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The hydrology of areas of low precipitation — L'hydrologie des
régions à faibles précipitations (Proceedings of the Canberra
Symposium, December 1979; Actes du Colloque de Canberra, décembre
1979): IAHS-AISH Publ. no. 128.
The measurement and description of rill erosion
R. J. LOCH Queensland Wheat Research Institute, Toowoomba,
Australia
Abstract. Rill erosion can be the major mechanism of soil loss
from sloping, cultivated land. The various methods of measuring and
describing rill erosion are discussed, using both published data
and results from simulated rainfall studies. To allow comparison of
various studies, and the development of predictive models, it is
essential to measure rill erosion rates and their change with time.
Observations of channel pattern, while seldom made, could be of
value in interpreting results.
Mesure et description de ltérosion en rigoles Résumé. L'érosion
en rigoles peut être le mécanisme le plus important agissant sur
les pertes de sol sur les terres cultivées à fortes pentes. Les
diverses méthodes de mesure et de description de l'érosion en
rigoles sont analysées en utilisant à la fois les données publiées
et les résultats obtenus avec les simulateurs de pluie. Pour
permettre les comparaisons des diverses études et la mise au point
de modèle de prédétermination il est essentiel de mesurer le taux
d'érosion en rigoles et ses changements dans le temps. Les
observations sur le schéma des rigoles, quoique rarement faites
seraient d'un grand intérêt pour l'interprétation des
résultats.
INTRODUCTION
Rill erosion is the most obvious mechanism of soil loss from
sloping, cultivated land (Fig. 1). Preliminary studies using
simulated rain on 22.5 m long field plots (Loch, 1978) have shown
that rill flow can carry much higher sediment loads than sheet
flow, and is probably the major soil erosion process on the
swelling clay soils of the Darling Downs area of Queensland. The
measurement and description of rill erosion is therefore an
important aspect of erosion research in the area. Published rill
erosion studies have shown considerable variation in methods of
measurement. The aim of this paper is to discuss the various
approaches used on the basis of both published data and results
obtained from simulated rainfall studies on the eastern Darling
Downs.
EROSION RATE - TIME CONSIDERATIONS
The rilling process is basically the development of a drainage
network. This development is usually limited to one or two runoff
events before cultivation removes the channels formed, usually
leaving slight depressions which serve to concentrate runoff from
subsequent storms. While all the phases of rill development —
initiation, elongation, elaboration, maximum extension and
abstraction — may not occur, there is little doubt that rill
erosion rate can depend on the stage of channel development that is
reached.
This is illustrated by Fig. 2, which shows a gradual decrease in
sediment concentration after a peak was reached at 26 min. Romkens
et al. (1975) and Parker (1976) have re-ported a similar decrease
in sediment yields from an initial peak concentration. This is
usually considered to be an initial channel-forming phase with the
rill being incised and loose, and easily eroded material being
removed.
The considerable variation in sediment concentration with time,
shown by Fig. 2 seems to be characteristic of rilling on the highly
erodible swelling clay soils of the Dar-ling Downs. This may be due
to random large inputs of sediment from bank collapse.
429
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430 R.J . Loch
7 ^ ,- - , .
• * '
'X
FIGURE 1. Severe rill erosion in cultivated land, eastern
Darling Downs area.
10
Z 9 LU
a e Z 7
1 6
I 5 LU
8 z 3 LU S 2
3 LU
m 1
-
//V Id.
A Awf / v y
• t y : / /
1 1 1 1 1
RUNOFF RATE
• *>
1 / v \ ^ w ^ SEDIMENT \
^ CONCENTRATION • 1
1 1 1 22 26 30 34 38
TIME IN MINUTES FROM START OF RAIN
2.0
1.8
1.6 8 z o t/1
0.8 5 cr
0.6 li-CS
z
0.4 g
0.2 -10 50
FIGURE 2. Sediment concentration and runoff rate for a 22.5 m
long rainulator plot on a black earth.
Meyer et al. (1975) measured total soil loss due to rill erosion
for two successive 30 min periods of rainfall. Such measurements,
using some form of flow divider, may over-come problems of the
variability of sediment loss. However, the lack of information on
changes in sediment concentration with time could limit the
usefulness of the data. This is particularly so when it is desired
to compare results from studies using periods of measurement of
different length. The development of predictive models also
requires data to be extended to runoff events of differing
duration.
The time periods used in published studies vary widely, not only
in their duration, but also in the stage of the rilling process
that they cover. Meyer etal. (1975) used a period of 30 min rain,
but did not specify the length of time taken for runoff to com-
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Rill erosion 431
mence. Loch (unpublished data) used a period of 19 min (chosen
for convenience) after sediment concentrations showed that rilling
had commenced. Young and Wiersma (1973) used a period of 30 min
after runoff had equilibrated, after 135 min of rainfall.
As rill drainage basins are small it is likely that runoff
events will be relatively brief. Measurement should obviously cover
the initial period when sediment concentrations are highest, and
the total length of time for which measurements are made may depend
on either local storm durations or on the need to describe changes
in rill erosion rate with time.
CHANNEL PATTERN
Channel pattern is not generally recorded in studies of rill
erosion, although it could be of value. Schumm (1977) presents
evidence for threshold slopes associated with the change from one
pattern to another — e.g. straight to meandering — and notes the
work of Edgar (1973) showing that these thresholds can be altered
by a change in discharge. Rills produced by the author on
rainulator plots with a slope of 0.04 remained straight with some
thalweg meander while discharge was less than 1.5 l./s. At
discharges greater than 1.5 l./s rills first developed a fan at
their outlet, and then a braided area up to 2 m wide several metres
upslope. A difference in discharge (e.g. between replicates) can
therefore result in a change in rill pattern and probably sediment
loss. Moreover, the development of rill pattern with time may help
to explain time-dependent changes in sediment loss.
F K J L R L 3. Ipslope section of rill - 22.5 m long rainulator
plot on a black earth.
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432 R.J. Loch TABLE 1. Channel pattern and sediment load data
for two soil types^
Channel pattern
Meander amplitude Sediment concentration Concentration of
sediment
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Rill erosion 433 the soils being quite marked (Figs. 3 and 4).
Bank stability could be a factor governing soil erodibility;
relating to the degree of consolidation of the cultivated layer.
Meyer et al. (1972) reported an effect of tilth on susceptibility
to rill erosion, but did not speculate on possible causes.
Observations of channel pattern may be very useful in recognizing
and interpreting tilth or soil type effects.
CONCLUSIONS
(1) When studies of rill erosion are undertaken, a measure of
erosion rate and its change with time appears to be more useful
than a measure of total sediment yield. Sampling for erosion rate
needs to be carried out as frequently as possible because of large
fluctuations in sediment concentration.
(2) Observations of rill pattern are likely to be of value in
interpreting the results obtained.
REFERENCES
Edgar, D. E. (1973) Geomorphic and hydraulic properties of
laboratory rivers. MS Thesis, Colarado State University, Fort
Collins, USA.
Loch, R. J. (1978) Use of a rainulator for studying runoff and
soil loss processes. Austral. Soc. Soil Set, NSW Branch, Soils
Conference, pp. 129-133 .
Meyer, L. D., Foster, G. R. and Romkens, M. J. M. (1972) Source
of soil eroded by water from upland slopes. In Proceedings of
Sediment Yield Workshop, pp. 177-189: USDA Sedimen-tation
Laboratory, Oxford, Mississippi, ARS-S-40.
Meyer, L. D., Foster, G. R. and Nikolov, S. (1975) Effect of
flow rate and canopy on rill erosion. Trans. Amer. Soc. Agric.
Engrs 18, no. 5, 905 -911 .
Parker, R. S. (1976) Experimental study of drainage system
evolution. Unpublished report, Colorado State University, Fort
Collins, USA.
Romkens, M. J. M., Nelson, D. W. and Roth, C. B. (1975) Soil
erosion on selected high clay sub-soils./. Soil Wat. Conserv. 30,
173-176.
Schumm, S. A. (1977) The Fluvial System: John Wiley, New York,
USA. Schumm, S. A. and Khan, H. R. (1972) Experimental study of
channel patterns. Geol. Soc. Amer.
Bull 83 ,1755-1770. Young, T. A. and Wiersma, J. L. (1973) The
role of rainfall impact in soil detachment and trans-
port. Wat. Resour. Res. 9, no. 6 ,1629-1636.
