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The influence of land management and soil characteristics on infiltration and the occurrence of overland flow
Journal of Hydrology 13 (1971) 163-181; © North-Holland Publishing Co., Amsterdam N o t to be reproduced by photoprint or microfilm without written permission from the publisher
THE INFLUENCE
OF LAND MANAGEMENT
CHARACTERISTICS OCCURRENCE
ON INFILTRATION OF OVERLAND
AND SOIL AND THE
FLOW
RODNEY C. HILLS* Abstract: Infiltration capacity is measured using cylinders and relates more closely to the treatment of a soil, especially the compaction of the surface layers, than to the natural soil characteristics. Overland flow is demonstrated with fluorescent dyes on low infiltration capacity sites with various soils and slopes. Relating infiltration measurements to local rainfall rates, after allowing for interception and surface storage, shows that at most only 30 %, and on many sites fewer than 10 %, of rainfalls can produce overland flow and then only on very restricted areas.
Introduction T h e role o f infiltration in the h y d r o l o g i c a l cycle has r e m a i n e d one o f conjecture, m a r r e d by a p a u c i t y o f field o b s e r v a t i o n , for longer t h a n is c o m m o n in scientific e n d e a v o u r . The reasons for this d i s r e g a r d b e c o m e app a r e n t when field investigations are u n d e r t a k e n : m e t h o d s o f m e a s u r e m e n t are crude a n d i n a c c u r a t e : the p o p u l a t i o n s are difficult to define: the s a m p l i n g errors are large. T w o p o i n t s s h o u l d be noted. The first is t h a t early m e t h o d s o f field m e a s u r e m e n t o f infiltration c a p a c i t y were f o u n d to be inaccurate and were, in a n y case, s e l d o m used with a carefully c o n s i d e r e d s a m p l i n g design. M o s t o f these studies were m a d e to assess erosion h a z a r d s 1) or moisture d e p l e t i o n u n d e r cattle p a s t u r e ~). F r o m the d a t a presented in p a p e r s such as these it is difficult to assess the significance o f low infiltration c a p a c i t y areas in the response o f a c o m p l e t e c a t c h m e n t to rainfall. The second p o i n t t h a t arises is a m o v e m e n t a w a y f r o m field infiltration c a p a c i t y m e a s u r e m e n t t o w a r d s the d e d u c t i o n o f infiltration c a p a c i t y f r o m a series o f factors in an overall water b a l a n c e : this is p a r t i c u l a r l y true o f the literature o f the last ten years. This has led to m a n y a s s u m p t i o n s a b o u t infiltration characteristics m a d e with the s u p p o r t o f sparce d a t a f r o m field m e a s u r e m e n t . It is with these p o i n t s in m i n d that this investigation was u n d e r t a k e n . The following terms will be used in the p a p e r and are defined below: 1) Infiltration capacity. " T h e m a x i m u m rate at which a given soil when in a given c o n d i t i o n can a b s o r b rain as it falls. ''z) * Present address: E.A. Meteorological Dept., P.O. Box 30259, Nairobi, Kenya. 163
164
RODNEY C. HILLS
2) Infiltration. Specifically, in this paper, the entry of water into the soil. 3) Overland flow. Any water flowing over the surface of the soil resulting from an excess of water which does not infiltrate. 4) Compound overland flow. As 3) above but compounded with overland flow increments from upslope. 5) Runoff: Any water flowing out of the area of interest, usually in channel form.
6) Storage. Water held in such ways that it stops, temporarily or permanently, moving through the hydrological cycle.
Method of measurement and sampling design Infiltration capacity was measured using 10 cm diameter steel cylinders, constructed of ~ in. gauge steel, driven 5 cm into the soil and fed from a calibrated feeder bottle maintaining a head of 5 cm over the soil. Details of the technique have been published elsewhere4). Allowance for lateral flow underneath the cylinder was made using an empirically determined correction technique described previously s). The aim of the investigation was to elucidate the interacting effects of soil characteristics and land management. The area in which the investigation took place (around Bristol in S.W. England) was stratified according to Soil Series and type of management or treatment. Within each stratum a sampling site was chosen with respect to its representative nature and accessability. At each site a clustered random sampling design was used containing 20 individuals per stratum. This sample size was a compromise between statistical precision and convenience, in that it enabled a site to be sampled in one day. The sites sampled constituted a wide range of conditions in the area. The Soil Series and subdivisions sampled are listed below, together with an abbreviated code by which the divisions will be referred to in the paper and a principle reference describing details of the profile : i) Worcester (Wf) 6). a) Heavy Worcester (WfH) 7). b) Light Worcester (WfL)7). ii) Greinton (Gc) 6). iii) Evesham (Ea)6). iv) Nibley (Ni)6). v) Haselor (H)7). vi) Spetchley (Sy) 6). vii) Charlton Bank (Cb)6). in terms of textural lightness and profile morphology, the more freely
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
165
draining soils were Gc, Ni, and H. The Wf group is often regarded as impeded but, at the sites investigated, the Wf subdivisions seemed to exhibit quite a light texture and would also fall close to the freely draining group. Series Ea, Sy and Cb would all be classed as impeded or poorly drained. The Cb Series being notably intractable. The land uses or treatments investigated were as follows: i) Woodland. Natural with no marked biotic influence. ii) Orchard. Trees planted for fruit and relatively undisturbed. iii) Pasture. Grassland, not grazed at time of sampling. iv) Pasture (light). Grazing just sufficient to keep down all vegetation other than grasses, lush and not unduly disturbed. v) Pasture (heavy). Grass grazed short. Cattle movements appeared to be seriously disturbing the soil structure. vi) Cultivated. Soil was disturbed and was about to be or had been planted. vii) Compacted. Soil was severely disturbed and compressed and structure was broken down. The group could be subdivided into "animal" and "vehicle" compaction. viii) It was useful to make a further distinction at any site between "bare" and "vegetated" - a distinction which becomes important in assessing interception and surface storage quantities over any surface. Two kinds of bare surfaces were sampled: those produced chemically (deliberately produced to aid some agronomic practice) and biotically produced (usually accidentally). The hydrological implications of chemically induced bare surfaces have already been described 8). Details of treatments are described in the Appendix. Analysis of results The infiltration data proved to be very heterogeneous. Not only was there pronounced spatial variation in infiltration values, but the sampled populations did not portray any systematic deviation from normality. The populations displayed heterogeneity among variances, both positive and negative skews and, in many cases, multimodality. In view of the problems of analysis presented by these data, it was decided to treat the populations as complete units rather than to attempt to determine a single useful parameter. There are two other reasons for this: 1) The spread of the data produced large sampling errors and mean figures could not be used to assess general infiltration characteristics with any useful confidence. 2) In the production of overland flow it is the relative proportions of soil
166
RODNEY C. HILLS
that display high and low infiltration capacities (in relation to local rainfall intensities) that are important. These proportions may occur in populations that have a variety of different forms. A simple survey of the total population enables one to estimate whether or not the site has a significantly large area with low infiltration capacity. It has been widely reported that measured infiltration capacities exhibit a falling rate with time3). In the experiments described here a variety of patterns were found and this also influenced the method of analysis. For each individual determination, infiltration capacity was measured at 5 rain intervals for the first 15 min and then at 15 min intervals for the remainder of the test which lasted between 30 and 120 min. Some determinations, even within a stratum, exhibited a falling rate while others showed a steady rate. This aspect of the data has yet to be studied in detail. For the present analysis a a mean figure for each individual determination was calculated for the complete run and, where sample means are quoted, these are simply the means of the twenty individual means. Mean initial and final infiltration capacities were also calculated. The initial rates were usually so high as to be far greater than British rainfall intensities. The final rates were lower than the mean figures but, if substituted for the means, were found to make no difference to the placing of the sites in the classification which follows. The data suggested that each soil treatment stratum has a mean infiltration curve with differentiating characteristics and that this must be considered if more detailed analysis is to take place. It has been mentioned that it is the relative proportions of high and low infiltration values that determine whether a site is likely to produce overland flow. The samples were therefore divided into four groups as follows: Group L Where 60~o or more of the individual infiltration capacity means were 0.01 cm/hr or less. Group II. Where 50~o or more of the individual means were 1.0 cm/hr or less (but excluding G r o u p I). Group IlL Where more than 50~o but less than 90~o of the individual means were greater than 1.0 cm/hr. Group IV. Where more than 90~o of the individual means were greater than 1.0 cm/hr. Figure 1 shows the sites and populations within these groups. This classification is used for two reasons: 1) Experience in the field and observation of natural storms suggested that, if about 60~o of the soil surface had a very low infiltration capacity, the site was very likely to produce overland flow during any rainlall. If 50~o or more of the surface had a slightly higher infiltration capacity, the site would produce overland flow during intense storms. If less than 50 ~o of the area had this
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
ii
[ wf H(9) ~: w
ii Wf sL(3) i ! I1
.:..Sy (2)w I ~ -% Wf H(9) I~ s I; mee °~
H(1)w
ii
•
•
000 •
•
OOlO
•
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ee
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.%, Ni(2)w I I
. . . .
•
•
C b(4)w i !
"o"1
2 3:5
e(5 /
89e10~1121314151617~19
rTlc~flrl infiltrqtion
Group I: Frequency histogram.
Fig. la.
w
COl3ocity (cms/hr)
•
h
w
~
•
•
~
~:~! .....
WfH (4)
|i •
o':
2
Fig. lb.
"3 4
•
•
oo
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•
m
•
5 6 7 'B 9 I0 11 12 13 14 15 1G 17 18 lg m~an infiltrotion copocity (cms / hr )
Group II: Frequency histogram.
167
168
RODNEY C. HILLS
WfH(4) w Sy(3)w Ni(1)w WfH(5) w Wf H(6) w H(2)w WIL(1) w Eo (3)w Cb(2)s Cb(4 )s W f L(5 ) W SyO )w Cb(1 )w imegn
Fig. lc.
WfH(3) w
•
infiltroDon
( cms, / hr )
c opocity
Group Ill: Frequency histogram.
I,
WfwH(1 ) Cb(3)w Wf H(2) w Ni(1)s Ni(3)S
I.
]q
Sy(2)s WfL(4) w Ea(1)s Cb(3)s CbC)s mean
infiltration
Fig. I d.
capacity
(cms/
hn)
Group IV: Frequency histogram.
slightly higher capacity (the remainder being greater) the site would only show surface storage and no overland flow and if more than 90~o of the area had high infiltration capacities, overland flow would not be produced and surface storage would be very small. 2) The numerical values in the classification system are used with reference to rainfall data. An analysis of the record of the Dyne's autographic
THE INFLUENCEOF LAND MANAGEMENTAND SOIL CHARACTERISTICS 169 raingauge at Long Ashton (Fig. 3), together with Bilham's 9) classification of rainfalls, suggested sensible values for high and low intensity falls and important infiltration capacities. The lowest value, 0.01 cm/hr, is at the limit of accuracy of rainfall intensity measurement (on the Dyne's chart). Any soil with an infiltration capacity lower than this can be expected to store water on the surface soon after any rain begins, provided evaporation rate does not exceed infiltration capacity. The higher limit is 1.0 cm/hr. This separates the short high intensity storms from the long low intensity winter storms. Storms of greater intensity than 1.0 cm/hr seldom last more than 30 rain. Storms of greater intensity than this lasting 10 rain or less fall into Bilham's "noteworthy" category, with a recurrence interval of 1.6 per annum ; whereas such storms lasting 30 rain would have a recurrence interval of 0.1 per annum. This intensity represents the boundary between quite unusual occurrences and the fairly common and the range between 0.1 cm/hr and 1.0 cm/hr contains most British falls. The infiltration capacity data is portrayed in Fig. 1. A frequency histogram for each site is drawn and is given a code which refers it to the list below. The suffix " w " or "s" refers to a winter or summer determination. These seasonal determinations were made to assess the significance of seasonal changes in soil moisture content. Further details of the sites are given in the Appendix. Site
Site characteristics
Ea(1)w Ea(2)w H(1)w Ni(2)w Ni(2)s Sy(2)w Cb(4)w
0.05 cm/hr Orchard, bare (chemical), compaction (vehicle) 0.9 As WfH(9)w 0.2 Cultivated,bare (chemical), compacted (vehicle) 0.0 Heavy pasture 0.0 Compacted (cattle), bare (biotic) 0.9 Heavy pasture 2.3 Compacted (cattle), vegetated 1.1 As Ni(2)w 0.8 Compacted (vehicle), part bare 2.3 Compacted (cattle), vegetated
Group H
WfL(3)w WfH(4)s WfH(7)w WfL(2) WfH(8)w
0.5 2.9 1.6 0.6 1.3
Cultivated,bare (chemical) Compaction (vehicle), part bare (biotic) Simazine 20, bare (chemical) Compaction, bare (biotic) Diuron, orchard, bare (chemical)
Group 111
WfH(5)w WfH(6) WfL(l)w
2.7 2.9 4.7
Simazine control Simazine 2½ Orchard
Group I
WfH(9)w
Mean value
WfH(9)s WfL(3)s
170
R O D N E Y C. H I L L S
Site
Group IV
Mean value
Site characteristics
WfL(5)w Ea(3)w H(2)w Ni(l)w Sy(l)w Cb(l)w Cb(2)s Cb(4)s WfH(4)w Sy(3)w
6.3 cm/hr 53 3.6 2.6 8.6 19.1 5.5 5.7 1.7 2.5
Cultivated Cultivated Cultivated Light pasture Orchard Woodland Woodland, compacted, part bare Heavy pasture, compacted Compaction (vehicle), part bare (biotic) Cultivated
WfH(1)w WfH(2)w WfH(3)w WfL(4)w Ea(1 )s Ni(1)s Ni(3)s Sy(2)s Cb(1)s Cb(3)s Cb(3)w
5.0 8.2 4.5 15.4 16.4 I 1.5 11.6 14.2 38.3 36.6 6.8
Woodland Orchard Orchard, compaction (vehicle) Cultivated,bare (chemical) Heavy pasture Light pasture Cultivated,vegetated Compaction(vehicle), part bare Woodland Light pasture Light pasture
The distribution diagrams hardly need further comment. In Group ! all the distributions are either centred on zero or show pronounced skewness with a distinct mode on or close to zero. In Group II the skewness is still apparent but is less pronounced. In Groups 1II and IV the patterns become more complex: the range of data increases and the variety of form among the distributions becomes noticeable. An analysis of these Groups by treatment and Soil Series was undertaken. When the sites in each group were cross classified according to "treatment" and "soil type", it was found (using Z2 at 0.05 probability) that grouping according to treatment (simply classified into disturbed/compaction and undisturbed/light pasture) was significant and grouping according to soil type (simply classified into light and heavy textured) was not significant. It was concluded that the treatment or management of the soil is more important in determining its infiltration capacity than its morphological characteristics.
Tests for overland flow production The hypothesis relating infiltration characteristics to treatment rather than to natural soil characteristics was tested under natural rainfall. The method used was a synthesis of two approaches used elsewhere.
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
171
Reynolds 1°) tested various dyes and found that Pyranine Conc. had good persistence characteristics in water tracing studies in soils. Dunn n) used fluorescent dyes to trace underground waters in caves and describes a technique of collecting dye on activated charcoal granules suspended in the water. The samples were elutriated in laboratory conditions with a 5~o solution of potassium hydroxide in ethyl alcohol. These two techniques were combined. The dye powder was sprinkled, at an intensity of 500 g/sq.m, inside a small triangular steel frame with sides 30 cm long, driven into the soil, apex upslope and the open end facing downslope. The upslope sides projected 5 cm above the soil surface, protecting the plot from overland flow from above, while if the plot itself generated overland flow the dye would be washed down over the soil surface. Activated charcoal granules were wrapped in a nylon mesh and spread across the path of water flowing downslope about 30-50 cm from the base of the frame. After suitable periods of rain the charcoal was collected and elutriated in the laboratory and fresh supplies of charcoal and dye were put down. When the elutriated samples were studied under a UV lamp it was quite easy to differentiate those samples which had collected dye from those which had not although it was not easy to determine the degree of fluorescence quantitatively. The results were therefore treated as dichotomous: either positive or negative-either producing or not producing overland flow. Six sites were studied on three slopes, one in each case compacted and one less disturbed. The sites are listed below: 1) Slope 2.5° (Soil WfL) a) Pathway, tractor compacted, thin grass cover, bare in patches. b) Orchard, lush grass growth. 2) Slope 6.5° (Soil WfH) a) Pathway, tractor compressed, lush grass cover, infrequently used. b) Orchard, lush grass over. 3) Slope 16.6 ° (Soil Ea) a) Artificial compaction by foot and further thinning of vegetation on pasture site. b) Pasture, cattle compacted, thin turf cover. The results of the experiment are drawn up in Fig. 2. Each number is the number of positive results for the site. When the totals are submitted to a X2 test the results are encouraging. At 0.05 probability, all the data from the undisturbed sites are significantly different from the expected frequency, regardless of slope, where the expected frequency is the maximum possible for the test (i.e. all sites producing overland flow). Two of the three compacted sites produced data which was not significantly different from the expected frequency. The third, which still produced substantial overland flow, was
172
RODNEY C. HILLS
found upon closer inspection to be rather poorly representative of its class, being very little disturbed and would have had a much greater interception loss than the other compacted sites. Plot 3b also produced interesting results in that it was a heavily poached pasture on a steep slope yet did not produce significant overland flow. This suggests that the poaching by cattle produced SiTE
16-6"
slope conqpoction
6,5' posture
2
compaction
2-5' orchard
compaction
orchard
0
O
2
0
1
2
3
2 ~u3
It=
3
cz
5
4
0
0
3
0
4
~
5
3
0
1
,:t
0
13
10
1
3
12
2
65
50
5
15
(30
10
TOTAL total as p~.~rcerl Of possible
Fig. 2.
Results of overland flow dye tests.
such a confusion of hoof marks on the soil surface that the amount of surface storage was increased to such a level that overland flow did not occur. These tests, together with observations made during natural storms12), suggest that infiltrometer cylinders may be used to assess natural rainfall infiltration and the probability of overland flow occurrence. A note on some other controlling factors
The conclusion that biotic interference, in the form of compaction, produces greater variations in infiltration capacity than natural variations among soils (at least among those sampled) was expanded by considering the following four factors: 1) The ecological status of the soil. 2) Bulk density. 3) Initial moisture content. 4) Rain beaten crusts. All have been described as significant influences on infiltration in the literature. Their significance here is assessed relative to the influence of "biotic interference." 1. THE ECOLOGICALSTATUSOF THE SOIL Sites where there is evidence of biotic compaction invariably appear to show poor ecological development. Since the total biomass in the soil is related
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
173
closely to its permeability, it is necessary to determine whether the significant factor reducing infiltration capacity is compaction of the surface or generally poor ecological development. Following the evidence produced by Guild la) suggesting that earthworms are closely related to the drainage characteristics of soils, the earthworm population of several sites was sampled (as an indicator of total biomass activity) using the formalin technique described by Rawl4). The data for total earthworm populations, classified by soil and treatment, were studied using analysis of variance after initial logarithmic transformation. Significant interaction was apparent and trends were difficult to differentiatel2). Data for total populations did not show a significant decline with increasing compaction although there was a slight fall. It seemed reasonable to conclude that, while earthworm populations were slightly influenced by compaction at the site (the result of disturbance, poorer food supply, and more difficult burrowing), the population reduction is small and the main reason for the low infiltration capacities is surface compaction rather than a lack of macro-pore spaces throughout the profile. 2. BULK DENSITY At eleven sites with a wide range of infiltration capacities bulk density samples were collected with a coring auger. Dry weight was calculated after oven drying for forty-eight hours at 105 °C and the internal dimensions of the coring auger were used to calculate the volume of the sample. The correlation coefficient between bulk density and infiltration was negative, small and insignificant. It seems likely that the low correlation coefficient resulted in part from that very factor that the analysis failed to reveal. It was extremely difficult to insert the auger into some of the compacted sites and a number of the samples were felt to be suspect. Certainly an intuitative assessment of bulk density (in terms of penetrability) would lead to the conclusion that bulk density is greater in the surface layers of compacted sites. Increased bulk density in such sites has been reported elsewhere15). It would therefore be reasonable to note a negative value for the correlation coefficient and to allow some latitude in interpreting its insignificance, in view of likely instrumental errors. 3. INITIAL SOIL MOISTURE
Research workers have consistently stressed the initial soil moisture content as a control over infiltration capacityl~). Only those working on the physics of infiltration 17) have pointed out that, at final infiltration, the effect of initial moisture content is unimportant. At all sites where infiltration capacity was measured initial soil moisture was determined from 20 samples taken from
174
RODNEY C. HILLS
the upper l0 cm of the profile. These were oven dried for 48 hr at 105°C and mean moisture content as a percentage of dry weight was calculated. The patterns of seasonal moisture changes in the various soils were quite complexl2). There was a close interaction between infiltration capacity and soil moisture content. Two generalisations could be made. The first was that changes in soil moisture level between winter and summei" were much smaller for compacted sites than for other sites. The second was that the changes in soil moisture were even smaller where the compacted site was not vegetated. There appears to be a complex causal relationship. If a site is bare and has a low infiltration capacity, there is little moisture entering the soil and there is no transpiration: moisture levels remain static throughout the year. If there is vegetation, there is greater biotic activity, the infiltration capacity fluctuates and so does the soil moisture content. It is difficult to see where the causal effect lies. Soil moisture is determined by infiltration capacity, itself largely determined by the treatment. The soil moisture level appears to be a minor influence on field infiltration capacity, being significant only within the broad limits of the treatment class but varying in importance with the texture of the soil. 4. RAIN BEAT CRUSTS Some research workers have suggested that a thin crust of impermeable compacted material may form on the soil surface under heavy rainfallX8). Studies of the energy of falling rain show that this is possiblel9). If such a crust had formed on some of the bare compacted sites described then the low infiltration capacities might have been due to a crust of this kind rather than to the effects of compaction. Tests were carried out at sites where this effect might have been possible. A standard set of infiltration tests was made together with a set of identical tests with the surface of the soil pierced randomly with a needle. There were no significant differences between the results of these tests suggesting that in Britain rainfalls are usually of insufficiently high intensity or fall with insufficiently large drops to cause surface crusts to form at least for the soils investigated.
Rainfall intensity and the generation of overland flow The infiltration data have been shown to relate closely to overland flow development and it is necessary to relate the infiltration capacity values to rainfall intensity figures for storms which affect the area studied. To make this possible the 24 hr charts from the Dyne's autographic raingauge at the Long Ashton Research Station were analysed for a 5 year period. The data were classified into units of fall by intensity and duration. Each unit was
175
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
termed a "rainfall event" and any rainfall event was defined simply as a period of rainfall of uniform intensity separated from any other such period by a dry spell. The total numbers of events are drawn up in Fig. 3. Rainfall events of this kind, grouped together, form longer storm periods of variable rainfall intensity. An arbitrary definition of a " s t o r m " or "rainfall period"
DURATION
'
2
.13
~
3
7
!
5 2 15 25 30 21 27 20 21 21 67
17 11 I 5 ~ 8 I "20 4
_ RAIN
OF 30 9 4 8 5 4 1 2 2 1 4
35 6 5 3 2 1
F L E V E N'T
F,
C
( MINS, 05
8
4
2 3 1 7
1 ,
3
-
h
'
1 .
, ----
Fig. 3. Rain events recorded at L o n g A s h t o n (1963-67 inc.) cross classified according to d u r a t i o n a n d intensity o f fall. N u m b e r s in cells are the total n u m b e r s of events in each class.
has not been undertaken. While it is acknowledged that the classification system used is deficient, in that it ignores evaporation and infiltration from storage sources in short periods between rainfall events, its simplicity makes it valuable at this stage in research. A straightforward comparison of rainfall rates and infiltration capacities is complicated by the fact that interception and surface storages must be satisfied before sufficient excess water is available to allow overland flow to begin. There is also a continuous evaporation loss from any standing water. Dismissing the latter and concentrating on interception and surface storage, the researcher is only partly aided by estimates of these quantities by other research workers. While a quantity of research into interception loss has taken place, losses have not been classified according to storm type or season and there is no data available on surface storage. An estimate has to be made of the initial quantity of water in any fall which must be discounted for overland flow generation as going immediately to satisfy these storages. More work has been undertaken for forests than for grasslands and it would seem reasonable to take a loss of 0.1 in. for any rain event as being necessary to satisfy the dry storage capacity of a forest cover and its understorey. This figure is based upon the review of interception data by Kitteredge z0) and on the estimate made for British conditions by Leyton,
7,
176
RODNEY C. H I L L S
Reynolds and Thompson zz). For grassland sites an estimate is more difficult to make but, taking the data for Molinia from Leyton, Reynolds and Thompson, a figure of 0.01 in. would seem reasonable. They show and comment upon the fact that a shrubby cover of heather or bracken increases this figure to nearly its forest equivalent but one must assume that for poor pasture the figure may be even lower. In attempting to estimate surface storage, the problem is much more difficult. One cannot, for instance, expect a natural increase in surface storage with an increasing vegetation cover. For example, a heavily grazed pasture may have only a very thin grass cover but, because of the pockets formed by hoofprints, it may have a great storage capacity. A vehicle track, on the other hand, may have an equally low infiltration capacity but may have little, or only moderate surface storage, depending upon the pattern of the tyre tread. Under thick vegetation it is reasonable to assume that surface storage levels are high, with a rough undisturbed soil surface created by the changes in subsurface volume associated with plant growth and animal movements. Given the estimates for interception which have been made by others and making arbitrary, but it is hoped reasonably inspired, estimates for surface storage, a set of figures are tabulated below which represent estimates of minimum rainfall which must fall on various surfaces before overland flow can
occur.
surface bare ground heavy grazing and cultivation light grazing shrub and woodland
interception
0.0 in 0.005 0.01 0.1
surfacestorage
total
0.05 in. 0.1 0.1 0.1
0.05 in. 0.105 0.11 0.2
lsopleths for equal falls of quantities 0.05, 0.1 and 0.2 in. are constructed in Fig. 3 (dashed lines EF, CH, CJ). The horizontal lines AB and C K represent important intensity divisions: 0.01 in/hr is at the limit of rainfall intensity measurement and is greater than the limit for important infiltration areas in G r o u p I and 0.4 in/hr (1.02 cm/hr) is just greater than the higher limit used in the infiltration site classification described earlier. The diagram can be used to compute, roughly, the probability of a site in a particular group producing overland flow. For example, sites falling within G r o u p I have a maximum probability of overland flow occurrence of 0.27. This is stated differently by saying that, for 27 ~o of the time when it is raining, these sites could be shedding overland f l o w - provided that the surface is bare of vegetation. This probability is computed by adding up all the events
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
177
beneath the minimum storage line (0.05 in.) and comparing this with the total number of rainfall events. If the soil surface has a thin vegetation cover the probability drops to 0.11 and if the vegetation cover is thick the probability drops even lower to 0.04. For G r o u p II sites, if 1 cm/hr is the critical infiltration capacity, for bare ground the overall probability of overland flow production is only 0.06 and for thin and heavy vegetation covers this drops to 0.02 and 0.01. For Groups III and IV it is not possible to compute the probabilities of overland flow production in this way but is is apparent that the likelihood of these sites producing overland flow is very small. Less than 30% of rain events, in the area, can be expected to produce overland flow and even then only on the restricted and rather unusual areas contained within G r o u p 1. Once the surface has a vegetation cover, even if only thin grass, the probability of overland flow occurring drops to 0. l and below. The data emphasize how misleading it may be to talk in terms of an overland flow contribution to stream discharge from every storm. The rainfall data suggest that two kinds of storm produce overland flow regularly. These are very high intensity falls lasting from 5 to 25 rain, probably summer storms, and very long storms of several hours of medium intensity. Given that the former occur in hot summer conditions, the evaporation loss may be higher than the estimates given here and, since the group is a small one anyway, it may be insignificant. The long medium intensity storms are usually associated with intense depressions during winter when interception and evaporation are small. Added to this is a problem of the rainfall classification adopted here because, if winter evaporation losses are low, events classed as individuals may not always have to satisfy initial storages. A number of individuals may be grouped into one long storm of slightly lower intensity with only one period of storage satisfaction at the beginning of the storm. The number of falls in this category producing overland flow may therefore be greater than is suggested. Conclusions
Most soils in Britain with their characteristic vegetation cover are capable of absorbing or storing the majority of British rainfalls. The zone of particcular danger is bare or lightly vegetated compacted ground when rain is falling at moderate intensities for long periods in winter. A further comment is necessary on the difference between overland flow production and river discharge. While certain areas can be expected to produce overland flow, the total pattern of such areas and especially their mutual contiguity must influence the ultimate composition of compound overland flow. For example, a small area of compaction around a farm gate at the top
178
RODNEY C. HILLS
of a hill may produce overland flow which may be absorbed downslope if the field is not heavily pastured. On the other hand, such a gateway compaction area, close to a drainage ditch in a valley bottom, may make a quick contribution to overland flow and river discharge. Similarly, an area compacted by cattle alongside a shelter hedge, running straight up and down a hillside, may provide a continuous channel for flowing water under intense storms. This analysis may therefore allow generalisations to be made about overland flow generation but this cannot automatically be assumed to relate to river discharge unless the ground pattern of the low infiltration capacity areas is studied and the distribution shown to be conducive to compound overland flow and runoff development. It has been assumed for many years that overland flow and runoff are causally related. The common methods of hydrograph analysis make this basic assumption. However, the significance of the surface contribution is not yet clear. Overland flow can be demonstrated on small plots, over short distances, but one cannot conclude, especially in rural areas with no artificial drainage, that this becomes a contribution to streamflow. Overland flow is caused by low infiltration capacities but the manipulation of the relationship into catchment hydrology needs experimental verification on a much wider scale.
Acknowledgements Most of this work was carried out during the tenure of a N.E.R.C. research award. The author's thanks are due to Prof. R. F. Peel of the Dept. of Geography in Bristol University who made the award available and provided research facilities. Dr. H. A. Osmaston, of the same Dept., gave continual assistance which is gratefully acknowledged. The Bristol Avon River Authority provided financial assistance, thanks are due to the Chief Engineer, Mr. F. Greenhaulgh and to Mr. R. Whitaker and Mr. J. Lavis for their help. At the Long Ashton Horticultural Research Station many members of the staff gave assistance. Special mention must be made of Mr. A. Stringer, Mr. K. G. Stott and Mr. G. E. Clothier. Dr. E. R. C. Reynolds of the Commonwealth Forestry Institute, Oxford, gave guidance on many occasions. Thanks must be extended to many others (notably local landowners) who in many ways, large and small, made the work possible.
Bibliography 1) J. T. Auten, The effect of forest burning and pasturing in the Ozarks on the water absorption of forest soils. U.S.D.A, Forest Service, Central States Expt. St., Note 16 (1934)
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
179
2) J. H. Stoeckeler, Trampling by livestcck drastically reduced infiltration rate of soil in open pine woods in south western Wisconsin. Lakes States Forest Expt. St., Tech. Note 556 (1959) 3) R. E. Horton, An approach toward a physical interpretation of infiltration capacity. Proc. Soil Sci. Soc. Amer. 5 (1940) 399-417 4) R. C. Hills, The determination of the infiltration capacity of field soils using the cylinder infiltrometer. British Geomorph. Res. Group, Tech. Bull. No. 3 (1970) 5) R. C. Hills, (in press). 6) D. C. Findlay, The soils of the Mendip district of Somerset. The Soil Survey of England and Wales, Harpenden (1965) 7) D. A. Osmond, A survey of the soils of the Long Ashton Research Station Farm. Annual Rept. of the Agric. and Hort. Res. St., Long Ashton, Bristol (1936) 8) R. C. Hills, The effect of agricultural treatment on the quantity of water available in the soil. In : The role of water in agriculture, Ed. J. A. Taylor, Univ. Coll. of Wales, Aherystwyth, Memorandum No. 12 (Pergamon Press, 1970) 9) E. G. Bilham, The classification of heavy falls in short periods. British Rainfall, H.M.S.O. (1935) 10) E. R. C. Reynolds, The percolation of rainwater through soil demonstrated by fluorescent dyes. J. Soil Sci. 17 (1966) 127-132 11) J. R. Dunn, Stream t r a c i n g - mid Appalachian region. National Spel. Soc. Bull. 2 (1963) 3 9 12) R. C. Hills, Infiltration measurement using cylinders as a method of assessing the influence of land management and soil type on the occurrence of overland flow. Unpublished Ph.D. Thesis, University of Bristol (1968) 13) W. J. McL. Guild, Earthworms and soil structure. In: Soil zoology. Proc. of Nottingham 2rid Easter School in Agric. Sci., Ed. D. K. E. Kevan (Butterworths, London, 1955) 14) F. Raw, Estimating earthworm populations by using Formalin. Nature, No. 4699 (1959) 1661-2 15) G. R. Free, G. M. Browning and G. W. Musgrave, Relative infiltration and related principal characteristics of certain soils. U.S.D.A., Tech. Bull. 729 (1940) 16) U. Schendal, Relations between the rate of infiltration and the water content of a Red Earth Loamy Sand to a depth of 25-30 cm. Z. Kulturtechnik, 3 (1962) 99-102 17) J. R. Philip, The theory of infiltration: 5, the influence of the initial moisture content. Soil Sci. 84 (1957) 329-339 18) F. L. Duley, Surface factors affecting the rate of intake of water by soils. Soil Sci. Proc. Amer. 4 (1939) 60 64 19) Meyer, et al., Collected papers on simulated rainfall for erosion research. Trans. A.S.A.E. 8 (1965) 63-75 20) J. Kitteredge, Forest influences (McGraw-Hill Inc., New York, 1948) 1st ed. 21) L. Leyton, E. R. C. Reynolds and F. B. Thompson, Rainfall interception in forest and moorland. In : Forest Hydrology. Proc. of a Nat. Sci. Found., Adv. Sci. Seminar, Penn. State Univ., August, 1965 (Pergamon Press, 1967)
Appendix G r i d r e f e r e n c e s ( B r i t i s h N a t i o n a l G r i d S y s t e m ) f o r t h e sites m e n t i o n e d in t h e p a p e r a r e l i s t e d : t h e n e a r e s t i d e n t i f y i n g t o w n o r v i l l a g e is a l s o g i v e n .
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RODNEY C. HILLS
Site code WfH(1) WfH(2) WfH(3) WfH(4) WfH(5) WfH(6) WfH(7) WfH(8) WfH(9) WfL(l) WfL(2) WfL(3) WfL(4) WfL(5) Ea(l) Ea(2) Ea(3) H(1) H(2) Ni(l ) Ni(2) Ni(3) Sy(1) Sy(2) Sy(3) Cb(l) Cb(2) Cb(3) Cb(4)
Location Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long A s h t o n Long A s h t o n Long A s h t o n Long A s h t o n Long Ashton Long Ashton Long Ashton Long Ashton Long A s h t o n Long Ashton Long Ashton Winterbourne Winterbourne Winterbourne Long Ashton Long Ashton Long Ashton Wickwar Wickwar Wickwar Wickwar
Grid Reference ST/534/697 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/539/697 ST/539/697 ST/538/697 ST/538/697 ST/538/697 ST/539/694 ST/540/694 ST/540/693 ST/538/693 ST/538/693 ST/651/813 ST/651/813 ST/651/813 ST/543/694 ST/543/694 ST/543/694 ST/742/876 ST/742/876 ST/754/882 ST/745/882
The site descriptions need some amplification with regard to treatment. Sites listed as woodland, orchard, pasture and cultivated are self descriptive. However, those described as "Simazine" or " D i u r o n " sites or perhaps (as WfH(9)) as "orchard, bare (chemical)" need expansion. On these plots the herbicides Simazine and Diuron were being applied at various intensities to produce a vegetation free surface which enabled easy movement of machinery and personnel. Two kinds of experiments were in progress, one where the aim was simply to study persistence of the herbicide with no maintained vegetation cover (these are simply labelled "Simazine" or " D i u r o n " in the paper) and those where the herbicides were being used among a field crop (labelled either "orchard" or "cultivated, bare (chemical)"). The treatments applied to these plots were as follows: WfH(5) Control plot untouched for 4 years.
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
WfH(6) WfH(7) WfH(8) WfH(9) WfL(3)
181
Treated with Simazine at 2½ lb/acre annually from 1963-67. Treated with Simazine at 20 lb/acre in 1963 and 1964. Orchard plot treated with Diuron at 6 lb/acre from 1960-67. As WfH(8). Blackcurrant bushes, treated with Simazine twice annually at ¼ lb/acre for 4 years. WfL(4) As WfL(3). All the infiltration tests were carried out between January, 1966, and March, 1968. All the plots listed above were maintained and used by members of the staff at the Long Ashton Research Station and infiltration tests were carried out on them with their permission. These treatments sustain large areas of bare ground which are very susceptible to rainbeat, vehicle compaction, overland flow and soil erosion. These effects have been described more fully in ref. 8.
THE INFLUENCE
OF LAND MANAGEMENT
CHARACTERISTICS OCCURRENCE
ON INFILTRATION OF OVERLAND
AND SOIL AND THE
FLOW
RODNEY C. HILLS* Abstract: Infiltration capacity is measured using cylinders and relates more closely to the treatment of a soil, especially the compaction of the surface layers, than to the natural soil characteristics. Overland flow is demonstrated with fluorescent dyes on low infiltration capacity sites with various soils and slopes. Relating infiltration measurements to local rainfall rates, after allowing for interception and surface storage, shows that at most only 30 %, and on many sites fewer than 10 %, of rainfalls can produce overland flow and then only on very restricted areas.
Introduction T h e role o f infiltration in the h y d r o l o g i c a l cycle has r e m a i n e d one o f conjecture, m a r r e d by a p a u c i t y o f field o b s e r v a t i o n , for longer t h a n is c o m m o n in scientific e n d e a v o u r . The reasons for this d i s r e g a r d b e c o m e app a r e n t when field investigations are u n d e r t a k e n : m e t h o d s o f m e a s u r e m e n t are crude a n d i n a c c u r a t e : the p o p u l a t i o n s are difficult to define: the s a m p l i n g errors are large. T w o p o i n t s s h o u l d be noted. The first is t h a t early m e t h o d s o f field m e a s u r e m e n t o f infiltration c a p a c i t y were f o u n d to be inaccurate and were, in a n y case, s e l d o m used with a carefully c o n s i d e r e d s a m p l i n g design. M o s t o f these studies were m a d e to assess erosion h a z a r d s 1) or moisture d e p l e t i o n u n d e r cattle p a s t u r e ~). F r o m the d a t a presented in p a p e r s such as these it is difficult to assess the significance o f low infiltration c a p a c i t y areas in the response o f a c o m p l e t e c a t c h m e n t to rainfall. The second p o i n t t h a t arises is a m o v e m e n t a w a y f r o m field infiltration c a p a c i t y m e a s u r e m e n t t o w a r d s the d e d u c t i o n o f infiltration c a p a c i t y f r o m a series o f factors in an overall water b a l a n c e : this is p a r t i c u l a r l y true o f the literature o f the last ten years. This has led to m a n y a s s u m p t i o n s a b o u t infiltration characteristics m a d e with the s u p p o r t o f sparce d a t a f r o m field m e a s u r e m e n t . It is with these p o i n t s in m i n d that this investigation was u n d e r t a k e n . The following terms will be used in the p a p e r and are defined below: 1) Infiltration capacity. " T h e m a x i m u m rate at which a given soil when in a given c o n d i t i o n can a b s o r b rain as it falls. ''z) * Present address: E.A. Meteorological Dept., P.O. Box 30259, Nairobi, Kenya. 163
164
RODNEY C. HILLS
2) Infiltration. Specifically, in this paper, the entry of water into the soil. 3) Overland flow. Any water flowing over the surface of the soil resulting from an excess of water which does not infiltrate. 4) Compound overland flow. As 3) above but compounded with overland flow increments from upslope. 5) Runoff: Any water flowing out of the area of interest, usually in channel form.
6) Storage. Water held in such ways that it stops, temporarily or permanently, moving through the hydrological cycle.
Method of measurement and sampling design Infiltration capacity was measured using 10 cm diameter steel cylinders, constructed of ~ in. gauge steel, driven 5 cm into the soil and fed from a calibrated feeder bottle maintaining a head of 5 cm over the soil. Details of the technique have been published elsewhere4). Allowance for lateral flow underneath the cylinder was made using an empirically determined correction technique described previously s). The aim of the investigation was to elucidate the interacting effects of soil characteristics and land management. The area in which the investigation took place (around Bristol in S.W. England) was stratified according to Soil Series and type of management or treatment. Within each stratum a sampling site was chosen with respect to its representative nature and accessability. At each site a clustered random sampling design was used containing 20 individuals per stratum. This sample size was a compromise between statistical precision and convenience, in that it enabled a site to be sampled in one day. The sites sampled constituted a wide range of conditions in the area. The Soil Series and subdivisions sampled are listed below, together with an abbreviated code by which the divisions will be referred to in the paper and a principle reference describing details of the profile : i) Worcester (Wf) 6). a) Heavy Worcester (WfH) 7). b) Light Worcester (WfL)7). ii) Greinton (Gc) 6). iii) Evesham (Ea)6). iv) Nibley (Ni)6). v) Haselor (H)7). vi) Spetchley (Sy) 6). vii) Charlton Bank (Cb)6). in terms of textural lightness and profile morphology, the more freely
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
165
draining soils were Gc, Ni, and H. The Wf group is often regarded as impeded but, at the sites investigated, the Wf subdivisions seemed to exhibit quite a light texture and would also fall close to the freely draining group. Series Ea, Sy and Cb would all be classed as impeded or poorly drained. The Cb Series being notably intractable. The land uses or treatments investigated were as follows: i) Woodland. Natural with no marked biotic influence. ii) Orchard. Trees planted for fruit and relatively undisturbed. iii) Pasture. Grassland, not grazed at time of sampling. iv) Pasture (light). Grazing just sufficient to keep down all vegetation other than grasses, lush and not unduly disturbed. v) Pasture (heavy). Grass grazed short. Cattle movements appeared to be seriously disturbing the soil structure. vi) Cultivated. Soil was disturbed and was about to be or had been planted. vii) Compacted. Soil was severely disturbed and compressed and structure was broken down. The group could be subdivided into "animal" and "vehicle" compaction. viii) It was useful to make a further distinction at any site between "bare" and "vegetated" - a distinction which becomes important in assessing interception and surface storage quantities over any surface. Two kinds of bare surfaces were sampled: those produced chemically (deliberately produced to aid some agronomic practice) and biotically produced (usually accidentally). The hydrological implications of chemically induced bare surfaces have already been described 8). Details of treatments are described in the Appendix. Analysis of results The infiltration data proved to be very heterogeneous. Not only was there pronounced spatial variation in infiltration values, but the sampled populations did not portray any systematic deviation from normality. The populations displayed heterogeneity among variances, both positive and negative skews and, in many cases, multimodality. In view of the problems of analysis presented by these data, it was decided to treat the populations as complete units rather than to attempt to determine a single useful parameter. There are two other reasons for this: 1) The spread of the data produced large sampling errors and mean figures could not be used to assess general infiltration characteristics with any useful confidence. 2) In the production of overland flow it is the relative proportions of soil
166
RODNEY C. HILLS
that display high and low infiltration capacities (in relation to local rainfall intensities) that are important. These proportions may occur in populations that have a variety of different forms. A simple survey of the total population enables one to estimate whether or not the site has a significantly large area with low infiltration capacity. It has been widely reported that measured infiltration capacities exhibit a falling rate with time3). In the experiments described here a variety of patterns were found and this also influenced the method of analysis. For each individual determination, infiltration capacity was measured at 5 rain intervals for the first 15 min and then at 15 min intervals for the remainder of the test which lasted between 30 and 120 min. Some determinations, even within a stratum, exhibited a falling rate while others showed a steady rate. This aspect of the data has yet to be studied in detail. For the present analysis a a mean figure for each individual determination was calculated for the complete run and, where sample means are quoted, these are simply the means of the twenty individual means. Mean initial and final infiltration capacities were also calculated. The initial rates were usually so high as to be far greater than British rainfall intensities. The final rates were lower than the mean figures but, if substituted for the means, were found to make no difference to the placing of the sites in the classification which follows. The data suggested that each soil treatment stratum has a mean infiltration curve with differentiating characteristics and that this must be considered if more detailed analysis is to take place. It has been mentioned that it is the relative proportions of high and low infiltration values that determine whether a site is likely to produce overland flow. The samples were therefore divided into four groups as follows: Group L Where 60~o or more of the individual infiltration capacity means were 0.01 cm/hr or less. Group II. Where 50~o or more of the individual means were 1.0 cm/hr or less (but excluding G r o u p I). Group IlL Where more than 50~o but less than 90~o of the individual means were greater than 1.0 cm/hr. Group IV. Where more than 90~o of the individual means were greater than 1.0 cm/hr. Figure 1 shows the sites and populations within these groups. This classification is used for two reasons: 1) Experience in the field and observation of natural storms suggested that, if about 60~o of the soil surface had a very low infiltration capacity, the site was very likely to produce overland flow during any rainlall. If 50~o or more of the surface had a slightly higher infiltration capacity, the site would produce overland flow during intense storms. If less than 50 ~o of the area had this
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
ii
[ wf H(9) ~: w
ii Wf sL(3) i ! I1
.:..Sy (2)w I ~ -% Wf H(9) I~ s I; mee °~
H(1)w
ii
•
•
000 •
•
OOlO
•
•
•
ee
•
.%, Ni(2)w I I
. . . .
•
•
C b(4)w i !
"o"1
2 3:5
e(5 /
89e10~1121314151617~19
rTlc~flrl infiltrqtion
Group I: Frequency histogram.
Fig. la.
w
COl3ocity (cms/hr)
•
h
w
~
•
•
~
~:~! .....
WfH (4)
|i •
o':
2
Fig. lb.
"3 4
•
•
oo
•
•
m
•
5 6 7 'B 9 I0 11 12 13 14 15 1G 17 18 lg m~an infiltrotion copocity (cms / hr )
Group II: Frequency histogram.
167
168
RODNEY C. HILLS
WfH(4) w Sy(3)w Ni(1)w WfH(5) w Wf H(6) w H(2)w WIL(1) w Eo (3)w Cb(2)s Cb(4 )s W f L(5 ) W SyO )w Cb(1 )w imegn
Fig. lc.
WfH(3) w
•
infiltroDon
( cms, / hr )
c opocity
Group Ill: Frequency histogram.
I,
WfwH(1 ) Cb(3)w Wf H(2) w Ni(1)s Ni(3)S
I.
]q
Sy(2)s WfL(4) w Ea(1)s Cb(3)s CbC)s mean
infiltration
Fig. I d.
capacity
(cms/
hn)
Group IV: Frequency histogram.
slightly higher capacity (the remainder being greater) the site would only show surface storage and no overland flow and if more than 90~o of the area had high infiltration capacities, overland flow would not be produced and surface storage would be very small. 2) The numerical values in the classification system are used with reference to rainfall data. An analysis of the record of the Dyne's autographic
THE INFLUENCEOF LAND MANAGEMENTAND SOIL CHARACTERISTICS 169 raingauge at Long Ashton (Fig. 3), together with Bilham's 9) classification of rainfalls, suggested sensible values for high and low intensity falls and important infiltration capacities. The lowest value, 0.01 cm/hr, is at the limit of accuracy of rainfall intensity measurement (on the Dyne's chart). Any soil with an infiltration capacity lower than this can be expected to store water on the surface soon after any rain begins, provided evaporation rate does not exceed infiltration capacity. The higher limit is 1.0 cm/hr. This separates the short high intensity storms from the long low intensity winter storms. Storms of greater intensity than 1.0 cm/hr seldom last more than 30 rain. Storms of greater intensity than this lasting 10 rain or less fall into Bilham's "noteworthy" category, with a recurrence interval of 1.6 per annum ; whereas such storms lasting 30 rain would have a recurrence interval of 0.1 per annum. This intensity represents the boundary between quite unusual occurrences and the fairly common and the range between 0.1 cm/hr and 1.0 cm/hr contains most British falls. The infiltration capacity data is portrayed in Fig. 1. A frequency histogram for each site is drawn and is given a code which refers it to the list below. The suffix " w " or "s" refers to a winter or summer determination. These seasonal determinations were made to assess the significance of seasonal changes in soil moisture content. Further details of the sites are given in the Appendix. Site
Site characteristics
Ea(1)w Ea(2)w H(1)w Ni(2)w Ni(2)s Sy(2)w Cb(4)w
0.05 cm/hr Orchard, bare (chemical), compaction (vehicle) 0.9 As WfH(9)w 0.2 Cultivated,bare (chemical), compacted (vehicle) 0.0 Heavy pasture 0.0 Compacted (cattle), bare (biotic) 0.9 Heavy pasture 2.3 Compacted (cattle), vegetated 1.1 As Ni(2)w 0.8 Compacted (vehicle), part bare 2.3 Compacted (cattle), vegetated
Group H
WfL(3)w WfH(4)s WfH(7)w WfL(2) WfH(8)w
0.5 2.9 1.6 0.6 1.3
Cultivated,bare (chemical) Compaction (vehicle), part bare (biotic) Simazine 20, bare (chemical) Compaction, bare (biotic) Diuron, orchard, bare (chemical)
Group 111
WfH(5)w WfH(6) WfL(l)w
2.7 2.9 4.7
Simazine control Simazine 2½ Orchard
Group I
WfH(9)w
Mean value
WfH(9)s WfL(3)s
170
R O D N E Y C. H I L L S
Site
Group IV
Mean value
Site characteristics
WfL(5)w Ea(3)w H(2)w Ni(l)w Sy(l)w Cb(l)w Cb(2)s Cb(4)s WfH(4)w Sy(3)w
6.3 cm/hr 53 3.6 2.6 8.6 19.1 5.5 5.7 1.7 2.5
Cultivated Cultivated Cultivated Light pasture Orchard Woodland Woodland, compacted, part bare Heavy pasture, compacted Compaction (vehicle), part bare (biotic) Cultivated
WfH(1)w WfH(2)w WfH(3)w WfL(4)w Ea(1 )s Ni(1)s Ni(3)s Sy(2)s Cb(1)s Cb(3)s Cb(3)w
5.0 8.2 4.5 15.4 16.4 I 1.5 11.6 14.2 38.3 36.6 6.8
Woodland Orchard Orchard, compaction (vehicle) Cultivated,bare (chemical) Heavy pasture Light pasture Cultivated,vegetated Compaction(vehicle), part bare Woodland Light pasture Light pasture
The distribution diagrams hardly need further comment. In Group ! all the distributions are either centred on zero or show pronounced skewness with a distinct mode on or close to zero. In Group II the skewness is still apparent but is less pronounced. In Groups 1II and IV the patterns become more complex: the range of data increases and the variety of form among the distributions becomes noticeable. An analysis of these Groups by treatment and Soil Series was undertaken. When the sites in each group were cross classified according to "treatment" and "soil type", it was found (using Z2 at 0.05 probability) that grouping according to treatment (simply classified into disturbed/compaction and undisturbed/light pasture) was significant and grouping according to soil type (simply classified into light and heavy textured) was not significant. It was concluded that the treatment or management of the soil is more important in determining its infiltration capacity than its morphological characteristics.
Tests for overland flow production The hypothesis relating infiltration characteristics to treatment rather than to natural soil characteristics was tested under natural rainfall. The method used was a synthesis of two approaches used elsewhere.
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
171
Reynolds 1°) tested various dyes and found that Pyranine Conc. had good persistence characteristics in water tracing studies in soils. Dunn n) used fluorescent dyes to trace underground waters in caves and describes a technique of collecting dye on activated charcoal granules suspended in the water. The samples were elutriated in laboratory conditions with a 5~o solution of potassium hydroxide in ethyl alcohol. These two techniques were combined. The dye powder was sprinkled, at an intensity of 500 g/sq.m, inside a small triangular steel frame with sides 30 cm long, driven into the soil, apex upslope and the open end facing downslope. The upslope sides projected 5 cm above the soil surface, protecting the plot from overland flow from above, while if the plot itself generated overland flow the dye would be washed down over the soil surface. Activated charcoal granules were wrapped in a nylon mesh and spread across the path of water flowing downslope about 30-50 cm from the base of the frame. After suitable periods of rain the charcoal was collected and elutriated in the laboratory and fresh supplies of charcoal and dye were put down. When the elutriated samples were studied under a UV lamp it was quite easy to differentiate those samples which had collected dye from those which had not although it was not easy to determine the degree of fluorescence quantitatively. The results were therefore treated as dichotomous: either positive or negative-either producing or not producing overland flow. Six sites were studied on three slopes, one in each case compacted and one less disturbed. The sites are listed below: 1) Slope 2.5° (Soil WfL) a) Pathway, tractor compacted, thin grass cover, bare in patches. b) Orchard, lush grass growth. 2) Slope 6.5° (Soil WfH) a) Pathway, tractor compressed, lush grass cover, infrequently used. b) Orchard, lush grass over. 3) Slope 16.6 ° (Soil Ea) a) Artificial compaction by foot and further thinning of vegetation on pasture site. b) Pasture, cattle compacted, thin turf cover. The results of the experiment are drawn up in Fig. 2. Each number is the number of positive results for the site. When the totals are submitted to a X2 test the results are encouraging. At 0.05 probability, all the data from the undisturbed sites are significantly different from the expected frequency, regardless of slope, where the expected frequency is the maximum possible for the test (i.e. all sites producing overland flow). Two of the three compacted sites produced data which was not significantly different from the expected frequency. The third, which still produced substantial overland flow, was
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RODNEY C. HILLS
found upon closer inspection to be rather poorly representative of its class, being very little disturbed and would have had a much greater interception loss than the other compacted sites. Plot 3b also produced interesting results in that it was a heavily poached pasture on a steep slope yet did not produce significant overland flow. This suggests that the poaching by cattle produced SiTE
16-6"
slope conqpoction
6,5' posture
2
compaction
2-5' orchard
compaction
orchard
0
O
2
0
1
2
3
2 ~u3
It=
3
cz
5
4
0
0
3
0
4
~
5
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1
,:t
0
13
10
1
3
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65
50
5
15
(30
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TOTAL total as p~.~rcerl Of possible
Fig. 2.
Results of overland flow dye tests.
such a confusion of hoof marks on the soil surface that the amount of surface storage was increased to such a level that overland flow did not occur. These tests, together with observations made during natural storms12), suggest that infiltrometer cylinders may be used to assess natural rainfall infiltration and the probability of overland flow occurrence. A note on some other controlling factors
The conclusion that biotic interference, in the form of compaction, produces greater variations in infiltration capacity than natural variations among soils (at least among those sampled) was expanded by considering the following four factors: 1) The ecological status of the soil. 2) Bulk density. 3) Initial moisture content. 4) Rain beaten crusts. All have been described as significant influences on infiltration in the literature. Their significance here is assessed relative to the influence of "biotic interference." 1. THE ECOLOGICALSTATUSOF THE SOIL Sites where there is evidence of biotic compaction invariably appear to show poor ecological development. Since the total biomass in the soil is related
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
173
closely to its permeability, it is necessary to determine whether the significant factor reducing infiltration capacity is compaction of the surface or generally poor ecological development. Following the evidence produced by Guild la) suggesting that earthworms are closely related to the drainage characteristics of soils, the earthworm population of several sites was sampled (as an indicator of total biomass activity) using the formalin technique described by Rawl4). The data for total earthworm populations, classified by soil and treatment, were studied using analysis of variance after initial logarithmic transformation. Significant interaction was apparent and trends were difficult to differentiatel2). Data for total populations did not show a significant decline with increasing compaction although there was a slight fall. It seemed reasonable to conclude that, while earthworm populations were slightly influenced by compaction at the site (the result of disturbance, poorer food supply, and more difficult burrowing), the population reduction is small and the main reason for the low infiltration capacities is surface compaction rather than a lack of macro-pore spaces throughout the profile. 2. BULK DENSITY At eleven sites with a wide range of infiltration capacities bulk density samples were collected with a coring auger. Dry weight was calculated after oven drying for forty-eight hours at 105 °C and the internal dimensions of the coring auger were used to calculate the volume of the sample. The correlation coefficient between bulk density and infiltration was negative, small and insignificant. It seems likely that the low correlation coefficient resulted in part from that very factor that the analysis failed to reveal. It was extremely difficult to insert the auger into some of the compacted sites and a number of the samples were felt to be suspect. Certainly an intuitative assessment of bulk density (in terms of penetrability) would lead to the conclusion that bulk density is greater in the surface layers of compacted sites. Increased bulk density in such sites has been reported elsewhere15). It would therefore be reasonable to note a negative value for the correlation coefficient and to allow some latitude in interpreting its insignificance, in view of likely instrumental errors. 3. INITIAL SOIL MOISTURE
Research workers have consistently stressed the initial soil moisture content as a control over infiltration capacityl~). Only those working on the physics of infiltration 17) have pointed out that, at final infiltration, the effect of initial moisture content is unimportant. At all sites where infiltration capacity was measured initial soil moisture was determined from 20 samples taken from
174
RODNEY C. HILLS
the upper l0 cm of the profile. These were oven dried for 48 hr at 105°C and mean moisture content as a percentage of dry weight was calculated. The patterns of seasonal moisture changes in the various soils were quite complexl2). There was a close interaction between infiltration capacity and soil moisture content. Two generalisations could be made. The first was that changes in soil moisture level between winter and summei" were much smaller for compacted sites than for other sites. The second was that the changes in soil moisture were even smaller where the compacted site was not vegetated. There appears to be a complex causal relationship. If a site is bare and has a low infiltration capacity, there is little moisture entering the soil and there is no transpiration: moisture levels remain static throughout the year. If there is vegetation, there is greater biotic activity, the infiltration capacity fluctuates and so does the soil moisture content. It is difficult to see where the causal effect lies. Soil moisture is determined by infiltration capacity, itself largely determined by the treatment. The soil moisture level appears to be a minor influence on field infiltration capacity, being significant only within the broad limits of the treatment class but varying in importance with the texture of the soil. 4. RAIN BEAT CRUSTS Some research workers have suggested that a thin crust of impermeable compacted material may form on the soil surface under heavy rainfallX8). Studies of the energy of falling rain show that this is possiblel9). If such a crust had formed on some of the bare compacted sites described then the low infiltration capacities might have been due to a crust of this kind rather than to the effects of compaction. Tests were carried out at sites where this effect might have been possible. A standard set of infiltration tests was made together with a set of identical tests with the surface of the soil pierced randomly with a needle. There were no significant differences between the results of these tests suggesting that in Britain rainfalls are usually of insufficiently high intensity or fall with insufficiently large drops to cause surface crusts to form at least for the soils investigated.
Rainfall intensity and the generation of overland flow The infiltration data have been shown to relate closely to overland flow development and it is necessary to relate the infiltration capacity values to rainfall intensity figures for storms which affect the area studied. To make this possible the 24 hr charts from the Dyne's autographic raingauge at the Long Ashton Research Station were analysed for a 5 year period. The data were classified into units of fall by intensity and duration. Each unit was
175
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
termed a "rainfall event" and any rainfall event was defined simply as a period of rainfall of uniform intensity separated from any other such period by a dry spell. The total numbers of events are drawn up in Fig. 3. Rainfall events of this kind, grouped together, form longer storm periods of variable rainfall intensity. An arbitrary definition of a " s t o r m " or "rainfall period"
DURATION
'
2
.13
~
3
7
!
5 2 15 25 30 21 27 20 21 21 67
17 11 I 5 ~ 8 I "20 4
_ RAIN
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F L E V E N'T
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C
( MINS, 05
8
4
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3
-
h
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, ----
Fig. 3. Rain events recorded at L o n g A s h t o n (1963-67 inc.) cross classified according to d u r a t i o n a n d intensity o f fall. N u m b e r s in cells are the total n u m b e r s of events in each class.
has not been undertaken. While it is acknowledged that the classification system used is deficient, in that it ignores evaporation and infiltration from storage sources in short periods between rainfall events, its simplicity makes it valuable at this stage in research. A straightforward comparison of rainfall rates and infiltration capacities is complicated by the fact that interception and surface storages must be satisfied before sufficient excess water is available to allow overland flow to begin. There is also a continuous evaporation loss from any standing water. Dismissing the latter and concentrating on interception and surface storage, the researcher is only partly aided by estimates of these quantities by other research workers. While a quantity of research into interception loss has taken place, losses have not been classified according to storm type or season and there is no data available on surface storage. An estimate has to be made of the initial quantity of water in any fall which must be discounted for overland flow generation as going immediately to satisfy these storages. More work has been undertaken for forests than for grasslands and it would seem reasonable to take a loss of 0.1 in. for any rain event as being necessary to satisfy the dry storage capacity of a forest cover and its understorey. This figure is based upon the review of interception data by Kitteredge z0) and on the estimate made for British conditions by Leyton,
7,
176
RODNEY C. H I L L S
Reynolds and Thompson zz). For grassland sites an estimate is more difficult to make but, taking the data for Molinia from Leyton, Reynolds and Thompson, a figure of 0.01 in. would seem reasonable. They show and comment upon the fact that a shrubby cover of heather or bracken increases this figure to nearly its forest equivalent but one must assume that for poor pasture the figure may be even lower. In attempting to estimate surface storage, the problem is much more difficult. One cannot, for instance, expect a natural increase in surface storage with an increasing vegetation cover. For example, a heavily grazed pasture may have only a very thin grass cover but, because of the pockets formed by hoofprints, it may have a great storage capacity. A vehicle track, on the other hand, may have an equally low infiltration capacity but may have little, or only moderate surface storage, depending upon the pattern of the tyre tread. Under thick vegetation it is reasonable to assume that surface storage levels are high, with a rough undisturbed soil surface created by the changes in subsurface volume associated with plant growth and animal movements. Given the estimates for interception which have been made by others and making arbitrary, but it is hoped reasonably inspired, estimates for surface storage, a set of figures are tabulated below which represent estimates of minimum rainfall which must fall on various surfaces before overland flow can
occur.
surface bare ground heavy grazing and cultivation light grazing shrub and woodland
interception
0.0 in 0.005 0.01 0.1
surfacestorage
total
0.05 in. 0.1 0.1 0.1
0.05 in. 0.105 0.11 0.2
lsopleths for equal falls of quantities 0.05, 0.1 and 0.2 in. are constructed in Fig. 3 (dashed lines EF, CH, CJ). The horizontal lines AB and C K represent important intensity divisions: 0.01 in/hr is at the limit of rainfall intensity measurement and is greater than the limit for important infiltration areas in G r o u p I and 0.4 in/hr (1.02 cm/hr) is just greater than the higher limit used in the infiltration site classification described earlier. The diagram can be used to compute, roughly, the probability of a site in a particular group producing overland flow. For example, sites falling within G r o u p I have a maximum probability of overland flow occurrence of 0.27. This is stated differently by saying that, for 27 ~o of the time when it is raining, these sites could be shedding overland f l o w - provided that the surface is bare of vegetation. This probability is computed by adding up all the events
T H E I N F L U E N C E OF L A N D M A N A G E M E N T A N D SOIL C H A R A C T E R I S T I C S
177
beneath the minimum storage line (0.05 in.) and comparing this with the total number of rainfall events. If the soil surface has a thin vegetation cover the probability drops to 0.11 and if the vegetation cover is thick the probability drops even lower to 0.04. For G r o u p II sites, if 1 cm/hr is the critical infiltration capacity, for bare ground the overall probability of overland flow production is only 0.06 and for thin and heavy vegetation covers this drops to 0.02 and 0.01. For Groups III and IV it is not possible to compute the probabilities of overland flow production in this way but is is apparent that the likelihood of these sites producing overland flow is very small. Less than 30% of rain events, in the area, can be expected to produce overland flow and even then only on the restricted and rather unusual areas contained within G r o u p 1. Once the surface has a vegetation cover, even if only thin grass, the probability of overland flow occurring drops to 0. l and below. The data emphasize how misleading it may be to talk in terms of an overland flow contribution to stream discharge from every storm. The rainfall data suggest that two kinds of storm produce overland flow regularly. These are very high intensity falls lasting from 5 to 25 rain, probably summer storms, and very long storms of several hours of medium intensity. Given that the former occur in hot summer conditions, the evaporation loss may be higher than the estimates given here and, since the group is a small one anyway, it may be insignificant. The long medium intensity storms are usually associated with intense depressions during winter when interception and evaporation are small. Added to this is a problem of the rainfall classification adopted here because, if winter evaporation losses are low, events classed as individuals may not always have to satisfy initial storages. A number of individuals may be grouped into one long storm of slightly lower intensity with only one period of storage satisfaction at the beginning of the storm. The number of falls in this category producing overland flow may therefore be greater than is suggested. Conclusions
Most soils in Britain with their characteristic vegetation cover are capable of absorbing or storing the majority of British rainfalls. The zone of particcular danger is bare or lightly vegetated compacted ground when rain is falling at moderate intensities for long periods in winter. A further comment is necessary on the difference between overland flow production and river discharge. While certain areas can be expected to produce overland flow, the total pattern of such areas and especially their mutual contiguity must influence the ultimate composition of compound overland flow. For example, a small area of compaction around a farm gate at the top
178
RODNEY C. HILLS
of a hill may produce overland flow which may be absorbed downslope if the field is not heavily pastured. On the other hand, such a gateway compaction area, close to a drainage ditch in a valley bottom, may make a quick contribution to overland flow and river discharge. Similarly, an area compacted by cattle alongside a shelter hedge, running straight up and down a hillside, may provide a continuous channel for flowing water under intense storms. This analysis may therefore allow generalisations to be made about overland flow generation but this cannot automatically be assumed to relate to river discharge unless the ground pattern of the low infiltration capacity areas is studied and the distribution shown to be conducive to compound overland flow and runoff development. It has been assumed for many years that overland flow and runoff are causally related. The common methods of hydrograph analysis make this basic assumption. However, the significance of the surface contribution is not yet clear. Overland flow can be demonstrated on small plots, over short distances, but one cannot conclude, especially in rural areas with no artificial drainage, that this becomes a contribution to streamflow. Overland flow is caused by low infiltration capacities but the manipulation of the relationship into catchment hydrology needs experimental verification on a much wider scale.
Acknowledgements Most of this work was carried out during the tenure of a N.E.R.C. research award. The author's thanks are due to Prof. R. F. Peel of the Dept. of Geography in Bristol University who made the award available and provided research facilities. Dr. H. A. Osmaston, of the same Dept., gave continual assistance which is gratefully acknowledged. The Bristol Avon River Authority provided financial assistance, thanks are due to the Chief Engineer, Mr. F. Greenhaulgh and to Mr. R. Whitaker and Mr. J. Lavis for their help. At the Long Ashton Horticultural Research Station many members of the staff gave assistance. Special mention must be made of Mr. A. Stringer, Mr. K. G. Stott and Mr. G. E. Clothier. Dr. E. R. C. Reynolds of the Commonwealth Forestry Institute, Oxford, gave guidance on many occasions. Thanks must be extended to many others (notably local landowners) who in many ways, large and small, made the work possible.
Bibliography 1) J. T. Auten, The effect of forest burning and pasturing in the Ozarks on the water absorption of forest soils. U.S.D.A, Forest Service, Central States Expt. St., Note 16 (1934)
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
179
2) J. H. Stoeckeler, Trampling by livestcck drastically reduced infiltration rate of soil in open pine woods in south western Wisconsin. Lakes States Forest Expt. St., Tech. Note 556 (1959) 3) R. E. Horton, An approach toward a physical interpretation of infiltration capacity. Proc. Soil Sci. Soc. Amer. 5 (1940) 399-417 4) R. C. Hills, The determination of the infiltration capacity of field soils using the cylinder infiltrometer. British Geomorph. Res. Group, Tech. Bull. No. 3 (1970) 5) R. C. Hills, (in press). 6) D. C. Findlay, The soils of the Mendip district of Somerset. The Soil Survey of England and Wales, Harpenden (1965) 7) D. A. Osmond, A survey of the soils of the Long Ashton Research Station Farm. Annual Rept. of the Agric. and Hort. Res. St., Long Ashton, Bristol (1936) 8) R. C. Hills, The effect of agricultural treatment on the quantity of water available in the soil. In : The role of water in agriculture, Ed. J. A. Taylor, Univ. Coll. of Wales, Aherystwyth, Memorandum No. 12 (Pergamon Press, 1970) 9) E. G. Bilham, The classification of heavy falls in short periods. British Rainfall, H.M.S.O. (1935) 10) E. R. C. Reynolds, The percolation of rainwater through soil demonstrated by fluorescent dyes. J. Soil Sci. 17 (1966) 127-132 11) J. R. Dunn, Stream t r a c i n g - mid Appalachian region. National Spel. Soc. Bull. 2 (1963) 3 9 12) R. C. Hills, Infiltration measurement using cylinders as a method of assessing the influence of land management and soil type on the occurrence of overland flow. Unpublished Ph.D. Thesis, University of Bristol (1968) 13) W. J. McL. Guild, Earthworms and soil structure. In: Soil zoology. Proc. of Nottingham 2rid Easter School in Agric. Sci., Ed. D. K. E. Kevan (Butterworths, London, 1955) 14) F. Raw, Estimating earthworm populations by using Formalin. Nature, No. 4699 (1959) 1661-2 15) G. R. Free, G. M. Browning and G. W. Musgrave, Relative infiltration and related principal characteristics of certain soils. U.S.D.A., Tech. Bull. 729 (1940) 16) U. Schendal, Relations between the rate of infiltration and the water content of a Red Earth Loamy Sand to a depth of 25-30 cm. Z. Kulturtechnik, 3 (1962) 99-102 17) J. R. Philip, The theory of infiltration: 5, the influence of the initial moisture content. Soil Sci. 84 (1957) 329-339 18) F. L. Duley, Surface factors affecting the rate of intake of water by soils. Soil Sci. Proc. Amer. 4 (1939) 60 64 19) Meyer, et al., Collected papers on simulated rainfall for erosion research. Trans. A.S.A.E. 8 (1965) 63-75 20) J. Kitteredge, Forest influences (McGraw-Hill Inc., New York, 1948) 1st ed. 21) L. Leyton, E. R. C. Reynolds and F. B. Thompson, Rainfall interception in forest and moorland. In : Forest Hydrology. Proc. of a Nat. Sci. Found., Adv. Sci. Seminar, Penn. State Univ., August, 1965 (Pergamon Press, 1967)
Appendix G r i d r e f e r e n c e s ( B r i t i s h N a t i o n a l G r i d S y s t e m ) f o r t h e sites m e n t i o n e d in t h e p a p e r a r e l i s t e d : t h e n e a r e s t i d e n t i f y i n g t o w n o r v i l l a g e is a l s o g i v e n .
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RODNEY C. HILLS
Site code WfH(1) WfH(2) WfH(3) WfH(4) WfH(5) WfH(6) WfH(7) WfH(8) WfH(9) WfL(l) WfL(2) WfL(3) WfL(4) WfL(5) Ea(l) Ea(2) Ea(3) H(1) H(2) Ni(l ) Ni(2) Ni(3) Sy(1) Sy(2) Sy(3) Cb(l) Cb(2) Cb(3) Cb(4)
Location Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long Ashton Long A s h t o n Long A s h t o n Long A s h t o n Long A s h t o n Long Ashton Long Ashton Long Ashton Long Ashton Long A s h t o n Long Ashton Long Ashton Winterbourne Winterbourne Winterbourne Long Ashton Long Ashton Long Ashton Wickwar Wickwar Wickwar Wickwar
Grid Reference ST/534/697 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/534/696 ST/539/697 ST/539/697 ST/538/697 ST/538/697 ST/538/697 ST/539/694 ST/540/694 ST/540/693 ST/538/693 ST/538/693 ST/651/813 ST/651/813 ST/651/813 ST/543/694 ST/543/694 ST/543/694 ST/742/876 ST/742/876 ST/754/882 ST/745/882
The site descriptions need some amplification with regard to treatment. Sites listed as woodland, orchard, pasture and cultivated are self descriptive. However, those described as "Simazine" or " D i u r o n " sites or perhaps (as WfH(9)) as "orchard, bare (chemical)" need expansion. On these plots the herbicides Simazine and Diuron were being applied at various intensities to produce a vegetation free surface which enabled easy movement of machinery and personnel. Two kinds of experiments were in progress, one where the aim was simply to study persistence of the herbicide with no maintained vegetation cover (these are simply labelled "Simazine" or " D i u r o n " in the paper) and those where the herbicides were being used among a field crop (labelled either "orchard" or "cultivated, bare (chemical)"). The treatments applied to these plots were as follows: WfH(5) Control plot untouched for 4 years.
THE INFLUENCE OF LAND MANAGEMENT AND SOIL CHARACTERISTICS
WfH(6) WfH(7) WfH(8) WfH(9) WfL(3)
181
Treated with Simazine at 2½ lb/acre annually from 1963-67. Treated with Simazine at 20 lb/acre in 1963 and 1964. Orchard plot treated with Diuron at 6 lb/acre from 1960-67. As WfH(8). Blackcurrant bushes, treated with Simazine twice annually at ¼ lb/acre for 4 years. WfL(4) As WfL(3). All the infiltration tests were carried out between January, 1966, and March, 1968. All the plots listed above were maintained and used by members of the staff at the Long Ashton Research Station and infiltration tests were carried out on them with their permission. These treatments sustain large areas of bare ground which are very susceptible to rainbeat, vehicle compaction, overland flow and soil erosion. These effects have been described more fully in ref. 8.