effects of rock fragment cover on soil infiltration, interrill runoff and erosion

Effects of rock fragment cover on soil in®ltration,interrill runoff and erosion

A . C E R D AÁ

Centro de Investigaciones sobre Deserti®cacioÂn-CIDE (CSIC, Universitat de ValeÁncia, Generalitat Valenciana), CamõÂ Real, s/n, 46470

Albal, ValeÁncia, Spain

Summary

Considerable attention has been paid recently to the in¯uence of surface rock fragments on hydrological

and erosional processes, although much of this research has been done on disturbed soils under laboratory

conditions. I have studied the effects of rock fragments on soil in®ltration, runoff and erosion under ®eld

conditions using simulated rainfall on bare areas of natural soils within typical Mediterranean scrubland

characterized by patchily distributed vegetation. Sample areas were chosen where rock fragments cover

more than half the surface within unvegetated patches. Twenty experiments were carried out by applying

rain at an intensity of 55 mm h±1 for 60 minutes. This approach shows that rock fragments (i) retard

ponding and surface runoff, and (ii) give greater steady-state in®ltration rates and smaller interrill runoff

discharges, sediment concentrations and interrill erosion rates. A second set of six experiments was

carried out by applying rainfall at an intensity of 55 mm h±1 for two runs of 60 minutes. The second run

was initiated 10 minutes after the ®rst. During this interval, surface rock fragments were removed in order

to measure their effects on in®ltration, interrill runoff and erosion rates. In this way, I showed that water

and soil losses are reduced by the rock fragments. After the removal of rock fragments the steady-state

in®ltration rate diminished from 44.5 to 27.5 mm h±1 and the runoff coef®cient, sediment concentration

and erosion rates were, respectively, 3, 33 and 39 times greater than they were before the rock fragments

were removed.

Introduction

Poesen & Lavee (1994) point out that in erosion experiments

considerable attention has been paid to the role played by ®ne

particles, but much less attention has been devoted to the

effects of the coarsest soil particles (> 2 mm). Nevertheless,

recently there has been a growing interest in soils containing

abundant rock fragments (Abrahams & Parsons, 1991, 1994;

Bunte & Poesen, 1993, 1994; Poesen et al., 1994, 1997, 1998;

van Wesemael et al., 1995, 1996; Poesen & Bunte, 1996).

Such soils are widespread globally, particularly around the

Mediterranean Sea, where they often occupy more than 60% of

the land (Poesen, 1990). They cover a signi®cant area of land

in other countries including the USA (Miller & Guthrie, 1984).

Thus, there is a need for more quantitative information on

the effects of rock fragments on hydrological and soil

degradation processes, so we can improve models aiming to

predict the effects of land-use changes on these soils (Poesen

& Lavee, 1994). Most of the research dealing with the effects

of rock fragments on soil processes is related to agricultural

land, whereas rangelands have been much less studied. My

research has been done on rangeland on which vegetation is

distributed as a patchy mosaic of bare and vegetated zones. In

particular, I have chosen bare patches of soil with the sparsest

of vegetation, where surface rock fragments are the main

protection against raindrop impact and overland ¯ow erosion.

On the vegetated patches, in®ltration is rapid and runoff and

erosion are negligible (CerdaÁ, 1997a).

The most widely used method to study the effects of rock

fragments on soil erosion has been to use simulated rainfall

under laboratory conditions on disturbed soils. Few experi-

ments have been carried out under natural conditions. I have

therefore chosen rainfall simulation experiments because they

are more rapid, ef®cient, controlled and adaptable than natural

rainfall research (Meyer, 1994). Rainfall simulation experi-

ments are suitable for the study of infrequent heavy

precipitation events such as those that occur under semiarid

conditions in southeastern Spain (CerdaÁ, 1996). My research

has been done under natural ®eld conditions at a design rainfall

intensity of 55 mm h±1 applied on 0.24-m2 plots on bare areas

in one Mediterranean scrubland dominated by Quercus

coccifera and Pistacia lentiscus. Similar patchily distributed

rangeland vegetation is found widely in semiarid areas

(Tongway & Ludwig, 1990; Rostagno et al., 1991; SaÂnchezE-mail: [email protected]

Received 23 August 1999; revised version accepted 7 August 2000

European Journal of Soil Science, March 2001, 52, 59±68

# 2001 Blackwell Science Ltd 59

& PuigdefaÁbregas, 1994; Dunkerley & Brown, 1995; CerdaÁ,

1997b; Bochet et al., 1999), and little is known about the

effects of rock fragments on soil hydrology and erosion. A

study of the bare areas, the main source of sediment and

runoff, will provide some insight into the hydrological and

erosional behaviour of rangeland and thus into its manage-

ment.

Materials and methods

Quanti®cation of rock fragments

Rock fragments are stones and soil particles 2 mm or larger in

diameter and include all material that has horizontal dimen-

sions smaller than a pedon (Miller & Guthrie, 1984). Three

variables commonly are used to express the amount of rock

fragments in the topsoil (surface cover, volume and mass of

rock fragments). I have chosen the rock fragment cover of soil

surface, Rfc, to characterize the experimental plots because it

can be measured accurately enough and is very suitable for

experimental design (Poesen & Lavee, 1994). The rock

fragment cover was measured accurately for each plot using

vertical photographs of the surface. I mapped the rock

fragment cover from the photographs and later measured their

area using a planimeter.

Study area

The site chosen in Valencia province, southeast Spain

(38°55¢N, 0°27¢¢W) (Figure 1) has a Mediterranean climate

characterized by dry-hot summers and wet-warm spring,

autumn and winter. The annual average precipitation is

688 mm, which falls mainly in autumn. The mean monthly

temperature of the hottest month (August) is 26.7°C and that of

the coldest month (February) is 9.9°C. Frost is unusual.

Figure 1 Location of the study area.

Table 1 Initial soil moisture content, rock fragment cover, soil organic matter content, particle-size distribution, total bulk density and calcium

carbonate content at 0±3 cm depth for the 20 plots studied

Soil moisture content Rock fragment Organic Particle-size distribution Bulk Calcium

(0±1 cm) (4±6 cm) cover matter Clay Silt Sand density carbonate

Plots _________________________________________________________________ /% ________________________________________________________________ /g cm±3 /%

1 3.9 6.5 50.26 2.04 59.56 20.52 19.92 1.26 6.70

2 1.4 3.2 51.36 1.96 53.56 22.21 24.23 1.19 5.32

3 0.6 1.7 55.65 2.01 57.65 22.03 20.32 1.29 7.30

4 0.7 1.3 59.68 1.85 53.25 21.88 24.87 1.09 7.10

5 1.2 2.1 60.21 3.01 53.93 25.31 20.76 1.20 6.23

6 1.1 3.4 64.25 1.66 55.15 21.61 23.24 1.14 6.30

7 1.2 3.7 65.25 4.26 58.27 19.59 22.14 1.12 7.16

8 0.9 7.9 67.55 2.87 56.49 19.79 23.72 1.08 5.40

9 2.1 4.3 71.21 4.32 54.23 23.56 22.21 1.03 3.60

10 2.1 3.1 72.65 2.15 58.69 19.16 22.15 1.08 6.54

11 2.1 5.1 74.26 3.45 54.23 26.12 19.65 1.12 5.70

12 2.5 7.0 75.65 4.06 53.98 21.72 24.30 1.04 4.32

13 2.1 3.2 80.24 3.24 57.31 23.28 19.41 0.99 3.56

14 1.2 3.7 82.34 2.99 55.10 22.32 22.58 0.87 2.68

15 2.1 4.6 84.12 3.45 54.82 20.88 24.30 0.92 6.60

16 2.1 1.9 85.36 4.62 56.23 24.36 19.41 1.05 3.45

17 2.1 3.8 90.25 3.85 56.35 21.07 22.58 1.03 5.26

18 0.7 0.6 95.34 4.59 54.89 20.81 24.30 0.98 5.24

19 1.1 2.9 97.87 3.76 55.36 22.36 22.28 1.09 4.69

20 2.2 3.7 96.57 4.95 55.55 21.87 22.58 1.01 3.48

Mean 1.7 3.7 73.94 3.25 55.73 22.02 22.25 1.08 5.33

SD 1.87 14.2 1.04 1.80 1.81 1.79 0.11 1.43 0.11

60 A. CerdaÁ

# 2001 Blackwell Science Ltd, European Journal of Soil Science, 52, 59±68

The soils are Lithosols and Luvisols. Vegetation cover is

70±75% and distributed as patches of Quercus coccifera and

Pistacia lentiscus. Between these patches dwarf shrubs

including Thymus vulgaris, Fumana ericoides and

Globularia alypum are found. Most of the ground between

the patchy vegetation is, however, littered with rock fragments

that cover 70±80% of these areas. Crusted soil (15±20%) and

vegetation (5±10%) are less extensive. The overall slope

pro®le is convex±straight±concave, and the underlying rock is

homogeneous limestone. I chose a moderate (straight) south-

facing slope facet to minimize the in¯uence of variations in

slope angle, soil characteristics, vegetation cover and soil

moisture, all of which change with geomorphological position.

Rainfall simulation experiments and soil analysis

The rainfall simulation experiments were carried out during

summer 1991 after a period of very dry weather. Sites

representative of the interpatch areas were chosen that had

negligible vegetation and litter cover, slope angles ranging

from 12° to 17°, and rock fragment cover varying between

50% and 100% (Table 1). Twenty-six plots were selected.

I used the sprinkler rainfall simulator described by CerdaÁ

et al. (1997) at an intensity of 55 mm h±1 over a 1-m2 area for

1 h. Runoff was measured from 0.24-m2 plots in the centre of

each area (Figure 2). Distilled water was used because the

erosional and hydrological response of semiarid soils is

in¯uenced by the chemical composition of the rain (Agassi

et al., 1994). Three variables were measured: time to ponding

from the start of the application (Tp), time to surface runoff

(Tr) and time to runoff outlet (Tro). Time to ponding was

measured when 40% of the surface showed ponds on ¯at or

concave microsurfaces. Runoff occurred without previous

ponding on the steeper microsurfaces, though it could be

detected as a shine on such areas before runoff started. Such

visual determinations identify the areas where the top few

millimetres of the soil are saturated. To ensure uniformity one

person made these assessments for the whole set of experi-

ments.

The intervals from Tp to Tr, from Tr to Tro and from Tp to Tro

are key to understanding the mechanics of Hortonian runoff

generation because they give the delay and the velocity of the

overland ¯ow. When interrill runoff appeared, the discharge

from the plots was measured at 1-minute intervals. The steady-

state in®ltration rate, Fc, was determined for each test, and the

runoff coef®cient, Rc, was measured from the runoff curve.

Runoff sediment concentration, Sc, was sampled every

10 minutes. The samples were later dried and the sediment

content weighed.

Linear regressions were ®tted to deduce the effect of the

rock fragments on the hydrological and erosional variables.

The linear regression equation, Sc = a + b(time), was ®tted to

the ®eld data to determine the tendency (increase, + b;

decrease, ±b) with time and thus the changes in runoff

sediment concentration (Tsc) over time. The erosion rate (Er)

was calculated from the total runoff discharge and the mean

runoff sediment concentration. This standard experiment was

applied to 20 plots with varied rock fragment cover.

Figure 2 Plot surface before (a) and after (b) removal of the rock fragments.

Effects of rock fragments on soil erosion 61


After data from the 20 experiments were analysed, six

further controlled experiments were done to quantify the effect

of rock fragment removal. These experiments had three stages:

1 a standard application (55 mm h±1 lasting for 60 minutes);

2 the plots were then left for 10 minutes without rainfall,

during which period the rock fragments were removed (Figure

2). Figure 2 shows a plot before (a) and after (b) the removal of

surface stones;

3 stage 1 was repeated.

Steady-state in®ltration rates were reached before the ends of

both runs, so comparison of these values indicates the

in¯uence of rock fragments on in®ltration. However, the

runoff and the erosion rates are determined partly by the

different initial conditions: dry for the ®rst run and wet for the

second.

Several workers have studied the frequency and duration of

natural rainfall events in eastern Spain, and they show that

storms similar to my experimental ones have a return period of

4±5 years. ElõÂas & Ruiz (1977) show that rainfall events of 1 h

duration with intensities of 80±110 mm h±1 occur on average

every 10 years.

The measured in®ltration values (precipitation rates minus

runoff rates) were ®tted to the Horton equation to determine

the steady-state in®ltration rate as explained by CerdaÁ (1996).

Soil samples to determine the moisture content were taken

before the experiments at 0±2 cm and 4±6 cm depths. The soil

water content was measured by gravimetry. The organic matter

content (Walkley & Black method), grain-size distribution

(USDA classi®cation), bulk density (ring method), and

calcium carbonate content (Bernard calcimetry) were mea-

sured for each plot.

Results

The comparison with the rock fragment cover (Rfc) by linear

regression with hydrological and erosional parameters (Tp, Tr,

Tro, Fc, Rc, Er, Sc, Tsc) gives an idea of the relationships

between them.

Rock fragments and soil characterization

On the sites studied, most rock fragments rest on the soil

surface, and rock fragments partly or completely embedded

within the soil are few. Rock fragments have a mean density of

2.5 g cm±3 and their moisture content at saturation (by mass) is

1.3%. Attention has been paid mainly to the surface cover

because of its wide distribution. Rock fragment cover was on

average 74%, and ranged from 50 to 98% on the 20

experimental plots studied.

The top 0±3 cm of soil is characterized because of its

in¯uence on the hydrological and erosional response of

unvegetated soils. The texture is sandy loam, with calcium

carbonate content less than 7.3% (5.3 on average), total bulk

density ranges from 0.87 to 1.29 g cm±3 (1.08 g cm±3 on

average) and soil organic matter varies between 1.66 and

4.95% (Table 1). See Torri et al. (1994) for the relation

between the rock content fragments and soil bulk density of

the ®ne soil. The cover of rock fragments in¯uences the

underlying soil by increasing organic matter content and

preventing the formation of crusts. This gives a negative

relation between the rock fragment content and the soil bulk

density. On the other hand, there are no effects of rock

fragments on soil texture. Figure 3 shows these relationships.

The soil was very dry before the experiments. The topsoil

(0±1 cm depth) everywhere had less than 3.9% moisture and

the mean value was 1.7%. At 4±6 cm depth, the soil moisture

content ranged from 0.6 to 7.9% (average 3.7%) (see Table 1).

Figure 3 Relationships between rock fragment cover and soil

calcium carbonate, organic matter and bulk density.

62 A. CerdaÁ


It should be emphasized that the soil moisture contents during

summer 1991 were at their lowest with little spatial variability.

Under such conditions the soil water distribution did not affect

the spatial variability of runoff and in®ltration.

Time to ponding (Tp), runoff initiation (Tr) and runoff outlet

(Tro)

The main conclusion from the experiments is that rock

fragment cover delays initiation of runoff. During application,

the ®rst surface change is the onset of ponding. The time to

ponding (Tp) ranged from 97 to 612 s, being much the fastest

on the least covered soils (Figure 4). Ponding arose mainly

where the soil was crusted and it was found that in plots

protected by stones rainfall in®ltrated more readily because of

the greater porosity and aggregation developed around the

stones. Both surface runoff and runoff outlet started later on

the more densely covered soils (Figure 4). Surface runoff

occurred after 140±910 s and the runoff outlet started slightly

later after 215±1274 s.

The more the stone covered the soil, the greater the delay in

runoff generation. The time from Tp to Tr (Tr ± Tp, time

necessary to move the ponding downslope) was greater on the

more stone covered soils. Tr ± Tro and Tro ± Tp had the same

trend. This shows that the times necessary to transform the

ponding into runoff and the surface runoff into runoff outlet

increase with greater rock fragment cover due to the greater

soil roughness (Figure 4).

Steady-state in®ltration rates and runoff coef®cient

The positive relationships between rock fragment cover and

the runoff initiation show that soils densely covered by rock

fragments are more readily in®ltrated. This is con®rmed by the

properties related to soil hydrology: the steady-state in®ltration

rates (Fc) and the runoff coef®cient (Rc). The steady-state

in®ltration rates range from 25 mm h±1 (50% of Rfc) to

55 mm h±1, which occurred on three plots ranging from 80 to

95% of Rfc. The greatest in®ltration rate (Fc) values always

occurred on the more covered soils, and the greater runoff rates

(Rc) values were found on the less covered soils, which ranged

from 0 to 41% (Figure 5).

Runoff sediment concentration and erosion rates

Coverage of rock fragments determines the amount of

sediment detachment because it protects the soil surface

against raindrop impact. This is shown by the reduction of the

Figure 4 Relationships between rock fragment cover and time to

ponding (Tp), time to runoff (Tr), time to runoff outlet (Tro) and the

differences between these parameters (Tr ± Tp; Tro ± Tr; Tro ± Tp).



sediments removed by the runoff (Sc) when rock fragment

cover (Rfc) increases (Figure 5). Sediment concentration is a

good measure of the soil erodibility because it identi®es the

soil's susceptibility to detachment and transport by the agents

of erosion. Evidently, rock fragments protect the soil from

erosion (Figure 5).

Sediment concentration trends

During the experiments, the runoff sediment concentration was

measured at time intervals of 10 minutes. All the experiments

show that sediment concentration falls from the start of the

runoff until the end of the 60 minutes rainfall application. The

Tsc is close to zero (no temporal changes) on the most covered

soils, whereas the bare surfaces have values of ±9. Figure 5

shows the relation between temporal changes of the sediment

concentration (Tsc) and the rock fragment coverage (Rfc).

Changes caused by removal of rock fragments

Six sites were selected to investigate the effects of rock

fragment removal on soil hydrological and erosional pro-

cesses. The plots chosen were in the middle of a south-facing

slope similar to the 20 plots previously used (compare Tables 1

and 2). Soil organic matter content ranges from 2.1 to 5.4%.

The texture is sandy loam, the calcium carbonate content is

always less than 8.56% and the bulk densities range from 1.0

to 1.3 g cm±3 (Table 2). The comparison between the

hydrographs and the sediment concentration of both runs

shows the in¯uence of the rock fragments on soil hydrology

and erosion. The soil moisture content at the start was small

due to the summer drought (1.7% at surface and 3.7% at 4±

6 cm depth), and there were no signi®cant differences in

dryness within plots (Table 3).

Hydrological and erosional response before and after the

rock fragment removal

The initial soil moisture content was much greater before the

second application than before the ®rst. At 0±2 cm depth the

gravimetric soil moisture content was 2±17%. At 4±6 cm depth

it was 3±15%. Ponding occurred much earlier on the second

run because the initial soil moisture content was greater. This

resulted in much faster runoff on the second run than in the

®rst. Nevertheless, some variables indicate that the removal of

rock fragment affected the hydrological and the erosional

Figure 5 Relationships between the rock fragment cover and the

steady-state in®ltration rate (Fc), runoff coef®cient (Rc), sediment

concentration (Sc), erosion rate (Er) and the changes in sediment

concentration during the experiment (Tsc) for plots both with and

without no-runoff plots.

64 A. CerdaÁ


behaviour of the soil. The steady-state in®ltration rate was

reduced from 44.5 to 27.5 mm h±1 after removal. These rates

are not affected by the initial soil moisture content as the

in®ltration rate stabilized after 20 minutes of rain (Figure 6).

The decrease in Fc and the faster Tp gives increased runoff

during the second run. On average, the Rc rose from 12 to 38%,

and the interrill erosion increased from 2 to 78 g m2 h±1.

Although the increase of the erosion rates is due partly due to

the faster runoff, the main cause is the marked increase in soil

erodibility. The sediment concentration increased from

0.26 g l±1 to 8.41 g l±1. The greater amount of sediment

available after the removal of rock fragments resulted in a

34-fold increase in sediment removed and quicker reduction of

the available sediment (Tsc changes from ±0.26 to ±16.14). At

the beginning of the second run the runoff was turbid with

more than 20 g sediment l±1. After the initial ¯ush of erosion,

the Sc diminished to a steady-state concentration of 3 g l±1

(Figure 6).

Table 2 Soil organic matter content, particle-size distribution, total bulk density and calcium carbonate for the six plots from which stones

were removed

Organic matter Particle-size distribution Total bulk Calcium carbonate

content Clay Silt Sand density content

Plot /% _____________________________________ /% ____________________________________ /g cm±3 /%

1 5.36 55.36 22.46 22.18 1.09 5.36

2 3.98 56.39 20.37 23.24 1.04 4.25

3 4.56 52.65 21.00 26.35 1.12 6.59

4 2.58 53.78 25.99 20.23 1.15 7.45

5 3.75 54.29 20.36 25.35 1.21 8.56

6 2.06 58.29 21.25 20.46 1.26 7.60

Mean 2.80 55.45 22.53 22.01 1.21 7.87

SD 1.23 2.02 2.14 2.51 0.08 1.59

Table 3 Initial rock fragment cover and gravimetric soil moisture contents at 0±1 and 4±6 cm depth, time to ponding (Tp), runoff coef®cient

(Rc), steady-state in®ltration rates (Fc), sediment concentration (Sc), erosion rates (Er) and tendency of the sediment concentration (Tsc) for runs

(a) and (b) of the six plot experiment. Run (a) was done under natural conditions, run (b) after the removal of rock fragments and under wet

conditions. See Figure 6

Rock fragment Initial soil moisture

cover content Tp Rc Fc Sc Er Tsc

Plot /% /% /% /s /% /mm h±1 /mg l±1 /g m2 h±1

Before

1a 91.45 4.12 3.26 512 0.00 55.00 0.00 0.00 ±

2a 88.65 3.02 3.21 383 0.00 55.00 0.00 0.00 ±

3a 84.27 2.65 3.45 465 1.26 55.00 0.00 0.00 ±

4a 76.58 1.14 2.91 277 16.65 43.83 0.36 2.60 ±0.56

5a 65.28 1.20 2.38 225 23.32 26.70 0.67 4.20 ±0.72

6a 58.36 2.01 2.65 156 30.25 31.03 0.53 4.94 ±0.27

Mean 77.43 2.36 2.98 336 11.91 44.43 0.26 1.96 ±0.26

SD 13.28 1.15 0.41 140.02 13.31 12.88 0.30 2.27 ±0.23

After

1b 4.26 21.45 24.56 161 13.29 35.12 8.39 80.17 ±18.23

2b 6.54 17.25 16.69 86 29.63 38.22 5.65 53.19 ±19.52

3b 2.98 13.62 14.25 105 28.45 34.99 9.56 44.62 ±9.35

4b 2.45 18.36 16.25 128 53.26 17.94 5.89 66.70 ±15.24

5b 4.26 19.25 14.26 83 46.91 20.07 8.69 86.51 ±12.25

6b 4.75 19.56 13.25 88 58.64 18.66 12.26 134.15 ±22.23

Mean 4.21 18.25 16.54 109 38.36 27.50 8.41 77.56 ±16.14

SD 1.44 2.66 4.14 30.74 17.37 9.53 2.46 31.89 4.79



Discussion

The in¯uence of rock fragments on soil hydrology and erosion

can be summarized as follows. A stony surface favours more

rapid in®ltration and deeper penetration of applied water. This

is because the contact between the stones and the soil matrix

favours a faster and deeper ¯ow (Poesen et al., 1990). The rock

fragments can all intercept large quantities of rain (El Boushi

& Davis, 1969) and absorb part of it, especially where the rock

fragments are weathered (Childs & Flint, 1990). The greater

the cover of rock fragments, the more temperate is the soil

climate. This in turn favours faunal activity and better

development of root and macropore systems, and thus

in®ltration.

The soil surface is rough where it is covered with rock

fragments, thus runoff is slowed and in®ltration enhanced.

Abrahams & Parsons (1994) demonstrated that the hydraulics

of interrill overland ¯ows under semiarid and arid conditions

are greatly affected by surface rock fragments. On cultivated

land, overland ¯ow is reduced from 40.6 cm s±1 on bare soils to

2.54 cm s±1 on soils covered with fragments (Meyer et al.,

1972). In areas between rock fragments the ponds formed are

deeper, thus the water column pressure is greater and thereafter

in®ltration takes place more quickly and penetrates more

deeply. The delayed initiation of ponding and runoff on the

soil covered by rock fragments is due mainly to the in¯uence

of the fragments on soil structure by increasing the surface

roughness.

Coverage of rock fragments reduces the erosivity of the rain,

and the runoff carries relatively little sediment, both of which

favour faster in®ltration and reduce pore clogging. The cover

prevents the soil surface from sealing and crusting, but various

other factors such as position, size and cover determine the

degree of its in¯uence on in®ltration, runoff and erosion

(Poesen & Lavee, 1994). It also favours in®ltration, the

reduction of runoff and erosion losses because the soil beneath

the rock fragments is richer in organic matter, better

aggregated, more stable and less dense, and there are more

macropores. My ®ndings con®rm the results of Lamb &

Chapman (1943) who found a positive effect of rock fragments

on soil moisture, temperature, erosion and evaporation. Others

note an increase in in®ltration and a reduction of runoff and

erosion (Grant & Struchtemeyer, 1959; Adams, 1966;

Tromble, 1976; Box, 1981; Agassi & Levy, 1991), under both

laboratory and ®eld conditions (Yair & Lavee, 1976; SaÂnchez

& Wood, 1987; Lavee & Poesen, 1991; Poesen & Lavee,

1991; Valentin, 1994). Nevertheless, other authors found a

negative relation between the rock fragment cover and the soil

hydrological response (Wilcox et al., 1988; Abrahams &

Parsons, 1991). The effect of rock fragment position in the

pro®le on runoff production (Poesen et al., 1990; Poesen &

Ingelmo, 1992) needs to be emphasized. This could be the

reason for the contrasting effects of rock fragment cover on

in®ltration and runoff found by different researchers.

Ingelmo-Sanchez et al. (1980) noted reduced evaporation

rates on soils covered by rock fragments. Corey & Kemper

(1968) found that the surface stoniness is effective under very

heavy rain over short times because of the ready in®ltration

and the subsequent reduction of the evaporation by the rock

mulch. The kind of intense infrequent rain under

Mediterranean and semiarid conditions in southeast Spain

means that rock fragment coverage can be the key factor in the

management of cultivated and natural soils. Probably most of

the in®ltrated rainfall is protected against evaporation by the

rock fragment cover, which in fact controls the soil water

regime (van Wesemael et al., 1996).

Finally, rock fragment cover protects the soil from erosion

by wind by protecting the soil immediately below the rock

Figure 6 Runoff hydrographs and sediment concentration changes for plot 4. Experiment 1 shows the ®rst run with rock fragment cover, and

Experiment 2 is the run made after removing the cover.

66 A. CerdaÁ


fragments against de¯ation. Soil erosion by water is markedly

reduced because runoff and rainfall detachment are dimin-

ished. My results con®rm studies under cultivated ®eld

conditions (Chow & Rees, 1995) and under laboratory

conditions (Poesen, 1985), but under ®eld conditions differ-

ences are even more marked.

Conclusions

Surface rock fragments retarded ponding and surface runoff,

increased steady-state in®ltration rates and diminished runoff

discharge, sediment concentrations and erosion rates. Rock

fragments enhanced the water percolation and reduced the

erosion by curbing erodibility and runoff. These ®ndings have

implications for erosion modelling and soil conservation under

Mediterranean and semiarid climatic conditions. Similar soils

to those studied here are widespread in other Mediterranean

areas so my conclusions can be of wide application.

Acknowledgements

This research was partly supported by the project IFD-97-0551

and a contract from the Ministerio de EducacioÂn y CieÂncia. I

thank J.M. Hodgson, R. Webster and two anonymous referees

for improving the script and editing the language.

References

Abrahams, A.D. & Parsons, A.J. 1991. Relation between in®ltration

and stone cover on a semiarid hillslope, southern Arizona. Journal

of Hydrology, 122, 49±59.

Abrahams, A.D. & Parsons, A.J. 1994. Hydraulics of interrill overland

¯ow on stone-covered desert surfaces. Catena, 23, 111±140.

Adams, J.E. 1966. In¯uence of mulches on runoff, erosion and soil

moisture depletion. Soil Science Society of America Proceedings,

30, 101±114.

Agassi, M. & Levy, G.J. 1991. Stone cover and rain intensity: effects

on in®ltration, erosion and water splash. Australian Journal of Soil

Research, 29, 565±575.

Agassi, M., Shainberg, I. & Van der Merwe, D. 1994. Effect of water

salinity on inter-rill erosion and in®ltration: laboratory study.

Australian Journal of Soil Research, 32, 595±601.

Bochet, E., Rubio, J.L. & Poesen, J. 1999. Modi®ed topsoil islands

within patchy Mediterranean vegetation in SE Spain. Catena, 38,

23±44.

Box, J.E. 1981. The effects of surface slaty fragments on soil erosion

by water. Soil Science Society of America Journal, 45, 111±116.

Bunte, K. & Poesen, J.W.A. 1993. Effects of rock fragment covers on

erosion and transport of noncohesive sediment by shallow overland

¯ow. Water Resources Research, 29, 1415±1424.

Bunte, K. & Poesen, J. 1994. Effects of rock fragment size and cover

on overland ¯ow hydraulics, local turbulence and sediment yield on

an erodible soil surface. Earth Surface Processes and Landforms,

19, 115±135.

CerdaÁ, A. 1996. Seasonal variability of in®ltration rates under

contrasting slope conditions in southeast Spain. Geoderma, 69,

217±232.

CerdaÁ, A. 1997a. Seasonal changes of the in®ltration rates in a

Mediterranean scrubland on limestone. Journal of Hydrology, 198,

209±225.

CerdaÁ, A. 1997b. The effect of patchy distribution of Stipa

tenacissima L. on runoff and erosion. Journal of Arid

Environments, 36, 37±51.

CerdaÁ, A., IbaÁnÄez, S. & Calvo, A. 1997. Design and operation of a

small and portable rainfall simulator for rugged terrain. Soil

Technology, 11, 161±170.

Childs, S.W. & Flint, A.L. 1990. Physical properties of forest soils

containing rock fragments. In: Sustained Productivity of Forest

Soils (eds S.P. Gessel, D.S. Lacate, G.F. Weetman & R.F. Powers),

pp. 95±121. University of British Columbia, Faculty of Forestry,

Vancouver, BC.

Chow, T.L. & Rees, H.W. 1995. Effects of coarse-fragment content

and size on soil erosion under simulated rainfall. Canadian Journal

of Soil Science, 75, 227±232.

Corey, A.T. & Kemper, W.D. 1968. Conservation of Soil Water by

Gravel Mulches. Colorado State University Hydrology Paper No

30, Fort Collins, CO.

Dunkerley, D.L. & Brown, K.J. 1995. Runoff and runon areas in a

patterned chenopod shrubland, arid western New South Wales,

Australia: characteristics and origin. Journal of Arid Environments,

30, 41±55.

El Boushi, I.M. & Davis, S.N. 1969. Water retention characteristics of

coarse rock particles. Journal of Hydrology, 8, 431±441.

ElõÂas, J. & Ruiz, L. 1977. Precipitaciones MaÂximas en EspanÄa.

Ministerio de Agricultura, Madrid.

Grant, W.J. & Struchtemeyer, R.A. 1959. In¯uence of the coarse

fraction in two Maine potato soils on in®ltration, runoff and

erosion. Soil Science Society of America Proceedings, 23, 391±396.

Ingelmo-Sanchez, F., Cuadrado-Sanchez, S. & Blanco de Pablos, A.

1980. EvaporacioÂn de agua en suelos de distinta textura. Anuario

del Centro de EdafologiõÂa y BiologõÂa Aplicada de Salamanca, 6,

255±280.

Lamb, J.J. & Chapman, J.G. 1943. Effect of surface stones on erosion,

evaporation, soil temperature and soil moisture. Journal of the

American Society of Agriculture, 35, 567±572.

Lavee, H. & Poesen, J.W.A. 1991. Overland ¯ow generation and

continuity on stone-covered soil surfaces. Hydrological Processes,

5, 345±360.

Meyer, L.D. 1994. Rainfall simulators for soil erosion research. In:

Soil Erosion Research Methods, 3rd edn (ed. R. Lal), pp. 83±103.

Soil and Water Conservation Society (Ankeny)/St. Lucie Press,

Delray Beach.

Meyer, L.D., Johnson, C.B. & Foster, G.R. 1972. Stone and woodchip

mulches for erosion control on construction sites. Journal of Soil

and Water Conservation, 27, 264±269.

Miller, F.T. & Guthrie, R.L. 1984. Classi®cation and distribution of

soils containing rock fragments in the United States. In: Erosion

Productivity of Soils Containing Rock Fragments (eds J.D. Nichols,

P.L. Brown & W.J. Grand), pp. 1±6. Special Publication No 13,

Soil Science Society of America, 13Madison, WI.

Poesen, J. 1985. Surface sealing on loose sediments: the role of

texture, slope and position of stones in the top layer. In: Assessment

of Soil Surface Sealing and Crusting (eds D. Callebaut, D. Gabriels



& M. De Boodt), pp. 354±362. Department of Physics, State

University of Ghent, Gent.

Poesen, J. 1990. Erosion process research in relation to soil erodibility

and some implication for improving soil quality. In: DegradacioÂn y

RegeneracioÂn del Suelo en Condiciones Ambientales

MediterraÂneas (eds J. Albadalejo, M.A. Stocking & E. DõÂaz),

pp. 159±170. CSIC-CEBAS, Murcia.

Poesen, J.W.A. & Lavee, H. 1991. Effects of size and incorporation of

synthetic mulch on runoff and sediment yield from interrills in a

laboratory study with simulated rainfall. Soil and Tillage Research,

21, 209±223.

Poesen, J. & Ingelmo-SaÂnchez, F. 1992. Runoff and sediment yields

from topsoils with different porosity as affected by rock fragment

cover and position. Catena, 19, 451±474.

Poesen, J. & Lavee, H. 1994. Rock fragments in top soils: signi®cance

and processes. Catena, 23, 1±28.

Poesen, J. & Bunte, K. 1996. The effects of rock fragments on

deserti®cation processes in Mediterranean environments. In:

Mediterranean Deserti®cation and Land Use (eds C.J. Brandt &

J.B. Thornes), pp. 247±269. John Wiley & Sons, Chichester.

Poesen, J., Ingelmo-SaÂnchez, F. & MuÈcher, H. 1990. The hydrological

response of soil surfaces to rainfall as affected by cover and

position of rock fragments in the top layer. Earth Surface Processes

and Landforms, 15, 653±671.

Poesen, J.W., Torri, D. & Bunte, K. 1994. Effects of rock fragments

on soil erosion by water at different spatial scales: a review.

Catena, 23, 141±166.

Poesen, J., van Wesemael, B., Govers, G., MartõÂnez-FernaÂndez, J.,

Desmet, P., Vandaele, K. et al. 1997. Patterns of rock fragment

cover generated by tillage erosion. Geomorphology, 18, 183±197.

Poesen, J., van Wesemael, B., Bunte, K. & SoleÂ, A. 1998. Variation of

rock fragment cover and size along semiarid hillslopes: a case-

study from southeast Spain. Geomorphology, 23, 323±335.

Rostagno, C.M., Del Valle, H.F. & Videla, L. 1991. The in¯uence of

shrubs on some chemical and physical properties of an aridic soil in

north-eastern Patagonia, Argentina. Journal of Arid Environments,

20, 179±188.

SaÂnchez, C.E. & Wood, M.K. 1987. The relationship of soil surface

roughness with hydrologic variables on natural and reclaimed range

land in New Mexico. Journal of Hydrology, 94, 345±354.

SaÂnchez, G. & PuigdefaÁbregas, J. 1994. Interactions of plant growth

and sediment movement on slopes in a semi-arid environment.

Geomorphology, 9, 243±260.

Tongway, D.J. & Ludwig, J.A. 1990. Vegetation and soil patterning in

semi-arid mulga lands of Eastern Australia. Australian Journal of

Ecology, 15, 23±34.

Torri, D., Poesen, J., Monaci, F. & Busoni, E. 1994. Rock fragment

content and ®ne soil bulk density. Catena, 23, 65±71.

Tromble, J.M. 1976. Semiarid rangeland treatment and surface runoff.

Journal of Range Management, 29, 251±255.

Valentin, C. 1994. Surface sealing as affected by various rock

fragment covers in West Africa. Catena, 23, 87±97.

van Wesemael, B., Poesen, J. & de Figueiredo, T. 1995. Effects of

rock fragments on physical degradation of cultivated soils by

rainfall. Soil and Tillage Research, 33, 229±250.

van Wesemael, B., Poesen, J., Kosmas, C.S., Danalatos, N.G. &

Nachtergaele, J. 1996. Evaporation from cultivated soils containing

rock fragments. Journal of Hydrology (Amsterdam), 182, 65±82.

Wilcox, B.P., Wood, M.K. & Tromble, J.M. 1988. Factors in¯uencing

in®ltrability of semiarid mountain slopes. Journal of Range

Management, 41, 197±206.

Yair, A. & Lavee, H. 1976. Runoff generative process and runoff

yield from arid talus mantled slopes. Earth Surface Processes, 1,

235±247.

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