EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 9. 371-381 (1984)

SURFACE RUNOFF AND SEDIMENT YIELD IN THE ARDECHE RANGELANDS

J. M. ROELS Department of Physical Geography, State University of Utrecht, Heidelberglaan 2, P.O. Box 80.1 IS, 3508 TC Utrecht,

The Netherlands

Received 16 May 1983 Revised 22 January 1984

ABSTRACT

This paper presents a case study of runoff and sediment generation under Submediterranean rangeland conditions (Ardkhe drainage basin, France). Measurements indicate that on a rough hillslope interrill runoff and sediment are not produced uniformly over the slope surface. It is observed that runoff concentrates immediately in non-permanent interrill flow paths, which under average storm conditions vary in length from 1.0 to 125m. Long interrill flow paths may eventually become permanent. These permanent flow paths, called pre-rills, are introduced as a new source area, and are considered to be the initial stage in the development of rills. Along pre-rills considerable quantities of runoff and sediment are carried away.

This study also shows that calculations based on interrill, pre-rill, and rill runoff will only have significance if storm and soil conditions are specified in detail. It is concluded from a correlation analysis between the runoff volume and the amount of soil loss on a storm-by-storm basis that the runoff volume alone cannot explain the amount of sediment that is generated in each source area; soil availability is an additional factor that must be taken into account.

KEY WORDS Source area Runoff volume Soil loss

INTRODUCTION

Soil eroded from upland areas comes from rills (runoff microchannels) and interrill areas (the slope surface between these channels (Meyer, 1979)). It is usually assumed that interrill erosion is a relatively uniform removal of soil whereas rill erosion is a locationally selective removal caused by concentrated runoff (Meyer, Foster, and Romkens, 1975; Meyer, 1979). However, Roels and Jonker (1983) demonstrate that under Ardtche rangeland conditions interrill erosion is not uniform. The coefficients of variation (CV) that they found for random interrill erosion samples ranged from 20 to 68 per cent, depending on the sample size. From these CV values they concluded that interrill erosion is also a locationally selective displacement of soil, probably caused by concentrated but unchannelled flow. This conclusion indicates that the conventional concept of the uniformity of interrill erosion may have to be reappraised. If interrill erosion on rough rangeland slopes is caused by concentrated runoff, slope length becomes an important factor, as it is with rill erosion. This implies that under these conditions analytical models in which the influence of slope length on soil erosion is deduced from the ratio of uniform interrill to rill erosion (Foster and Meyer, 1975; Foster et al., 1977; Moldenhauer and Foster, 1981) are no longer useful. Perhaps even the mechanisms of interrill detachment and transport need to be reconsidered.

The purpose of this study is first of all to measure the slope lengths for both concentrated interrill and rill runoff and to evaluate the dependence of the runoff volume on these lengths. Since runoff generation is influenced by the amount (and intensity) of rainfall and by the intake rate of the soil, these factors are taken into account in the description of runoff accumulation as a function of slope length.

01 97-9337/84/O40371-1 l$Ol. 10 0 1984 by John Wiley & Sons, Ltd.

372 J. M. ROELS

Neglecting the specific mechanisms of detachment and transport, the next step is to correlate the runoff volume, as explained by slope length, rainfall, and soil characteristics, with the amount of soil loss. It is assumed that an increase in the runoff volume causes an increase in the detachment and transport capacity of the flow. As a result the sediment yield of the area will increase. In this paper this expected positive correlation between the runoff volume and soil loss is quantified and the efficacy of the correlation technique is evaluated.

STUDY AREA AND METHODS

The study was carried out in the rangelands of the Ardkche drainage basin (France). The research area is limited to a part of a subcatchment slope (Figure 1). It covers 8,250m’. Average annual rainfall is 1036mm (de HCdouville, 1980). The bedrock stratification consists of alternating limestones and mark (S.C.G., 1967) with a gentle dip (4 to 5’) towards the southeast. The local soil is a Lithic Xerochrept (USDA, 1975). The vegetation of the study area is described as a Xerobromion grassland containing scattered shrubs (Wolkinger and Plank, 198 1).

The slope is divided into an upper predominantly interrill slope segment and a lower rilled slope segment. The soil loss from the upper slope segment differs significantly from that of the lower slope segment. Basic slope segment characteristics are summarized in Table I.

On the natural rangeland slopes there are no cultivation practices to smooth the soil surface. Runoff will concentrate and rills can develop purely as a result of topographical irregularities and extensive natural surface roughness and not as a result of tillage. In the Ardkche rangelands therefore rills are permanent erosion phenomena, having a depth from 10 to 60cm, a minimum length of 15m and a mean width of 50cm.

Since the flow lines are not always parallel to the slope because of irregular topography, open plots instead of bounded plots are used to measure both rill and interrill erosion. These open plots consist of several separate but adjoining, modified Gerlach troughs (Gerlach, 1967). The troughs, having a width of 50cm, are made of aluminium and are attached to the soil surface by means of concrete. They drain into plastic containers. Some of the troughs are installed under the rills to obtain reliable rill erosion data. Separate from these measurements interrill erosion is gauged in adjacent troughs. Sixty troughs are placed in the upper and lower slope segment. They are lined up ‘en echelon’ to ensure a clear run to each collector from the top of the slope (cf. Morgan, 1980). On the upper slope segment interrill erosion is measured in 59 troughs and rill erosion in one trough, while on the lower slope segment interrill erosion is measured in 55 troughs and rill erosion in five. Both the

Figure 1. Location of the study area (after Roels and Jonker, 1983)

RUNOFF AND SEDIMENT GENERATION 373

Table I. Some basic slope segment characteristics (after Roels and Jonker, 1983)

Factor Variable Upper slope segment Lower slope segment

Topography distance to the crest* slope form slope anglet

clay content organic matter content % area covered by chalk flints bulk density saturated hydraulic conductivity2

Vegetation % area covered by canopy

Soil soil mantle depth2

30 slightly convex

15.1 26 43

2.7

,< 15 1280

23.8 x lo-"

80

60 slightly concave

18.5 26 45

1.1

Q 35 1407

13.5 x lo-"

45

* distance trough-crest; 7 mean slope angle of profile unit having a length of 25 m; 1 the median value; in all other cases the mean values are presented.

trough design, the sampling procedures used and their accuracy are described in detail in Roels and Jonker (1983).

RUNOFF ACCUMULATION

When the rainfall intensity exceeds the infiltration capacity, the excess rainfall fills surface depressions. When these depressions are full the excess precipitation spills downslope. Instead of coalescing as an irregular sheet the water concentrates immediately in flows due to the extreme surface roughness. The courses of these ephemeral shallow flows are called interrill flow paths. During a storm the development of several interrill flow paths is observed. Depending on the rainfall intensity and the duration of the storm on the one hand and on the local microtopography and infiltration rate on the other hand, interrill flow paths combine with other interrill flow paths to form a small chain of flow paths. These chains of flow paths may have different lengths during different storms but their location remains the same.

An increase in the amount of rainfall causes an increase in the length of the chains of interrill flow paths. The growth of these lengths is intermittent. The excess rainfall will first cause an increase in the depth of flow; this in turn causes submerging of topographical irregularities and surface roughness barriers and eventuallv an increase in the flow path length.

It is observed that storms with a high rainfall intensity but moderate duration and storms with a moderate rainfall intensity and longer duration (hereafter called average storm conditions) ultimately produce interrill flow path chains of comparable length. For each storm a large spatial variation in interrill flow path lengths is measured. Under average storm conditions these lengths vary from 1 to 20 m. Storm events with (very) high intensity and (very) long duration eventually produce a more continuous flow along the slope profile. The lengths of interrill flow path chains were not measured during these storms.

The accumulation of concentrated interrill runoff in unchannelled flow paths is completely different from the accwdat ion of rill runoff in permanent microchannels. The interrill runoff response to rainfall is relatively rapid; the runoff volume increases discontinuously due to the intermittent growth of the interrill flow path length. The rill runoff response is delayed appreciably due to microchannel storage. But when all reservoirs are filled, the runoff flows downslope through the entire channel. Supplementary input is provided by several interrill flows. Moreover, as a consequence of the convexxoncave slope profile (see Table I), the upper slope segment is a source of return-flow for the rills in the lower slope segment.

At the end of a rainstorm the interrill flow path chains generally break up. No significant interrill after-flow is produced, but there is considerable rill after-flow. However, some of the interrill flow path chains do produce

3 74 J. M. ROELS

after-flow. It follows that these flow path chains must be permanent. Repeated detailed observation showed that these specific interrill flow path chains have a permanent section which is at least 7.5 or 5 m long (for the lower and upper slope segment respectively, under average storm conditions). These permanent interrill flow paths, which do not look like microchannels but which carry away after-flow, are called pre-rills. The pre-rill flow paths are an important stage in the development of a rill. Contrary to Emmett (1970) it is concluded that on natural hillslopes rills can develop gradually and need not be the result of accelerated erosion. The pre-rill stage is the essential initial stage in this gradual rill development. Emmett (1970) however does describe the occurrence of flow concentrations dictated by microtopography without the appearance of rills. Perhaps these flow paths may be interpreted as pre-rills.

Consequently, three source areas are introduced here: interrill, pre-rill, and rill areas. Table I1 reviews the frequency distributions of the flow paths and the microchannel lengths of these three source areas under average storm conditions.

The interrill flow path length distributions displayed in Table I1 exhibit a marked positive skewness; there are few very long flow paths and many small flow paths. The overlapping distributions of the interrill and pre- rill lengths show that long interrill flow paths become permanent only gradually. Also the transition from the pre-rill to the rill stage is gradual. If rilling proceeds (as in the lower slope segment) the length of the rills surpasses that of the pre-rill flow paths. Because all runoff is concentrated, the lengths of either the flow path chain or the microchannel will have an effect on the runoff volume. The rate of growth of the runoff volume with increasing length will be higher for the permanent flow paths and microchannels (pre-rills and rills) than it is for the non-permanent ephemeral interrill flow paths. The rill runoff volumes will be somewhat reduced due to the large microchannel storage capacity of the rills. In the following section some runoff volumes of each source area are presented.

INTERRILL, PRE-RILL, AND RILL RUNOFF VOLUMES

In order to interpret the influence of the length and permanence of flow paths on the runoff volume one has to take into account factors which determine runoff generation: the intake rate of the soil and the amount of rainfall (or rain intensity and duration).

The intake rate of the soil depends largely on the crusting process under impact of raindrops. The crust develops in two stages. At first small disaggregated particles fill the pore space at the soil surface (cf. Farres,

Table 11. Frequency distribution of interrill and pre-rill flow path lengths and rill microchannel lengths

Lower slope segment Upper slope segment Length flow path frequency flow path frequency

(m) interrill pre-rill rill interrill pre-rill rill

< 2.5 2'5-5.0 543-73 7'5-10.0

10.0-1 2.5

15.0-17.5 17.5-20.0 20.0-22.5 22.5-25.0 25.0-215 27.5-30.0 30.0-35.0 35G-40.0 40W5.0

12.5-1 5.0

> 45.0

35 8 3 1 2 1 1

1 2 1

47 5 1 2 1

1 1

1

1

1 1 1 1

1

RUNOFF AND SEDIMENT GENERATION 375

6.0.

-~

20-

10.

1978). The next stage is the compaction by drop impact of the soil aggregates and their residuals (cf. McIntyre, 1958).

A vane borer with eight blades (blade-height = 0.5 cm; vane diameter = 2.5 cm) was used to measure the compaction of the soil. The mean of 35 vane measurements was calculated. Progressive compaction of the soil during three consecutive storms (8,9, 10) is illustrated by the mean vane values given in Table 111. This table shows that significant compaction of the soil can occur in less than three weeks: most of the vane recordings have doubled in value. A smaller increase in vane values is measured at vegetated sites (4 and 8) where the soil is protected against drop impact.

As a result of the compaction of the soil surface the rate at which water can enter the soil is reduced and the runoff response to a unit amount of rainfall is consequently increased. In Figure 2 this decrease in the intake rate of the soil is displayed (March-April 1981, storm nos. 8,9 and 10). Since no agricultural activities take place in the study area, the crust development can only be altered by a natural cause. Due to frost action in autumn and winter and desiccation in spring and summer many cracks form in the crust, causing an increase in the intake rate of the soil. Erosion of the crust by overland flow was observed only during high intensity rainstorms of long duration. Figure 2 shows all these trends in the variation over time of the intake rate of the soil. Rainfall intensities exceeding these rates eventually cause at least 70 per cent of the troughs to fill with overland flow.

Besides the soil intake rate, the runoff volume also depends on rainstorm characteristics. A storm classification is developed which is based on the excess rainfall amount (EA, in mm) and the peak intensity during a five minute period (ZM, in mm.hr - '). These two variables are multiplied to give the compound energy factor (EAZM mm2.hr-'). Four different storm types are distinguished with energy increasing from A to D. The EAIM index is less than 150 rnm2.hr-' for storm type A, it varies between 150 and 500 mm2.hr-' for

- Frost action

crust ermion

i tZZ4 crusting

. . . . . . 0.. . ..

Table 111. Mean vane values, measured at eight sites. During all tests pF = 2

Upper slope segment vane values in N/m2 ( x loe4)

Lower slope segment vane values in N/m2 ( x

site 15-3 22-3 04-4 1981 site 15-3 22-3 04-4 1981

1 0.86 1.20 2.08 5 0.76 080 1.75 2 1.05 1.09 2.11 6 1.18 0.96 2.09 3 1.30 1.52 2.06 7 0.84 1.31 1.92 4 2.12 2.12 2.32 8 0.94 1.03 1.59

Figure 2. Variations over time of soil intake rate values (Ic) in m / h r

376 J. M. ROELS

storm type B, between 500 and 1500 mm2.hr-' for storm type C, and between 1500 and 6000 mm2.hr-' for storm type D.

Runoff volumes were accurately gauged during only twelve storms. The collected runoff data have been classified on the basis of storm energy: five storms of type A, four of type B, two of type C, and only one of type D were recorded. In addition, for each storm type all measured runoff volumes were classified by source area. None of these runoff values per source area and storm type was distributed normally.

If runoff values are assumed to be proportional to flow path or microchannel lengths a normal distribution is not to be expected, because these lengths are not distributed normally themselves (see Table 11). However, a logarithmic transformation of the runoff values may yield a normal distribution. The frequency distributions of the transformed runoff volumes (by taking their logarithms to base e), classified by source area and storm type, are depicted in Table IV. From this table it is concluded that the runoff volume increases with increasing storm energy. For storm type A, there is no significant difference in the runoff volume of the source areas. Irrespective of the length of the flow path or microchannel, little runoff is generated. The rill runoff volume will also be reduced due to microchannel storage. With increasing storm energy it is possible to differentiate between the interrill runoff volumes and the runoff volumes of the pre-rills and rills, the volumes of the latter being much higher. This demonstrates the effect of the flow path length and stresses the significance of the permanence of this flow path. Because the pre-rills, which are shorter than the rills, produce runoff volumes comparable to rill runoff volumes, it can be concluded that their storage capacity is much smaller than that of the rills.

The second conclusion from Table IV is that despite the transformation of runoff volumes, the data still deviate from a normal distribution. The scatter is due to the combination of several different runoff distributions, belonging to different storms, in each storm type class. The runoff response to rainfall depends largely on the momentary value of the intake rate of the soil, which varies considerably (see Figure 2). The influence of the soil intake rate hinders the determination of the influence of the length factor on the runoff volume.

This description of runoff generation and accumulation has several important implications for erosion research. First of all, interrill flow path lengths appear to vary considerably both with respect to location and time. On rough slopes they may have lengths of up to about 10 m; pre-rill flow paths are even longer. Since the conventional bounded interrill plots have a fixed length, which is usually less than 2.5 m, these plots cannot provide reliable overland flow information for such slopes. Instead, plots at least 20 m long are needed to study interrill and pre-rill flows.

Secondly, in conceptual erosion models interrill erosion results primarily from the detachment and transport effects of raindrop impact. Therefore interrill erosion is only slightly affected by the location of the slope. Detachment by interrill flow is neglected. The capacity of the flow to transport (raindrop) detached particles is assumed to be limited (Foster and Meyer, 1975; David and Beer, 1975; Meyer, 1979,1981; Morgan and Morgan, 1981). However, in view of the measured runoff volumes, the potential of pre-rill flow to detach and transport soil particles cannot be neglected and also that of interrill flow must be considered. Since the flow is concentrated both interrill and pre-rill erosion models will require a slope length factor.

Thirdly, logarithmically transformed runoff volumes, classified by source area and storm type, are not distributed normally due to a large temporal variation in the intake rate of the soil. This deviation from the normal distribution impedes not only a comparison of the mean runoff volume from a source area for several different storm types, but it also hinders all calculations based on the mean runoff volume classified by storm type and source area. Consequently, runoff values can only be used effectively if the source area, storm conditions, and the intake rate of the soil are specified in detail. This means that only interrill, pre-rill, and rill runoff volumes of separate storms can be used.

Following this last-mentioned implication, runoff values of separate storms were correlated with soil loss.

THE RELATION BETWEEN THE RUNOFF VOLUME AND SOIL LOSS

The relation between the runoff volume and soil loss is expressed by the general equation: log Q, = d o g Qw + b

Tabl

e IV.

Freq

uenc

y di

strib

utio

n of

the

uppe

r an

d lo

wer

slo

pe s

egm

ent r

unof

f vo

lum

es fo

r fou

r st

orm

type

s

Upp

er s

lope

seg

men

t st

orm

type

Lo

wer

slo

pe s

egm

ent

stor

m t

ype

A

B

C

D

A B

C

D

(4

) (2

) (1

) (5

) (4

) (2

) (1

) (5

) R

unof

f vol

ume

Run

off v

olum

e (e

log

trans

form

ed)

Run

off v

olum

e fr

eque

ncy

(e lo

g tra

nsfo

rmed

) R

unof

f vol

ume

freq

uenc

y ~~

~

CLA

SS

IR

PR

R

IR

PR

R

IR

PR

R

IR

PR

R

CLA

SS

IR

PR

R IR

PR

R

IR

PR

R

IR

PR

R

2.

30-

2.65

83

4 1

22

2.30

- 2.

65

48

1 2

2.65

- 3.

00

92

6

2.65

- 3.

00

16

1

2 3.

00-

3.35

19

10

3.00

- 3.3

5 2

11

3.

35-

3.70

38

2

6 2

1 3.

35 3

.70

21

11

4

3.70

- 4.

05

12

2 3.

70-

405

17

3 2

4.05

- 4.

40

13

11

6

4.05

- 4.

40

10

22

1

4.40

- 4.7

5 10

15

1

4.40

- 4.

75

15

3 5

4.75

- 5.

10

16

3 22

4.

75-

5.10

8

13

13

z

5.10

- 54

5 26

15

1

1 5.

10-

545

13

12

5

8 5.

45-

5.80

14

2

1 10

1

3 5.

45-

5.80

6

4 ?!

21

2 5.

80-

615

52

14

9

5.80

- 6.

15

13

41

9

2 $

6.15

- 65

0 16

2

15

2 7

6.15

- 6.

50

20

1

1

19

5 U

6.

50-

6.85

5

1

14

1 16

2

6.50

- 68

5 9

13

20

6

6 6.

85-

7.20

4

2

44

2

15

1 2

6.85

- 7.

20

16

2 El 0

C

39

2 1

12

B 8 f 8

7.20

- 7.5

5 3

15

10

1

16

2 9

7.20

- 7.5

5 5

3

26

6 1

20

1 7.

55-

7.90

1

1

14

6 75

5- 7

.90

2 2

21

8

3 12

1

3

7.90

- 82

5 11

1

13

7.90

- 8.

25

23

10

3

2 13

1

7 8.

25- 8.60

2 9

1

9 8.

25-

860

4 7

15

8.

60-

8.95

12

8

8.60

- 89

5 1

51

1

17

8.

95-

9.30

1

1 2

1

8.95

- 9.

30

2 2

22

7

5 9.

30-

9.65

1

1

2 9.

30-

9.65

2

13

4

4

7

1000

-103

5 2

1

1000

-103

5 2

2 5

4

1

2 1

21

10

3510

70

1 10

35-1

070

1 1

1

1070

-11.

05

1070

-11.

05

12

2

1

3 11

.05-

11.4

0 1

11

11

.05-

1 1.

40

1

22

1

4 11

40-1

1.75

1

1

11.4

0-11

.75

12

2

1

11.7

5-12

.10

1 11

.75-

12.1

0 4

1 1

12.1

0-12

.45

12.1

0-1 2

.45

1

1 12

.45-

12.8

0 12

.45-

12.8

0 1

1 2

Bet

wee

n br

acke

ts: n

umbe

r of

sto

rms

used

. O

rigin

al data

in m

l. e-

loga

rithm

ic tr

ansf

orm

atio

n of d

ata.

C

lass

wid

th =

035

. IR

= in

terr

ill a

rea;

nu

mbe

r of

col

lect

ors

for

uppe

r an

d lo

wer

slo

pe s

egm

ent;

54,4

8 re

sp.

PR =

pre

-rill

are

a ;

num

ber

of c

olle

ctor

s fo

r up

per

and

low

er s

lope

seg

men

t: 5,

7 re

sp.

R

= ri

ll ar

ea

; nu

mbe

r of

col

lect

ors f

or u

pper

and

low

er s

lope

seg

men

t: 1,

5 re

sp.

4

9.65

-100

0 1

1

1

9.65

-100

0 3

4 1

16

z w

4

4

378 J. M. ROELS

where Q, = the amount of soil loss, Q, = the runoff volume, and a, b = coefficients. A positive correlation is expected, because an increase in the runoff volume causes an increase in the detachment and transport capacity of the flow and hence an increase in soil loss. The calculated regression equations are shown in Table V.

Due to the limited number of troughs gauging pre-rill and rill runoff and sediment, no correlation between runoff volume and soil loss was tested for the separate source areas. But in three of the logarithmic Q, - Q, relationships from Table V, which are shown in Figures 3,4, and 5, the interrill, pre-rill, and rill runoff and sediment values are depicted separately.

Table V indicates that within the same class of storm types the runoff and soil loss values of separate storms appear to be different and their relations cannot be compared either. Therefore it is absolutely essential to consider both runoff and soil loss data on a storm-by-storm basis. Moreover, in view of the considerable variation in correlation coefficients of the regression equations given in Table V, it is concluded that the runoff volume is not the predominant factor in sediment delivery. A detailed description of the conditions under which the runoff and the sediment are produced helps to explain this conclusion.

In the cases of storms 7 and 11 (see Table V) the crust was cracked and crumbled by frost action and desiccation respectively (see Figure 2). As a result, many loose soil particles became available. A low rain intensity (storm type A) combined with a high intake rate of the soil left only very little overland flow. In the rills both water and sediment were stored in the microchannels. In the interrill and pre-rill area less runoff was generated but more small soil particles were available. Since the interrill and pre-rill storage capacity is much smaller than that of the rills, these small particles were collected. Consequently a small range in runoff values is combined with a relatively wide range of soil loss values, which gives low correlation coefficients (see Figure 3).

In the cases of the medium energy rain storms 12,13, and 14, (storm type B) increasing crust development was observed (see Figure 2). As a result, the intake rate of the soil decreased, which led to increasing amounts of runoff. In the interrill area a wide range of increased flow rates existed as a function of the length of the flow path chains. Despite the sediment-limited conditions due to progressive crusting of the soil, a relatively large volume of sediment could still be produced per unit volume of runoff. In other words the sediment production was relatively efficient. Pre-rill and rill runoff volumes increased rapidly and greatly exceeded the interrill runoff volumes. Due to the decreasing availability of sediment and detachment-limited conditions, however, pre-rills and rills had a relatively low efficiency of sediment production. Consequently, there is a large scatter in runoff rates and a smaller variation in soil loss values, which leads to a moderate degree of positive association (see Figure 4).

At the beginning of the high energy storm 8 (storm type C, see Table V) the soil surface was hardly sealed (see Figure 2). A runoff response of high energy but of moderate volume was observed. In all source areas relatively large amounts of sediment were available. In the interrill area, the runoff transport capacity is less than or equal to the supply of particles detached by the raindrops and by the flow itself. In the pre-rills and rills the high runoff energy ensures considerable detachment of soil. This erosion efficiency guarantees a high correlation between the runoff volume and the soil loss (see Figure 5).

At the beginning of the low energy storm 9 (type A, see Table V) fewer soil particles were available because they had been removed during the preceding storm. The soil surface was progressively sealed (see Figure 2 and Table 111) and consequently a considerable runoff response to a unit amount of rainfall was found. This low

Table V. Equations describing the logarithmic relations between the runoff volume (Q,; in ml) and soil loss (Qs; in g)

Storm Storm Lower slope segment type number

Upper slope segment

~ ~ ~ ~ ~ _ _ _ _ _ ~ ~ ~ ~ ~~

A 7 log Q, = 0.25 log Q, -047 (r = 0.15) log Q, = 0.54 log Q, - 1.29 (r = 0.16) C 8 log Q, = 1.30 log Q, - 3.37 (r = 0.90) log Q, = 1.61 log Q, - 457 (r = 0.90)

A 11 log Q, = 0.26 log Q, - 1.02 (r = 0.16) log Q, = 0.38 log Q, - 1.29 (r = 0.20)

B 13 log Q, = 0.47 log Q, - 1.34 (r = 0.64) log Q, = 0.72 log Q, - 2.33 (r = 0.66) B 14 log Q, = 0.22 log Q, - 0.25 ( r = 0.51) log Q, = 0.38 log Q , - 0.85 (r = 0.47)

A 9 log Q, = 0.99 log Q, - 2.99 (r = 0.81) log Q, = 0.97 log Q, - 2.71 (r = 0.86)

B 12 log Q, = 0.41 log Q, - 0.97 (r = 0.56) log Q, = 0.67 log Q, - 1.75 (r = 0.51)

RUNOFF AND SEDIMENT GENERATION 379

Lower slope segment

. . 02

01

* Rill area P Pre-rill area

lnterrill area

2 5 10 20 50 100 200 Q~

2oo 100 L 20 --

lo -

5 -p t - * t

2 - # p. * p'* .

0 2.-

01 P , , . I . , , , I , , Jl 0'2 0 5 1 2 5

20 --

lo -

5 -p t - * t

2 - # p. * p'* .

0 2.-

01 P , , . I . , , , I , , Jl 0'2 0 5 1 2 5 20 60 100 200 Ow,,]

Figure 3. The relation between soil loss and runoff volume for storm 7: upper slope segment: log Q, = 0.54 log Q, - 1.29 (r = 0.16); lower slope segment: log Q, = 0.25 log Q, - 0.47 ( r = 0-15)

05

0 2

i. * Rill area P Pre-rill area

2 5 10 20 50 100 200 Q ~ ~ ~ , ,

01 = , ,_, , ,, , , , I , , , , , , I , , , , , I , , , , , , , 01 0'2 05 i 2 6 i b 20 50 ioo 2b0 dwll,

Figure 4. The relation between soil loss and runoff volume for storm 1 3 upper slope segment: log Q, = 0.72 log Qw - 2.33 (r = 0.66); lower slope segment: log Q, = 047 log Qw - 1.34 ( r = 064)

energy flow was not capable of detaching soil particles-few particles were left to the transported anyway. The erosion conditions were unfavourable in all source areas. This also resulted in a high correlation between runoff volume and sediment yield.

From this description it can be concluded that in explaining sediment delivery one must take into account both the runoff volume and the availability of soil particles. Yair and Lavee, in their analysis of sediment delivery under arid hillslope conditions, come to a similar conclusion (Yair and Lavee, 1981).

380 J. M. ROELS

Figure 5. The relation between soil loss and runoff volume for storm 8 upper slope segment: log Q, = 1.61 log Qw - 4.57 ( r = 0.90); lower slope segment: log Q, = 1.30 log Qw - 3.37 ( r = 590)

CONCLUSIONS

The assumption that interrill flow is produced uniformly over a hillslope breaks down under rough rangeland slope conditions. It is observed that runoff concentrates immediately, due to surface roughness, in non- permanent flow paths. Under average storm conditions the flow paths can be as long as 12.5 m. Long interrill flow paths may eventually become permanent. As a result of this permanence the runoff volume increases considerably. Along the permanent flow paths, called pre-rills, after-flow is carried away. Pre-rills are considered to be an important initial stage in the development of rills. Under natural conditions on rough slopes pre-rills gradually change into rills. They do not look like microchannels. Therefore they cannot easily be mapped; they only can be traced after a microtopography survey.

Because interrill and pre-rill runoff on rough slopes is concentrated and because it can run downslope over considerable distances, the potential of the flow to detach and transport soil must be considered. This implies that the conventional conceptual interrill erosion models have to be adapted to such slope conditions. The model modification will have to be accompanied by further data collection and interpretation. If these data are to be reliable the size of the conventional interrill plots will have to be enlarged, because the flow pattern must be allowed to develop naturally.

Runoff generation and accumulation, as a function of rainfall characteristics, the soil intake rate and slope length, is highly variable both with respect to time and location. As a result, calculations based on runoff data will only be significant if source area, storm and soil conditions are specified in detail.

The runoff volume is not the predominant factor in the sediment delivery. The availability of soil particles, usually neglected in soil erosion models, is an equally important factor determining the amount of soil loss. Also the availability of soil particles varies considerably from storm to storm (or even within a storm), and with respect to location. Therefore a correlation analysis between the runoff volume and the amount of soil loss will only be meaningful if both runoff generation and accumulation and sediment availability are specified according to storm, soil and source area.

A comparison of the conditions of sediment delivery by rills and pre-rills shows that in the latter source area more soil particles are available and less sediment is stored within the flow path. Consequently, in a pre-rill area a unit amount of soil loss is produced by a smaller volume of runoff than in a rill area. Therefore much emphasis should be placed on both the detection of pre-rills and the description of the mechanisms of pre-rill erosion.

RUNOFF AND SEDIMENT GENERATION 381

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