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Soil Science Society of America Journal 66:1404-1406 (2002)
© 2002 Soil Science Society of America

COMMENTS & LETTERS TO THE EDITOR

Second reply to Kinnell's comments on "Vertical Hydraulic Gradient and Run-on Water and Sediment Effects on Erosion Process and Sediment Regimes"

C. Huang*,a, L. D. Nortona and F. Zhengb

a USDA-ARS National Soil Erosion Research Laboratory 1196 SOIL Bldg., Purdue University West Lafayette, IN 47907-1196
b Institute of Soil and Water Conservation 26 Xinong Road Yangling, Shaanxi P.R. China, 712100

* chihua{at}purdue.edu

Abbreviations: FD, flow detachment • FT, flow transport • RD, raindrop detachment • RIFT, raindrop induced flow transport • ST, splash transport • DDL, dynamic deposition layer • H, surface protection coefficient • KSM, erodibility of the soil material • KDDL, erodibility of the dynamic deposition layer

Prologue: We encourage the readers to read the first set of Kinnell's comments and our reply (Kinnell, 2001; Huang et al., 2001) together with this discussion.

In the two comments Kinnell made (Kinnell, 2001; Kinnell, 2002) on our paper (Zheng et al., 2000), Kinnell used his unvalidated erosion theory to show that our experimental procedure and data analysis might be flawed if we altered our experiment and obtained different results. Without data, Kinnell theorized that the run-on sediment from the feeder box are temporally stored in a dynamic deposition layer (DDL) and this distinct layer has a different erodibility (presumably higher) than the original soil (see Eq. 1 of Kinnell, 2002). Under this hypothesis, Kinnell argued that we might have incorrectly analyzed the sediment feed effects since not all the feed sediments had been transported out of the test box when we collected samples after the feed was removed. We do not see the potential for any fruitful results from this kind of discussion because: (i) we are not interested in conducting additional experiments nor was it our objective to either prove or disprove Kinnell's argument; (ii) we do not have any means to quantify the processes (i.e., RD, FD, ST, RIFT, and FT) and parameters (i.e., raindrop size distribution, flow depth, flow velocity, particle velocity distribution, DDL, active zone, H, KSM, KDDL) that are necessary to respond to Kinnell's comments (Huang et al., 2001); and (iii) if Kinnell is interested in challenging our results, he can simply conduct the necessary experiments and present the results through a regular journal publication.

Since we do not have any means to separate the feeder sediment from the sediment eroded from the ‘original’ soil in the test box to ensure that all feeder sediment had been transported off the test box after the feed was removed, we will not be able to address the residual effect questioned by Kinnell. Nevertheless, we would like to point out the potential difficulty in testing Kinnell's theory and present some peripheral data that show Kinnell's proposition is incorrect and may not exist in our experiment.

We believe that erosion and deposition (or alluvium) are part of the soil forming processes, and with time, different soils eventually develop at different landscape positions. Therefore, the soil is a composite of particles from different origins collected through time. In Kinnell's model, the soil is separated into previously detached material (or DDL) and ‘original’ soil. But, Kinnell failed to specify how and when the previously detached and deposited material becomes the ‘original’ soil as time progresses. Mechanical disturbances such as tillage operations also create a disturbed layer with a mixture of clods (‘original’ soil?) and disintegrated soil particles and aggregates (DDL?) on the surface. The question is, given a soil surface at some state during or after the action of rain or other mechanical disturbance, how can one separate the detached material from the ‘original’ soil and define soil erodibility for these supposedly different portions? This would be impossible to do.

During the experiment, we collected run-off samples from the test box first without the feeder input, then with feeder input, and finally after the removal of feed. The sample timing was designed for an apparent steady run-off discharge in each of the without-feed and with-feed stages. Whether the sediment discharge had reached a steady state was an experimental outcome (see Fig. 2 of Zheng et al., 2000) rather than an experimental requirement. In the data analysis, we used the average of the four sediment samples before the feed and two samples after the feed as the sediment discharge without feed and compare it with sediment discharge with feeder input. By doing this, we averaged out the unsteadiness for a comparison of the runon sediment effects. If Kinnell's argument that a residual effect from the sediment feed is affecting our results, then the sediment discharges collected immediately after the feed will be greater than those before the feed.



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  		Fig. 2. Comparison of sediment concentrations from the test box before and after the sediment feed for all runs made in the study of Zheng et al. (2000).

 
From our data shown in Table 3 of Zheng et al. (2000), the test box was under the deposition regime for only one out of the 23 treatments tested when the box was at 5% slope, drainage condition, under 50 mm h-1 rain with greatest sediment feed (i.e., 0% cover on the feeder box). This would be the only situation having the residual sediment feed effect because of the net deposition on the test box with the feeder sediment. When the text box is under net erosion with sediment in-feed, we would not expect to see the residual sediment feed effect when the test box is considered as a whole unit, although localized deposition of the feed material may occur. In Fig. 1 , we plotted the net deposition and erosion from the test box for two replicate runs under this specific condition. Based on Kinnell's argument, the erosion rate immediately after the removal of the greatest sediment feed while the box was under net deposition would be much higher than the rest of the without-feed situations. This was clearly not the case in both replicate runs.



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  		Fig. 1. Deposition and erosion in the text box from two replicate runs under 5% slope, 50 mm h-1 rainfall and drainage condition with ({triangleup}) and without ({circ}) sediment feed with the average feed rate denoted by solid squares.

 
Additionally, we plotted sediment concentration averaged from the last two samples immediately before feeder input against the average concentration from the two samples taken after the feeder input for all the runs we made (Fig. 2) . If Kinnell's assertion were correct, we would expect the data to fall above the 1:1 line, i.e., greater sediment yield after the feed than before the feed. Instead, the data were scattered around the 1:1 line to ~70 to 75 kg m-3 and then fell below the 1:1 line indicating the sediment discharges after the feed were in fact less then those before the feed at high sediment yields. As we have illustrated in Fig. 2 of Zheng et al. (2000), sediment discharge decreased with time under severe erosion, causing the data to fall below the 1:1 line in Fig. 2.

In conclusion, it is neither our interest nor objective to prove or disprove Kinnell's unvalidated erosion theory. Because of the lack of measurement techniques to quantify physical processes actually occurring in the multiple-box system, we have treated the box system as a sequence of ‘black’ boxes and focused our efforts on the sediment input and output fluxes between the feeder and test boxes. We cannot stop Kinnell's imaginary inquisition into what had happened inside these ‘black’ boxes. But, we do wish that when Kinnell posted challenges to a research effort, he would have done it through a rigorous scientific protocol instead of using unsubstantiated ‘if ... then ...’-type proposition in unrefereed Letter-to-the-Editor. Soil erosion is a science that can only be advanced through experimentation, not arguments based on hypothetically derived theories and assumptions. This kind of challenging approach only detracts from the progress of science. The abuse of the Letter-to-the-Editor forum only diminishes the already low credibility of the author in the field of erosion science.

Received for publication August 28, 2001.

REFERENCES





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