecedent moisture conditions is considered. Moreover, if the potential influence of other soil properties is taken into consideration, a somewhat greater range ofmeasured rainwash erosion at this scale of measurement may be expected.
4.
DISCUSSION
It has been established in this study that the antecedent soil moisture content is extremely important in influencing the rate of measured rainwash. In general, rainwash increases exponentially with antecedent moisture content. The following discussion is concerned with the mechanisms by which soil moisture influences rainwash erosion. The commonly considered mechanism is runoff generation. When other influencing factors are held constant, a higher antecedent moisture content leads to a reduced infiltration rate and reduced time lag to occurrence ofoverland flow. Thus, total runoffand rainwash are linear functions of antecedent moisture content. These relationships are apparent in the data collected in this study (Fig. 2 & 3). However, such relationships are complicated by soil crustability and aggregation characteristics. Soil crustability is known to be an index of soil stability (e.g. DE PLOEY & IVlUCHER 1981). Crust formation has been reported to be strongly influenced by the size and stability of surface aggregates (FARRES 1978). Thus, for unstable soils, the timing of crust formation is clearly important in determining runoffand rainwash rates. In the present study, surface crusts were not observed during the field experiments. Crusts were found at the end of some laboratory rainfall simulation tests for samples derived from the cultivated Guelph soil but it is unlikely that they have affected the field rainwash data. The size of soil aggregates is an important soil property influencing measured rainwash, as is found in the experimental results of the Font loam soil (Table 3). Other studies have shown that the amount of measured runoffand rainwash varies with the size distribution of surface aggregates (e.g. LUK 1983). Laboratory data collected by HARTMANN & DE BOODT (1974) demonstrate that aggregate size and aggregate stability vary with the soil moisture content of the previous wetting cycle. Unfortunately, the soil moisture content of the previous wetting cycle was not monitored in the present study, and thus the total effect of soil aggregation cannot be evaluated. Another soil property to be considered is soil shear strength. CRUSE & LARSON (1977), followed by AL-DURRAH & BRADFORD (1981, 1982) have shown that splash detachment as determined by the drop-test method is a function of soil shear strength. DE PLOEY & MUCHER (1981) reported that for a group of Belgian soils, the index C510, or the change in moisture content between 5 and 10 blows on a standard liquid limit curve is a suitable index of soil stability. In this study, initial laboratory tests were conducted on 'undisturbed' field samples by using a mechanised torvane. The results of these tests show a wide scatter which is mostly attributable to variations in soil bulk density and the density of rootlets. To overcome these problems, remoulded samples were used. A constant volume of air-dried soil commensurate with the average field bulk density of the selected soils was compressed in a mechanical corn-
SOIL MOISTURE AND RAINWASH EROSION
137
Torsional shear strength (kPa)
100
10
I
I
I
I
I
0
10
20
30
40
Moislure conlenl
50
(%)
Fig. 4: Relationship between torsional shear strength and moisture content, Guelph silt loam (A) and Font loam (F-I).
pactor. These samples which have a diameter of 54 mm and thickness of 30 mm were saturated overight before they were dried to a wide range of moisture levels and then tested in the mechanised torvane. Results of these tests (Fig. 4) show that for both the Guelph silt loam and the Font loam, torsional shear strength decreases exponentially as soil moisture increases. The rate of reduction in shear strength, however, shows an abrupt increase at 29.5% and 26.4% moisture content for the Guelph and the Font soils respectively. These turning points approximate their liquid limits which are 32.8% and 29.0% respectively as determined by the conventional liquid limit apparatus. Thus, it can be suggested that increases in rainwash erosion in response to higher antecedent moisture may be related to soil shear strength reduction, particularly at the higher moisture content when the liquid limit is equalled or exceeded. A case in point is the test results from the Font loam samples (Fig. 3) where the sharp increases in rainwash are shown to relate to soil samples with an antecedent moisture that is very close to their liquid limit (as determined by the vane shear test). From the above discussion, it is obvious that the influence of soil moisture content on rainwash erosion is complicated by responses in the 'intermediate' factors, particularly r u n -
138
LUK
offcapacity and shear strength which relate to the force and the resistance, respectively, that are involved in the erosion process. The interactive effects concerning the force ofand resistance to erosion are extremely important and must be properly considered if attempts at modelling the erosion process are to be successful. Finally, the quantitative result obtained in this study must be considered in the proper perspective. It is quite apparent that the magnitude of the soil moisture effect varies with many factors. Further experiments should attempt to evaluate this effect for a range of rainfall erosivity, soil type and plot size.
5.
CONCLUSIONS
It has been observed in a series of field rainfiall simulation experiments that along with soil bulk density and aggregate size, the antecedent moisture content is a significant lhctor contributing to variations in measured rainwash erosion. Extrapolated data suggest that rainwash differs by 4 to 5 times ifthe full range of antecedent moisture content is considered. For the soils tested which are cohesive in nature, the impact ofantecedent moisture content is not confined to runoffgeneration, but also shear strength reduction. Future experinaents should be conducted to evaluate the relative importance of these different effects tbr a range of rainfall erosivity, soil type and plot size. ACKNOWLEDGEMENTS The research project o n which this paper is based was financially supported by the Natural Sciences and Engineering Research Council of Canada which is gratefully acknowledged, Access to the research sites was provided by the University of Guelph and the Halton Region Conservation Authority. Research assistants were Messrs. Robert Irvine, Glenn Darras and Christine Roncato. REFERENCES
AL-DURRAH, M.M. & BRADFORD, J.M. (1981): New methods ofstudying soil detachment due to waterdrop impact. Soil Sci. Soc. Am. J. 45, 949-953. AL-DURRAH, M.M. & BRADFORD, J.M. (1982): Parameters for describing soil detachment due to single waterdrop impact. Soil Sci. Soc. Am. J. 46, 836-840. ARMAN, A., POPLIN, J.K. & AHMAD, N. (1975): Study ofthe vane shear. Proc. Conf. on In Situ Measurement of Soil Properties. Am. Soc. Cir. Engin. 1, 93-120. BALL, B.C. & O'SULLIVAN, M.F. (1982): Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments. J. Soil Sci., 33, 609-622. BAVEIL LD. (1937): Rainfall characteristics of Missouri in relation to runoffand erosion. Soil Sci. Soc. Amer. Proc. 2, 533-536. BRYAN, R.B. & LUK, S.H. (198 I): Laboratory experiments on the variation ofsoil erosion under simulated rainfall. Geoderma 26, 245-265. BURROUGH, P.A. (1983): Multiscale sources ofspatial variation in soil. I. The application of fractal concepts to nested levels ofsoil variation. J. Soil Sci., 34, 577-597. CAMPBELL, I.A. (1970): Erosion rates in the Steveville Badlands, Alberta. Can. Geogr. 14, 202-216. CHORLEY, R.J. (1959): The geomorphic significance ofsome Oxford soils. Am. J. Sci. 257, 503-515. CRUSE, R.M. & LARSON, W.E. (1977): Effect ofsoil shear stength on soil detachment due to raindrop impact. Soil Sci. Soc. Am. J. 41,777-781. DE PLOEY, J. & MIJCHER, H.J. (1981): A consistency index and rainwash mechanism on Belgian loamy soils. Earth Surf. Proc. Landforms 6, 319-330. ELLISON, W.D. (1945): Some effects of raindrops and surfiace flow on soil erosion and infiltration. Am. Geophy. Union Trans. 26, 415-429.
SOIL MOISTURE AND RAINWASHEROSION
139
FARRES, P. (1978): The role of time and aggregate size in the crusting process. Earth Surl: Proc. 3, 243-254. GABRI ELS, D. & DE BOODT, M. ( 1974): Relationship between moisture content, aggregate Ibrmation and aggregate stability of a loam soil treated with a soil conditioner in various concentrations. CATENA 2, 23-30. GILLESPIE, J.E., W1CKLUND, R.E. & MILLER, M.H. (1972): The Soils of Halton County. Ontario Soil Survey Report 43, Ontario Dept. Agri. Food, Toronto. HARTMANN, R. & DE BOODT, M. ( 1975): The influence ofthe moistu re content, texture and organic matter on the aggregation ofsandy and loamy soils. Geoderma 1I, 53-62. HOFFMAN, D.W., MATTH EWS, B.C. & W1CKLUN D, R.E. ( 1963): Soil Survey of Wellington County. Ontario Soil Survey Report 35, Ontario Dept. Agri. Food, Toronto. HORTON, R.E. (1945): Erosional development of streams and their drainage basins: hydro-physical approach to quantitative morphology. Geol. Soc. Amer, Bull. 56, 275-370. LAWS, J.O. & PARSONS, D.A. (1943): The relation of rain drop size to intensity. Am. Geophys. Union Trans. 24, 452-459. LUK, S.H. (1975): Soil Erodibilityand Erosion in Part ofthe Bow River Basin, Alberta, Canada. Unpublished Ph.D. Thesis, Univ. Alberta, Edmonton. LUK, S. H. ( 1982): Variability ofrainwash erosion within small sample areas. Thorn, C. (Ed.): Space and Time in Geomorphology. Allen & Unwin, London, 243-268. LUK. S.H. (1983): Effect ofaggregate size and microtopography on rainwash and rainsplash erosion. Z. Geomorph. 27, 283-295. LUK, S.H. & MORGAN, C. (198 I): Spatial variations ofrainwash and runoffwithin apparently homogeneous areas. CATENA 8, 383-402. MULLINS, C.E. & FRASER, A. (1980): Use of the drop-cone penetrometer on undisturbed and remoulded soils at a range ofsoil-water tensions. J. Soil Sci. 31, 25-32. N EAL, J. H. ( 1937): The effect ofthe degree of slope and rainl~allcharacteristicson ru noffand soil erosion. Soil Sci. Soc. Am. Proc. 2, 525-532. O'SULLIVAN, M.F. & BALL, B.C. ( 1982): Acomparison offive instruments for measuring soil strength in cultivated and uncultivated cereal seedbeds. J. Soil Sci., 33, 597-608. PAUL, C. L. & DE VRIES, J. (I 979): Prediction of soil strength from hydrologicand mechanical properties. Can. J, Soil Sci. 59, 301-311. SOONS, J.M. ( 1971): Factors involved in soil erosion in the southern Alps, New Zealand. Z. Geomorp h. 15, 460-470. UTOMO, W.H. & DEXTER, A.R. (198 la): Soil friability. J, Soil Sci. 32, 203-213. UTOMO, W.H. & DEXTER, A.IL (1981b): Age hardening of agricultural top soils. J. Soil Sci. 32, 335-350. UTOMO, W.H. & DEXTER, A.R. (1982): Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil. J. Soil Sci. 33, 623-637. YODER, R.E. (1936): A direct method of aggregate analysis of soils, and a study of the physical nature oferosion losses. J. Am. Soc. Agron. 28, 337-35 I.
Address of author: Dr. Shiu-hung Luk, Department of Geography, University of Toronto, Erindale Campus Mississauga, Ontario, Canada L5L 1C6
Vol.
12,
129-139
Braunschwcig 1985
EFFECT OF ANTECEDENT SOIL MOISTURE CONTENT ON RAINWASH EROSION Shiu-hung Luk, Toronto SUMMARY The effect of antecedent soil moisture on rainwash erosion was evaluated in a series of field experiments by using 0.84 m" erosion plots and, the modified Toronto simulator which delivered artificial rain at an intensity of 50 mm hr -~ (0.38 J m-~s-~kinetic energy). It was observed that antecedent moisture content is a significant factor contributing to variations in measured rainwash eroson. Extrapolation from the collected data suggests that rainwash differs by 4 to 5 times if the full range ofantecedent moisture content is considered. For the soils tested which are cohesive in nature, the effect of antecedent moisture is not confined to enhanced runoff capacity, but also shear strength reduction.
1.
INTRODUCTION
Results from mini-plot experiments under simulated rainfall indicate that rainwash erosion varies with different samples of the same soil in the laboratory (BRYAN & LUK 1981) and with different plots at the same site (LUK & MORGAN 1981, LUK 1982). Since the experimental results were obtained under the most practicably 'constant' conditions, the observed differences in erosional response were referred to as ~inherent soil loss variability'. It was suggested that this variability can be attributed to soil surface characteristics such as microreliefand microtopography, antecedent soil moisture content, soil shear strength and aggregation characteristics (LUK 1982, 1983). In addition, variabilities which are unexplained may be of a random nature and they are considered to be scale-dependent (BURROUGH 1983). The present study is primarily concerned with the effect of antecedent soil moisture content on rainwash erosion. The relationship between soil moisture content and runoff generation capacities as well as soil strength will also be discussed. The significance of soil moisture in relation to soil erosion has been considered for a long time (e.g. BAVER 1937) but much of the emphasis of the soil moisture l~actorhas been on its effects on infiltration and runoff generation rather than on shear strength and aggregation characteristics (e.g. NEAL 1937, ELLISON 1934, HORTON 1945). Recent work beginning with CHORLEY (1959) has revealed the potential influence of soil strength on erosion. Both cone penetration resistance (PAUL & DE VRIES 1979) and torsional shear resistance (ARMAN et al. 1975) have been considered suitable measurements. Other methods have been tested and results compared (O'SULLIVAN & BALL 1982). The cone penetration resistance was found to be linearly dependent on soil water tension within a limited range of moisture tensions close to saturation (PAUL & DE VRIES 1979, MULLINS & FRASER 1980). Measurements using other methods have produced similar results (UTOMO & DEXTER 1981b, BALL & O'SULLIVAN 1982). Another aspect is the relationship between shear strength and measured erosion. By using single drops, CRUSE & LARSON (1977) and subsequently AL-DURRAH & BRADFORD (1981, 1982) have ISSN 0341 - 8162 Coourioht IQRfi bu G A T F N A V F R I A ~
n - R R R 9 ('~remllnnen. Re~feAI W ~ = r m = n l l
130
LUK
observed significant relationships between soil shear strength and splash detachment in the laboratory. Relationships involving rainwash erosion have yet to be established. For aggregation characteristics, the influence of soil moisture on aggregate formation and aggregate stability has been evaluated (GABRIELS & DE BOODT 1974, HARTMANN & DE BOOT 1975). Recently, UTOMO & DEXTER (198 l a, 1982) have investigated the complex effects of wetting and drying on soil aggregation characteristics. The precise effect of wetting and drying was shown to relate to many factors, including tillage.
o~
E o
-m-
1
O.
Drop diameter
(mm)
Fig. 1: Drop size frequency for simulated rainfall compared with natural rainfall of the same intensity (50 mm hr-l).
2.
PROCEDURES
The modified Toronto pattern rainfall simulator (LUK 1982)was used in this study. The simulated rainfall was sprayed from nozzles (1.19 m m aperture) located on six radial arms which were rotated atop two sections of T.V. antenna towering. Each radial arm is 2.54 m in length; the speed of rotation is 4 rpm which produces an average wetting frequency of 2.5 s; and there are 27 nozzles on each arm. The 'rainfall' was sprayed at a water pressure of 80 kPa on an annular 18 m 2 in area, permitting 4 to 6 erosion plots e~ich 3 × 3 ft (0.84 m 2) to be wet-
SOIL MOISTURE AND RAINWASH EROSION
131
Tab. 1: BASIC SOIL PROPERTIES Soil Properties Sand % (0.063 - 2 mm) Silt % (0.002 - 0.063 ram) Clay % ( < 0.002 mm) Organic matter (%) Water stable aggregates % > 2 mm >0.5 mm Water-stable aggregates % (with sand correction)
-~ 37 51 12
Guelph s ---
5.21 40.3 73.8 > 2 mm 32.7 > 0.5 mm 72.3
0.41
'g 35 49 16
Font s
3.48
--0.21
10.5 7.5
48.1 71.0
13.0 11.2
--
28.2 48.2
-
2: sample mean; s: sample standard deviation
ted. With an apical flail height of 7.16 m above plot surfaces, drop sizes ranging from 0.4 to 5.8 mm, and a rainfall intensity of 50.8 m m hr -~, the simulator achieved a kinetic energy delivery rate of 0.38 J m -2 s -l (Fig. 1), which is practically 100% of the average amount of energy found in natural storms with the same intensity (LAWS & PARSONS 1943). Two soils in southern Ontario: the Font loam and the Guelph silt loam which are both Groy-Brown Luvisols (GILLESP1E et al. 1972, HOFFMANN et al. 1963) were selected for rainfall simulation studies. Selected properties of these soils are listed in Table 1. At each site, the surface which was mostly covered by grass was unilbrmly scalped to a depth of 35-40 mm by using a manual 'sod-cutter'. This helps to eliminate the influence of surface vegetation and most root systems. Four to six 0.84 m 2 plots were then established in the wetted annular by inserting 15 cm high steel boards to a depth ofapproximately 15 m m on the upslope and the two sides of the plot. At the downslope edge, a Gerlach-type trough was installed and securely attached to 15-1itre capacity plastic buckets. In total, 12 sets of data were collected for each of the Font and the Guelph soils. For the Font soil, three adjacent sites each with four plots were tested while for the Guelph, only two sites each with six plots were used. The rainfall simulator was calibrated at the Erindale Campus of the University of Toronto by using 20 7.5-cm gauges. It was found that the areal variation in rainfall amounts produced by the machine has a coefficient of variation of less than 10% of the target rainfall under calm conditions ( < 8 km hr-l). However, when winds > 8 km hr -I were present, it was observed that the wetted annular was disturbed, the finest drops were removed from the area subjected to the simulated rainfall, and the areal uniformity ofrainfall intensity was also distorted. To minimize experimental errors induced by wind interference, the following procedure was followed. Days with high winds were avoided. When light to moderate winds occur ( < 15 km hr-l), wind-screens made of open-weave polyethylene with dimensions of 7.5 x 4 m were erected on the up-wind side of the site shortly before the beginning of the experiments. At the end of the experiments, rainfall data obtained from the four 7.5-cm gauges located at the corners of each plot were examined. Plots which had not received an average amount of rainflall within + 10% of the target (25.4 m m in 30 minutes) were discarded. In addition, a correction factor ( (target rainfall) / (actual rainfiall) ) was applied to the accepted plot data. In this study, various soil characteristics were determined as follows. Gravimetric soil
132
LUK
moisture samples were collected at the two upslope corners of each plot immediately before the beginning of the experiment by coring to a depth of 1.5 cm. Microtopography was determined by using a specially designed microreliefgauge which is described as follows. Seven cross pieces in an aluminium frame were perforated at a regular spacing of 10 cm so that 49 aluminium rods of I cm in diameter and 69 cm in length can be lowered through the perforations onto the soil surface. When properly levelled, the length of the rods extending above the top edge of the frame, as measured by a vernier depth gauge, indicates the relative heights of the 49 sample points on the ground surface. A surface roughness index (R t ) similar to the one used in LUK (1982) was then computed by the following formula:
Rt =
n-
where h a is measured height for each ofthe 49 sample points, h is mean measured height and n is the number of sample points. Resistance of surface soil to torsional shear stress (or torsional shear strength) was determined by using a hand-held Pitcon torvane with a vane diameter of 19 mm and height of 29 ram. Bearing capacity, which is closely related to unconfined compressive strength, was measured by a Vicksburg-type proving ring penetrometer. Soil bulk density was determined by extracting 54-ram cores l - 2.5 cm below the soil surl:ace. in the laboratory, aggregate size distribution was measured by using the standard wet-sieving technique (YODER 1936). Air-dried samples were passed through an 8-ram sieve before wetted by direct immersion and wet-sieved for 30 minutes (amplitude of 15 mm and stroke of 40 per minute). Torsional shear strength was tested in a mechanised torvane with a vane diameter and height of 12.5 mm.
3.
RESULTS
Ten sets of collected data on antecedent moisture, rainwash and runoffwere accepted and they are presented in Figures 2 and 3. The positive relationship observed between rainwash and runoff (Fig. 2) is consistent with previous findings (e.g. LUK & MORGAN 1981 ). There are also clear evidences of a positive relationship between rainwash and antecedent moisture but the scatter of points suggest possible interference by variations of other soil properties. A case in point is plot 2-4 for the Guelph soil where a relatively high rainwash was observed at an antecedent moisture of only 7.2% (Fig. 3). Inspection of the data on surPace soil properties show that this plot has a particularly high bulk density of 1.37 compared to the mean of 1.21 g cm -3. The soil surface properties which were determined tbr each plot are presented in Table 2. As indicated by the C.V. levels, there are considerable variations ofthese properties which probably have affected the measured rainwash at the selected sites. Therelbre, a suitable statistical technique is required to isolate the effects of these properties and to reveal the inlluence of antecedent moisture. Multiple regression techniques were used Ibr this purpose. In the regression analysis, measured rainwash erosion constitutes the dependent wlriable and antecedent moisture along with surface bulk density, surt~ace aggregate size,
SOIL MOISTUREAND RAINWASHEROSION
133
Rainwash. Wr (g) 800
400
!
D
200
Wr = --98 2 8 + 34 60 Fin
10
15
20
150
100
50
0
.0
i
i
I
.5
1_0
1.5
Runoll. Fin (dm 3 )
Fig. 2: Relationship between rainwash and runoff, Guelph silt loam (A) and Font loam (I-q).The regression coefficients are significant at the 0.05 level.
slope steepness and surface roughness are the independent wlriables. Both untranslbrmed and lgl0-transformed rainwash were used in separate computations. The best regression equations for the Guelph and the Font soils are presented in Table 3 (Equations 3.1,3.2, 3.4 and 3.5). it can be seen that significant results were obtained using either the untranstbrmed and the Igl0-transtbrmed rainwash as the dependent wiriable. The R2 values achieved w~ry between 88 and 96% and the coefficients of determination are significant at the 0.05 level. These wdues compare very favourably with results obtained in previous studies (e.g. CAMPBELL 1970, SOONS 1971, LUK 1975) where minwash was measured at the same plot scale but under natural rainfall. For all the equations computed, antecedent moisture was selected as one of the significant independent wlriables (at the 0.05 level), thus confirming its important influence on rainwash erosion. Moreover, other significant independent wlriablcs which pertain to the soil surlhce characteristics were also selected, reflecting that the local variations
134
LUK
Rain~ash (g) 6O0
400
(3
200
O
O D
0
100 2-4
&
50 ak dk
0
I
I
10
20
30
Anlecedent rnoisture (%)
Fig. 3: Relationship between rainwash and antecedent moisture, Guelph silt loam (1) and Font loam (r--I).Plot 2-4 is labelled. The arrow refers to the liquid limit of the Font soil as determined by the vane shear test.
in soil properties are also signilicant. To demonstrate the precise influence of the soil moisture filctor, Equations 3.2 and 3.5 with lgl0-transformed rainwash as the dependent wiriable (Table 3) were selected and residual equations (Eq. 3.3 and 3.6) were computed by substituting the mean of bulk density and water-stable aggregates > 2 mm into Equations 3.2 and 3.5 respectively. The residual equations were then used to estimate rainwash erosion rates under dilTerent antecedent moisture conditions (Table 4). These conditions include the saturation state which occurs during the spring-melt period, and the "wilting point' (or approximately 15 bars moisture tension) which may exist during the summer season. The percent soil moisture used for these estimations is based on field moisture samples collected at different times of the year. For comparison purposes, the ratio of estimated rainwash relating to the saturated state
SOIL MOISTURE AND RAINWASH EROSION Tab. 2:
135
SOIL SURFACE CHARACTERISTICS Guelph
Soil chanlcteristics Antecedent moisture (%)
~ 14.9
s 5.8
Water-stable aggregates (%) > 2 mm >0.5ram
40.3 73.8
10.5 7.5
.
Font
c'v.* 2 0.39 21.6 0.26 0.10
s 3.3
48.1 71.0
c.v. 0.15
13.0 0.27 11.2 0.16
Bulk density (gcm -3)
1.21
0.07
0.06
1.30
0.05 0.04
Surlhce roughness (ram)
2.63
0.66
0.25
3.64
0.51 0.14
Slope angle (°)
4.56
1. I 1
0.24
8.09
1.56 0.19
3,14
0.09
Torsional shear strength (kPa)
33.1
23.7
1.70 0.07
Bearing capacity(kPa) 533.2 100,0 0.19 331.6 17.4 0.05 * Coefficient of variation = ~/s where-ff is sample mean and s is sample standard deviation. Tab. 3:
Equation Number
REGRESSION ANALYSIS RESULTS
Dependent Constant Variable Term
GUELPH SILT LOAM 3. I Wr -- 392.37. 3.2 Ig~oWr -1.636 3.3 Igl0Wr 1.368 FONT LOAM 3.4 Wr 3.5 Igl0Wr 3.6 Igl0Wr
286.979 2.394 1.946
Partial 0d
2.5210 0.0179 0.0179
Regression Ys
Coefficient Gt
353.94 2.485
-- 0.6522
R2
Remarks
0.963 0.888 ys =
13.9576 0.02339 0.02339
-- 5.8742 -- 0.00931
1.209 g c m -3
0.882 0.903 G t = 48.102%
0d: antecedent soil moisture content (%); Ys : soilbulk density (g cm-3); G t : water-stable aggregates > 2 mm (%); Wr: rainwash (g). For equation 3.1,3.2, 3.4 and 3.5, all the partial regression coefficients and coefficients ofdetermination are signilicant at the 0.05 level. Tab. 4:
EXTRAPOLATED RAINWASH EROSION FOR SELECTED MOISTURE CONSTANTS
Soil
Wr at 0dw (g)
Wr at 0ds (g)
Wr(0ds ) Wr(O dw )
Guelph Silt Loam Font Loam
35.29 151.43
149.47 761.90
4.2 5.0
Extrapolation based on Equation 3.3 and 3.6; 0dw, 0ds : soil moisture content near the wilting point and saturation; for Guelph silt loam, 0dw = 10%, 0ds = 45%; for Font loam, 0dw = 10%, 0ds = 40%.
136
LUK
to that relating to the wilting point was computed for each of the two soils tested. This ratio was found to vary between 4 and 5 (Table 4). This result demonstrates that fora selected field site where the soil properties can be assumed to be relatively 'homogeneous', measured rainwash erosion according to the specific experimental conditions described above may differ by as much as five times if the full range ofantecedent moisture conditions is considered. Moreover, if the potential influence of other soil properties is taken into consideration, a somewhat greater range ofmeasured rainwash erosion at this scale of measurement may be expected.
4.
DISCUSSION
It has been established in this study that the antecedent soil moisture content is extremely important in influencing the rate of measured rainwash. In general, rainwash increases exponentially with antecedent moisture content. The following discussion is concerned with the mechanisms by which soil moisture influences rainwash erosion. The commonly considered mechanism is runoff generation. When other influencing factors are held constant, a higher antecedent moisture content leads to a reduced infiltration rate and reduced time lag to occurrence ofoverland flow. Thus, total runoffand rainwash are linear functions of antecedent moisture content. These relationships are apparent in the data collected in this study (Fig. 2 & 3). However, such relationships are complicated by soil crustability and aggregation characteristics. Soil crustability is known to be an index of soil stability (e.g. DE PLOEY & IVlUCHER 1981). Crust formation has been reported to be strongly influenced by the size and stability of surface aggregates (FARRES 1978). Thus, for unstable soils, the timing of crust formation is clearly important in determining runoffand rainwash rates. In the present study, surface crusts were not observed during the field experiments. Crusts were found at the end of some laboratory rainfall simulation tests for samples derived from the cultivated Guelph soil but it is unlikely that they have affected the field rainwash data. The size of soil aggregates is an important soil property influencing measured rainwash, as is found in the experimental results of the Font loam soil (Table 3). Other studies have shown that the amount of measured runoffand rainwash varies with the size distribution of surface aggregates (e.g. LUK 1983). Laboratory data collected by HARTMANN & DE BOODT (1974) demonstrate that aggregate size and aggregate stability vary with the soil moisture content of the previous wetting cycle. Unfortunately, the soil moisture content of the previous wetting cycle was not monitored in the present study, and thus the total effect of soil aggregation cannot be evaluated. Another soil property to be considered is soil shear strength. CRUSE & LARSON (1977), followed by AL-DURRAH & BRADFORD (1981, 1982) have shown that splash detachment as determined by the drop-test method is a function of soil shear strength. DE PLOEY & MUCHER (1981) reported that for a group of Belgian soils, the index C510, or the change in moisture content between 5 and 10 blows on a standard liquid limit curve is a suitable index of soil stability. In this study, initial laboratory tests were conducted on 'undisturbed' field samples by using a mechanised torvane. The results of these tests show a wide scatter which is mostly attributable to variations in soil bulk density and the density of rootlets. To overcome these problems, remoulded samples were used. A constant volume of air-dried soil commensurate with the average field bulk density of the selected soils was compressed in a mechanical corn-
SOIL MOISTURE AND RAINWASH EROSION
137
Torsional shear strength (kPa)
100
10
I
I
I
I
I
0
10
20
30
40
Moislure conlenl
50
(%)
Fig. 4: Relationship between torsional shear strength and moisture content, Guelph silt loam (A) and Font loam (F-I).
pactor. These samples which have a diameter of 54 mm and thickness of 30 mm were saturated overight before they were dried to a wide range of moisture levels and then tested in the mechanised torvane. Results of these tests (Fig. 4) show that for both the Guelph silt loam and the Font loam, torsional shear strength decreases exponentially as soil moisture increases. The rate of reduction in shear strength, however, shows an abrupt increase at 29.5% and 26.4% moisture content for the Guelph and the Font soils respectively. These turning points approximate their liquid limits which are 32.8% and 29.0% respectively as determined by the conventional liquid limit apparatus. Thus, it can be suggested that increases in rainwash erosion in response to higher antecedent moisture may be related to soil shear strength reduction, particularly at the higher moisture content when the liquid limit is equalled or exceeded. A case in point is the test results from the Font loam samples (Fig. 3) where the sharp increases in rainwash are shown to relate to soil samples with an antecedent moisture that is very close to their liquid limit (as determined by the vane shear test). From the above discussion, it is obvious that the influence of soil moisture content on rainwash erosion is complicated by responses in the 'intermediate' factors, particularly r u n -
138
LUK
offcapacity and shear strength which relate to the force and the resistance, respectively, that are involved in the erosion process. The interactive effects concerning the force ofand resistance to erosion are extremely important and must be properly considered if attempts at modelling the erosion process are to be successful. Finally, the quantitative result obtained in this study must be considered in the proper perspective. It is quite apparent that the magnitude of the soil moisture effect varies with many factors. Further experiments should attempt to evaluate this effect for a range of rainfall erosivity, soil type and plot size.
5.
CONCLUSIONS
It has been observed in a series of field rainfiall simulation experiments that along with soil bulk density and aggregate size, the antecedent moisture content is a significant lhctor contributing to variations in measured rainwash erosion. Extrapolated data suggest that rainwash differs by 4 to 5 times ifthe full range of antecedent moisture content is considered. For the soils tested which are cohesive in nature, the impact ofantecedent moisture content is not confined to runoffgeneration, but also shear strength reduction. Future experinaents should be conducted to evaluate the relative importance of these different effects tbr a range of rainfall erosivity, soil type and plot size. ACKNOWLEDGEMENTS The research project o n which this paper is based was financially supported by the Natural Sciences and Engineering Research Council of Canada which is gratefully acknowledged, Access to the research sites was provided by the University of Guelph and the Halton Region Conservation Authority. Research assistants were Messrs. Robert Irvine, Glenn Darras and Christine Roncato. REFERENCES
AL-DURRAH, M.M. & BRADFORD, J.M. (1981): New methods ofstudying soil detachment due to waterdrop impact. Soil Sci. Soc. Am. J. 45, 949-953. AL-DURRAH, M.M. & BRADFORD, J.M. (1982): Parameters for describing soil detachment due to single waterdrop impact. Soil Sci. Soc. Am. J. 46, 836-840. ARMAN, A., POPLIN, J.K. & AHMAD, N. (1975): Study ofthe vane shear. Proc. Conf. on In Situ Measurement of Soil Properties. Am. Soc. Cir. Engin. 1, 93-120. BALL, B.C. & O'SULLIVAN, M.F. (1982): Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments. J. Soil Sci., 33, 609-622. BAVEIL LD. (1937): Rainfall characteristics of Missouri in relation to runoffand erosion. Soil Sci. Soc. Amer. Proc. 2, 533-536. BRYAN, R.B. & LUK, S.H. (198 I): Laboratory experiments on the variation ofsoil erosion under simulated rainfall. Geoderma 26, 245-265. BURROUGH, P.A. (1983): Multiscale sources ofspatial variation in soil. I. The application of fractal concepts to nested levels ofsoil variation. J. Soil Sci., 34, 577-597. CAMPBELL, I.A. (1970): Erosion rates in the Steveville Badlands, Alberta. Can. Geogr. 14, 202-216. CHORLEY, R.J. (1959): The geomorphic significance ofsome Oxford soils. Am. J. Sci. 257, 503-515. CRUSE, R.M. & LARSON, W.E. (1977): Effect ofsoil shear stength on soil detachment due to raindrop impact. Soil Sci. Soc. Am. J. 41,777-781. DE PLOEY, J. & MIJCHER, H.J. (1981): A consistency index and rainwash mechanism on Belgian loamy soils. Earth Surf. Proc. Landforms 6, 319-330. ELLISON, W.D. (1945): Some effects of raindrops and surfiace flow on soil erosion and infiltration. Am. Geophy. Union Trans. 26, 415-429.
SOIL MOISTURE AND RAINWASHEROSION
139
FARRES, P. (1978): The role of time and aggregate size in the crusting process. Earth Surl: Proc. 3, 243-254. GABRI ELS, D. & DE BOODT, M. ( 1974): Relationship between moisture content, aggregate Ibrmation and aggregate stability of a loam soil treated with a soil conditioner in various concentrations. CATENA 2, 23-30. GILLESPIE, J.E., W1CKLUND, R.E. & MILLER, M.H. (1972): The Soils of Halton County. Ontario Soil Survey Report 43, Ontario Dept. Agri. Food, Toronto. HARTMANN, R. & DE BOODT, M. ( 1975): The influence ofthe moistu re content, texture and organic matter on the aggregation ofsandy and loamy soils. Geoderma 1I, 53-62. HOFFMAN, D.W., MATTH EWS, B.C. & W1CKLUN D, R.E. ( 1963): Soil Survey of Wellington County. Ontario Soil Survey Report 35, Ontario Dept. Agri. Food, Toronto. HORTON, R.E. (1945): Erosional development of streams and their drainage basins: hydro-physical approach to quantitative morphology. Geol. Soc. Amer, Bull. 56, 275-370. LAWS, J.O. & PARSONS, D.A. (1943): The relation of rain drop size to intensity. Am. Geophys. Union Trans. 24, 452-459. LUK, S.H. (1975): Soil Erodibilityand Erosion in Part ofthe Bow River Basin, Alberta, Canada. Unpublished Ph.D. Thesis, Univ. Alberta, Edmonton. LUK, S. H. ( 1982): Variability ofrainwash erosion within small sample areas. Thorn, C. (Ed.): Space and Time in Geomorphology. Allen & Unwin, London, 243-268. LUK. S.H. (1983): Effect ofaggregate size and microtopography on rainwash and rainsplash erosion. Z. Geomorph. 27, 283-295. LUK, S.H. & MORGAN, C. (198 I): Spatial variations ofrainwash and runoffwithin apparently homogeneous areas. CATENA 8, 383-402. MULLINS, C.E. & FRASER, A. (1980): Use of the drop-cone penetrometer on undisturbed and remoulded soils at a range ofsoil-water tensions. J. Soil Sci. 31, 25-32. N EAL, J. H. ( 1937): The effect ofthe degree of slope and rainl~allcharacteristicson ru noffand soil erosion. Soil Sci. Soc. Am. Proc. 2, 525-532. O'SULLIVAN, M.F. & BALL, B.C. ( 1982): Acomparison offive instruments for measuring soil strength in cultivated and uncultivated cereal seedbeds. J. Soil Sci., 33, 597-608. PAUL, C. L. & DE VRIES, J. (I 979): Prediction of soil strength from hydrologicand mechanical properties. Can. J, Soil Sci. 59, 301-311. SOONS, J.M. ( 1971): Factors involved in soil erosion in the southern Alps, New Zealand. Z. Geomorp h. 15, 460-470. UTOMO, W.H. & DEXTER, A.R. (198 la): Soil friability. J, Soil Sci. 32, 203-213. UTOMO, W.H. & DEXTER, A.IL (1981b): Age hardening of agricultural top soils. J. Soil Sci. 32, 335-350. UTOMO, W.H. & DEXTER, A.R. (1982): Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil. J. Soil Sci. 33, 623-637. YODER, R.E. (1936): A direct method of aggregate analysis of soils, and a study of the physical nature oferosion losses. J. Am. Soc. Agron. 28, 337-35 I.
Address of author: Dr. Shiu-hung Luk, Department of Geography, University of Toronto, Erindale Campus Mississauga, Ontario, Canada L5L 1C6