A methodology to study rain splash and wash processes under natural rainfall

Transcription

1 HYDROLOGICAL PROCESSES Hydrol. Process. 17, (2003) Published online 21 November 2002 in Wiley InterScience ( DOI: /hyp.1154 A methodology to study rain splash and wash processes under natural rainfall A. I. J. M. Van Dijk,* L. A. Bruijnzeel and E. H. Eisma Department of Geo-Environmental Sciences, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam Abstract: The complex interactions between rainfall-driven erosion processes and rainfall characteristics, slope gradient, soil treatment and soil surface processes are not very well understood. A combination of experiments under natural rainfall and a consistent physical theory for their interpretation is needed to shed more light on the underlying processes. The present study demonstrates such a methodology. An experimental device employed earlier in laboratory studies was used to measure downslope rain splash and splash-creep, lateral splash, upslope splash and rainfall-driven runoff transport (wash) from a highly aggregated clay-rich oxisol exposed to natural rainfall in West Java, Indonesia. Two series of measurements were made: the first with the soil surface at angles of 0, 5, 15 and 40 ; and the second all at an angle of 5 but with different tillage and mulching treatments. A number of rainfall erosivity indices were calculated from rainfall intensity measurements and compared with measured transport components. Overall storm kinetic energy correlated reasonably well with sediment transport, but much better agreement was obtained when a threshold rainfall intensity (20 mm h 1 ) was introduced. Rain splash transport measurements were interpreted using a recently developed theory relating detachment to sediment transport. Furthermore, a conceptually sound yet simple wash transport model is advanced that satisfactorily predicted observed washed sediment concentrations. The lack of replication precluded rigorous assessment of the effect of slope and soil treatment on erosion processes, but some general conclusions could still be drawn. The results stress the importance of experiments under conditions of natural rainfall. Copyright 2002 John Wiley & Sons, Ltd. KEY WORDS rain splash; sheet wash; soil erosion; rainfall erosivity; threshold energy; modelling INTRODUCTION Rain splash soil detachment and transport by impacting rain drops is an important first step in soil erosion. Unconcentrated (sheet) runoff usually does not have enough power to actively detach and entrain soil particles (Rose, 1993), but particles detached by rain splash may subsequently be transported by the flow (Kinnell, 1990; Hudson, 1995). On short steep slopes (e.g. bench terrace risers) rain splash may be the dominant transport mechanism (Van Dijk, 2002; Van Dijk et al., in press). Physical understanding of the processes causing detachment and transport by falling rain drops has improved considerably over the last two decades, mainly by laboratory experiments (Poesen and Savat, 1981; Ghadiri and Payne, 1986, 1988; Poesen and Torri, 1988; Sharma and Gupta, 1989; Salles and Poesen, 2000). Unfortunately, the results of laboratory studies are not readily translated to field situations. For example, rainfall kinetic energy has been related to rain splash in both natural and laboratory studies (Kneale, 1982; Salles and Poesen, 2000), but the manner in which kinetic energy was varied and estimated differs between the two experiment types. Changes in simulated rainfall kinetic energy are normally achieved by using (uniform) drops of different size or fall height, while rainfall intensity is often high (e.g. Free, 1960; Quansah, 1981). By contrast, natural rainfall kinetic energy depends primarily on the rain drop size distribution (although * Correspondence to: Dr A. I. J. M. Van Dijk, Department of Geo-Environmental Sciences, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam. dija@geo.vu.nl Received 23 October 2001 Copyright 2002 John Wiley & Sons, Ltd. Accepted 5 April 2002

2 154 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA moderated by wind) and in field studies is often not measured but estimated using empirical relationships with rainfall intensity (see Van Dijk et al., 2002a for a recent review). A comparable difference relates to the apparent existence of a rainfall kinetic energy threshold observed in both laboratory and field experiments. Individual water drops have to overcome a critical kinetic energy threshold before they are actually able to dislodge and transport soil particles (Sharma and Gupta, 1989; Salles et al., 2000) and, at first sight, this appears to agree well with findings that use of a threshold intensity (or corresponding kinetic energy) correlates better to sediment transport measurements than does gross kinetic energy (e.g. Hudson, 1965; see discussion). The moments of natural rain drop size distributions increase with rainfall intensity in a gradual manner, however (despite considerable temporal variation, see Van Dijk et al., 2002a). No sudden increase in the proportion of drops with higher than threshold kinetic energy occurs, therefore, and even very low intensity rainfall will contain some erosive rain drops. This renders the existence of a threshold kinetic energy for natural rainfall difficult to explain in physical terms. Finally, in most rain splash experiments splash transport is measured and this depends on both detachment (expressed on an area basis) and splash distance (Van Dijk et al., 2002b). Laboratory experiments by Riezebos and Epema (1985) indicated that the average distance over which particles are splashed depends on the height from which (artificial) drops fall, and this further complicates extrapolation of laboratory experiments to natural conditions. A similar case can be made with regard to rainfall-driven runoff transport, or wash. Laboratory experiments have greatly improved the physical understanding and mathematical description of wash processes (Moss, 1988; Kinnell, 1990; Proffitt and Rose 1991; Heilig et al., 2001), but most experiments involved artificial rainfall at a constant rate and uniform soil material. Natural rainfall and field soils present added complexity whereas, in addition, experimental boundary conditions (rainfall rate, water inflow or outflow rates, flow depth and velocity) can no longer be controlled outside the laboratory and, in fact, probably become closely related. The problems outlined above clearly demonstrate that to understand rainfall-driven erosion processes experiments under natural rainfall are indispensable. The current study presents such a methodology, using experimental soil trays modified slightly from the design used by Wan et al. (1996) in laboratory studies. The soil trays were filled with a highly aggregated clay soil exposed to the natural rainfall regime prevailing in West Java, Indonesia, during two consecutive periods covering 17 and 15 storms, respectively. After each event, different components of sediment transport were measured separately. During the first period, the devices were placed at four different angles to investigate the effects of slope on splash and wash processes, whereas during the second period, the effects of mulch cover and frequent tillage were studied. Various rainfall erosivity indices proposed in the literature were tested as predictors of splash and wash transport for this specific combination of soil and rainfall regime. A mathematical theory relating splash transport to detachment (Van Dijk et al., 2002b) is employed to interpret splash measurements, and a simple conceptual wash transport model is advanced. Given the high variability of erosion processes expected under (semi-)natural conditions and the lack of replication for the various slopes and treatments, a rigorous statistical assessment of their effects was not feasible, but some interesting observations could still be made. MATERIALS AND METHODS Site and soil Experiments were conducted within the framework of the Cikumutuk Hydrology and Erosion Research Project (CHERP) in the small (125 ha) but steep agricultural Cikumutuk catchment about 60 km east of Bandung, in the volcanic uplands of West Java, Indonesia. All experiments were carried out between 19 February and 9 April 1999 near the project laboratory ( S; W) at an altitude of 580 m above sea

3 SPLASH AND WASH UNDER NATURAL RAINFALL 155 level. The area receives an annual rainfall of c mm, with a drier season (average monthly rainfall less than 60 mm) generally extending from June until September. The soil used was a kaolinitic silty clay with very little sand (c. 70% clay, 30% silt) classified as a Haplorthox (USDA nomenclature) and developed in Pleistocene and Holocene andesitic tuffs. Material used in the experiments was collected from the top 10 cm of the beds of reverse sloping bench terraces subject to seasonal rain-fed cropping for more than six years. The soil had a median dispersed particle size of 1Ð2 µm and was highly aggregated, with crumbly sub-angular aggregates sometimes attaining a diameter of more than 8 mm and a median dry- and wet-sieved aggregate size of 3Ð2 and 1Ð1 mm, respectively. Oven-dry bulk density was 920 kg m 3 (st. dev. š90), with high porosity (64 š 8%) and saturated hydraulic conductivity ( mm h 1 ). More details about research context, site and soil properties are given in Van Dijk (2002). Experimental design The combined splash and runoff collecting system used was modified slightly from the design by Wan et al. (1996) (see Figure 1). It consisted of a central soil tray with dimensions of 0Ð60 ð 0Ð30 ð 0Ð10 m 3 (L ð W ð H), with sediment collectors attached to all sides, to separately measure sediment transport by wash and the respective directional components of splash on a sloping surface. Runoff flowed over an apron attached to the soil tray and into a small runoff guiding trough at the downslope end of the soil tray (Figure 1b). Unlike the design by Wan et al. (1996), the present collecting system had a separate piece of flattened U-shaped metal to cover the runoff collector and intercept any splashed sediment. The downslope splash board was roofed to prevent sediment from being splashed back onto the soil. The runoff collector lid rested on the apron of the wash collector with four small extensions, leaving a slot of 5 mm height for the runoff to enter. During preliminary trials, substantial amounts of coarse aggregates were splashed or pushed through this slot by rain drops and ended up on the apron. A small portion of these aggregates was eventually washed into the runoff collecting trough, obscuring the distinction between washed and splashed sediment. To separate most of this splash-creep sediment (cf. Moeyersons, 1975) from washed sediment proper, the runoff collecting trough was covered with 2 mm nylon mesh. Any sediment remaining in the trough was added to the splash-creep sediment fraction. This might result in an overestimate of wash transport, because fine sediment not initially washed ended up in the container, or an underestimate, because some sediment did not reach the runoff container but remained on the apron or in the runoff guiding trough (Figure 1b, see discussion). Upslope, lateral and downslope splashed material ended up in separate collectors equipped with splash boards. The splash collectors were attached to the soil tray in such a way that the boards could always be positioned vertically, the collectors were detachable from the splash boards to facilitate sampling (Figure 1b). Cover strips were added to guide all sediment from the boards into the collectors. Unlike the device used by Wan et al. (1996), the lateral and upslope splash boards were not equipped with roofing, as this might interfere with rain falling on the soil. Small perforations were made c. 1 cm above the bottom of the splash collectors, however, to allow drainage of excess rain water, while at the same time maintaining a water layer preventing water and sediment from splashing back onto the soil or moving in suspension. Experimental procedures Two series of experiments were conducted with four devices each; the first series was carried out between 19 February and 20 March 1999 and covered 17 storms, whereas the second series was conducted from 23 March to 9 April 1999 during 15 storms. Before each series of measurements, a c. 4-cm thick layer of coarse (8 30 mm) gravel was placed on the bottom of each soil tray to facilitate drainage through holes at the downslope end of the soil tray (Figure 1b). On top of the gravel, a wet piece of cloth was placed and the tray was filled with soil to the apron of the wash collector and up to 2 3 mm below the rims of the splash collectors. The soil used was not dried or sieved (although large litter was excluded) and had an estimated

4 156 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA Figure 1. (a) Overall and (b) longitudinal cross-sectional view of the combined splash and runoff collection system used in the present study as modified from Wan et al. (1996). Drawings are not to scale, for full explanation see text

5 SPLASH AND WASH UNDER NATURAL RAINFALL 157 initial moisture content of c. 55% in both measuring series, but remained well aggregated. The soil was not artificially wetted prior to the experiments, but after the first substantial storm some soil material was added to re-attain the original level, after which the measurements were started. For the first series of measurements, the four devices were placed at angles of 0, 5, 15 and 40, respectively. In the second series, all devices were placed at an angle of 5 ; the soil of two devices was partially covered with mulch, while a third was tilled after each event and the fourth was left untouched for reference. Actual mulch cover provided by partly decomposed maize stalks and leaves was determined by digital analysis of overhead photographs, at 35% and 55%, respectively (both accurate within 3%). Tillage was done after each storm to break up any crusts that might have formed, using a small rake with nails protruding 3 cm. Sediment transport components were sampled after each storm. First, the water volume in the runoff container was measured to the nearest millilitre, after which the water and sediment were transferred to a beaker. Some coagulating agent (Al 2 (SO 4 ) 3 aq ) was added and after the sediment had settled the supernatant water was decanted. Sediment adhering to the splash boards or to the cover of the runoff guide (Figure 1b) was transferred into the corresponding collectors, using a small paintbrush (when dry) or a wash bottle (when wet). Next, the collectors were taken out and sediment was washed into crucibles, as was splash-creep sediment on the apron and in the runoff trough. The splashed and suspended washed sediment was dried to constant mass at 80 C and allowed to cool and attract air moisture (increasing mass by 1 4%) before mass was measured to the nearest 0Ð001 g. Rainfall characteristics and erosivity indices Storm rainfall depth was measured using a standard rain gauge (100 cm 2 orifice), while rainfall intensity was measured with a custom-built tipping bucket-logger system, and calibrated and resampled into 5-min intervals following methods outlined in Van Dijk (2002). A number of erosivity indices proposed in the literature were calculated for each individual storm using the rainfall intensity data. The indices and corresponding equations are listed in Table I and include storm rainfall depth (P in mm), storm kinetic energy flux (E K in J m 2 ), the amount of rain falling at an intensity higher than a threshold intensity R 0 (P R0 in mm), the kinetic energy corresponding to that amount of rainfall (E R0 Table I. Rainfall erosivity indices as used in the present study and their method of calculation. P (mm) denotes rainfall depth, R (mm h 1 ) rainfall intensity, 1t (h) time interval length, E K (J m 2 ) storm kinetic energy flux, e K (J m 2 mm 1 ) kinetic energy load, R 0 (mm h 1 ) threshold rainfall intensity, R 30 and R 5 (mm h 1 ) maximum rainfall intensity for 30- and 5-min periods, respectively, and b an empirical coefficient Description Equation (i) Storm rainfall depth (mm) (ii) Storm kinetic energy (J m 2 ) (iii) Amount of rainfall falling at higher than threshold intensity (mm) (iv) Kinetic energy of rainfall falling at higher than threshold intensity (J m 2 ) (v) Power function of rainfall intensity P E K D n e K R1t i id1 R>R 0 P R0 D R1t i id1 R>R 0 E R0 D e K R1t i id1 R b D (vi) EI 30 index (Wischmeier and Smith, 1978) EI 30 D E K R 30 (vii) AI m index (Lal, 1976) AI m D PR 5 n R b 1t i id1

6 158 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA in J m 2 ), rainfall intensity raised by an (optimized) power ( R b ), the USLE rainfall or R factor (EI 30 ; Wischmeier and Smith, 1978) and the AI m index of Lal (1976). More complex erosivity indices, involving rain drop size and drop momentum or kinetic energy, have been proposed (inter alia Meyer, 1965; Riezebos and Epema, 1985; Salles and Poesen, 2000), but to calculate these for natural rainfall would require detailed knowledge of the corresponding rain drop size distributions, and this was not available. Rainfall kinetic energy was estimated using a general relationship between rainfall intensity (R in mm h 1 ) and kinetic energy load (e K in J m 2 mm 1 ) proposed in Van Dijk et al. (2002a). To calculate the EI 30 index, storm kinetic energy was also estimated using the set of equations proposed by Wischmeier and Smith (1978), although this will not have affected results much (cf. Van Dijk et al., 2002a). A number of erosivity indices involve the use of a threshold rainfall intensity or empirical coefficient (cf. Table I). These were evaluated using two different approaches. Firstly, coefficient values were optimized by least squares for each individual series of sediment transport component measurements, using the Levenberg Marquardt method. For each device, this yielded five different coefficient values (i.e. for upslope, lateral and downslope splash, splashcreep and wash transport, respectively) and, therefore, 40 values in total (two measured periods with four devices each). The median of these 40 values was subsequently used to calculate erosivity indices again. In both approaches the coefficient of determination (r 2 ) associated with a one-parameter linear regression between event sediment transport amounts and storm erosivity index was used as a measure of performance. Interpretation of rain splash transport components Upslope, lateral and downslope splash and splash-creep transport rates (all in g m 1 ) across the boundaries of the soil tray were calculated by dividing the amount of splashed sediment by the (projected) length of that particular side. From these, gross downslope splash transport (q g ) was calculated as downslope splash plus splash-creep, whereas upslope transport was subtracted from these to derive net downslope transport (q n ). Dividing transported amounts by rainfall erosivity yielded estimates for lateral, gross and net downslope splash transportability (T lat, T g and T n, respectively, with units depending on the erosivity index used). In Van Dijk et al. (2002b) an expression was derived relating splash transport (q in g m 1 ) across a boundary to soil detachment on an area basis ( in g m 2 ) by assuming a radial exponential redistribution of splashed particles: q D 3 1 where 3 (in m) is the weighed average splash length. The (projected) splash distance changes on a slope because of changes to the splash process and the effect of the slope itself on splash trajectories (Ghadiri and Payne, 1986, 1988; Van Dijk et al., 2002b). The current experimental design did not produce measurements of average splash distance, but field measurements suggested values of 3 ³ 1 cm for all-sided splash on sub-horizontal (2 3 ) bench terrace beds and 3 ³ 12 cm for downslope splash on terrace risers, respectively (Van Dijk et al., in press). If a change in slope only affects the component of splash length in the direction of the slope gradient, and not that parallel to the slope, then lateral splash length should equal the value of 3 on a horizontal surface for all slopes. Tentatively making this assumption and using a value of 3 D 1 cm, Equation (1) was used with lateral transportability (T lat ) values to obtain initial estimates of soil detachability (D, the unit depending on the erosivity index used). Interpretation of runoff and wash transport Measured storm runoff volumes were divided by the projected area of soil to obtain total event runoff depth Q tot (in mm) and the spatially variable infiltration model (SVIM) of Yu et al. (1997) was used to estimate soil infiltration characteristics from these. The model assumes an exponential spatial distribution of infiltration rates and successfully simulated runoff from (sections of) bench terraces in the study area (Van Dijk, 2002).

7 SPLASH AND WASH UNDER NATURAL RAINFALL 159 After an initial addition infiltration (F 0 in mm) has taken place, runoff equals rainfall excess and can be expressed as a one-parameter function of rainfall intensity (R): Q D R I m [1 e R/I m ] 2 where Q (in mm h 1 ) is the instantaneous runoff rate and I m (in mm h 1 ) the average maximum infiltration rate, reached when the entire area generates runoff (Yu et al., 1997). For each of the eight data sets a single value of I m, optimized by least squares using the Levenberg Marquardt method, and rainfall intensity data were used to model runoff depth. Model performance was expressed by model efficiency (ME; Nashand Sutcliffe, 1970). A simple conceptual model of wash transport was based on the recognition that the maximum possible concentration of detached particles in rain water, directly after impact, is given by the ratio of total detachment () and rainfall (P) for the event. If it is assumed that a constant washed fraction j of the detached sediment settles slowly enough to be transported across the downslope end of the soil area before settling, then event soil loss by wash (M wash ingm 2 ) may be approximated by: M wash D c wash Q tot D j P Q tot D j DE P Q tot 3 where c wash (in kg m 3 ) is the concentration of washed sediment in runoff and E is the chosen erosivity index. On a soil area of limited extent, Equation (3) introduces an error, because material splashed from the soil area is not available for wash transport, but this error is demonstrably very small in the present case. Presumably, j will be dependent on the size distribution of detached particles, but it may also vary as a function of slope gradient, length and roughness (Moss, 1988; Kinnell, 1990; Proffitt and Rose, 1991; Heilig et al., 2001). It is acknowledged that the assumption of j being constant regardless of rainfall pattern and intensity probably represents a simplification of reality. Detachability values derived from lateral splash transport were used together with j values optimized using the Levenberg Marquardt method. Sediment concentrations and wash transport amounts modelled with Equations (2) and (3) were compared to those observed. Model performance was investigated by calculating Nash Sutcliffe model efficiency (ME), both in terms of sediment concentration and wash transport amounts for events. For comparison, calculations were also made using modelled runoff and a single optimized overall sediment concentration for each device. RESULTS Rainfall and performance of erosivity parameters Rainfall patterns during the two measuring periods are shown in Figure 2. The first series of measurements (19 February 20 March 1999, 22 days) counted 17 storms of 1Ð0 38Ð8 mm, totalling 322 mm with a total kinetic energy of 6234 J m 2. The second series of measurements (23 March 9 April 1999, 16 days) included 15 storms of 4Ð2 57Ð0 mm producing 366 mm of rainfall with a kinetic energy of 8199 J m 2.Maximum 5- and 30-min rainfall intensity were 81 and 51 mm h 1, respectively, during the first period and 147 and 101 mm h 1 during the second. The average coefficients of determination (r 2 ) for the one-parameter linear regression equations between event sediment transport components and storm erosivity indices are listed in Table II. The best result overall was obtained using the kinetic energy of rainfall above an optimized threshold intensity (r 2 D 0Ð58), giving the best predictions for 22 of the 40 data sets. Optimized threshold values for individual data sets were 2 24 mm h 1, with the most extreme values still resulting in rather low r 2 values and being associated with low overall transport amounts (upslope splash on a 40 slope and of splash from the mulched soil). Applying the median (and average) optimized threshold intensity value of 20 mm h 1 throughout did not decrease the

8 160 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA Series I Series II Storm depth (mm) Feb 28-Feb 8-Mar 16-Mar 24-Mar 1-Apr 9-Apr Figure 2. Patterns of daily rainfall during the two experimental periods in 1999 average coefficient of determination much (r 2 D 0Ð57), whereas the number of best predictions when using a single coefficient value increased to 26 out of 40 (right-hand column in Table II). The second best performance was obtained by using the amount of rainfall at intensities higher than a threshold value (r 2 D 0Ð55, best performance for eight out of 40 data sets). The median, average and range of these optimized threshold intensities were very similar to those found for kinetic energy, but using the median value of 20 mm h 1 throughout reduced the coefficient of determination to r 2 D 0Ð49. A similar degree of association (r 2 D 0Ð54) was found for a power function of (5-min) rainfall intensity (R b ), but the optimized power values varied widely (b D 1Ð0 3Ð0). Using the average and median value (1Ð7 š 0Ð4) throughout resulted in an average correlation that was somewhat less (r 2 D 0Ð50). Total storm kinetic energy (E K ), the AI m index, the USLE R factor (EI 30 ) and storm rainfall depth (P) performed less under the studied conditions (r 2 D 0Ð35 0Ð44; Table II). Splash transport Cumulative sediment transport components for individual devices and lateral, gross and net downslope transportability values are listed in Table III. Despite the lack of replication, trends in splash transport with slope gradient conformed with expectations: both splash-creep and downslope transport increased with slope, whereas upslope transport decreased. Lateral splash did not show any obvious trend. Simultaneously with the Table II. Performance of the various rainfall erosivity indices listed in Table I in explaining the variance in 40 data sets, covering five sediment transport components from four experimental soil trays during two periods (see text for further explanation) Index Coefficient optimized Parameter statistics Median value used Average r 2 N best fit Median Average (st. dev.) Range Average r 2 N best fit E R0 0Ð (š3) Ð57 26 P R0 0Ð (š5) Ð49 5 R b 0Ð Ð7 1Ð7 (š0.4) 1Ð0 3Ð0 0Ð50 1 E K 0Ð44 0Ð44 7 AI m 0Ð41 0Ð41 EI 30 0Ð36 0Ð36 P 0Ð35 0Ð35 1

9 SPLASH AND WASH UNDER NATURAL RAINFALL 161 Table III. Summary of measurements and modelling of various rain splash and wash transport components from eight experimental devices, with different slope and treatments, during two consecutive periods (MC D mulch cover fraction). Listed for each device are: the various components of splash transport, gross and net downslope and lateral transportability and detachability estimated from this, observed runoff, runoff coefficient, average sediment concentration in runoff and wash transport, relative contribution of wash transport to total downslope sediment transport, optimized average maximum infiltration rate (I m ), associated runoff model efficiency (ME), average absolute difference between simulated and observed storm runoff depth and relative difference between cumulative amounts, optimized washed fraction (j), wash transport model efficiency (ME) for event values of washed sediment concentration and transport, respectively, relative difference between cumulative observed and predicted wash transport, model efficiency associated with using modelled runoff with a single optimized sediment concentration, and the associated difference between cumulative simulated and observed transport Slope Series I Series II Treatment (P D 332 mm; E 20 D 3324 J m 2 ) (P D 366 mm; E 20 D 6090 J m 2 ) Tilled MC D 0Ð35 MC D 0Ð55 Splash transport (g m 1 ) splash-creep 80Ð8 299Ð3 374Ð8 913Ð7 373Ð0 240Ð6 96Ð5 34Ð5 downslope 170Ð1 315Ð5 309Ð9 820Ð4 288Ð5 106Ð1 39Ð5 28Ð1 lateral 212Ð8 203Ð3 139Ð1 207Ð7 145Ð7 64Ð3 30Ð7 13Ð4 upslope 255Ð9 71Ð7 21Ð5 1Ð6 85Ð6 35Ð7 33Ð7 14Ð7 net downslope 5Ð0 543Ð1 663Ð1 1732Ð5 576Ð0 311Ð0 102Ð3 48Ð0 Transportability (g m 1 kj 1 ) gross downslope (T 20,g ) 75Ð Ð7 108Ð6 56Ð9 22Ð3 10Ð3 net downslope (T 20,n ) 1Ð5 163Ð4 199Ð5 521Ð2 94Ð6 51Ð1 16Ð8 7Ð9 lateral (T 20,lat ) 64Ð0 61Ð1 41Ð9 62Ð5 23Ð9 10Ð6 5Ð0 2Ð2 detachability (D 20,g J 1 ) a 13Ð4 12Ð8 8Ð8 13Ð1 5 2Ð2 1Ð0 0Ð5 Runoff and wash transport total runoff depth (mm) 2Ð5 42Ð5 29Ð2 36Ð9 60Ð0 88Ð4 44Ð5 16Ð9 runoff coefficient (%) 0Ð75 12Ð8 8Ð8 11Ð1 16Ð0 24Ð2 12Ð2 5Ð0 sediment conc. in runoff (g L 1 ) 1Ð33 5Ð28 6Ð75 8Ð62 2Ð51 1Ð65 0Ð91 0Ð71 wash transport (g m 1 ) 2Ð0 134Ð6 118Ð3 159Ð1 90Ð4 87Ð3 24Ð2 7Ð2 wash/total transport b 100% c 19Ð9% 15Ð1% 8Ð4% 13Ð6% 21Ð9% 19Ð1% 13Ð0% Runoff modelling infiltration rate (I m ;mm h 1 ) model efficiency (ME) 0Ð52 0Ð69 0Ð68 0Ð84 0Ð53 0Ð54 <0 0 avg. absolute error (mm) 0Ð1 1Ð0 0Ð9 2Ð1 1Ð8 3Ð2 2Ð3 0Ð9 diff. cum. runoff (sim.-obs.) 2% 3% 5% 6% 16% 6% 39% 37% Wash transport modelling washed fraction (j, %) 0Ð9 3Ð0 4Ð4 4Ð1 2Ð3 4Ð5 4Ð9 8Ð1 ME for concentration <0 0Ð62 0Ð63 0Ð72 <0 <0 <0 <0 ME for transport <0 0Ð84 0Ð80 0Ð90 0Ð61 <0 <0 <0 diff. cum. transp. (sim.-obs.) 16% 1% 11% 12% 22% 16% 4% 31% ME using average conc. d <0 0Ð77 0Ð57 0Ð72 0Ð43 <0 <0 <0 diff. cum. transp. (sim.-obs.) 14% 13% 12% 25% 14% 5% 42% 49% using average conc. a Derived assuming average lateral splash length of 1 cm (see text). b Total meaning gross downslope splash and wash transport. c Theoretical value. d Relating to wash transport.

10 162 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA increase in downslope splash transport, upslope transport decreased very rapidly with slope. Regular tillage and mulch cover had a marked effect on splash transport: lateral and net downslope splash from the soil tilled after every storm was 44 54% of that from the bare control, whereas this ratio was 18 21% and 8 9% on the soil with 35% and 55% mulch cover, respectively (cf. Table III). Gross and net downslope splash transportability (T 20,g and T 20,n, respectively, g m kj 1 ) relating transport to the best performing erosivity index E 20 (i.e. E R0 with R 0 D 20 mm h 1 ) increased with slope, from 76 and 2 gmkj 1, respectively at 0, to 522 and 521 g m kj 1 at 40 slope. The first series of experiments produced lateral transportability values of g m kj 1, but at 24 g m kj 1, the value derived for the control treatment in the second experiment was much lower. Downslope splash transportability at 5 also differed considerably between the first and the second experiment (cf. Table III). Using an average lateral splash length of 1 cm and lateral transportability values yielded detachability values of g J 1 for the first series, but repeating the exercise for the untreated soil in the second series of measurements gave a much lower value of 7Ð5 gj 1 (Table III). Runoff and wash transport Table III also lists the results of measurements and modelling of runoff and wash transport. Cumulative runoff coefficients were 9 13% during the first series (except for the horizontal slope producing only 0Ð75% of incident rainfall) and 5 24% in the second series. The lowest runoff coefficient was associated with the 55% mulch cover treatment, while the highest runoff coefficient was found for the tilled soil (Table II). Resulting I m values were mm h 1, except for the two mulch treatments (309 mm h 1 and 839 mm h 1 )and the horizontal soil (1375 mm h 1 ). The SVI model predicted runoff satisfactorily except for the mulched soils. Excluding these, model efficiency was 0Ð52 0Ð84 and cumulative measured and modelled runoff depths differed by less than š16% (Table III). The difference for the mulched soils was greater (37 38%) but absolute differences on an event basis were no larger than those for the other devices at ½5 slope (0Ð9 2Ð3 mm versus 0Ð9 3Ð2 mm; Table III). Use of an initial infiltration (F 0 ) did not improve model performance. Sediment concentrations from the horizontal device (1Ð3 gl 1 ) were notably lower than for the inclined devices during the first experiment (5Ð3 8Ð6 gl 1 ). On the other hand, concentration from the bare control soil at 5 in the second experiment was lower by a factor of two compared to the first experiment (2Ð5 versus 5Ð3 gl 1 ). At 1Ð7 gl 1, sediment concentration in runoff from the tilled soil was 66% lower than from the control, while application of 35% and 55% of soil cover reduced concentrations to 37% and 28% of the bare soil, respectively (Table III). Apart from the horizontal soil, the absolute magnitude of wash transport did not appear to change much with slope, whereas net downslope splash increased, and hence the relative importance of wash transport in total downslope transport decreased with slope, from 14 20% for untreated soil at 5 slope, to 15% at 20 slope, to 8% at 40 slope. In the second series the relative contribution by wash transport was 13 22% without a clear pattern (Table III). Lateral splash transportability (T 20,lat ) and average sediment concentration (c wash ) for the eight devices are compared in Figure 3a. There appears to be a reasonably well-defined, proportional relationship between the two for slopes ½5, which offers some support for the modelling approach followed (cf. Equation (3)). As illustrated in Figure 3b, agreement between observed and predicted sediment concentrations in surface wash was satisfactory. In all cases, Equation (3) explained more of the variations in observed event wash transport than modelled runoff depth with a single optimized concentration only, although model efficiency was less than zero for four devices (Table III). Total wash transport amounts were generally estimated with reasonable accuracy, however ( 31% to C16%), while model performance improved as the magnitude of wash transport increased. At 2Ð3 4Ð4%, optimized values of the washed fraction (j) for the untreated soil were of comparable magnitude (except for the horizontal soil), whereas for the mulched soils somewhat higher values of 4Ð9 8Ð1% resulted.

11 SPLASH AND WASH UNDER NATURAL RAINFALL 163 Average sediment concentration,c (g l -1 ) C avg = 0.12 T 20,lat r 2 = 0.85 horizontal tray Lateral splash transportability, T 20,lat (g m kj -1 ) a. b. Predicted sediment concentration in runoff (g l -1 ) 100 1: (I) (II) MC=0.35 MC= Observed sediment concentration in runoff (g l -1 ) Figure 3. Comparison of (a) average sediment concentration in runoff with lateral transportability by splash (the horizontal device was not included in the regression); (b) modelled versus observed event sediment concentrations for six devices (the horizontal and tilled devices are not shown for reasons explained in the text; MC denotes mulch cover fraction and 5 (I) and (II) refer to the first and second experimental series, respectively) DISCUSSION Rainfall erosivity indices For the currently studied combination of soil and rainfall, use of the kinetic energy of rainfall falling at an intensity higher than 20 mm h 1 produced the best correlation between rainfall erosivity and sediment transport. Hudson (1965) concluded that the use of a threshold intensity (25 mm h 1 ) gave the best results for sediment splashed from cups in Zimbabwe than did gross kinetic energy, and this KE 25 index has since been used successfully in Tanzania (Rapp et al., 1972), and Malaysia (Morgan, 1974). For temperate climates, much lower threshold values have been suggested, e.g. 10 mm h 1 in Britain (Morgan, 1977), 6mmh 1 in Germany (Richter and Negendank, 1977) and 2Ð5 mmh 1 in the Netherlands (Van Asch and Epema, 1983). A possible physical explanation for such a threshold rainfall intensity may be related to soil water potential changes during a storm: soil splash has been shown to reach a maximum when the soil is at or near saturation and a thin water film develops (Sloneker and Moldenhauer, 1974; Ghadiri and Payne, 1986). Although maximum infiltration rates in excess of 70 mm h 1 were found in the present study (Table III), the spatially distributed theory behind Equation (2) suggests that partial saturation would also have occurred at lower rainfall rates (Yu et al., 1997). Sources of error and variability Contrary to theoretical expectation, lateral splash, upslope splash and the sum of downslope splash and splash-creep transport from the horizontal device were not all equal, although the differences were not very large: the smallest amount (lateral splash; 213 g m 1 ) was 17% less than the largest ( upslope splash; 256gm 1 ). Similar differences presumably occurred in other cases and may be explained by the possibility that the vertical distance between the tray rim and the soil surface was not equal across the entire area and for all storms (cf. Kinnell, 1974). Sediment transport itself resulted in a slight lowering of the soil surface calculated to be 0Ð2 4Ð5 mm overall after the various experiments (cf. Table III; using a bulk density of 950 kg m 3 ) but probably more at the upslope end of inclined devices, whereas the downslope side had a rim several millimetres lower than on the other sides to allow the passage of surface runoff. The effect of these errors overall is not thought to be very large, however. Splash transport measured from confined areas

12 164 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA (such as the soil trays) represents an underestimate depending on the ratio of average splash length over tray size (Van Dijk et al., 2002b). Correction factors listed in the latter study for the current combinations of tray size and expected splash length (1 12 cm) suggest underestimations to be small overall, however, except perhaps for downslope splash on the steepest slope that may have been underestimated by up to c. 10%. A greater problem was to separate splash-creep and washed sediment; the possible error associated with this was visually estimated to be up to 20% of actual wash transport amounts. The greatest variability in sediment transport was probably caused by temporal changes in surface conditions. An important issue in studies of transport from confined soil areas relates to the possible downslope movement and eventual depletion of a more readily erodible fraction. Field studies have shown that finer aggregates are more transportable than coarser ones, particularly on steep slopes (Van Dijk et al., in press). As a result, a coarse, less erodible pavement may develop and, indeed, detachment and sediment transport from the devices rather quickly resulted in the formation of a splash pavement of coarse aggregates on top of a more or less crusted layer (cf. Savat and Poesen, 1977). The formation of a splash pavement and associated decrease of soil transportability with time were also observed on larger areas in field studies, however, and can be attributed to the high turnover rate of soil under the prevailing tropical rainfall, resulting in sorting for size and crust formation even without depletion of finer aggregates (Wiegman, 1999; Van Dijk et al., in press). Lateral transportability for soil at 5 slope was much higher in the first series of experiments than in the second series. Part of this difference may be related to the fact that more and longer dry spells occurred during the first period (cf. Figure 2), which occasionally led to initial cracking not observed during the second period (cf. Bryan, 1996). Field studies on the same soil suggest that drying and cracking plays a role in the formation of new, loose aggregates (Wiegman, 1999). Runoff and transport by wash presumably also were affected by drying and cracking. Summarizing, the measurements were possibly affected by gradual depletion of a more readily erodible fraction, but this effect could not be separated from surface lowering and the natural formation of a splash pavement. The effect of these processes can be reduced by decreasing the length of the experiment, but this leads to problems of its own as it would prevent natural surface processes from taking place during and between storms (cf. Bryan, 1996; Sutherland et al., 1996). Splash transport Lateral transportability was g m kj 1 for untreated soils This compares well to values derived from field splash cup measurements, decreasing from g m kj 1 at the beginning of the rainy season to about 13gmkJ 1 after 331 mm of rain (Van Dijk et al., in press). Assuming the average splash length of 1 cm derived in the splash cup experiments yields detachability estimates of 7Ð5 20gJ 1 for untreated soil in the present case. Comparison with the literature is difficult because of differences in methods to estimate storm erosivity and detachment rates from measured sediment transport (cf. Van Dijk et al., in press). Detachability values reviewed by Poesen and Torri (1988) were 0Ð19 10 g J 1 using total storm kinetic energy, and values calculated this way for the current study are at the high end of this scale, 6 11 g J 1. If detachability, lateral splash length and the fraction splashed downslope do not change with slope, then the ratio of downslope over lateral splash transport corresponds with the ratio of splash lengths downslope and on a horizontal plane, respectively (cf. Van Dijk et al., 2002b). Again using a horizontal splash length of 1 cm yields downslope splash lengths increasing from 3 5 cm on a 5 slope, to 8 cm at 40. As the fraction splashed downslope presumably did increase with slope in reality (cf. Ghadiri and Payne, 1986, 1988), these values are likely to represent overestimates and as such are rather low when compared to an average minimum (projected) downslope splash length of about 12 cm measured on bare terrace risers at slope (Van Dijk et al., in press). Runoff and wash transport Runoff measurements did not suggest any clear influence of slope on maximum average infiltration rate (I m ) and values derived for untreated sloping soil ( mm h 1 ) are within the range found for field runoff

13 SPLASH AND WASH UNDER NATURAL RAINFALL 165 plot studies ( mm h 1 ; Van Dijk, 2002). The markedly higher infiltration rates observed on soils with mulching also correspond well with field observations, which suggested I m to increase by % after mulching (Van Dijk, 2002). The advanced wash transport model produced plausible estimates of washed sediment concentration and transport. Both the proportionality between lateral splash transport amounts and washed sediment concentrations and the consistency in derived values of the washed fraction (j) provide further support for the approach taken. The modest importance of wash transport (8 22% of total downslope sediment transport) contrasts with observations for similar experiments and soils by Wan et al. (1996), who found this fraction to be as much as 93% on a 5 slope, decreasing to 42% on a 20 slope. The differences can be attributed to the higher artificial rainfall intensity and the less permeable Hawaiian soil (Wan et al., 1996). Effect of tillage and mulching Splash transport components for the tilled soil were 42 52% of those from the bare control (Table III). Tillage caused aggregates to break up and smear, thereby forming a crust-like layer appearing more compact and less permeable than for the other treatments. Runoff from the tilled soil was also greater than that from the untreated soil, especially during the first storms (data not shown). The reduction of rainfall-driven erosion by mulch cover was also observed in field measurements of splash transport on terrace beds (Van Dijk et al., in press) and of sediment transport from sections of terrace beds and risers (Van Dijk, 2002). These indicated an exponential decrease of transport with increasing cover described well by the relationship of Laflen and Colvin (1981): MF D e amc 4 where MF is the mulch factor, i.e. the ratio of soil loss from a surface with a fraction MC of mulch cover over that without mulch, and a is an empirical exponent (usually between 3 and 7; Laflen and Colvin, 1981; Morgan, 1986). Values of a calculated from measured gross and net downslope total, splash and wash transport, respectively, were 3Ð8 4Ð9 (4Ð5 š 0Ð3 on average). This compares well with values found for splash on terrace beds (3Ð9 4Ð1; Van Dijk et al., in press) and values based on rainfall-driven sediment transport from sections of terrace bed with mulch cover (<2Ð5; Van Dijk, 2002). Both studies indicated that the mulch was partially decomposed and buried by sediment within a few months, however, and hence became less effective. CONCLUDING REMARKS Experiments under natural rainfall are essential to improve our understanding of rainfall-driven splash and wash transport processes. Although the lack of replication precluded a more rigorous analysis of the effect of slope, tillage and mulch cover on these processes, the presented methodology was demonstrated to be suitable for making such observations. The interpretation of splash measurements would be aided by measurements of average splash lengths in different directions on a slope. In future experiments these may be obtained by dividing the device s splash collecting containers into parallel compartments, or by simultaneous separate experiments following approaches outlined in Van Dijk et al. (2002b). A conceptually sound yet simple wash transport model was advanced as a first attempt to translate detailed laboratory process knowledge into a physically-based model for use under natural conditions. Further research is needed to include factors determining the relationship between sediment concentration and splash detachability in this model, and the manner in which it is influenced by soil and rainfall characteristics. A combination of experiments and consistent, physical splash, runoff and wash models such as presented here should be able to shed more light on the precise nature of these interactions.

14 166 A. I. J. M. VAN DIJK, L. A. BRUIJNZEEL AND E. H. EISMA ACKNOWLEDGEMENTS This work was conducted within the framework of the Cikumutuk Hydrology and Erosion Research Project (CHERP, near Malangbong, West Java, Indonesia. Albert van Dijk was supported by a grant from the Netherlands Foundation for the Advancement of Tropical Research (WOTRO, grant no. W76-193). The authors extend their gratitude to the students of the Vrije Universiteit and the field staff of CHERP who participated in the experiments. REFERENCES Bryan RB Erosional response to variations in interstorm weathering conditions. In Advances in Hillslope Processes, Vol. I, Anderson MG, Brooks SM (eds). Wiley: Chichester; Free GR Erosion characteristics of rainfall. Agricultural Engineering 41: Ghadiri H, Payne D The risk of leaving the soil unprotected against falling rain. Soil and Tillage Research 8: Ghadiri H, Payne D The formation and characteristics of splash following raindrop impact on soil. Journal of Soil Science 39: Heilig A, DeBruyn D, Walter MT, Rose CW, Parlange J-Y, Steenhuis TS, Sander GC, Hairsine PB, Hogarth WL, Walker LP Testing a mechanistic soil erosion model with a simple experiment. Journal of Hydrology 244: Hudson NW The influence of rainfall on the mechanics of soil erosion with particular reference to Southern Rhodesia. MSc thesis, University of Cape Town. Hudson NW Soil Conservation. Batsford: London; 391 pp. Kinnell PIA Splash erosion: some observations on the splash-cup technique. Soil Science Society of America Proceedings 38: Kinnell PIA The mechanics of raindrop-induced flow transport. Australian Journal of Soil Research 28: Kneale WR Field measurements of rainfall drop size distribution, and the relationship between rainfall parameters and soil movement by rainsplash. Earth Surface Processes and Landforms 7: Laflen JM, Colvin TS Effect of crop residue on soil loss from continuous row cropping. Transactions of the American Society of Agricultural Engineers 24: Lal R Soil erosion problems on an Alfisol in western Nigeria and their control. Monograph 1, International Institute of Tropical Agriculture, Ibadan, Nigeria; 208 pp. Meyer LD Symposium of simulation of rainfall for soil erosion. Transactions of the American Society of Agricultural Engineers 8: Moeyersons J An experimental study of pluvial processes on granite gruss. Catena 2: Morgan RPC Estimating regional variations in soil erosion hazard in Peninsular Malaysia. Malaysian Nature Journal 28: Morgan RPC Soil Erosion in the United Kingdom: Field Studies in the Silsoe Area, National College of Agricultural Engineering Silsoe Occasional Paper No. 4, Silsoe. Morgan RPC Soil Erosion and Conservation. Longman Scientific & Technical: Harlow; 298 pp. Moss AJ Effects of flow-velocity variation on rain-driven transportation and the role of rain impact in the movement of solids. Australian Journal of Soil Research 26: Nash JE, Sutcliffe JV River flow forecasting through conceptual models Part I. A discussion of principles. Journal of Hydrology 10: Poesen J, Savat J Detachment and transportation of loose sediments by raindrop splash. Part II. Detachability and transportability measurements. Catena 8: Poesen J, Torri D The effect of cup size on splash detachment and transport measurements. Part II. Field measurements. Catena Supplement 12: Proffitt APB, Rose CW Soil erosion processes. I. The relative importance of rainfall detachment and runoff entrainment. Australian Journal of Soil Research 29: Quansah C The effect of soil type, rain intensity and their interactions on splash detachment and transport. Journal of Soil Science 32: Rapp A, Axelsson V, Berry L, Murray-Rust DH Soil erosion and sediment transport in the Morogoro river catchment, Tanzania. Geografiska Annaler 54: Richter G, Negendank JFW Soil erosion processes and their measurement in the German area of the Moselle river. Earth Surface Processes 2: Riezebos HT, Epema GF Drop shape and erosivity. Part II: Splash detachment, transport and erosivity indices. Earth Surface Processes and Landforms 10: Rose CW Erosion and sedimentation. In Hydrology and Water Management in the Humid Tropics, Bonell M, Hufschmidt MM, Gladwell JS (eds). Cambridge University Press: Cambridge; Salles C, Poesen J Rain properties controlling soil splash detachment. Hydrological Processes 14: Salles C, Poesen J, Govers G Statistical and physical analysis of soil detachment by raindrop impact: rain erosivity indices and threshold indices. Water Resources Research 36: Savat J, Poesen J Splash and discontinuous runoff as creators of fine sandy lag deposits with Kalahari sands. Catena 4: Sharma PP, Gupta SC Sand detachment by single raindrops of varying kinetic energy and momentum. Soil Science Society of America Journal 53: