Soil Science Society of America Journal 65:953-956 (2001)
© 2001 Soil Science Society of America
COMMENTS & LETTERS TO THE EDITOR
Comments on "Vertical Hydraulic Gradient and Run-On Water and Sediment Effects on Erosion Processes and Sediment Regimes"
P.I.A. Kinnell
School of Resource, Environmental and Heritage Sciences Division of Science and Design University of Canberra ACT 2600 Australia
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INTRODUCTION
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Zheng et al. (2000) report results from experiments where sediment at different rates was fed into the top end of a 5-m-long test box containing soil being subjected to rainfall erosion using artificial rainfall. Data were collected under drainage and seepage conditions. They presented an analysis based on mass balances obtained from the runoff and sediment discharges collected from the feeder system (a smaller inclined surface of the same soil material), the test box without feeder input, and the test box with feeder input (their Table 2). For the drainage and lower (5%) slope conditions, Zheng et al. concluded that the erosion process in the test box changed from a deposition-dominated process to a transport-dominated process as the sediment concentration from the feeder box was decreased. When the slope was increased to 10% or the hydraulic condition changed to seepage condition, they concluded that the erosion process became transport-dominated.
The procedure used to generate the data analysed involved subjecting the inclined surfaces of the feeder and test boxes initially to 8 min of rain when the feeder box and the test box were separated, followed by a period of 7 min when the two surfaces were connected, after which the boxes were disconnected and data collected for a further 2 min. This initial sequence was performed with 100% of the soil surface in feed box being exposed to the rain. After that, the feed box exposure was subsequently reduced to 50, 25, and 0%, and the sequence repeated after each reduction.
The dominant detachment and transport system involved varied during some of the experiments. There are four detachment and transport systems involved in rainfall erosion.
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1 . Flow detachment–flow transport (FD–FT)
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Normally associated with erosion in rills, FD–FT has also been observed in sheet flow on high-strength crusted surfaces (Romkens, 2000, personal communication). On less stable surfaces, FD–FT leads to the development of micro-rills in interrill areas (e.g., in experiments by Meyer and Harmon, 1989).
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2 . Raindrop detachment–flow transport (RD–FT)
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RD-FT occurs in rain-impacted flow when cohesion in the soil surface is high enough to prevent detachment by flow but not sufficient to prevent detachment by raindrops impacting the flow and the flow shear stress or stream power is sufficient to transport detached material; RD-FT is a detachment-limited erosion system.
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3 . Raindrop detachment–raindrop induced flow transport (RD–RIFT)
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RD–RIFT occurs in rain-impacted flow when flow shear stress or stream power is not only insufficient to detach soil material from the surface of the soil mass but also insufficient to entrain loose soil material sitting on top of the soil surface. In RIFT, sediment transport is induced by raindrop impact lifting the loose material up into the flow and will not occur in the absence of either raindrop impact or flow (Kinnell 1990, 1993). RD–RIFT is a transport-limited system. As a result, a mobile layer of predetached material occurs on the soil surface when RIFT operates, and this has an impact on the erosion rate (Kinnell 1994).
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4 . Raindrop detachment–splash transport (RD-ST)
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RD–ST is common in sheet and interrill areas prior to the development of runoff. Splash transport is highly dependent on slope gradient and decreases rapidly as the depth of water on the surface increases. RD–ST is a transport-limited system, particulary when operating on large areas. Because RD–ST is a transporting-limiting system, like RD–RIFT, it fosters the development of a layer of predetached soil material sitting on the soil surface.
Figure 1
shows diagrammatically how the four detachment–transport systems vary with raindrop kinetic energy and stream power. The systems are not mutually exclusive. Because the critical shear stress or stream power that dictates the change between RD–FT and RD–RIFT varies with particle size and density, RD–FT can operate simultaneously with RD–RIFT in the same flow. It is not uncommon for fine particles detached by RD to be transported by FT, while coarser particles are transported by RIFT. Also, splash may transport detached material aerially to interill or sheet flow where it may then be transported along the line of flow by RIFT and/or FT. The detachment–transport systems that operate under such circumstances are RD-ST–RIFT and RD–ST–FT. Zheng et al. observed that rills developed 
30 min into the run when 100 mm h-1 rain was applied to the test surface inclined at 10% under drainage conditions. When rills are present and actively scouring, FD–FT can be expected to dominate. Prior to rilling on the 10% slope under 100 mm h-1 rain, RD–FT, a detachment-limited system, would have been dominant. The change in dominance between the RD–FT and FD–FT detachment–transport systems almost certainly led to the temporal increase in sediment delivery observed in the experiments with 100 mm h-1 rain on the 10% slope. In contrast, the dominance of RD–RIFT in the experiments with 500 mm h-1 rain on the 10% slope, and the 50 and 100 mm h-1 rains on the 5% slope, resulted in little temporal increase in sediment delivery during the experiments. However, it would seem that the analytical approach used by Zheng et al. may not be not particularly well suited for studying the effect of the varying the sediment feed into the test box.
The test box was 5 m long and the first samples during the experiments when the input feed was applied were taken 2 to 3 min after the input feed began. In the case of a sample being taken at the 3-min mark, only feed particles travelling at or greater than 28 mm s-1 would be collected. If the last sample was collected 4 min later, then it would contain feeder particles travelling at 12 mm s -1 or more. While the next two samples collected in the sequence were collected when the feed was removed, they would contain feeder particles travelling between 12 and 10, and 10 and 9 mm s-1, respectively, if the timing sequence was maintained. Four more samples without feeder input were taken before the next time the input feed was applied. The sediment delivered from the test box during the samples associated with the new feed would contain a mixture fast-moving particles from the new feed and slow-moving particles from the first feed. The situation becomes more and more complicated as the series of experiments on the one surface progresses. The fact that RD–ST dominated the 30-min pretreatment adds a further complication. As the data used in the analysis are not for equilibrium conditions, is the mass balance approach adopted by the authors in their Table 2 appropriate?
Not withstanding the concern about non-steady conditions associated with the data, an analysis based on the sediment concentration data provides another approach to analyzing data from rainfall erosion experiments. The theory behind using sediment concentrations is associated with observations that, at least for RIFT, sediment delivery varies directly with flow velocity (Kinnell, 1990; Fox and Bryan, 1999) while variations in flow depth have little effect in very shallow flows. Consequently, for constant rainfall conditions, the effect of soil surface conditions on sediment delivery can be examined through sediment concentration in most RD–RIFT systems.
Figure 2
shows the ratios of the test box sediment concentrations with infeed to those without during the drained experiments in relation to the cover on the feeder box. A decline in the ratio occurs in association with the decline in feeder input. A ratio of 1.0 is associated with no effect. It is evident from Fig. 2 that the apparent impact of the feeder input is greater for 50 mm h-1 rainfalls than for 100 mm h-1 rainfalls. Also, the apparent effect of the feeder input is not particularly influenced by the change in slope gradient despite rills occurring with the 100 mm h-1 rainfalls on the 10% slope. A value of 1.0 for the ratio occurs for the 100 mm h-1 rain on the 5% slope, and this indicates that, at least for this condition, the feeder input in terms of both water and sediment produced by the 100% cover had no impact on the sediment discharge. This is not the case for the 50 mm h-1 rains. The failure of the ratio to reach 1.0 for the 100% cover with the 50 mm h-1 rains indicates the change in flow rate produced by feeder input had some impact in sediment concentration under the 50 mm h-1 rains but not under the 100 mm h-1 rains. Why this is so warrants further study. Experiments where the sediment feed sequence is reversed together with a control using a clear water feed throughout may be useful in this context.
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Fig. 2. Ratios of the test box sediment concentrations with infeed to sediment concentrations without infeed for 50 mm h-1 rain on 5% (50/5) and 10% (50/10) slopes and for 100 mm h-1 rain on 5% (100/5) and 10% (100/10) slopes in relation to feeder box cover in the Zheng et al. experiments.
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Received for publication July 20, 2000.
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REFERENCES
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- Fox, D.M., and R.B. Bryan. 1999. The relationship of soil loss by interrill erosion to slope gradient. Catena 38:211–222.
- Kinnell, P.I.A. 1990. The mechanics of raindrop induced flow transport. Aust. J. Soil. Res. 28:497–516.
- Kinnell, P.I.A. 1993. Sediment concentrations resulting from flow depth–drop size interactions in shallow overland flow. Trans. ASAE 36:1099–1103.
- Kinnell, P.I.A. 1994. The effect of predetached particles on erosion by shallow rain-impacted flow. Aust. J. Soil Res. 31:127–142.
- Meyer, L.D., and W.C. Harmon. 1989. How row-sideslope length and steepness affect sideslope erosion. Trans. ASAE 32:639–644.
- Zheng, F., C. Huang, and D. Norton. 2000. Vertical hydraulic gradient and run-on water and sediment effects on erosion processes and sediment regimes. Soil Sci. Soc. Am. J. 64:4–11.[Abstract/Free Full Text]
Reply to "Comment on ‘Vertical Hydraulic Gradient and Run-On Water and Sediment Effects on Erosion Processes and Sediment Regimes’ "
Chi-hua Huang,
L.Darrell Norton and
Fen-li Zheng
b USDA-ARS National Soil Erosion Research Laboratory 1196 SOIL Bldg., Purdue University West Lafayette, IN 47907-1196
c Institute of Soil and Water Conservation 26 Xinong Road Yangling, Shaanxi P.R. China 712100
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INTRODUCTION
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We appreciate Dr. Kinnell's interest in our work on the sediment mass balance relationship under different near-surface hydraulic gradients (Zheng et al., 2000). Kinnell's comment contains three points: (i) to describe processes occurred during our experiment by a set of detachment-transport regimes based on kinetic energy of the rain and stream power of the flow; (ii) to point out our experiment might not reach equilibrium—hence, our analyses of the mass balance scenario were inappropriate; and (iii) to quantify the runon water and sediment effects by a sediment concentration ratio. We offer our response accordingly.
To clarify, we would like to define detachment as the initiation of soil material into motion from its stationary position and transport is the process of moving already detached material. Therefore the detachment process precedes the transport process. Another process Kinnell failed to describe and occurs during rainfall is deposition, defined as the return of soil material in motion to a stationary position on the surface. In a very simple way, we can define sediment delivery or transport as the difference between detachment and deposition. We agree that detachment (i.e., raindrop [RD] and flow [FD]) and transport (i.e., splash [ST], raindrop induced flow transport [RIFT], and flow [FT]) processes defined by Kinnell occur simultaneously during rainfall events. When we measure sediment discharge at an outlet, sediments are in fact transported by all three mechanisms (ST, RIFT, and FT), and the dominance of these transport mechanism shifts from ST to FT as the contributing area is increased. In the meter-sized test plots or boxes, one can simply use the combination of splash collector and surface cover (i.e., to remove raindrop impact effects) to partition the total sediment delivery by these three transport mechanisms. Nevertheless, partitioning the total sediment discharge based on its detachment mechanism is not a simple matter. A soil material can be detached by either RD or FD; transported by a combination of ST, RIFT, and FT; deposited back to the surface; and redetached again by RD or FD. Therefore, sediment discharged at the outlet may have gone through several cycles of detachment–transport–deposition from its original location. With this understanding in mind, the four detachment–transport systems proposed by Kinnell can only be viewed as a small portion of many possible detachment–transport–deposition scenarios occurring on the surface. The deficiency of Kinnell's detachment–transport concept is well illustrated by our data when the deposition process became dominant under high sediment feeds.
In terms of the equilibrium condition issue, we believe Kinnell is confused about the definitions of equilibrium and steady-state. Equilibrium is defined as the equal balance of one process by its reverse process. Under this definition, erosion never reaches equilibrium, because it is an irreversible process. Steady state is a condition when a process remains constant with time. On an eroding surface during the rainfall event, sediment delivery may or may not reach a steady state. We illustrated this time dependency in Fig. 2 of our paper. Regardless of whether a process reaches its equilibrium or steady state or not, one can always perform a mass balance analysis. In our multiple box study, we were only interested in the mass balance relationship based on the total sediment flux into and out of the 5-m test box as the surface hydrologic condition was changed. Kinnell incorrectly argued that such a mass balance could not be performed when the system was not at steady state because of different sediments travelling at different velocities. One can certainly perform a mass balance analysis for sediments with different sizes or travel velocities, if such data sets are available. Our study was designed to examine the total sediment flux in and out of the test box. Whether the eroded sediments or the soil surface became coarser or finer as rain progressed was not an objective of our study.
Lastly, we would like to point out that Kinnell's proposal of using a sediment concentration ratio to quantify the runon water and sediment effects produces little new information and possibly a misinterpretation of the physical processes. Sediment concentration, the ratio of sediment mass flux to runoff water flux, is not a good measure for sediment mass balance when the water flux is changing as we apply rainfall and inflow onto the study box. This is why we presented our analyses on sediment and water fluxes separately. With our data set, we were able to separate runon water and runon sediment effects (Tables 3 and 4 of Zheng et al., 2000). Our data under the drained condition showed a relatively constant runon water effect since the runon water flux was somewhat constant and a significantly decreasing runon-induced sediment detachment was measured as the runon sediment was increased (Fig. 1
, plotted from Table 3 of Zheng et al., 2000). Increased rainfall intensity or slope steepness increased the runon water and sediment effects. The threshold sediment feed can be viewed as the condition when the feed sediment did not cause any additional detachment nor deposition on the test surface. This threshold condition is illustrated in Fig. 1 by the crossing point at the y-axis, that is, 
10 kg h-1 feed under 50 mm h-1 rain and 19 kg h-1 feed under 100 mm h-1 rain at 5% slope. Contrarily, Kinnell used the ratio of sediment concentration from concentrations with infeed to those without as a measure of runon water and sediment effects and defined a ratio of one as having no runon effects. The greater the ratio beyond one, the more the runon effects. Kinnell's analyses showed three entirely different trends: (i) a decreased runon effect with an increased rainfall intensity, (ii) runon effect not sensitive to changes in slope steepness, and (iii) a decreased runon effect with a decreased sediment feed. This contradictory finding is also illustrated in Fig. 1 where we marked situations for the least or no runon effect (A) and the most runon effect (B) from Kinnell's analysis. Obviously, this discrepancy in runon effects between Kinnell's and our analyses is due to the different definition that Kinnell chose to use but failed to define. Besides the ambiguity of Kinnell's runon effect, we would like to point out two major flaws in Kinnell's analysis. First, the sediment concentration ratio will not separate the runon water and runon sediment effects, while such effects can be identified easily with a flux-based analysis. Second, the use of a feeder surface cover as a dependent variable to quantify the runon effects has no physical meaning, since we merely adjusted the surface cover to obtain different levels of sediment feed rate. Different levels of sediment feed can also be obtained by either adjusting the rainfall intensity or the slope of the feeder box. The runon effect depends on the amount of water and sediment applied to the test box, not on the surface cover of the feeder box.
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Fig. 1. Runon sediment effect as a function of sediment feed rate for the drained condition under different rainfall and slope conditions. A and B denote conditions of least and most runon effects from Kinnell's analysis.
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We believe we have developed a multiple-box system that can be used to quantify sediment mass balance relationships under variable surface hydrologic conditions. Furthermore, we believe the flux-based mass balance approach is sound and can be used to identify runon water and runon sediment effects which occur in hillslopes. We look forward to seeing more research initiatives and experimental results to expand our knowledge base on how surface condition affects the erosion process scenario.
Received for publication September 14, 2000.
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REFERENCES
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- Zheng, F., C. Huang, and L.D. Norton. 2000. Vertical hydraulic gradient and run-on water and sediment effects on erosion processes and sediment regimes. Soil Sci. Soc. Am. J. 64:4–11.
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P.I.A. Kinnell
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C. Huang, L. D. Norton, and F. Zheng
Second reply to Kinnell's comments on "Vertical Hydraulic Gradient and Run-on Water and Sediment Effects on Erosion Process and Sediment Regimes"
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TOP
INTRODUCTION
1 . Flow detachment-flow...
c(loose) = critical stream power required to transport loose (predetached) soil particles.