Applied Geography (19S5). 5. 143-X

Soil erosion, climatic vagary and agricultural change on the Downs around Lewes and Brighton, autumn 1982

J. Boardman

Department of Humanities and Countryside Research Unit, Brighton Polytechnic, Falmer, Brighton, BNl 9PH, England

and D. A. Robinson

Geography Laboratory, University of Sussex, Falmer, Brighton BNl 9QN, England

Abstract

Farmland on the Downs between Lewes and Brighton suffered severe erosion during the autumn of 1982. The erosion was widespread and affected a variety of topographic situations, but it was confined to areas of arable land and recently-sown grass leys. Erosion on the scale recorded during the autumn of 1982 has never previously been recorded from this area. Three major sites of erosion are described and explanations for the erosion are sought through an analysis of rainfall conditions experienced during autumn 1982 and in recent changes in agricultural land use on the Downs. It is concluded that, whilst total rainfall and the intensity of rainstorms were both unusually high, similar events are likely to recur several times a century. Evidence is presented which suggests that the ploughing up of permanent pasture, the removal of field boundaries and the increased adoption of autumn-sown cereals have all contributed to the onset of severe erosion. It appears that a major re-activation of erosion on the Downs may be commencing which threatens the long-term viability of farming in the area.

Introduction

Regions with temperate maritime climates have probably the lowest rates of natural erosion in the world (Saunders and Young 1983) and, traditionally, soil erosion by running water has never been considered a major agricultural hazard in lowland Britain. Despite this, several recent studies have shown that significant rates of soil erosion do occur within this region under a variety of agricultural land usages and on a variety of soil types (Morgan 1975; 1977; Evans 1978; Evans and Nortcliff 1978; Reed 1979; Boardman 1983). Records of erosion are predominantly from areas of arable cultivation on sandy and silty soils of low structural stability in the Midlands and East Anglia. This paper presents detailed information on, and discussion of, soil erosion that has recently affected a number of sites on the chalk downlands of East Sussex. This is an area and an environment from which serious occurrences of contemporary erosion have never been previously recorded although there are records of erosion affecting chalk soils elsewhere in the UK, in Cambridgeshire for example (Evans and Morgan 1974) and in the Yorkshire Welds (Foster 1978).

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The South Dotvns in East Sussex are overlain by ;I comples of thin rendzina soils of the Icknield. Upton and Andover series, bvhich predominate on the upper slopes and the steeply sloping areas, and thicker brown calcareous earths (Coombe series) on the lower, more gentle slopes and in the valley bottoms (Avery 1980). On valley,-side slopes, soil A horizons are usually less than 30 cm thick. The soils all contnln an unusually high proportion of silt due to the inclusion of loess and, as a result, they tend to become structurally weak when wet and are susceptible to erosion (Morgan 19SO). Despite this, erosion has never been considered a major hazard to contemporary farming on the Downs. However. after scattered occurrences of erosion in the 197Os, notably in the very wet autumn of 1976, erosion on an unprecedented scale occurred in the autumn of 1982. In 29 km’ of arable land between Brighton and Lewes, rilling and sheetwash of soils occurred at 66 sites (Fig. 1). In some locations, the surface run-off of soil-laden water caused localized flooding of nearby roads, dwellings and urban areas (Boardman et al. 19S3). The erosion occurred in a variety of topographic locations and was not confined solely to steep valley sides. Erosion was recorded on slopes inclined at less than 5 degrees and in relatively flat-floored. low-gradient, valley bottoms. A description of the thr ee major sites where erosion occurred gives some idea of its scale and variable character.

R;Iethodology and terminology

In the following examples, estimates of the rates of erosion and soil loss have been calculated either from measurement of the volume of the material escnvated from rills, or from the volume of debris deposited in fans. As such they are minimum figures for soil loss and total losses are undoubtedly much greater. Rills have been mapped in the field, and from aerial photographs flown specially for the purpose. The volume of the rills teas estimated by surveying cross-sections at 5-m intervals along each rill. Soil bulk densities were measured to convert the volume loss to weight loss. The volume of the fans was obtained by surveying their area and estimating their depth tvith a network of auger probes. Conversion of volume to weight was by use of a standard estimate of 1.5 g cm-” for the bulk density of the loosely compacted flinty gravel in the fans. Ponded infiltration rates ivere obtained by use of ring infiltrometers conformin g to the design recommended by Hills (1970). Water-stable aggregates greater than 0.5 mm (WSA ~0.5) ivere obtained by wet sieve analysis (Grieve 1979).

Throughout this paper all the eroded channels are referred to as rills. immaterial of their dimensions. Rills are generally defined as continuous channels of narrow width and shallow depth which can be obliterated by ploughing or lveathering of the surface, Lvhilst the term gully is restricted to larger, generally permanent features (Soil Survey Staff 1951; Morgan 1979; Evans 19SOa). All the eroded channels described in this paper were infilled by the late spring of 1983 by the collapse of their side-walls and re-cultivation of the eroded areas by the farmers. Thus, although many were of sufficiently impressive dimensions to possibly justify their description as gullies, they are all referred to as rills on the basis of their ephemeral nature.

Bevendean (TQ 357072)

The most spectacular erosion occurred on the south~vard-facing slopes of the upper section of a westward-draining dry valley on the eastern outskirts of Brighton (Fig.

J. Boardman and D. A. Robitwotr 245

sites of erosion

site of erosion detailed in text .,

and area covered by map .

. . ..\S?

--- limit of Chalk downland

.. 74, ” contour height in metres i .,.,

buift up area ‘.

:

:/,t g : :

:..,.,, : : 0 ‘. .,: ‘:. ‘. .“,‘,‘, .., ., : ‘: _ ‘. “.,,

km

Figure 1. Sites of soil erosion on the Downs between Lewes and Brighton, autumn 1982

2). This valley has suffered from erosion and flooding in the past and a series of five sedimentation and storm-water storage dams have been constructed in the lower part of the valley to try to protect the Brighton suburb of Bevendean from occasional inundation by soil-laden floodwater. Erosion was most intense on 10 ha of a Iarger field that enclosed almost the entire south-facing valleyside slopes of the headward section of the valley. The maximum length of slope in the affected area was 400 m and the steepest slope angle recorded was 1.5 degrees. Some 10 000 m of rills developed in this field between 20 September and 25 October. The rills were

216 Soi/ erosion. clitmzric vagary and agric~rtlctral chilifgr or1 Iile Soltth D0~~n.s

up to 300 m in length, 0.8 m deep and I-- 7 m wide. The estimated loss of soil from within the rills alone in this period of five weeks was 100 t ha-‘. Many rills incised through the entire depth of soil to the chalk beneath. Flints up to 380 cm3 weighing up to 1 kg were ripped out of the rills and deposited in large fans consisting of coarse flint and chalk rubble at the base of the slopes. Finer flint and chalk material was carried along the floor of the valley where it was deposited up to 500 m downstream, whilst much of the clay and silty soil material was carried down to the sedimentation dams over 1 km downstream.

major rills

shallow rills and trails of gravel

contour height in metres ,zz.....

metres 150

“. . . . ,,

Figure 2. The riil system at Bevendean, December 1982

J. Bourdtnan and D. A. Robitmtl 717

Prior to the onset of erosion, the field had been used for cereals for several years but was ploughed and harrowed in the late summer of 1952 and, in September, was sown with a grass ley and rolled. Serious rilhng commenced during heavy_ rainfall on 20 September and developed rapidly thereafter. By late October, the r&s were spaced at an average of every 4 m along the steepest section of slope (Fig. 2). The rilling was most prevalent on the steep section of slope immediately above the basal concavity. It was at this point, also, that the rills were most deeply incised. Rills rarely converged to form branching networks but existed as discrete channels from their head to the base of the slope. However, many of these discrete rills bifurcated downslope either into two or more distinct channels or, more frequently, into numerous, shallowly incised channels some of which petered out whilst others crossed over into neighbouring deeply incised rills. Some of these shallow rills may have formed during the initial stages of rilling before the master channels attained ascendency, but most appeared to be overflow channels where water flow had overtopped the sides of the major rills. These ‘overspill’ channels were particularly common on sections of slope where the gradient lessened.

Many rills or sections of rills were not aligned down the steepest angle of slope, at right angles to the contours, but lay at a slightly oblique angle (Fig. 2). This orientation was clearly related to the pattern of grass sowing and rolling vvhich had produced a pronounced rectangular pattern, paralleling the north and west boundaries of the field, in the compression of the ground and germination of seedlings. The initial flow of water appeared to have followed depression zones left by tractor wheels and the rollers with the maximum angle of slope exerting its directional influence at a later stage, probably when the depth of water flow exceeded the depth of the depressions or the limited water transport capacity of the incipient rills.

In November and December, the rills remained occasionally active during exceptionally heavy rainstorms. On 7 December, for example, an hour after a storm commenced, water moving at velocities of up to I.5 m s-t, carrying almost 5000 mg I-’ of sediment, was recorded in the main exit channel from the eroded field. However, the growing grass provided an increasingly dense cover to the remaining areas of uneroded soil and there was little further extension of the rill system. In mid-December, keen winter frosts began to occur and collapse of the vertical walls of the rills became prevalent resulting in the partial infilIing of many of the channels. In spring, infilhng was completed by the farmer and the rilfed areas were resown with grass.

Highdown (TQ 398112)

This site received the most publicity and caused most controversy because soil and water discharging from arable land blocked drains and flooded houses and roads on the western outskirts of the county town of Lewes. Protective and remedial works had to be carried out by the District and County Councils at a cost of over El2 000 (Boardman and Stammers 1984). The flooding was generated by run-off and erosion from a large field of 20.6 ha on the immediate periphery of the built-up area of the town (Fig. 3). The field was drilled with wheat on the 5 November and erosion commenced on 27 November during a heavy rainstorm.

The field occupies the gentle, saucer-shaped head of a shallowly incised, eastward-draining dry valley, the lower parts of which are occupied by a housing estate. On the arable field, rainfall impact destroyed the surface structure of the soil and the dispersed particles of fine soil quickly formed a relatively impermeable

248 Sod rrosiorr, clirmric vaynry and ugriculturcll cl~rrngr on tire Solld~ Dowm

soil crust. On the gently sloping crestal areas. and in wheelings and cultivation hollows aligned along the contours, surface ponding of water was widespread, and surface wash was a major cause of erosion. Rill development was more limited than at Bevendean and those which developed were much more shallowly incised. Rilling was concentrated on the steeper slopes of the convexity, below the slope crest where slopes were inclined at or above 11 degrees. The direction of many of the rills was controlled by the direction of drillin g and by wheelings of agricultural vehicles which in general were down the maximum slope. Most of the rain falling onto the 20.6 ha catchment was fed by a network of minor rills into a major one along the centre of the valley, which carried water southeastwards and discharged it towards houses lower down the valley.

f-- main areas of rilling \_-=-- /

- major valley-bottom rills \ arable ‘F/

4--- path of water through gardens and into street

L’

drainage system

76 contour height in metres

0 metres

Figure 3. Detail of soil erosion site at Highdown, Lewes

By early December there existed an extensive system of shallow rills covering most of the field; response to rainfall was rapid with flow occurring in a pipe discharging into Highdown Road on the Highdown housing estate less than 30 minutes after the commencement of storms (see Fig. 3). As the area of crusted soils increased, the density of rills increased and even relatively minor rainfall events of magnitudes regularly experienced every. winter led to major discharges from the field (e.g. on 13 January a major flood discharge was generated by a rainfall of just 6 mm over three hours). Compared to Bevendean, the slope convexity at Highdown is much closer to the crest and therefore water volumes and velocities in individual rills were lower. Although large amounts of water reached the major rill from all parts of the field, this was located on a low-angle, valley-bottom slope and

J. Boardman and D. A. Robinson 249

little incision occurred, the rill rarely attaining a depth of 0.3 m. Despite this, rates of run-off and erosion during heavy rainfall were very considerable with run-off in excess of 50 I s-i carrying 20 g I- ’ of sediment being recorded, (i.e. a loss of more than 1 tonne of soil every 16.6 minutes). The eroded soil material was predominantly composed of fine silt, clay and organic matter. Some sand and flint gravel built up a fan at the exit of the field and spreads of sand partly buried vegetables in allotments behind the houses. A trench excavated to protect the houses was infilled with some 14 tonnes of sediment during a period of less than two weeks at the beginning of December.

Erosion also occurred at ten sites in the adjacent valley of Houndean on similar soils under similar land use, and in response to the same pattern of rainfall. The soil lost from these sites was small compared to Highdown because of the smaller size of fields and therefore more limited generation of run-off. Another factor which was probably significant at most sites was the lack of a wide slope crest acting as a water storage area (Evans 1980b; Boardman 1984).

Breaky Bottom (TQ 399038)

Breaky Bottom is a deeply incised, meandering dry valley which runs eastwards towards the Ouse valley. Some of the slopes exceed 20 degrees, which is exceptionally steep for cultivation even by contemporary standards on the Downs. The soils are very stony. In October, run-off on these slopes eroded a series of shallow rills which converged downslope towards the head of the valley floor. Erosion was concentrated around two semi-circular valley heads while rilling of the more southerly one was the most serious (Fig. 4). Here, on slopes in excess of 20 degrees, sown with winter cereals in the early autumn of 1982, about 45 rills developed of mean size 30 X 0.3 x 0.2 m. Estimated soil loss from the rills alone was 81 m3 (i.e. about 35 t ha-’ valley floor and a further 15 m 3

. Over 12 m3 of flinty gravel was spread out on the of similar material was trapped behind a fence. In

the more northerly valley a large fan contained 40 m3 of soil material, including flints up to 1000 cm3 in size weighing 2.5 kg. In total, the fans contained at least 90 tonnes of coarse sand and gravel. Fines were carried in suspension down the valley. On 13-14 October, Breaky Bottom vineyard and farmhouse, which lie a kilometre downvalley, were extensively flooded and there was widespread deposition of silt on the valley floor. Further inundations later in the autumn were avoided by the building of earth banks and ditches across the valley floor, upvalley of the farm.

Rainfall conditions

The rainfall characteristics most likely to induce erosion of arable soils in Britain remain uncertain. Studies at Silsoe led Morgan (1980) to suggest that rainfalls of high intensity were important and he proposed that the sum kinetic energy of all rain falling at intensities greater than 10 mm hr-’ (K.E. >lO), was a reasonable indicator of the erosivity of rainfall in Britain. However, Morgan has also emphasized the importance of moderate rainfall events and the duration and total volume of rainfall received (Morgan 1977: 30, 34). An analysis of records of soil erosion in lowland England by Evans (1980b) has suggested that erosion can occur during rainfalls of relatively low magnitude if soils have been pre-saturated by antecedent rainfall, and Evans and Nortcliff (1978: 188) suggested that, in a British context, ‘the amount of rain which falls may be more important than intensity of rainfall’. Reed (1979:43) has shown that on certain compacted soils the erosion

*/.

metres

‘. /---I- / Breaks 1 . .

f- areas of rilling

e main route of water

**A deposition, gravel fans

- field boundaries

---- field boundaries removed

~6 contour height in metres

Figure 4. Detail of soil erosion site at Breaky Bottom

threshold level can be as low as 1 mm hr-’ in rainfall events supplying 10 mm of total rainfall.

The autumn of 1982 was particularly wet over the South Downs. The characteristics of the rainfall that fell during this period and the probable return periods for rainfaIls of similar magnitudes have been discussed in detail by Browne and Robinson (1984). After a dry summer, heavy rainfall commenced on 20 September and continued intermittently throughout October, November and December (Fig. 5). Monthly rainfall totals were well above average for each of the last three months of 1982 with October, when more than twice the average rainfall for this month was recorded, being particularly wet. October 1982 was the third wettest since daily records began at Lewes in 1931, and it was during this month in particular that much of the erosion occurred. The combined rainfall total at Lewes for the last three months of 1982 was 493.4 mm, which is the second highest ever recorded and compares to a long-term average for this period of 258.9 mm. As a result of this rainfall, soils remained saturated or near saturated for most of the period from early October onwards.

However, it was not just the total volume of rainfall which was unusual, but also the intensity of rainfalts recorded within this period. If S mm hr-’ is taken as a convenient measure of high-intensity rainfall, intensities equal to, or greater than, this threshold occurred for 76 individual Z-minute periods in the autumn of 1982 (Table I). On the basis of the 14 years for which continuous autographic raingauge

J. Boardman and D. A. Robinson 251

15 dsyr

35

30

j 25

=, 20

-6 ; 15 c

IO

5

0

10 1 5

40 -

3s -

30 -

2 s 25-

f 20-

; z 15- L

lo-

October

.I.

” .... 1.1.

.

:::: :j: -“il : 1:: :j: .... .... ..... ..... . . ::. ..... ..... ..... ..... ..... ..... ..... ........ ..... ........ .... ..... ........ ..... ......... ..... ........ .:.:

..... ......... ..... ........ .......

..... ......... .....

15 20 25 30 days

November

15

days

30 - December l-l

25 - ::. ::. ::_

::. ::.

days

Figure 5. Daily rainfall at Southover, Lewes, September-December 1982

data is available, this is more than twice the average for this period of the year. The maximum 15minute intensity recorded at Leues was equivalent to an intensity of 30 mm hr-’ and the maximum intensity sustained for 60 minutes was 11.5 mm hr-i.

The extent to which the rainfall in autumn 1982 was exceptional remains uncertain. On the basis of the daily rainfall records, the return period for the occurrence of rainfall equal to, or greater than, the total recorded for the Iast three months of 1982 is approximately 25 years. Return periods for individual months are much lower, varying from approximately 2.5 years for September to 13 years for October (Browne and Robinson 1984). It is meaningless to calculate return periods for the intense, short-period rainfall events because of the limited number of years for which data are available. Nevertheless, examination of the available records shows that, in 19S2, there were more than twice as many periods of intense rainfall (i.e. an intensity >5 mm hr-’ sustained for 15 minutes) than were recorded in 10 out of the previous 13 years (Table 1).

Table 1. Freyency of rainfall intensities equivalent to, or greater than, 5 mm hr- for 15. 30 and 60-minute durations, Southover, Lewes

Year 15 mins 30 mins 60 mins

1969 25 16 7 1970 30 16 4

1971 11 8 1972 26 11 5 1973 19 10 6 1971 4s 21 6 1975 25 13 4 1976 66 35 11 1977 33 15 5 1978 21 6 2 1979 29 14 6 1980 31 19 8 1981 3s 22 6 1982 76 42 15

Mean 3-N 17.7 6.6

Thus, the autumn of 19S2 was undoubtedly very wet with a high frequency of intense rainfall. However, equivalent or greater volumes of rainfall have occurred in previous years, notably in 1960, which is the wettest autumn on record (553.5 mm of rainfall between 1 October and 31 December), when there is no evidence to suggest that erosion occurred on any scale approaching that of 19S2. It is possible that the intensity and duration characteristics of the rainfall encountered in earlier years, before autographic records were available, were significantly different to the autumn of 1982, but there is overwhelming evidence that a major contributor to changes in the extent and severity of erosion has been a change in agricultural land-use practice.

Agricultural change

Since 1939, the chalk Downs of East Sussex have been transformed from

J. Bonrdrnnn nnd D. A. Robinson 253

predominantly pastureland to extensive arable. In the 1930s only about 20 per cent of the East Sussex Downs were arable. Today. the figure has increased to around SO per cent. In the past decade, and especially in the past five years, there has been a further change, with farmers switching increasingly to autumn-sown cereals and grass. This leaves the potentially erodible chalk soils exposed to the erosive impact of rainfall throughout autumn and winter, which is the wettest period of the year (Potts and Browne 1953). Each of the three sites described in detail, in common with most other sites where erosion was recorded, were on cultivated ground drilled with either winter cereals or ley grass in the autumn of 1982.

In addition, as the size and tractive power of farm vehicles has increased from year to year, field boundaries have been eliminated, field sizes enlarged and increasingly steep slopes have gone under the plough. At Bevendean for example, the upper slopes of the field which suffered severe rilling in 1982 originally comprised a separate field, but removal of a boundary and the amalgamation of this upper field with the tower increased the uninterrupted length of valley-side slope from 220 m to 400 m (see Fig. 2). Simiiarly, at Highdown, the majority of the present cultivated field was unenclosed rough pasture at the time of the First Land Utilisation Survey in the 1930s. The subsequent removal of a hedge boundary and ploughing of the rough pasture has today extended the area of cultivated land from about 8 ha to 20.6 ha (see Fig. 3). At Breaky Bottom the most severe erosion occurred on a slope recently increased from 130 to 260 m in length by the removal of a field boundary.

Physical characteristics of eroded sites

Examination of the physical characteristics of the 66 sites that suffered erosion in 1982 suggests that slope gradient is not of prime importance in determining the occurrence and intensity of erosion. It appears that the size of contributing area and the length of uninterrupted slope are probably more important controlling factors. These two characteristics are major determinants of the volume of run-off generated by a given rainfall and both are related to the size and shape of fields. The enlargement of fields such as those at Bevendean, Highdown and Breaky Bottom means many field boundaries, which once interrupted the flow of run-off down long valleyside slopes, have now gone. The boundaries that have been removed were frequently at or near to the break of slope between convexity and crest and their removal now aliows uninterrupted run-off of water stored on the crestal area (Evans and Nortcliff 1978). This is the case at both Bevendean and Breaky Bottom. In addition, the preparation of seed-beds produces smooth, unprotected soil surfaces with very little depression storage (Evans 1980a). The problem is intensified by the widespread practice of pioughing, drilling and rolling down, rather than across, the maximum slope, thereby creating lines of flow for run-off water.

In addition, the breakdown of soil particles to produce a fine-tilth seed-bed, increases the likelihood of surface compaction and the disaggregation and dispersal of fine soil particles by falling raindrops. The dispersed particles are washed into the surface interstices of the soii where they decrease the porosity and lead to the formation of a relatively impervious surface crust or cap. In cultivated chalk soils with high silt contents, crusting may be accentuated by a lowering in the organic matter content of the soils for bonding the particles together. Under old pasture, organic carbon levels in downland soils are in the range 7-11 per cent but this can be reduced to between 2.5 and 4 per cent under permanent cultivation (Hodgson

25-i Soil erosion, clinratic vagary and ugricdtmd cimzge or1 the Solull Dowrzs

1967). The loss is progressive. Results of analyses from the Houndean area (Table 2) suggest that the reduction in organic carbon resulting from the conversion of large areas of grassland to arable over the past 10 years will continue for many years with corresponding decreases in the stability of the soils (H. Brown, Pers. Comm.).

Table 2. Levels of organic carbon in downland soils around Houndean. Lewes, 1982-83

Land-use $40 organic carbon’

(S random samples from within each land-use block) Mean

Woodland-scrub Permanent grassland Converted to arable

post- 197s Converted to arable

1955-1978 Arable pre-1915

6.5 6-4 6.0 6.2 6.0 6.5 6-3 6.3 6,3 12.4 12.0 11.9 17.2 114 11.8 11.6 12.2 11.9

10.5 11.2 9.8 11.1 11.0 10.9 S.6 10.9 10.5

$6 5.7 4.9 5.3 50 5.5 6.0 5.6 5.5 34 2-5 3.2 3.1 2.5 3.3 3.1 3.8 3-1

a Organic carbon by the Walkley and Black method (Wnlkley and Black 1934. Smith and Atkinson 1975)

Greenland rt nl. (1975), have suggested that soils only become markedly unstable when the percentage of organic carbon drops below 2 per cent. This rarely occurs in these chalk soils even after many years of continuous arable cultivation, yet the soils undoubtably become highly erodible. This is almost certainly because the soils contain a high component of loess which becomes extremely unstable when flooded (Catt 1978). The combination of a high content of fine silt and reduced levels of organic carbon appear to combine to make these chalk soils particularIy susceptible to dispersion and crusting whenever the soil surface becomes saturated with water during heavy or persistent rainfall. Once crusting has occurred, infiltration is reduced and even quite minor rainfall events may be sufficient to generate surface run-off and erosion. The problem is further accentuated by soil compaction resulting from the passage of heavy agricultural machinery and, in the case of grass leys, from rolling of the ground surface. Wheelings produced by agricultura1 vehicles, particularly those associated with the spreading of herbicides on almost bare fields in winter, frequently act as loci for erosion. Under permanent grassland, average ponded infiltration rates for chalk soils range from 70 to 110 mm hr-’ (Oakes 1977). In contrast, at Bevendean, ponded infiltration rates ranged from 30 to 45 mm hr-‘, increasing to 40-80 mm hr-’ when the upper 2.5 cm of compacted crust were removed. None of these infiltration rates is likely to result in surface run-off under the rainfat conditions encountered, but it is difficult to relate these ponded infiltration rates to those that actually occur during natural rainfall. However, they do give some indication of the relative decrease in infiltration, and therefore the increased likelihood of the generation of erosive run-off which follows both the conversion of pasture to arable and the development of crusts on exposed arable soils.

Permanent cultivation also reduces the aggregate stabifity of the soils. Under grassland, measurements of the percentage of water-stable aggregates greater than O-5 mm, average 15-20 per cent for rendzina soils and 25-30 per cent for brown calcareous earths. In fields known to have been under continuous cultivation for periods of five years or more, such as those at Highdown and Bevendean, these

J. Boardman and D. A. Robinson 2%

figures are reduced to as low as 2-4 per cent. The percentage of water-stable aggregates within a soil has been shown to be a reasonably good indicator of the relative resistance of a soil to dispersion and erosion (Bryan 196s). Thus, not only is more run-off generated when soils are under permanent cultivation, but the particulate dispersal and erodibility of the soils also increases.

In winter, frosts normally disrupt any compacted surface crusts and improve the infiltration properties of the soils. However, in 19S2. late autumn and early winter were extremely mild and the first hard ground frost did not occur until mid-December (10 December), and widespread disruption of the soil surface did not occur until well into January 1983. As a result, once the soil surface was compacted by heavy rainfall in late September and October, the initiation of run-off during intense rainfalls was a regular occurrence for the remainder of the autumn. Localized compaction and surface sealing was also a function of the use of agricultural vehicles at these sites, often when soils were saturated.

The results of erosion

On the Downs, surface water flow generated by storms is short lived and intermittent in both time and space. As a result, although some of the soil eroded from arable fields in the autumn of 1982 was transported out of the downland environment, most was redeposited within the Dovvns. Most soil was lost from valleyside slopes and much of the eroded soil was redeposited on the floors of the dry valleys that dissect the chalk uplands, although some localized erosion did occur along some valley bottoms. The net result of this process is a further thinning of the already thin rendzina soils of the slopes and a thickening and enriching of the deeper and more fertile soils of the valley bottoms Lvhich are, however, very limited in total area. Thinning of the soil cover on the slopes decreases the agricultural value of the land due to increased droughtiness and a lowering of fertility. Crops tend to germinate more slowly, require more fertilizer and are slower to mature on these thin soils (Evans and Morgan 1974). In dry summers, the poorer quality of the crops on the thinner soils is clearly visible.

Morgan (1980) has suggested that a soil loss of 2 t ha-’ yr-’ is the maximum that should be considered acceptable for sandy and chalky soils in southern England. On the Downs, where weathering of the underlying chalk produces very little debris for incorporation within the soil body, even this low rate of erosion is undoubtably too great if the already thin cover of soil is not to be further depleted. Indeed, much of the original post-glacial soil material is totally irreplacable for it comprises wind-blovvn loess deposited on the Downs during the Devensian glacial period (Catt 1978). Despite this, the loss of soil from rill incision alone for just the three autumn months of 1982 for the 66 sites surveyed varied from less than 5 t ha-’ to 100 t ha-i, and at Bevendean the rate was 50 times the annual maxima suggested as acceptable by Morgan. At this site, the soil depth varies between 10 and 30 cm and the soil lost was equivalent to a surface lowering of 7.5 mm over the entire area of eroded field. It would only take between 13 and 20 erosive events of this magnitude to remove the whole depth of solum at this site.

Conclusion

The present-day soil cover of the Downs represents the much-altered, eroded remnants of a thicker, humose-rich woodland soil that developed over the area in the first half of the Flandrian. There is abundant evidence that forest clearance and

256 Soil erosion, clitoric b’agary LUI~ agriclfltcfrtrl chcmge on rile Solrtll Do~,tl.s

cultivation of the Downs from Neolithic times onward led to erosion of much of the original soil and its redeposition in valley bottoms within the Downs (Robinson and Williams 1983). In late Roman and Saxon times cultivation receded from extensive areas of the higher and steeper downland, which reverted to grass pasture. At the present time, the evidence suggests that a renewal of erosion is occurring as a result of the re-conversion of the Downs from pastureland to an area of estensive cereal cultivation. The dangers of erosion are further exacerbated by the increasing adoption of autumn-sown cereals in preference to spring-sown cereals, thereby exposing extensive areas of vulnerable soil to the heavy autumn rains.

Much of the soil eroded from the slopes and re-deposited in the valley bottoms during the earlier erosive period was fine grained. loess-rich, topsoil. In contrast, contemporary erosion appears to be transportin g a much greater proportion of coarse flint and chalk rubble. This may represent a change in the nature and intensity of the erosive processes involved, possibly a change from dominant surface wash to rilling. or it may be a reflection of the coarser, residual character of the soils currently being eroded. In some cases it results from actual erosion of the chalk rock underlying the soil. At Bevendean, for example. approximately 1 I per cent by volume of the material eroded by the rills was excavated from the underlying chalk.

How frequently erosion on the scale and intensity witnessed in autumn 19S2 is likely to recur remains uncertain. If it was the volume of rainfall during individual months that was primarily responsible, then equivalent erosive events can be expected to occur every few years. If it was the frequency of high-intensity rainfall that was more important then the return period may be rather longer. perhaps only once every 10 or 20 years. If it was the combination of a high volume of persistent rainfall and high intensity storms then reoccurrence may be much less common. Nevertheless, as long as the acreage of cereals and ley grassland continues to expand, as field boundaries are removed and excessively steep slopes are ploughed up, and as long as autumn-sown cereals remain economically attractive. the likelihood of erosive events of equal or greater magnitude to that experienced in 1982 will continue to increase. Early cultivators of the Downs could claim ignorance as mitigation for leaving later generations with only the denuded remnants of the original covering of woodland soils. To allow the re-introduction, and to encourage financially a farming system that is likely: to totally denude parts of the Downs of their productive agricultural capacity within a few decades, would appear to be a wilful neglect of the lessons of the past.

Acknowledgements

The authors are grateful to Dr R. Evans for his helpful comments on an earlier draft of this paper, and to H. Brown (Brighton Polytechnic) for the use of previously unpublished data. The diagrams were drawn by Sue Rowlands.

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