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Assessment of the interrill and rill contributions to total soil loss from an upland field plot
Geomorphology, 1 (1988) 343-354
343
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ASSESSMENT OF THE INTERRILL AND RILL CONTRIBUTIONS TO TOTAL SOIL LOSS FROM AN UPLAND FIELD PLOT G. GOVERS and J. POESEN* Laboratory of Experimental Geomorphology, Catholic University of Leuven, Redingenstraat 16 B, 3000 Leuven (Belgium) (Received December 29, 1987; accepted after revision June 22, 1988)
Abstract Govers, G. and Poesen, J., 1988. Assessment of the interrill and rill contributions to total soil loss from an upland field plot. Geomorphology, 1: 343-354. Erosion measurements were carried out on an upland field plot in the Loam Region of Belgium. The sediment being detached by splash on interrill areas is transported to the channel system mainly by interrill wash. Over the entire 7500 m=' field plot, rill (and gully) erosion is more important than interrill erosion. However, the relative importance of interrill erosion varies in time and in space, due to changes of the interrill surface characteristics and the activation of sidewall and gullying processes in the channel network. These factors should be taken into account in a realistic rill-interrill erosion model.
Introduction At present, few available field data permit the separation of rill ( a n d / o r gully) and interrill contributions to sediment removal from arable land. The scarcity of this type of field data is a major factor inhibiting the validation and further development of recently developed physically based models describing upland erosion by water (e.g., Foster, 1982; Rose et al., 1983). Knowledge of the contribution of rill and interrill processes to total soil loss has also some practical interest. If the ratio between rill and interrill erosion were known, even approximately, it would be possible to obtain a quick estimate of total erosion by making a volumetric recording of rill patterns. Plot measure*Both authors affiliated to the National Fund for Scientific Research (Belgium).
0169-555X/88/$03.50
ments will be necessary when interrill erosion has to be evaluated separately. In this paper an attempt is made to quantify the contribution of different sediment sources to total soil loss from an experimental field plot, located in Huldenberg, Belgium. The final estimates are necessarily based on extrapolations in time and in space. This leads to some inevitable uncertainty, but nevertheless, the study yields some interesting results. Materials and methods
The field site The field site of Huldenberg is located 15 km SSW of Leuven in the Belgian Loam Belt (Fig. 1, 2). The field, with a surface area of ca. 7500 m 2, is a south-facing convex-concave slope strip of about 50 m wide. Maximum slope is about
© 1988 Elsevier Science Publishers B.V.
344
Fig. 2. View on the Huldenberg experimental field. 25% (Fig. 1 ). The field varies markedly in lithological and pedological characteristics (Govers, 1987). It was conventionally tilled on 15 November 1983 and was kept clear of vegetation thereafter using herbicides. The observations discussed here took place from 15 November 1983 till 28 December 1984. Observations concerned rill and gully as well as interrill erosion processes. As rill and gully erosion processes have been described in detail elsewhere (Govers, 1987), attention here focuses on interrill erosion and the construction of a sediment budget.
Definition and measurement of rill and gully erosion
20 •
~ ,,
N
0
5
10m
Fig. 1. Map of the field site with location of the interrill plots and the channel network on 3/10/84. Contours are in m above sea-level, numbers 1-17 refer to transects.
The evolution of the rill and gully system was established by periodic surveys from 15 November 1983 to 3 October 1984 of rill and gully positions and dimensions along fifteen transects every 10 m downslope (Fig. 1 ). Flowlines where concentrated water flow had taken place without noticeable incision were also mapped. Three types of erosion were distinguished in the channel system (Fig. 3): (a) Rills were initially formed by hydraulic erosion of the cohesive bed (HRE). Its contribution to total channel erosion at a given cross section was estimated by multiplying bed width and depth of the rill.
345
/ ,,',//
/? I
/s,,'/
/
//
]///I 1/#]
RSW
Fig. 3. Definition sketch illustrating the various erosion subprocesses. (b) Mass transport processes on the sidewalls of the rills (RSW) caused subsequent widening of the rill. (c) Gully erosion (GE) occurred on the eastern half of the steepest slope section, where the channel bed consisted of Tertiary sands and was totally noncohesive when saturated. The upslope boundary of the gullies was formed by a clear headcut, so that their separate contribution to total channel erosion could easily be calculated.
Definition and measurement of interrill erosion It is generally accepted that sediment detachment on interrill areas is primarily caused by raindrop impact (e.g., Meyer et al., 1975; Kirkby, 1980; Gilley et al., 1985 ). The detached sediment can be transported downslope in three ways (Fig. 3 ): (a) There is a net downslope sediment flux because splash distances in the downslope direction are greater than in the upslope direction and because more detached sediment is ejected downslope than upslope (DSP) (b) Detached sediment can directly be splashed into the channel system and subsequently be evacuated by concentrated flow
(RSP) (c) Splash-detached sediment can be transported into the rill and gully system by interrill wash (IRW)
Splash detachment was measured using circular splash cups with a diameter of 5.2 cm. Real splash detachment is underestimated when calculated as the division of total mass of sediment caught in the cup by the splash cup surface area, as the contribution of the area within the cup is not taken into account (Poesen and Torri, in press; Torri and Poesen, 1988). Therefore, the masses of splashed sediment collected were multiplied by 1.25 in order to eliminate the effect of the splash cup diameter. From 15 November 1983 till 15 November 1984 measurements on splash detachment were collected on 20 sites randomly distributed over the field. From 2 October till 28 December 1984 splash detachment was measured within the interrill wash plots. Direct downslope movement of sediment by splash (DSP) was estimated from measured detachment rates using the model developed by Poesen and Savat (1981) and improved by Poesen (1985). To estimate RSP from detachment data, the relative amount of sediment splashed from a point source into an infinitely long channel of given width has to be known. This can be determined using procedures similar to those developed by Savat and Poesen (1981) to calculate real splash amounts and distances from tray data. Numerical integration of these point values over a sufficiently wide distance orthogonal to the channel allows then the calculation of the percentage of the material detached by drop impact per unit surface of interrill area that is received per unit surface of rill bottom. This procedure can be repeated for various channel widths and splash distances so that a nomograph can be constructed (Fig. 4). The nomograph can then directly be used to ascertain the amount of splash into a rill if the splash detachment per unit surface of interrill area, mean splash distance, and channel width are known. The relative amount of splash received by a rill bed is primarily dependent on the rill width and secondly on the mean splash distance (Fig. 4). On the field, mean splash distance was assumed
346 %
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0 10
0
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~0
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width of rift channel (cm)
Fig. 4. Nomograph to determine the relative splash amount caught by the rill bed (compared to an interrill area) as a function of mean splash distance (.~) and rill width.
to be 20 cm. Smits (1982) found that mean weighted splash distances of loamy soil materials varied from 16 to 24 cm and were almost independent of surface roughness and moisture content. Interrill wash erosion (IRW) was measured on 20 small bounded plots, installed at various locations on the field (Figs. 1 and 5). The surface areas of the plots varied between 0.5 and 0.66 m 2. Measurements took place from 2 October till 28 December 1984. As the field surface was totally covered with a crust at this time, tillage operations were evaluated by turning over the soil on 8 plots with a spade (Fig. 1 ). Each plot was equipped with a gutter and a
vessel to collect runoff water and sediment. The gutter was covered in order to prevent the collection of rainfall on the gutter surface and direct insplash (Fig. 5). The length of the plots (ca. 0.9 m) was smaller than the length of the mean flowpath on the interrill areas, which, according to visual observations, was of the order of a few meters. The plots were visited weekly, rainfall and interrill runoff volume being recorded after every important rainfall event. At the same time, representative runoff samples were taken to determine sediment concentration. Soil loss was calculated as the product of runoff voume and sediment concentration. In each plot a circular splash cup with a diameter
347
Fig. 5. View on an interrill plot.
of 5.2 cm was placed. The splash data permitted the calculation of a delivery ratio (DR), being the ratio between the mass of sediment effectively removed from the interrill plot by interrill wash to the mass of sediment detached by drop impact.
Rill and gully erosion Spatial variability of hydraulic rill erosion is to a large extent dependent on topographic factors, although slope and length exponents vary with time (Govers, 1987). Temporal evolution of total sediment production by H R E was strongly determined by higher magnitude events
on 3 February, 6 February and 28 August 1984, with rainfall amounts of 8, 13 and 36 mm at intensities of 50, 50 and 80 mm h -1, respectively. These three events, with a precipitation total of 57 mm, produced about 65% of total HRE during the observation period. Spatial variations in the hydrological and soil mechanical properties of the topsoil-subsoil complex made conditions for mass wasting most favourable on the eastern part of the midslope section, where a sandy loam topsoil with a thickness of ca. 30 cm overlays a compact, impervious subsoil, causing local saturation and consequent instability. On this part of the field, deep-seated sidewall failures were the most important. On the steepest slope section, where the plough layer had better structural characteristics, mass transport occurred mainly by soil fall. Mass movement on rill sidewalls was most active in the first moist period after the higher magnitude event of 28 August 1984. However, soil fall was also triggered by crack formation during the dry spring period. On 3 February 1984, gullies formed on the eastern part of the steepest slope section (transect 14) where Tertiary sand formations are very near the surface. Headcut retreat and gully deepening were only observed during higher magnitude events, while widening occurred mainly during dry periods when the sand lost all cohesion.
TABLE 1 Soil losses (t) due to different erosion processes and cumulative precipitation (P in ram) on different dates: direct splash to the rill and gully system (RSP), interrill wash (IRW), total interrill erosion (TIRE), hydraulic rill erosion (HRE), rill sidewall processes (RSW), gully erosion (GE), total rill and gully erosion (TRGE) and cumulative precipitation (P)
RSP IRW TIRE HRE RSW GE TRGE Total P (mm)
15/11/83
30/01/84
0 0 0 0 0 0 0 0 0
0.1 3.1 3.2 4.4 0.3 0.0 4.7 7.9 178
03/02/84 0.3 9.9 10.2 21.5 2.7 2.3 26.5 36.7 208
06/02/84
29/08/84
03/10/84
0.6 14.6 15.2 34.9 5.4 4.4 44.7 59.9 259
2.0 29.2 31.2 51.8 18.4 11.6 81.8 113.0 559
2.4 34.1 36.5 65.8 47.9 16.7 130.5 167.0 721
348 100 %-
-100%
RSP-
IRW-
RSW SO-
- so
HRE---
30 Jon.'8~
03 Ft b.'84
06 Feb.'8/,
28 Aug:B/,
03 0ct.'84
Fig. 6. Relative contribution of various subprocesses to total cumulative sediment output on five dates. TABLE 2 Soil losses (t) due to different erosion subprocesses per slope strip (extending 5 m upslope and 5 m downslope from each transect) on 30/01/84 and 3/10/84 Transect
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3/10/84
30/01/84 RSP
IRW
HRE
RSW
RSP
IRW
HRE
RSW
0.0 0.002 0.002 0.002 0.004 0.006 0.007 0.006 0.005 0.005 0.004 0.006 0.006 0.005 0.005
0.07 0.21 0.14 0.13 0.20 0.24 0.28 0.32 0.25 0.23 0.18 0.23 0.21 0.22 0.22
0.0 0.006 0.009 0.022 0.047 0.10 0.13 0.27 0.31 0.70 0.70 0.58 0.60 0.54 0.44
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.035 0.084 0.042 0.047 0.0 0.052 0.019
0.0 0.037 0.056 0.059 0.14 0.18 0.23 0.25 0.22 0.24 0.18 0.23 0.21 0.21 0.20
0.71 1.42 1.35 1.14 1.77 1.87 2.01 2.11 1.72 2.38 2.65 3.94 4.65 4.64 1.70
0.0 0.062 0.52 1.36 1.74 2.33 2.94 4.96 6.59 7.67 6.60 6.90 10.26 9.40 4.46
0.0 0.0 0.05 0.55 0.67 0.75 1.59 2.83 7.74 9.55 7.66 6.45 5.19 3.70 1.07
On the western half of the steepest slope section, where a loamy well-structured topsoil was present over a compacted Bt-horizon, relative intensive pipe erosion was observed. Many pipe segments, though not all, were initiated by mole activity. Pipes often turned into rills due to roof
collapse. The digging of the moles also supplied loose material to the existing rills. Soil loss due to animal activity in this part of the field was estimated to be of the order of a few tonnes. Generally, hydraulic rill erosion ( H R E ) was the dominant channel erosion process, al-
349 though its relative importance decreases with time (Table 1, Fig. 6). Sidewall processes were the most important sediment source in the channel system from 29 August till 3 October 1984. The contribution of the gullies to total sediment output from the channel system remained relatively stable after 3 February 1984; they produced ca. 10% of total channel erosion, although their total length was only 45 m, compared to a total channel length of ca. 2000 m. At the end of the observation period, gully erosion was dominant on transect 14, while sidewall processes were the most important on the mid-slope section, where deep-seated failures occurred (Table 2). Interrill erosion
Direct downslope movement by rainsplash
(DSP) As the total amount of sediment detached from 15 November 1983 till 3 October 1984 was ca. 15.2 kg m -2, total sediment discharge on the steepest slope section was m a x i m u m 2.41 kg m - 1of contour length, which corresponds to an upslope soil loss of less t h a n 0.02 kg m -2, taking into account an upslope slope length of 130 m. Furthermore, Poesen (1986) showed that during m a n y events splash transport is directed upslope due to the influence of southwesterly winds. This process is thus a relatively unimportant sediment transport agent.
Transport into the rill and gully system by splash (RSP) Using rill and gully width data, we estimated that 2.4 t of soil material were directly splashed into the rill and gully system (Table 1). The amount of sediment lost by RSP increased from the upper field boundary to transect 6, as in this area the number of rills as well as the mean rill width increased with distance downslope (Table 2 ). Further downslope, soil loss by RSP remained more or less constant, because the
increase in mean rill width compensated for the decrease in the number of rills (Table 2). Sediment lost from the field by this process represented only 2% of total soil loss (Table 1 ).
Transport into the rill and gully system by interriU wash (IR W) Although the delivery ratio of the interrill plots shows a high random variation, the mean delivery ratio of the tilled plots is significantly higher (p < 0.01 ) t h a n the mean delivery ratio of the crusted plots (Fig. 7). This can be explained by the fact that sediment e n t r a i n m e n t by interrill wash on low to intermediate slopes is sizeselective, whereby fines are preferably removed. After a sufficiently long period of time, a coarse residue is left on the surface, which can still be detached by splash but cannot be transported by interrill flow, causing a decrease of DR. Interrill erosion is then detachment-limited for the fine fractions, but transport-limited for coarser material. On a sufficiently steep slope all detached size fractions can be transported by the runoff, so that the DR remains high. There is abundant evidence that IRW on low to moderate slopes is a size-selective process. Gilley and Gee (1976) found that sediment eroded from a tilled silty clay loam on 13% slope only contained 3% sand, while the original soil had a sand content of 12%. The size distribution of the material eroded from a nontilled plot was about equal to that of the parent material, but total soil loss on the nontilled plots was markedly lower. Verhaegen (1984), using a rainfall intensity of 65 m m h - 1, found that wash material eroded from a 1 m long and 0.20 m wide flume filled with a soil containing 68% sand, contained only 20% sand on a 0.087 slope. However, on a 17% slope the size distribution of the wash-transported material was about equal to that of the parent material. Govers (1985) showed that erosion of loamy material by flowing water is size-selective as long as the (grain) shear velocity of the water is below
350 :~ 1.5-
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Fig. 7. Delivery ratio as a function of slope for tilled a n d crusted interrill plots. T h e curve indicates t h e tentative relationship between DR a n d slope for crusted plots.
3 cm s - 1. No clear influence of raindrop impact on the grain size distribution of the exported material was found. A runoff rate of 10 mm h - 1 (which was exceeded 3-4 times during the observation period) corresponds to a unit discharge of 0.3 cm 3 cm -1 s - l ) , assuming an interrill length of 2 m and water flow over 20% of the total width. A shear velocity of 3 cm s - 1 can then be reached on a 16% slope. The time taken for a freshly tilled surface with a high DR and selective erosion to evolve into a crusted surface with a low DR and nonselective erosion as well as the steady-state value of DR will depend on many factors, including the size distribution of grains and aggregates of the parent material, the depth of the active splash layer, the hydraulic characteristics of the interrill wash, the ratio between selective wash loss and
nonselective direct splash loss, the degree of disturbance by animal activity, frost action and cracking. The present data indicate that a steady state will be reached only after a considerable amount of precipitation and sediment removal. The DR of the tilled plots showed no tendency to decrease within the observation period with 163 mm of rainfall. On the contrary, the mean DR of the tilled plots increased during the first half of the observation period due to the increasing runoff production as crusting proceeded. This contrasts with the findings with respect to runoff generation. Mean runoff production on the tilled plots equalled mean runoff production on the crusted plots after about 150 mm of rain (Poesen and Govers, 1986 ). After tillage operations, equilibrium conditions are much
351 more rapidly established for interrill runoff production t h a n for interrill wash erosion. Different crust characteristics determine erosion and hydrological properties. This may have important implications with respect to crust evolution description. The fact that IRW can be highly selective also has implications for the influence of slope on interrill erosion. The slope dependency of interrill erosion varies with time: it is relatively small in the case of a freshly tilled seedbed, when fine, transportable material is dominant on the surface everywhere. As crusting and the formation of a coarse lag deposit proceed at a slopedependent rate, the influence of slope may become more important, as indicated by Fig. 7. Interrill wash measurements were limited to a relatively short period. In order to estimate the contribution of I R W to sediment removal for the period during which the rill and gully system was monitored, the following assumptions were made: (a) From 15 November 1983 till 5 February 1984 the DR was, everywhere on the field, assumed to be equal to the mean DR of the tilled plots (DR=0.725). (b)From 5 February 1984 till 3 October 1984, the DR was assumed to be dependent of slope, according to the tentative curve on Fig. 5. (c) Effective contribution was limited to those interrill areas located within 1 m on each side of a flowline, rill or gully. The assumed change from the first situation (tilled surface) to the second (crusted surface) is certainly too abrupt. It is only partly justified by the occurrence of the first higher magnitude event 3 February 1984, causing large amounts of soil loss, both on interrill areas and in the rill and gully system, and a drastic change of interrill surface microtopography. Surface microtopography did not change in any significant way after this date (Govers and Poesen, 1986). Based on the foregoing assumptions, total IRW could be calculated using splash detachment data collected from 15 October 1983 till 15 October 1984. Calculations were performed
per transect, taking into account the spatial variability of splash detachment and the decrease of interrill area due to rill development (Tables 1 and 2). Interrill wash appears to be by far the most important sediment removal agent on interrill areas (Table 1, Fig. 6). It is estimated that in total more t h a n 34 t of soil material were removed by IRW, corresponding to an ablation of ca. 3 mm, as the bulk density of the plough layer was about 1.35 g cm -~. However, the intensity of IRWwas highly variable in space. On the upper part of the slope only 2.4-3 kg m -2 was removed by IRW, while on the steepest slope I R W evacuated ca. 10 kg m -2, corresponding to an ablation of ca. 7 m m (Table 2). These data indicate that interrill erosion alone can cause soil losses far above generally accepted soil loss tolerance levels, at least if an efficient sediment evacuation system is present. Erosion survey data which are solely based on visual observations (e.g. on air photographs) may therefore severely underestimate the erosion hazard. Discussion
Most of the erosion on the field occurred in the rill and gully system. Only 22% of total sediment removed came from interrill areas. The relative importance of interrill erosion decreases with time from 46 to 22% (Fig. 6). This means that, on normally cultivated land, which is only vulnerable to erosion for a shorter period, interrill erosion may be relatively more important. The fact that the relative importance of interrill erosion decreases with time is not a general phenomenon. Collins and Dunne (1986) found that the erosion of tephra produced by the Mount St Helens eruption of 1980 occurred initially mainly by rills, but the importance of channel erosion decreased relatively rapidly with time. From the data presented by Mtkawa et al. (1987), which were collected on short plots (10 m), established on a tropic alfisol in Tanzania, it can be deduced that also in this case
352
the relative importance of rill erosion decreased with time. Mtkawa et al. do not discuss this issue, but Collins and Dunne explained the decreasing importance of channel erosion by two factors: the permeability of the interrill areas increased with time, mainly due to the stripping of the silty top layer and to frost action and incision of the deepest rills was blocked when the bed reached the more resistant original land surface. We observed the reverse trend on our site. Infiltrability of the interrill areas generally decreased in time, due to progressive sealing and compaction of the plough layer, although the crust was sometimes disturbed by animal activity or cracking (Govers and Poesen, 1986; Poe-
sen and Govers, 1986). Therefore, obliteration of the rills was very rare. Furthermore, gullying occurred on the eastern part of the slope because the resistance to runoff erosion of the sandy subsoil was much lower than that of the sandy loamy topsoil. Thus, on the Huldenberg site, as well as in the case of tephra erosion reported by Collins and Dunne, temporal evolution of interrill surface properties and variations of resistance to runoff erosion in the soil profile are some of the major controls on the balance between rill and interrill erosion. On the Huldenberg site, stability of rill sidewalls is also a very important factor. Also, topographic factors are important. Interrill erosion dominates on the upper part of the slope, as channel ero-
TABLE 3 Percentages of total interrill and rill erosion due to rill erosion as reported in the literature Source
Country
Period of observation
Soil type
Kerenyi (1985)
Hungary
Feb 1979
Brown stony forest soil Fine sandy soil Freshly deposited tephra --
~vlorgan et al. U.K. ( 1987 ) Smith and Swanson U.S.A. ( 1987 )
--
Mikhailov (1949, cited by Morgan, 1986) Zachar (1982)
--
U.S.S.R.
1980-1981
Slope ( To)
2-12 -0-45
Plot size
Surface state
Percent of total interrill and rill erosion due to rilling
Field length 20-1100 m --
Cultivated
14
Cultivated
20-50
2.4 km 2
Bare
37
23-30
Czechoslovakia After cloudburst - 13-15 on May 23 1958 Lootens (1983) Zaire Rain season Clay soil 0.51981-82 1.5 McCooland George U.S.A. (1983) Mtkawa et al. Nigeria 2 rainy seasons Medium to light 4.5(1987) 1985 textured 6.5 alfisol Gabriels et al. Belgium 20 Oct 1974Loamy sand 3 (1977) 20 Dec 1974 This study Belgium 15 Nov 1983Loamy to 0-13 3 Oct 1984 sandy loamy
51-63
0.6 ha
10mX2m
1.4 ha 150m × 50m
Potatoe field
70
Cultivated, bare Cultivated
73 75-95
Cultivated 67-99 maize, cowpea, fallow Cultivated, 87 seedbed Cultivated, 78 bare
353
sion is much more dependent on slope length and steepness. On 30 J a n u a r y 1984, interrill erosion was dominant up to transect 8, but on 3 October 1984 rill erosion was more important t h a n interrill erosion downslope from transect 4 (Table 2). On 3 October 1984, the ratio (interrill erosion)/(rill and gully erosion) is very low for transect 14, where the gullies were located. Some authors assumed a more or less fixed ratio between rill and interrill erosion in order to calculate total soil loss from rill survey data (e.g. McCool and George, 1983). It is obvious that such a fixed ratio does not exist; the balance between interrill and rill erosion processes is dependent on the factors discussed above, as well as on others, like the spatial organisation of the channel network, climatic and vegetation effects, etc. This explains some of the variability of the ratios reported in the literature (Table 3 ).
Conclusions On the field plot, sediment detachment on interrill areas occurs mainly by splash, while interrill wash is the most effective sediment transport agent. Only a minor a m o u n t of soil is removed by direct splash into the rill and gully system. However, interrill erosion is certainly not always detachment-linked. Due to the fact that sediment removal from a freshly tilled soil is size-selective, a coarse residue is formed at the surface which can no longer be transported by interrill wash. As a consequence the DR generally decreased in time and became more dependent on the slope angle; only on the steepest part of the plot a DR near 1 was maintained. Different surface characteristics determine sediment removal and runoff yield. Runoff yield reaches a steady state after a much smaller amount of rainfall. This is important with respect to crust characterisation as well as physically-based erosion modelling. Approximately 22% of total soil lost came from interrill areas. The relative importance of
interrill erosion decreases with time and with slope steepness, not only because of the slopedependent decrease of the DR, but also because of the activation of sidewall processes and gullying in the channel network. Other factors may also influence the ratio of interrill erosion to rill and gully erosion. The use of a fixed ratio is therefore not recommended.
Acknowledgement The authors wish to express their gratitude to Eng. J. Meersmans, L. Cleeren and R. Geeraerts for their technical assistance. The manuscript was much improved thanks to the comments of two anonymous referees.
References Collins, B.D. and Dunne, T., 1986. Erosion of tephra from the 1980 eruption of Mount St. Helens. Geol. Soc. Am. Bull., 97: 896-905. Foster, G.R., 1982. Modeling the erosion process. In: C.T. Haan (Editor), Hydrologic Modeling of Small Watersheds. Am. Soc. Agric. Eng., St. Joseph, pp. 297-370. Gabri~ls, D., Pauwels, J.M. and De Boodt, M., 1977. A quantitative rill erosion study on a loamy sand in the hilly region of Flanders. Earth Surf. Processes, 2: 257259. Gilley, J.E. and Gee, G.W., 1976. Particle size distribution of eroded spoil materials. North Dakota Farm Research, 34: 35-36. Gilley, J.E., Woolhiser, D.A. and McWorther, D.B., 1985. Interrill soil erosion Part I: Development of model equations. Trans. A.S.A.E., 28: 147-153. Govers, G., 1985. Selectivity and transporting capacity of thin film flows in relation to rill erosion. Catena, 12: 3549. Govers, G., 1987. Spatial and temporal variability in rill development processes at the Huldenberg experimental site. In: R.B. Bryan (Editor), Geomorphic Significance of Rill Development. Catena, Suppl., 8: 17-34. Govers, G. and Poesen, J., 1986. A field-scale study of surface sealing and compaction on loam and sandy loam soils Part I. Spatial variability of surface sealing and crusting. In: F. Callebaut, D. Gabri~ls and M. De Boodt (Editors), Assessment of Soil Surface Sealing and Crusting, Proc. Symp. Ghent, pp. 171-182. Kerenyi, A., 1985. Surface evolution and soil erosion as reflected by measured data. In: M. Pecsi (Editor), Environmental and Dynamic Geomorphology. Akad. Klado, Budapest, pp. 79-84.
354 Kirkby, M.J., 1980. Modelling water erosion processes. In: M.J. Kirkby and R.P.C. Morgan (Editors), Soil Erosion. Wiley, Chichester, pp. 183-216. Lootens, M., 1983. Erosion agricole acc~lerde sur sol nu au Shaba MSridonial (Za'ire). Ann. Fac. Sci Lubumbashi, 3: 1-6. McCool, D.K. and George, D.K., 1983. A second-generation adaption of the Universal Soil Loss Equation for Pacific Northwest drylands. A.S.A.E., Pap. 83-2066, 20 pp. Meyer, L.D., Foster, G.R. and RSmkens, M.J.M., 1975. Source of soil eroded by water from upland areas. In: Present and Prospective Technology for Predicting Sediment Yields and Sources. U.S. Agric. Res. Serv. Rep. ARS-S-40, pp. 177-189. Morgan, R.P.C., 1986. Soil Erosion and Conservation. Longman, London, p. 29. Morgan, R.P.C., Martin, L. and Noble, C.A., 1987. Soil erosion in the United Kingdom: a case-study from midBedfordshire. Silsoe College, Occ. Pap. 14, 58 pp. Mtkawa, P.W., Lal, R. and Sharma, R.B., 1987. An evaluation of the universal soil loss equation and field techniques for assessing soil erosion on a tropical alfisol in western Nigeria. Hydrol. Processes, 1: 199-209. Poesen, J., 1985. An improved splash transport model. Z. Geomorphol., 29: 139-211. Poesen, J., 1986. Field measurements of splash erosion in order to validate a splash transport model. Z. Geomorphol., Suppl., Band 58: 81-91. Poesen, J. and Govers, G., 1986. A field-scale study of surface sealing and crusting on loam and sandy loam soils. Part II. Impact of soil surface sealing and compaction on water erosion processes. In: F. Callebaut, D. Gabri~ls
and M. De Boodt (Editors), Assessment of Soil Surface Sealing and Crusting. Proc. Symp. Ghent, pp. 183-193. Poesen, J. and Savat, J., 1981. Detachment and transportation of loose sediment by raindrop splash II. Detachability and transportability measurements. Catena, 8: 19-41. Poesen, J. and Torri, D., 1988. The effect of cup size on splash detachment and transport measurements. I. Field measurements. Catena Suppl., 12:113-126. Rose, C.W., Williams, J.R., Sander, G.C. and Barry, D.A., 1983. A mathematical model of soil erosion and deposition processes. I. Theory for a plane land element. Soil Sci. Soc. Am., J., 47: 991-995. Savat, J. and Poesen, J., 1981. Detachment and transportation of loose sediments by raindrop splash I. The calculation of absolute data on detachability and transportability. Catena, 8: 1-17. Smith, R.D. and Swanson, F., 1987. Sediment routing in a small drainage basin in the blast zone at Mt. St. Helens, Washington, U.S.A. Geomorphol., 1: 1-13. Smits, M., 1982. De invloed van de bewerkingswijze van bodems en de vegetatie op de spaterosie. Unpl. M.Sc. Thesis, Catholic University of Leuven, 115 pp. Torri, D. and Poesen, J., 1988. The effect of cup size on splash detachment and transport measurements. II. A theoretical approach. Catena Suppl. 12: 127-137. Verhaegen, T., 1984. Labo-experimenten en terreinwaarnemingen in verband met de erosiegevoeligheid van lemige bodems. Unpl. Ph.D. Thesis, Catholic University of Leuven, 213 pp. Zachar, D., 1982. Soil Erosion. (Developments in Soil Science, 10.) Elsevier, Amsterdam.
343
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
ASSESSMENT OF THE INTERRILL AND RILL CONTRIBUTIONS TO TOTAL SOIL LOSS FROM AN UPLAND FIELD PLOT G. GOVERS and J. POESEN* Laboratory of Experimental Geomorphology, Catholic University of Leuven, Redingenstraat 16 B, 3000 Leuven (Belgium) (Received December 29, 1987; accepted after revision June 22, 1988)
Abstract Govers, G. and Poesen, J., 1988. Assessment of the interrill and rill contributions to total soil loss from an upland field plot. Geomorphology, 1: 343-354. Erosion measurements were carried out on an upland field plot in the Loam Region of Belgium. The sediment being detached by splash on interrill areas is transported to the channel system mainly by interrill wash. Over the entire 7500 m=' field plot, rill (and gully) erosion is more important than interrill erosion. However, the relative importance of interrill erosion varies in time and in space, due to changes of the interrill surface characteristics and the activation of sidewall and gullying processes in the channel network. These factors should be taken into account in a realistic rill-interrill erosion model.
Introduction At present, few available field data permit the separation of rill ( a n d / o r gully) and interrill contributions to sediment removal from arable land. The scarcity of this type of field data is a major factor inhibiting the validation and further development of recently developed physically based models describing upland erosion by water (e.g., Foster, 1982; Rose et al., 1983). Knowledge of the contribution of rill and interrill processes to total soil loss has also some practical interest. If the ratio between rill and interrill erosion were known, even approximately, it would be possible to obtain a quick estimate of total erosion by making a volumetric recording of rill patterns. Plot measure*Both authors affiliated to the National Fund for Scientific Research (Belgium).
0169-555X/88/$03.50
ments will be necessary when interrill erosion has to be evaluated separately. In this paper an attempt is made to quantify the contribution of different sediment sources to total soil loss from an experimental field plot, located in Huldenberg, Belgium. The final estimates are necessarily based on extrapolations in time and in space. This leads to some inevitable uncertainty, but nevertheless, the study yields some interesting results. Materials and methods
The field site The field site of Huldenberg is located 15 km SSW of Leuven in the Belgian Loam Belt (Fig. 1, 2). The field, with a surface area of ca. 7500 m 2, is a south-facing convex-concave slope strip of about 50 m wide. Maximum slope is about
© 1988 Elsevier Science Publishers B.V.
344
Fig. 2. View on the Huldenberg experimental field. 25% (Fig. 1 ). The field varies markedly in lithological and pedological characteristics (Govers, 1987). It was conventionally tilled on 15 November 1983 and was kept clear of vegetation thereafter using herbicides. The observations discussed here took place from 15 November 1983 till 28 December 1984. Observations concerned rill and gully as well as interrill erosion processes. As rill and gully erosion processes have been described in detail elsewhere (Govers, 1987), attention here focuses on interrill erosion and the construction of a sediment budget.
Definition and measurement of rill and gully erosion
20 •
~ ,,
N
0
5
10m
Fig. 1. Map of the field site with location of the interrill plots and the channel network on 3/10/84. Contours are in m above sea-level, numbers 1-17 refer to transects.
The evolution of the rill and gully system was established by periodic surveys from 15 November 1983 to 3 October 1984 of rill and gully positions and dimensions along fifteen transects every 10 m downslope (Fig. 1 ). Flowlines where concentrated water flow had taken place without noticeable incision were also mapped. Three types of erosion were distinguished in the channel system (Fig. 3): (a) Rills were initially formed by hydraulic erosion of the cohesive bed (HRE). Its contribution to total channel erosion at a given cross section was estimated by multiplying bed width and depth of the rill.
345
/ ,,',//
/? I
/s,,'/
/
//
]///I 1/#]
RSW
Fig. 3. Definition sketch illustrating the various erosion subprocesses. (b) Mass transport processes on the sidewalls of the rills (RSW) caused subsequent widening of the rill. (c) Gully erosion (GE) occurred on the eastern half of the steepest slope section, where the channel bed consisted of Tertiary sands and was totally noncohesive when saturated. The upslope boundary of the gullies was formed by a clear headcut, so that their separate contribution to total channel erosion could easily be calculated.
Definition and measurement of interrill erosion It is generally accepted that sediment detachment on interrill areas is primarily caused by raindrop impact (e.g., Meyer et al., 1975; Kirkby, 1980; Gilley et al., 1985 ). The detached sediment can be transported downslope in three ways (Fig. 3 ): (a) There is a net downslope sediment flux because splash distances in the downslope direction are greater than in the upslope direction and because more detached sediment is ejected downslope than upslope (DSP) (b) Detached sediment can directly be splashed into the channel system and subsequently be evacuated by concentrated flow
(RSP) (c) Splash-detached sediment can be transported into the rill and gully system by interrill wash (IRW)
Splash detachment was measured using circular splash cups with a diameter of 5.2 cm. Real splash detachment is underestimated when calculated as the division of total mass of sediment caught in the cup by the splash cup surface area, as the contribution of the area within the cup is not taken into account (Poesen and Torri, in press; Torri and Poesen, 1988). Therefore, the masses of splashed sediment collected were multiplied by 1.25 in order to eliminate the effect of the splash cup diameter. From 15 November 1983 till 15 November 1984 measurements on splash detachment were collected on 20 sites randomly distributed over the field. From 2 October till 28 December 1984 splash detachment was measured within the interrill wash plots. Direct downslope movement of sediment by splash (DSP) was estimated from measured detachment rates using the model developed by Poesen and Savat (1981) and improved by Poesen (1985). To estimate RSP from detachment data, the relative amount of sediment splashed from a point source into an infinitely long channel of given width has to be known. This can be determined using procedures similar to those developed by Savat and Poesen (1981) to calculate real splash amounts and distances from tray data. Numerical integration of these point values over a sufficiently wide distance orthogonal to the channel allows then the calculation of the percentage of the material detached by drop impact per unit surface of interrill area that is received per unit surface of rill bottom. This procedure can be repeated for various channel widths and splash distances so that a nomograph can be constructed (Fig. 4). The nomograph can then directly be used to ascertain the amount of splash into a rill if the splash detachment per unit surface of interrill area, mean splash distance, and channel width are known. The relative amount of splash received by a rill bed is primarily dependent on the rill width and secondly on the mean splash distance (Fig. 4). On the field, mean splash distance was assumed
346 %
0.80
\ \
0.70
\ \ \ \
0.60 \
.,.-
\
\ \
\\
\
\
\ \
0 50
\ \
\ \ \
¢:
\ \
\
\
\
\
O.L,O
\
\ \ \
\
\
\ "x
\
x,
"-
030 \ \ "~"
0 20
X
=
20cm
X" = loom
0 10
0
'
~0
10
'
~0
~0
10
width of rift channel (cm)
Fig. 4. Nomograph to determine the relative splash amount caught by the rill bed (compared to an interrill area) as a function of mean splash distance (.~) and rill width.
to be 20 cm. Smits (1982) found that mean weighted splash distances of loamy soil materials varied from 16 to 24 cm and were almost independent of surface roughness and moisture content. Interrill wash erosion (IRW) was measured on 20 small bounded plots, installed at various locations on the field (Figs. 1 and 5). The surface areas of the plots varied between 0.5 and 0.66 m 2. Measurements took place from 2 October till 28 December 1984. As the field surface was totally covered with a crust at this time, tillage operations were evaluated by turning over the soil on 8 plots with a spade (Fig. 1 ). Each plot was equipped with a gutter and a
vessel to collect runoff water and sediment. The gutter was covered in order to prevent the collection of rainfall on the gutter surface and direct insplash (Fig. 5). The length of the plots (ca. 0.9 m) was smaller than the length of the mean flowpath on the interrill areas, which, according to visual observations, was of the order of a few meters. The plots were visited weekly, rainfall and interrill runoff volume being recorded after every important rainfall event. At the same time, representative runoff samples were taken to determine sediment concentration. Soil loss was calculated as the product of runoff voume and sediment concentration. In each plot a circular splash cup with a diameter
347
Fig. 5. View on an interrill plot.
of 5.2 cm was placed. The splash data permitted the calculation of a delivery ratio (DR), being the ratio between the mass of sediment effectively removed from the interrill plot by interrill wash to the mass of sediment detached by drop impact.
Rill and gully erosion Spatial variability of hydraulic rill erosion is to a large extent dependent on topographic factors, although slope and length exponents vary with time (Govers, 1987). Temporal evolution of total sediment production by H R E was strongly determined by higher magnitude events
on 3 February, 6 February and 28 August 1984, with rainfall amounts of 8, 13 and 36 mm at intensities of 50, 50 and 80 mm h -1, respectively. These three events, with a precipitation total of 57 mm, produced about 65% of total HRE during the observation period. Spatial variations in the hydrological and soil mechanical properties of the topsoil-subsoil complex made conditions for mass wasting most favourable on the eastern part of the midslope section, where a sandy loam topsoil with a thickness of ca. 30 cm overlays a compact, impervious subsoil, causing local saturation and consequent instability. On this part of the field, deep-seated sidewall failures were the most important. On the steepest slope section, where the plough layer had better structural characteristics, mass transport occurred mainly by soil fall. Mass movement on rill sidewalls was most active in the first moist period after the higher magnitude event of 28 August 1984. However, soil fall was also triggered by crack formation during the dry spring period. On 3 February 1984, gullies formed on the eastern part of the steepest slope section (transect 14) where Tertiary sand formations are very near the surface. Headcut retreat and gully deepening were only observed during higher magnitude events, while widening occurred mainly during dry periods when the sand lost all cohesion.
TABLE 1 Soil losses (t) due to different erosion processes and cumulative precipitation (P in ram) on different dates: direct splash to the rill and gully system (RSP), interrill wash (IRW), total interrill erosion (TIRE), hydraulic rill erosion (HRE), rill sidewall processes (RSW), gully erosion (GE), total rill and gully erosion (TRGE) and cumulative precipitation (P)
RSP IRW TIRE HRE RSW GE TRGE Total P (mm)
15/11/83
30/01/84
0 0 0 0 0 0 0 0 0
0.1 3.1 3.2 4.4 0.3 0.0 4.7 7.9 178
03/02/84 0.3 9.9 10.2 21.5 2.7 2.3 26.5 36.7 208
06/02/84
29/08/84
03/10/84
0.6 14.6 15.2 34.9 5.4 4.4 44.7 59.9 259
2.0 29.2 31.2 51.8 18.4 11.6 81.8 113.0 559
2.4 34.1 36.5 65.8 47.9 16.7 130.5 167.0 721
348 100 %-
-100%
RSP-
IRW-
RSW SO-
- so
HRE---
30 Jon.'8~
03 Ft b.'84
06 Feb.'8/,
28 Aug:B/,
03 0ct.'84
Fig. 6. Relative contribution of various subprocesses to total cumulative sediment output on five dates. TABLE 2 Soil losses (t) due to different erosion subprocesses per slope strip (extending 5 m upslope and 5 m downslope from each transect) on 30/01/84 and 3/10/84 Transect
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3/10/84
30/01/84 RSP
IRW
HRE
RSW
RSP
IRW
HRE
RSW
0.0 0.002 0.002 0.002 0.004 0.006 0.007 0.006 0.005 0.005 0.004 0.006 0.006 0.005 0.005
0.07 0.21 0.14 0.13 0.20 0.24 0.28 0.32 0.25 0.23 0.18 0.23 0.21 0.22 0.22
0.0 0.006 0.009 0.022 0.047 0.10 0.13 0.27 0.31 0.70 0.70 0.58 0.60 0.54 0.44
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.035 0.084 0.042 0.047 0.0 0.052 0.019
0.0 0.037 0.056 0.059 0.14 0.18 0.23 0.25 0.22 0.24 0.18 0.23 0.21 0.21 0.20
0.71 1.42 1.35 1.14 1.77 1.87 2.01 2.11 1.72 2.38 2.65 3.94 4.65 4.64 1.70
0.0 0.062 0.52 1.36 1.74 2.33 2.94 4.96 6.59 7.67 6.60 6.90 10.26 9.40 4.46
0.0 0.0 0.05 0.55 0.67 0.75 1.59 2.83 7.74 9.55 7.66 6.45 5.19 3.70 1.07
On the western half of the steepest slope section, where a loamy well-structured topsoil was present over a compacted Bt-horizon, relative intensive pipe erosion was observed. Many pipe segments, though not all, were initiated by mole activity. Pipes often turned into rills due to roof
collapse. The digging of the moles also supplied loose material to the existing rills. Soil loss due to animal activity in this part of the field was estimated to be of the order of a few tonnes. Generally, hydraulic rill erosion ( H R E ) was the dominant channel erosion process, al-
349 though its relative importance decreases with time (Table 1, Fig. 6). Sidewall processes were the most important sediment source in the channel system from 29 August till 3 October 1984. The contribution of the gullies to total sediment output from the channel system remained relatively stable after 3 February 1984; they produced ca. 10% of total channel erosion, although their total length was only 45 m, compared to a total channel length of ca. 2000 m. At the end of the observation period, gully erosion was dominant on transect 14, while sidewall processes were the most important on the mid-slope section, where deep-seated failures occurred (Table 2). Interrill erosion
Direct downslope movement by rainsplash
(DSP) As the total amount of sediment detached from 15 November 1983 till 3 October 1984 was ca. 15.2 kg m -2, total sediment discharge on the steepest slope section was m a x i m u m 2.41 kg m - 1of contour length, which corresponds to an upslope soil loss of less t h a n 0.02 kg m -2, taking into account an upslope slope length of 130 m. Furthermore, Poesen (1986) showed that during m a n y events splash transport is directed upslope due to the influence of southwesterly winds. This process is thus a relatively unimportant sediment transport agent.
Transport into the rill and gully system by splash (RSP) Using rill and gully width data, we estimated that 2.4 t of soil material were directly splashed into the rill and gully system (Table 1). The amount of sediment lost by RSP increased from the upper field boundary to transect 6, as in this area the number of rills as well as the mean rill width increased with distance downslope (Table 2 ). Further downslope, soil loss by RSP remained more or less constant, because the
increase in mean rill width compensated for the decrease in the number of rills (Table 2). Sediment lost from the field by this process represented only 2% of total soil loss (Table 1 ).
Transport into the rill and gully system by interriU wash (IR W) Although the delivery ratio of the interrill plots shows a high random variation, the mean delivery ratio of the tilled plots is significantly higher (p < 0.01 ) t h a n the mean delivery ratio of the crusted plots (Fig. 7). This can be explained by the fact that sediment e n t r a i n m e n t by interrill wash on low to intermediate slopes is sizeselective, whereby fines are preferably removed. After a sufficiently long period of time, a coarse residue is left on the surface, which can still be detached by splash but cannot be transported by interrill flow, causing a decrease of DR. Interrill erosion is then detachment-limited for the fine fractions, but transport-limited for coarser material. On a sufficiently steep slope all detached size fractions can be transported by the runoff, so that the DR remains high. There is abundant evidence that IRW on low to moderate slopes is a size-selective process. Gilley and Gee (1976) found that sediment eroded from a tilled silty clay loam on 13% slope only contained 3% sand, while the original soil had a sand content of 12%. The size distribution of the material eroded from a nontilled plot was about equal to that of the parent material, but total soil loss on the nontilled plots was markedly lower. Verhaegen (1984), using a rainfall intensity of 65 m m h - 1, found that wash material eroded from a 1 m long and 0.20 m wide flume filled with a soil containing 68% sand, contained only 20% sand on a 0.087 slope. However, on a 17% slope the size distribution of the wash-transported material was about equal to that of the parent material. Govers (1985) showed that erosion of loamy material by flowing water is size-selective as long as the (grain) shear velocity of the water is below
350 :~ 1.5-
x
X titled plots • crusted ptots
~ 13121.11.00.90.8-
/ 0.7-
/
0.6-
Il •
0.50.~.-
x
/ /
x j/
J
/
/
/
/
/
/
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~f J
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0.20.10
o~
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Slope gradient
Fig. 7. Delivery ratio as a function of slope for tilled a n d crusted interrill plots. T h e curve indicates t h e tentative relationship between DR a n d slope for crusted plots.
3 cm s - 1. No clear influence of raindrop impact on the grain size distribution of the exported material was found. A runoff rate of 10 mm h - 1 (which was exceeded 3-4 times during the observation period) corresponds to a unit discharge of 0.3 cm 3 cm -1 s - l ) , assuming an interrill length of 2 m and water flow over 20% of the total width. A shear velocity of 3 cm s - 1 can then be reached on a 16% slope. The time taken for a freshly tilled surface with a high DR and selective erosion to evolve into a crusted surface with a low DR and nonselective erosion as well as the steady-state value of DR will depend on many factors, including the size distribution of grains and aggregates of the parent material, the depth of the active splash layer, the hydraulic characteristics of the interrill wash, the ratio between selective wash loss and
nonselective direct splash loss, the degree of disturbance by animal activity, frost action and cracking. The present data indicate that a steady state will be reached only after a considerable amount of precipitation and sediment removal. The DR of the tilled plots showed no tendency to decrease within the observation period with 163 mm of rainfall. On the contrary, the mean DR of the tilled plots increased during the first half of the observation period due to the increasing runoff production as crusting proceeded. This contrasts with the findings with respect to runoff generation. Mean runoff production on the tilled plots equalled mean runoff production on the crusted plots after about 150 mm of rain (Poesen and Govers, 1986 ). After tillage operations, equilibrium conditions are much
351 more rapidly established for interrill runoff production t h a n for interrill wash erosion. Different crust characteristics determine erosion and hydrological properties. This may have important implications with respect to crust evolution description. The fact that IRW can be highly selective also has implications for the influence of slope on interrill erosion. The slope dependency of interrill erosion varies with time: it is relatively small in the case of a freshly tilled seedbed, when fine, transportable material is dominant on the surface everywhere. As crusting and the formation of a coarse lag deposit proceed at a slopedependent rate, the influence of slope may become more important, as indicated by Fig. 7. Interrill wash measurements were limited to a relatively short period. In order to estimate the contribution of I R W to sediment removal for the period during which the rill and gully system was monitored, the following assumptions were made: (a) From 15 November 1983 till 5 February 1984 the DR was, everywhere on the field, assumed to be equal to the mean DR of the tilled plots (DR=0.725). (b)From 5 February 1984 till 3 October 1984, the DR was assumed to be dependent of slope, according to the tentative curve on Fig. 5. (c) Effective contribution was limited to those interrill areas located within 1 m on each side of a flowline, rill or gully. The assumed change from the first situation (tilled surface) to the second (crusted surface) is certainly too abrupt. It is only partly justified by the occurrence of the first higher magnitude event 3 February 1984, causing large amounts of soil loss, both on interrill areas and in the rill and gully system, and a drastic change of interrill surface microtopography. Surface microtopography did not change in any significant way after this date (Govers and Poesen, 1986). Based on the foregoing assumptions, total IRW could be calculated using splash detachment data collected from 15 October 1983 till 15 October 1984. Calculations were performed
per transect, taking into account the spatial variability of splash detachment and the decrease of interrill area due to rill development (Tables 1 and 2). Interrill wash appears to be by far the most important sediment removal agent on interrill areas (Table 1, Fig. 6). It is estimated that in total more t h a n 34 t of soil material were removed by IRW, corresponding to an ablation of ca. 3 mm, as the bulk density of the plough layer was about 1.35 g cm -~. However, the intensity of IRWwas highly variable in space. On the upper part of the slope only 2.4-3 kg m -2 was removed by IRW, while on the steepest slope I R W evacuated ca. 10 kg m -2, corresponding to an ablation of ca. 7 m m (Table 2). These data indicate that interrill erosion alone can cause soil losses far above generally accepted soil loss tolerance levels, at least if an efficient sediment evacuation system is present. Erosion survey data which are solely based on visual observations (e.g. on air photographs) may therefore severely underestimate the erosion hazard. Discussion
Most of the erosion on the field occurred in the rill and gully system. Only 22% of total sediment removed came from interrill areas. The relative importance of interrill erosion decreases with time from 46 to 22% (Fig. 6). This means that, on normally cultivated land, which is only vulnerable to erosion for a shorter period, interrill erosion may be relatively more important. The fact that the relative importance of interrill erosion decreases with time is not a general phenomenon. Collins and Dunne (1986) found that the erosion of tephra produced by the Mount St Helens eruption of 1980 occurred initially mainly by rills, but the importance of channel erosion decreased relatively rapidly with time. From the data presented by Mtkawa et al. (1987), which were collected on short plots (10 m), established on a tropic alfisol in Tanzania, it can be deduced that also in this case
352
the relative importance of rill erosion decreased with time. Mtkawa et al. do not discuss this issue, but Collins and Dunne explained the decreasing importance of channel erosion by two factors: the permeability of the interrill areas increased with time, mainly due to the stripping of the silty top layer and to frost action and incision of the deepest rills was blocked when the bed reached the more resistant original land surface. We observed the reverse trend on our site. Infiltrability of the interrill areas generally decreased in time, due to progressive sealing and compaction of the plough layer, although the crust was sometimes disturbed by animal activity or cracking (Govers and Poesen, 1986; Poe-
sen and Govers, 1986). Therefore, obliteration of the rills was very rare. Furthermore, gullying occurred on the eastern part of the slope because the resistance to runoff erosion of the sandy subsoil was much lower than that of the sandy loamy topsoil. Thus, on the Huldenberg site, as well as in the case of tephra erosion reported by Collins and Dunne, temporal evolution of interrill surface properties and variations of resistance to runoff erosion in the soil profile are some of the major controls on the balance between rill and interrill erosion. On the Huldenberg site, stability of rill sidewalls is also a very important factor. Also, topographic factors are important. Interrill erosion dominates on the upper part of the slope, as channel ero-
TABLE 3 Percentages of total interrill and rill erosion due to rill erosion as reported in the literature Source
Country
Period of observation
Soil type
Kerenyi (1985)
Hungary
Feb 1979
Brown stony forest soil Fine sandy soil Freshly deposited tephra --
~vlorgan et al. U.K. ( 1987 ) Smith and Swanson U.S.A. ( 1987 )
--
Mikhailov (1949, cited by Morgan, 1986) Zachar (1982)
--
U.S.S.R.
1980-1981
Slope ( To)
2-12 -0-45
Plot size
Surface state
Percent of total interrill and rill erosion due to rilling
Field length 20-1100 m --
Cultivated
14
Cultivated
20-50
2.4 km 2
Bare
37
23-30
Czechoslovakia After cloudburst - 13-15 on May 23 1958 Lootens (1983) Zaire Rain season Clay soil 0.51981-82 1.5 McCooland George U.S.A. (1983) Mtkawa et al. Nigeria 2 rainy seasons Medium to light 4.5(1987) 1985 textured 6.5 alfisol Gabriels et al. Belgium 20 Oct 1974Loamy sand 3 (1977) 20 Dec 1974 This study Belgium 15 Nov 1983Loamy to 0-13 3 Oct 1984 sandy loamy
51-63
0.6 ha
10mX2m
1.4 ha 150m × 50m
Potatoe field
70
Cultivated, bare Cultivated
73 75-95
Cultivated 67-99 maize, cowpea, fallow Cultivated, 87 seedbed Cultivated, 78 bare
353
sion is much more dependent on slope length and steepness. On 30 J a n u a r y 1984, interrill erosion was dominant up to transect 8, but on 3 October 1984 rill erosion was more important t h a n interrill erosion downslope from transect 4 (Table 2). On 3 October 1984, the ratio (interrill erosion)/(rill and gully erosion) is very low for transect 14, where the gullies were located. Some authors assumed a more or less fixed ratio between rill and interrill erosion in order to calculate total soil loss from rill survey data (e.g. McCool and George, 1983). It is obvious that such a fixed ratio does not exist; the balance between interrill and rill erosion processes is dependent on the factors discussed above, as well as on others, like the spatial organisation of the channel network, climatic and vegetation effects, etc. This explains some of the variability of the ratios reported in the literature (Table 3 ).
Conclusions On the field plot, sediment detachment on interrill areas occurs mainly by splash, while interrill wash is the most effective sediment transport agent. Only a minor a m o u n t of soil is removed by direct splash into the rill and gully system. However, interrill erosion is certainly not always detachment-linked. Due to the fact that sediment removal from a freshly tilled soil is size-selective, a coarse residue is formed at the surface which can no longer be transported by interrill wash. As a consequence the DR generally decreased in time and became more dependent on the slope angle; only on the steepest part of the plot a DR near 1 was maintained. Different surface characteristics determine sediment removal and runoff yield. Runoff yield reaches a steady state after a much smaller amount of rainfall. This is important with respect to crust characterisation as well as physically-based erosion modelling. Approximately 22% of total soil lost came from interrill areas. The relative importance of
interrill erosion decreases with time and with slope steepness, not only because of the slopedependent decrease of the DR, but also because of the activation of sidewall processes and gullying in the channel network. Other factors may also influence the ratio of interrill erosion to rill and gully erosion. The use of a fixed ratio is therefore not recommended.
Acknowledgement The authors wish to express their gratitude to Eng. J. Meersmans, L. Cleeren and R. Geeraerts for their technical assistance. The manuscript was much improved thanks to the comments of two anonymous referees.
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