proportionality of sediment load to flow and slope indicated by these results is consistent with current
understanding of rill erosion processes. Large and increasing dimensionless FT/C sediment load ratios
with soil moisture highlight the primary role of FT in rill erosion processes that increases with soil
moisture.
Local Measures of Erosion
We measured and characterized cross sections of the rills at 0.3L (24 cm) and 0.7L (55 cm) above
the outlet both before and after the flow event. The two norms used to measure cross-sectional change
were L2, the root-mean-square of the change in bed elevation resulting from the flow, and Linf, the
maximum bed elevation change at a point in each cross section. These measures are particular to these
specific locations, and might be influenced by local effects that are not representative of the complete
reach. To more fully characterize the extent of erosion in each bin, other local measures were obtained to
supplement the detailed cross-section measurements. Maximum channel width and maximum channel
depth at every 0.1L were measured at the conclusion of each experiment and averages are presented here.
L2 measures for both bins at 0.3L and the corresponding FT/C ratio are given in Figure 13 with
flow condition by slope and soil moisture series. Corresponding Linf results for this same cross section are
given in Figure 14. These same measures at 0.7L are given in Figures 15 and 16, respectively. Our initial
observation is that both measures are providing very similar information concerning cross-sectional
change. The norms indicate very small changes in the controls for both the mid and high soil moisture
series, with all norms less than 2.2 and 1 cm, respectively, for these series. The norms for the low soil
moisture series in the C bins are also generally small, with L2 < 2.5 cm and Linf < 3.6 cm. The upper
bound on these C norms decreased as soil moisture increased, indicating generally greater erosion
resistance at higher moisture content. The norms of both FT cross sections generally increase with
applied flow for the low and mid soil moisture series, and indicate generally enhanced erosion with slope.
At high soil moisture the slope dependence of cross-sectional change is again clear, but dependence on
flow rate is not apparent. The L2 and Linf FT/C ratios generally increase with soil moisture, but display no
clear dependence on flow or slope. Anomalously high ratios at 0.7L for the low moisture, high flow, and
low and high slope experiments indicate excessive erosion in that part of the FT bin relative to the same
location of the C bin. The ratios of the high soil moisture series are generally high, a result of very minor
erosion in the C bins. Greater relative erosion of the C bins in the high flow, high moisture experiments
produced the smallest ratios of the group.
Tables 7 and 8 give overall and group-averaged norms and their FT/C ratios for 0.3L and 0.7L,
respectively. L2 and Linf FT/C ratios for the overall experiment are equal (2.81) at 0.3L, and nearly equal
(4.94, 4.56) at 0.7L, with values much greater than 1 indicating a primary effect of the FT cycle on rill
erosion. At 0.3L the series-averaged ratios of the L2 and Linf measures are approximately equal for each
moisture series, and increase dramatically with soil moisture. This increase is due largely to significant
decreases in the C measures with increasing soil moisture. Rough equality of L2 and Linf FT/C ratios also
occurred at 0.7L, but the low soil moisture ratios were much larger and comparable to those at high
moisture. The high ratios at high moisture are again due to greatly decreased measures in the C bins.
Both norms show minor increases for increasing slope groupings at both locations, and corresponding
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