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The effect of mole drainage on the hydrological response of a swelling clay soil
Journal of Hydrology, 64 (1983) 205--223 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
205
[4] T H E E F F E C T O F M O L E D R A I N A G E ON T H E H Y D R O L O G I C A L RESPONSE OF A SWELLING CLAY SOIL
MARK ROBINSON and KEITH J. BEVEN Institute of Hydrology, Wallingford, Oxon OXI O 8BB (Great Britain)
(Received July 16, 1982; accepted for publication August 16, 1982)
ABSTRACT Robinson, M. and Beven, K.J., 1983. The effect of mole drainage on the hydrological response of a swelling clay soil. J. Hydrol., 64: 205--223. The results of a plot experiment to determine the effects of mole drainage on the hydrological response of a swelling clay soil under pasture are described. It is shown that the response is very dependent on antecedent moisture conditions, and that higher peak flows may be generated under relatively dry conditions on the (drier) drained plot. This appears to be related to the generation of flows in interpedal macropores supplying water to the mole drains. Under wet conditions, the hydraulic efficiency of the macropores is reduced by swelling of the clay and surface saturation develops on the undrained plot. This results in generally higher peak discharges than from the drained plot. Recession discharges and total water yields are almost always higher from the drained plot. The implications of this work are that it may be very difficult to detect or model the influence of clay land drainage on river flows.
INTRODUCTION In a r e c e n t review o f field drainage (Green, 1 9 7 9 , 1 9 8 0 ) it was s h o w n t h a t Britain is o n e o f t h e m o s t extensively d r a i n e d c o u n t r i e s in E u r o p e . T h e c u r r e n t rate o f land being drained each y e a r in E n g l a n d and Wales is 10 s ha. T h e m a j o r i t y o f this w o r k is carried o u t on soils with a high p r o p o r t i o n o f clay and low h y d r a u l i c c o n d u c t i v i t i e s leading to p r o b l e m s o f w a t e r l o g g i n g in w i n t e r ( R o b s o n a n d T h o m a s s o n , 1 9 7 7 ) . Drainage in Britain dates b a c k at least t o t h e Middle Ages, b u t field u n d e r d r a i n a g e o n l y b e c a m e significant in t h e 1 9 t h c e n t u r y . A l t h o u g h it is generally a c c e p t e d t h a t c h a n n e l i m p r o v e m e n t s increase flow peaks, t h e r e has been c o n s i d e r a b l e c o n t r o v e r s y regarding the e f f e c t o f field drainage o n river flows. A p a p e r a n d s u b s e q u e n t discussion at t h e I n s t i t u t i o n o f Civil Engineers in L o n d o n in 1861 s h o w e d t h a t s t r o n g views were held even t h e n with t h e p a r t i c i p a n t s arguing w h e t h e r drainage w o u l d increase t h e
0022-1694/83/$03.00
© 1983 Elsevier Science Publishers B.V.
206 rate of r u n o f f in storm periods, and hence the magnitude of flooding, or if the increased moisture storage available in the soil would reduce peak rates of flow (Bailey Denton, 1862). The controversy continues t o d a y with claims both that drainage increases flood risk (e.g., Howe et al., 1966; Ward, 1975) and reduces peak flows (e.g., Kendall, 1950; Nicholson, 1953). A report for the Ministry of Agriculture found it impossible to reach any definite conclusions due to the lack of observational data (M.A.F., 1951). The debate is of importance due to the possibility of changes in the incidence of flooding and also has implications for our ability to model catchm e n t behaviour, both in terms of flood estimation techniques for engineering design and in conceptual models of water flow in and on the soil. Drainage problems can usually be divided into two broad classes: "groundwater soils" which are reasonably permeable, but suffer waterlogging due to rising groundwater, and "surface water soils" which include many clays with low hydraulic conductivities and restricted water movement (e.g., Nicholson, 1953). Artificial drainage of groundwater soils may lower the water table and provide a significant available moisture store, but any such increase in clay soils will be limited. It has therefore been argued (Trafford, 1973; Bailey and Bree, 1981) that drainage of groundwater soils will tend to reduce peak discharges, while for surface-water soils the effect is uncertain. A number of field studies have indicated that intensive drainage of clay soils using mole drains can give a very peaky r u n o f f response, and (Trafford, 1973): "it seems hard to avoid the suspicion that drainage systems giving such a peaky runoff may well affect the river flows when significant parts of the catchment are so drained" Trafford suggested the winter period December--March (when the ground was wettest) was the time when the risk of increased flooding was greatest. PLOT INVESTIGATION To study the effect of mole drainage on the hydrological response of a swelling clay soil, and possible implications for arterial river flows, a small field drainage experiment was established on a lowland clay site in the Institute of Hydrology's River Ray experimental catchment near Grendon Underwood, Oxfordshire (51°52'N, 0°59'W, Fig. 1). The River Ray catchm e n t has been extensively drained over a period of ~ 30 years [Fig. 2, based on grant aid data supplied by the Ministry of Agriculture, Fisheries and F o o d ( M A F F ) ] . The type of drainage comprises mostly moles over pipe drains and although m a n y early schemes were of poor standards and not well maintained, later schemes have been carried out to good standards. The experimental site was on permanent pasture which had not previously been drained, and the land had not been ploughed in at least the last 60 years. Normal farming practices of grazing by cattle and growing hay were continued during this investigation. The soil is a tenacious clay formed
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on Oxford Clay and has been classified (Avery, 1973) as Evesham. This is a gleyed calcareous soil and its main drainage problem is surface ponding and waterlogging in upper horizons due to the low hydraulic conductivity of the topsoil and subsurface clay (e.g., Jarvis, 1973). The clay shrinks and cracks in dry summer months making it more permeable, but the cracks close up as the soils are wetted in a u t u m n and winter. The root zone is ~ 15--25 cm in depth. The chosen site has pronounced ridge and furrow topography c o m m o n in parts of southern Britain (e.g., Green, 1975). This was a c o m m o n traditional land management practice on poorly drained clay land and involved ploughing the soil into longitudinal ridges to provide improved local drainage. At the study site the ridges are 7--10 m apart, and are aligned directly downslope. An early study of land under such relief suggested it would control surface layer flow, with each ridge forming an effective barrier between furrows, and reducing plot "edge effects" (Childs, 1943). Two plots, each ~ 2 5 0 0 m 2 in area, were installed and instrumented in summer 1977 (Fig. 3). One remained undrained, while the other was moledrained along the ridge and furrows (at 45 cm depth and 2 m spacing). The upslope boundary to the plots was defined by a tile drain backfilled with gravel. Total flow was collected at the downslope end of each plot by a tile
209
drain backfilled with gravel (90 cm deep drain for the drained pl ot and 30 cm deep for the undrained plot). Each drain was led to a thin plate V-notch weir with 1:1 stage r e c or der and discharged into an existing ditch. Discharge was calculated using a standard formula (British Standards, 1965) which was verified by manual volumetric checks at low flows. In the first year of oper a t i on rainfall records were used from a m et eorological station run by the Institute of H ydrol ogy, 1.5 km to the west. In the following year a tipping-bucket raingauge was installed and a water-level sensor (Strangeways and Templeman, 1974) was added to each weir box, supplementing the chart recorders already in use. The water-level sensors and raingauge were c o n n e c t e d to a single data logger with synchronized recording at 5-min. intervals. Measurement of rainfall and flow at the site was restricted to t he winter m ont hs (October--April) since it has been claimed (e.g. Smith and Trafford, 1976) t h a t artificial drains rarely flowed in summer, and winter had been suggested as the main period of potential risk of increased flooding (Trafford, 1973). Breaks in the flow records occurred as a result of freezing of the weir boxes on occasions. A m ore serious problem limited to the first winter of 1 9 7 7 / 1 9 7 8 was backing up of water in the outfall ditch above the level of the plot weirs. A crest gauge installed in the ditch indicated that this effect was n o t always readily apparent in the recorded stage levels. This problem was, however, eliminated (in summer 1978) when the ditch was regraded.
SOIL MOISTURE M E A S U R E M E N T S
Soil moisture status on the drained and undrained plots was assessed using t e n s i o m e t e r and n e u t r o n pr obe measurements. The m e r c u r y m a n o m e t e r tensiometers were located in three groups: one consisting of four profiles on a ridge to f u rr ow slope in the drained site at depths of 5, 15, 25, 50 and 100 cm; one on a similar slope central to the undrained site with tensiometers at 15 and 100 cm; and one group of 100-cm tensiometers in a line parallel to the mole drains on the drained site. F o u r n e u t r o n probe access tubes were installed close to the banks of tensiometers on the drained site, and t w o more on the undrained site (Fig. 3). It was in ten de d to use the tensiometers at 1 0 0 c m depth in place of piezometers to indicate the dept h of the water table. Since tensiometers require only a small water flow to indicate a change in water potential, t h e y would be e x p e c t e d to give a b e t t e r indication of changes in the level o f saturation in this clay soil (see, e.g., Bouma et al., 1980). In fact, t he t e n s i o m e t e r measurements were of limited utility in this study. The f r e q u e n t occurrence o f frost in winter required r e peat ed purging of air from the system despite the use of protective insulation. Following purging it was shown that at depths below the r o o t zone, up to 48 hr. were necessary for potentials to equilibrate (Beven, 1980). In summer, potentials were generally
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below the measurement range o f the tensiometer cups ( ~ -- 80 kPa). The potential measurements that were obtained showed that on the drained site, the water table level indicated by the nearest tensiometer showing a positive pressure, generally rose over the winter period, but rarely penetrated the topsoil (Fig. 4a) and that there was a d o w n s l o p e gradient o f the water table towards the tile drain (Fig. 4b). These measurements
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must, however, be interpreted with care. Closer examination of the individual patterns of hydraulic head on each day suggested complex flow patterns with irregular (and sometimes reversed) hydraulic gradients both above and below the water table, even in winter with negligible evapotranspiration. It is felt that the tensiometers did not give a good indication of the flow field in this heavy clay soft. While the tensiometers may have given a good indication of local intra-pedal potentials, other evidence suggests that, certainly under storm conditions, the major flow pathways were interpedal fissures (see also Bouma et al., 1980). This point will be discussed further in a later section. The neutron probe measurements showed that the ridge sites were always drier than the furrow sites on both plots, and that the drained sites tended to be drier than the equivalent undrained sites (Fig. 5). The differences were greatest in the upper 30 cm of soil, with the clay below 50 cm on both sites tending towards constant moisture content throughout the winter. The annual range in total moisture content over the 10--95 cm depth range of the measurements was of the order of 150 mm of water (Fig. 5). The most obvious effect of the mole drains on soil moisture was in the observed patterns of surface saturation. This is shown, for example, in Fig. 6, a red-filtered aerial photograph of the plots taken on March 7, 1978. Areas of surface saturation and standing water show as dark tones and reveal the ridge and furrow topography quite clearly. The drained site and the mole
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213 surface. The difference between the plots was confirmed over a complete winter period by crest stage gauge measurements of m a x i m u m depth of surface water on both plots. No surface water was detected on the drained plot, while depths of up to 4 cm were measured in the furrows of the undrained plot. Surface flow detectors suggested that the occurrence of surface flow was related to complete saturation of the soil rather than to an "infiltration excess" mechanism (Beven, 1980). Surface flow was rare on the drained site.
WATER BALANCE DATA Breaks in the discharge data prevented a comparison of the overall water balance for the two plots, but balances could be obtained for periods beginning and ending with similar flow rates, and examples of such calculations are given in Table I. Data from the Institute of Hydrology's automatic weather station (Fig. 1) were used to derive Penman--Monteith estimates of potential evaporation. Actual evapotranspiration was then estimated using a simple moisture extraction model based on the root constant concept (Penman, 1949; Grindley, 1970). The root constant specifies a certain a m o u n t of soil moisture that can be extracted by plants w i t h o u t difficulty, so t h a t evapotranspiration is assumed to occur at the potential rate. Further moisture can only be extracted with increasing difficulty, and actual evapotranspiration is assumed to decrease with moisture deficit as a linear proportion of the potential rate until a specified m a x i m u m water capacity has been exhausted. Values of 50 mm r o o t constant and 125 mm available water capacity were used as reco m m e n d e d in Gardner (1981). Soil moisture values were obtained by neutron probe measurements in the plots. The indicated errors in the water balances are reasonably small given that t h e y are derived from measurements or estimates of four components, each subject to error. This supports the assumption that boundary errors were small and the measured plot discharges were representative of the hydrological response of the plots. Total volume of outflow from the drained plot was generally greater than from the undrained plot, primarily due to higher recession discharges. Seasonal changes in the relative water yields were apparent: in a u t u m n the {wetter) undrained plot at first gave a higher yield, but shortly after soil moisture deficits (SMD) had become zero, the drained plot had a higher yield (Fig. 7).
STORM HYDROGRAPHS Outflow hydrographs from the two plots are shown for a n u m b e r of storms in Fig. 8. There was no consistent relation between the peak discharges of the plots, with sometimes one plot yielding the higher storm
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peak and sometimes the other. Peak storm discharges for 39 storms are plotted in Fig. 9 and show a wide degree of scatter, although, there is an apparent t e n d e n c y for the undrained plot to give the higher peak for the smaller events, and the drained plot the higher peak for larger events. The variable pattern of peak storm r u n o f f from the plots indicates the complex pattern of processes operating, and the need perhaps to consider factors such as storm rainfall and ground state in attempting to interpret the observed behaviour. Rainfall volume and peak intensity were, however, found to have little correlation with the observed scatter of data points. To obtain a comparable index of soil moisture status for all storms (the neutron probe measurements were only available from 1977--79), soil moisture deficit (SMD) prior to each storm was calculated from the measured rainfall and estimated actual evapotranspiration. Notionally, SMD is the depth of water required to bring a soil to "field capacity". The SMD concept is clearly simplistic (see, e.g., the discussion in Gardner, 1981) and we recognize that moisture and actual evapotranspiration conditions will not be identical on the two plots. However, it serves here merely as a useful index of moisture status. A limitation of the SMD calculation scheme adopted here is that it assumes that excess water when SMD ~ 0 will drain away within a day. As a consequence small non-zero values of SMD can persist during rainless periods in winter. To avoid this problem an arbitrary figure of 5 mm was used to distinguish between storms on wet and dry ground. In fact it was found that storms could broadly be divided into those on ground at or above field capacity (SMD ~ 5 m m ) , when the undrained plot gave the higher storm discharge peak, and those on dry ground ( S M D ) 5 mm), when peak outflows from
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the moled plot were greater (Fig. 9). Exceptions included small events (generally with peak flows < 0.2 mm hr.- l ) for which the (wetter) undrained plot gave the higher maximum discharge irrespective of the SMD, together with two storms on ground estimated to be at field capacity, when the drained plot gave the higher peak. The first resulted from very low intensity rainfall (Fig. 8a) and closely followed a small storm for which the undrained plot peak was higher, and the second was a storm (Fig. 8d) that occurred when the SMD had reached zero only two days before.
W A T E R M O V E M E N T IN C L A Y S O I L S
Although the drained plot is drier than the undrained (based on the evidence of its higher yield, the tensiometer and neutron probe readings, and the absence of standing water), it can still yield comparable and even higher discharges and this provokes speculation on the nature of the movement of water in clay soils. In this respect the storm shown in Fig. 8d is of particular interest since it represents an extreme departure from the general broad similarity of response. Very intense rainfall (15 mm hr.- 1 ) resulted in a rapid response from
218 the drained plot, but n o t the undrained. The lack of r u n o f f from the undrained plot argues against overland flow as the main mechanism on the drained plot, and suggests t h a t rapid subsurface flow was occuring. This could n o t take place through the soil matrix, due to its low permeability, and could only have occurred by means of rapid flow through cracks and fissures in the soil. Evidence for such flow through clay soils has come primarily from the use of dyes to trace actual flow paths (e.g., Ritchie et al., 1972; Bouma and Dekker, 1978), but there is ample evidence for the importance of such macropores in water movements in field soils (see Beven and Germann, 1982). A dye tracing experiment was carried out on the drained plot in May 1979. Several doses of Rhodamine® dye were sprinkled onto the soil surface. A pit was dug to observe the path of the dye and it was noted there was general infiltration in the top 5 cm of the soil, but below this all movement of dye was confined to cracks between soil peds and earthworm channels. Such macropores were f o u n d to extend into the compact subsoil at over 50 cm. On one occasion dye was observed in a mole drain only 100 s after application at the surface, although previous applications might have established a network of water and dye-filled fissures from which water could have been displaced into the mole. Such a network of natural and artificial macropores could account for the very rapid response of the drained plot observed in Fig. 8d. The intense rain may have generated flow in the macropores down to the mole drains. On the undrained plot the absence of mole drains to provide a rapid link between the fissures and the lower collecting drain precluded such a rapid response of appreciable amounts of water. The higher moisture levels on the undrained plot would also tend to lead to earlier closing up of existing cracks than on the drained plot where the moles drain the fissures between storms and may help maintain the pedal soil structure. Most flow in the undrained plot presumably occurred as lateral flow in the upper, more permeable soil horizons (e.g., Trafford and Rycroft, 1973). Some water in the undrained plot may have found its way directly to the collecting tile drain, enabling its discharge to increase rapidly, but it was very limited in quantity (Fig. 8d). The limited extent of such a flow independent of mole drains helps explain the persistence of standing water on the undrained plot in winter. The crack cut by the tine of the mole plough remained visible t h r o u g h o u t the following winter and may have helped account for the higher flow from the drained plot in wet conditions shown in Fig. 8a. The importance of mechanically induced fissures in promoting a fast flow response to rainfall in mole drains has been demonstrated by Leeds--Harrison et al. (1982). It is suggested t h a t the following description of water movement is applicable to these plots. When the soil is dry (high SMD) both plots have significant storage available. On the drained plot, dependent on rainfall intensities and durations, water m a y be transmitted rapidly through macropores to the mole drains and consequently produce a very rapid response at the outflow.
219
The undrained plot would behave similarly, or by more general lateral flow in the topsoil layer, but the zone of influence of the collection system is limited by the absence of mole drains. However, w i t h o u t significant generation of macropore flows (small storms or very high deficits), the generally greater storage available in the drained plot may result in lower peak flows (see the wetting sequence of Fig. 8b). The dependence of peak flows on antecedent storage and rainfall intensities is also demonstrated by Fig. 8c and d. As the soil reaches "field capacity" and later water is standing in the furrows of the undrained plot, the clay will tend to swell and the cracks will start to close. They may not close completely on the drained plot, even after prolonged wetting, due to the action of the drains in removing excess water. However, there is a tendency for the discharge from the undrained plot to become less peaky. The average lag between the centroid of the rainfall and peak discharge for the drained plot was 1.5 hr. longer under wet conditions (with SMD ~< 5) than for SMD > 5. The major mechanisms of flow in the undrained plot under wet conditions will be lateral subsurface flow in the topsoil layer and surface flow in the saturated furrows. The result is generally higher peak discharges from the undrained plot when SMD ~< 5 (Figs. 8e, 8f and 9). Some data on the hydraulic characteristics of the soil at the site have been reported by Germann and Beven (1981). Their results suggest that macroporosity (small fissures and earthworm channels) may have a very important effect on flow rate even in a wetted sample. The apparent hydraulic conductivity of a large (25 cm diameter) core taken within the A--G0 horizon transition (20--35 cm depth) decreased from 1.3" 10 -4 cm s -1 at saturation, to 3 . 0 " 1 0 -s c m s -1 at a tension of ~ 0 . 5 k P a (drained porosity 0.045 cm 3 cm -3) to 4 . 0 " 1 0 -7 c m s -1 at a tension of 5 k P a (drained porosity 0 . 0 6 c m 3 cm-3 ).
D E T E R I O R A T I O N OF MOLE DRAINS
Mole drains are known to deteriorate over time, and the preceding interpretation of the plot data assumes the mole drains continued to function adequately over the 5-yr. study period. The life expectancy of mole drains ranges from a couple of years to over 10, and depends not only on soil type, but also on ground wetness at the time the moles are drawn, an SMD of at least 50 mm being recommended for successful mole drainage (Smith and Trafford, 1976). The performance of the mole drains was assessed by reference to a number of observations. Firstly, the difference in recession flows has already been noted as the most consistent difference between the plots. Storm discharge recessions from the drained plot were compared and there was no t e n d e n c y
220
for any change in rate of decline over the period. Secondly, the speed of response of storm flows was examined and there was found to be a small increase in "lag", defined as the time from the centroid of total rainfall to the hydrograph peak. The mean increase for the drained plot was ~ 0.57 hr. between 1977--1979 and 1980--1982, which was only slightly higher than the 0.45 hr. increase for the same storms on the undrained plot, and which was attributed largely to difference in storm profiles and antecedent conditions. Aerial photography in 1981 indicated that the drained plot did not have the standing water observed on adjacent undrained land. From the available evidence it would therefore appear the mole drains continued to function adequately over the study period, and although there may have been a small a m o u n t of deterioration, it was very limited. Evesham soils generally respond well to mole drainage, and properly formed mole drains in calcareous clays have a life expectancy of ~ 7 -10 yr. (Jarvis, 1973).
O B S E R V A T I O N S IN R E L A T I O N TO O T H E R S T U D I E S
It has been shown that the addition of mole drains above pipes in a clay soil gives higher peak rates of r u n o f f than from pipe drains alone (e.g., Trafford and Rycroft, 1973). Burke et al. (1974) compared r u n o f f from mole drained and undrained plots at Ballinamore, Ireland, and concluded that the hydrographs were very variable, though peak flows were in general higher from the moled plot. Interestingly, mole drainage appeared to delay the time of peak r u n o f f in that particular experiment. R y c r o f t and Massey (1975) analysed a number of events from Burke et al.'s study and while agreeing that the moled plot gave a later peak, paradoxically reached the opposite conclusion about peak flows, claiming the data indicated that they were lower from the mole drained plot. This disparity was noted by Bailey and Bree (1981) although t h e y could offer no explanation. The results from Grendon Underwood offer a possible reconciliation of these differing interpretations, since the hydrographs shown in Burke et al. (1974) were from summer events, while those examined by R y c r o f t and Massey (1975) were specifically confined to winter months, following Trafford's argument that winter months were the time of greatest risk of increased flooding. Cracking of the clays could account for higher mole drainage peaks in summer storms (Burke et al., 1974) while for winter storms, when structural cracks closed, the undrained plot could give higher peaks ( R y c r o f t and Massey, 1975). The data collected by Burke et al. have not been reworked in detail, but the soil at Ballinamore is known to be subject to some swelling and shrinkage, and it does appear that the mole flow peaks did exceed the undrained plot surface flow peaks when heavy rainfall occurred on dry ground in summer, and that the opposite occurred on wet ground in winter (W. Burke, pers. commun., 1982).
221 IMPLICATIONS OF FIELD DRAINAGE ON RIVER FLOWS Most drainage schemes are small in area --- 75% o f individual schemes in England and Wales being under 8 ha (Armstrong, 1978). Since only a small part of a given catchment would be drained in a particular year, and the installation of new drainage schemes would in part be balanced by the decay of old ones, Bailey and Bree (1981) have suggested that any effect on flood peaks would be unlikely. This is not necessarily correct, however, since it only means that it m a y be difficult to detect changes from year to year. The proportion of a catchment with artificial drainage may still be an important c a t c h m e n t parameter when, for example, comparing mean annual floods between catchments having differing proportions of drained land. Additionally, drainage work does n o t proceed at a constant rate, with large annual variations resulting from changes in factors such as the level of government grant available, and weather conditions (the rate of drainage work usually increases after a succession of very wet winters). This could give rise to large variations in the age distribution of the drainage schemes within an individual catchment. It may also take several years before the hydraulic properties of the soil stabilize after drainage (e.g., Nemec, 1976). The dependence of the differences in the hydrological response of the two plots on antecedent conditions that has been demonstrated in this study will make the detection and modelling of long-term changes due to drainage very difficult. Daffy rainfall and streamflow data for the years 1964--1975 were used to develop and test a lumped conceptual model for the River Ray (Eeles, 1978) and no evidence of a change in streamflow pattern due to drainage was found during that period (Eeles, pers. commun., 1980). Similar conclusions were reached by Beven (1980) from a study of m o n t h l y water balances and r u n o f f coefficients, and winter storm peaks, timing and r u n o f f coefficients based on hourly data.
CONCLUSIONS (1) Mole drainage of swelling clay soils may have a variable effect on hydrological response of an area depending on antecedent moisture status and, to a lesser extent, rainfall intensity. (2) If it is true that, as suggested here, discharge from the drained plots is critically dependent on the generation of macropore and fissure flows, and on the time-dependent hydraulic characteristics of those fissures, then modelling the effect of field drainage on river flows will be very difficult. Simple moisture accounting models that make no allowance for the bypassing of water through macropores in soils that are well below "field capacity", cannot be expected to model the demonstrated behaviour of the experimental plots.
222
(3) Mole drainage may be expected to reduce the magnitude of winter flood peaks, but may increase the magnitude of summer and autumn flood peaks before the soil has wetted up and the hydraulic efficiency of the macropores has been decreased by swelling of the clay.
ACKNOWLEDGEMENTS
Special thanks are due to William and John George who allowed the use of their land for the experiments. Thanks are also due to Liz Morris, Ken Blyth, Peter Germann and Rod Furnell who helped with the experimental work. This paper is published with the permission of the Director of the Institute of Hydrology.
REFERENCES Armstrong, A.C., 1978. A digest of drainage statistics. U.K. Min. Agric. Fish. Food, Field Drain Exp. Unit, Tech. Rep. 78/7. Avery, B.W., 1973. Soil classification in the Soil Survey of England and Wales. J. Soil Sci., 24: 324--338. Bailey Denton, J., 1862. On the discharge from under-drainage and its effects on the arterial channels and outfalls of the country. Proc. Inst. Civ. Eng., 21: 48--130. Bailey, A.D. and Bree, T., 1981. Effect of improved land drainage on river flows. In: Flood Studies Report -- five years on. Proc. Conf., Inst. Civ. Eng., Manchester, July 1980. Thomas Telford, London, 159 pp. Beven, K.J., 1980. The Grendon Underwood field drainage experiment. Inst. Hydrol., Wallingford, Rep. No. 65. Beven, K.J. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res. (in press). Bouma, J. and Dekker, L.W., 1978. A case study on infiltration into dry clay soil, 1. -Morphological observations. Geoderma, 20: 27--40. Bouma, J., Dekker, L.W. and Haans, J.C.F.M., 1980. Measurement of depth to water table on a heavy clay soil. Soil Sci., 130(5): 264--270. British Standards, 1965. Measurement of liquid flow in open channels. Br. Stand., BS 3680, Part 4A. Burke, W., Mulqueen, J. and Butler, P., 1974. Aspects of the hydrology of a gley on a drumlin. Irish J. Agric. Res., pp. 215--228. Childs, E.C., 1943. Studies in mole-draining -- interim report on an experimental drainage field. J. Agric. Sci., 33: 136--146. Eeles, C.W.O., 1978. A conceptual model of the estimation of historic flows. Inst. Hydrol., Wallingford, Rep. No. 55. Gardner, C.M.K. (Editor), 1981. The MORECS discussion meeting. Inst. Hydrol., Wallingford, Rep. No. 78. Germann, P. and Beven, K., 1981. Water flow in soil macropores, I. An experimental approach. J. Soil Sci., 32: 1--13. Green, F.H.W., 1975. Ridge and furrow, mole and tile. Geogr. J., 141 : 88--93.
223
Green, F.H.W., 1979. Field drainage in Europe: a quantitative survey. Inst. Hydrol., Wallingford, Rep. No. 57. Green, F.H.W., 1980. Current field drainage in northern and western Europe. J. Environ. Manage., 10: 149--53. Grindley, J., 1970. Estimation and mapping of evaporation. Symp. on World Water Balance, Int. Assoc. Hydrol. Sci., Publ. No. 92, pp. 200--213. Howe, G.M., Slaymaker, H.O. and Harding, D.M., 1966. Some aspects of the flood hydrology of the upper catchments of the Severn and Wye. Trans. Inst. Br. Geogr., 41: 33--58. Jarvis, M.G., 1973. Soils of the Wantage and Abingdon district. Mem. Soil Surv., G.B., Harpenden, 200 pp. Kendall, R.G., 1950. Land Drainage. Faber and Faber, London, 133 pp. M.A.F. (Ministry of Agriculture and Fisheries), 1951. Land drainage in England and Wales. Rep. Land Drain. Legislation Sub-Comm., H.M.S.O., London. Leeds-Harrison, P., Spoor, G. and Godwin, R.J., 1982. Water flow to mole drains. J. Agric. Eng. Res., 27: 81--91. Nemec, J., 1976. Time variability of hydraulic conductivity in heavy clay soils. Proc. Bratislava Symp. on Water in Heavy Clay Soils, 2: 81--92. Nicholson, H.H., 1953. The Principles of Land Drainage. Cambridge University Press, London, 165 pp. Penman, H.L., 1949. The dependence of transpiration on weather and soil conditions. J. Soil Sci., 1 : 74--89. Ritchie, J.T., Kissel, D.E. and Burnett, E., 1972. Water movement in undisturbed swelling clay soils. Soil Sci. Soc. Am. Proc., 36: 874--879. Robson, J.D. and Thomasson, A.J., 1977. Soil water regimes. Soil Surv. England--Wales, Harpenden, Tech. Monogr. No. 11. Rycroft, D.W. and Massey, W., 1975. The effect of field drainage on river flow. Ministry of Agriculture, Fisheries and Food. Field Drain. Exp. Unit, Tech. Bull. 75/9. Smith, L.P and Trafford, B.D., 1976. Climate and drainage. U.K. Min. Agric., Fish. Food, H.M.S.O., London, Tech. Bull. 3 4 , 1 1 9 pp. Strangeways. I.C. and Templeman, R.F., 1974. Logging river level on magnetic tape. Water Serv., 78: 57--60. Trafford, B.D., 1973. The relationship between field drainage and arterial drainage -theoretical aspects. U.K. Min. Agric. Fish. Food, Field Drain. Exp. Unit, Tech. Bull. 73/10. Trafford, B.D. and Rycroft, D.W., 1973. Observations on the soil water regimes in a drained clay soil J. Soil. Sci., 24: 380--391. Ward, R.C., 1975. Principles of Hydrology. McGraw-Hill, London, 2nd ed., 367 pp.
205
[4] T H E E F F E C T O F M O L E D R A I N A G E ON T H E H Y D R O L O G I C A L RESPONSE OF A SWELLING CLAY SOIL
MARK ROBINSON and KEITH J. BEVEN Institute of Hydrology, Wallingford, Oxon OXI O 8BB (Great Britain)
(Received July 16, 1982; accepted for publication August 16, 1982)
ABSTRACT Robinson, M. and Beven, K.J., 1983. The effect of mole drainage on the hydrological response of a swelling clay soil. J. Hydrol., 64: 205--223. The results of a plot experiment to determine the effects of mole drainage on the hydrological response of a swelling clay soil under pasture are described. It is shown that the response is very dependent on antecedent moisture conditions, and that higher peak flows may be generated under relatively dry conditions on the (drier) drained plot. This appears to be related to the generation of flows in interpedal macropores supplying water to the mole drains. Under wet conditions, the hydraulic efficiency of the macropores is reduced by swelling of the clay and surface saturation develops on the undrained plot. This results in generally higher peak discharges than from the drained plot. Recession discharges and total water yields are almost always higher from the drained plot. The implications of this work are that it may be very difficult to detect or model the influence of clay land drainage on river flows.
INTRODUCTION In a r e c e n t review o f field drainage (Green, 1 9 7 9 , 1 9 8 0 ) it was s h o w n t h a t Britain is o n e o f t h e m o s t extensively d r a i n e d c o u n t r i e s in E u r o p e . T h e c u r r e n t rate o f land being drained each y e a r in E n g l a n d and Wales is 10 s ha. T h e m a j o r i t y o f this w o r k is carried o u t on soils with a high p r o p o r t i o n o f clay and low h y d r a u l i c c o n d u c t i v i t i e s leading to p r o b l e m s o f w a t e r l o g g i n g in w i n t e r ( R o b s o n a n d T h o m a s s o n , 1 9 7 7 ) . Drainage in Britain dates b a c k at least t o t h e Middle Ages, b u t field u n d e r d r a i n a g e o n l y b e c a m e significant in t h e 1 9 t h c e n t u r y . A l t h o u g h it is generally a c c e p t e d t h a t c h a n n e l i m p r o v e m e n t s increase flow peaks, t h e r e has been c o n s i d e r a b l e c o n t r o v e r s y regarding the e f f e c t o f field drainage o n river flows. A p a p e r a n d s u b s e q u e n t discussion at t h e I n s t i t u t i o n o f Civil Engineers in L o n d o n in 1861 s h o w e d t h a t s t r o n g views were held even t h e n with t h e p a r t i c i p a n t s arguing w h e t h e r drainage w o u l d increase t h e
0022-1694/83/$03.00
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206 rate of r u n o f f in storm periods, and hence the magnitude of flooding, or if the increased moisture storage available in the soil would reduce peak rates of flow (Bailey Denton, 1862). The controversy continues t o d a y with claims both that drainage increases flood risk (e.g., Howe et al., 1966; Ward, 1975) and reduces peak flows (e.g., Kendall, 1950; Nicholson, 1953). A report for the Ministry of Agriculture found it impossible to reach any definite conclusions due to the lack of observational data (M.A.F., 1951). The debate is of importance due to the possibility of changes in the incidence of flooding and also has implications for our ability to model catchm e n t behaviour, both in terms of flood estimation techniques for engineering design and in conceptual models of water flow in and on the soil. Drainage problems can usually be divided into two broad classes: "groundwater soils" which are reasonably permeable, but suffer waterlogging due to rising groundwater, and "surface water soils" which include many clays with low hydraulic conductivities and restricted water movement (e.g., Nicholson, 1953). Artificial drainage of groundwater soils may lower the water table and provide a significant available moisture store, but any such increase in clay soils will be limited. It has therefore been argued (Trafford, 1973; Bailey and Bree, 1981) that drainage of groundwater soils will tend to reduce peak discharges, while for surface-water soils the effect is uncertain. A number of field studies have indicated that intensive drainage of clay soils using mole drains can give a very peaky r u n o f f response, and (Trafford, 1973): "it seems hard to avoid the suspicion that drainage systems giving such a peaky runoff may well affect the river flows when significant parts of the catchment are so drained" Trafford suggested the winter period December--March (when the ground was wettest) was the time when the risk of increased flooding was greatest. PLOT INVESTIGATION To study the effect of mole drainage on the hydrological response of a swelling clay soil, and possible implications for arterial river flows, a small field drainage experiment was established on a lowland clay site in the Institute of Hydrology's River Ray experimental catchment near Grendon Underwood, Oxfordshire (51°52'N, 0°59'W, Fig. 1). The River Ray catchm e n t has been extensively drained over a period of ~ 30 years [Fig. 2, based on grant aid data supplied by the Ministry of Agriculture, Fisheries and F o o d ( M A F F ) ] . The type of drainage comprises mostly moles over pipe drains and although m a n y early schemes were of poor standards and not well maintained, later schemes have been carried out to good standards. The experimental site was on permanent pasture which had not previously been drained, and the land had not been ploughed in at least the last 60 years. Normal farming practices of grazing by cattle and growing hay were continued during this investigation. The soil is a tenacious clay formed
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on Oxford Clay and has been classified (Avery, 1973) as Evesham. This is a gleyed calcareous soil and its main drainage problem is surface ponding and waterlogging in upper horizons due to the low hydraulic conductivity of the topsoil and subsurface clay (e.g., Jarvis, 1973). The clay shrinks and cracks in dry summer months making it more permeable, but the cracks close up as the soils are wetted in a u t u m n and winter. The root zone is ~ 15--25 cm in depth. The chosen site has pronounced ridge and furrow topography c o m m o n in parts of southern Britain (e.g., Green, 1975). This was a c o m m o n traditional land management practice on poorly drained clay land and involved ploughing the soil into longitudinal ridges to provide improved local drainage. At the study site the ridges are 7--10 m apart, and are aligned directly downslope. An early study of land under such relief suggested it would control surface layer flow, with each ridge forming an effective barrier between furrows, and reducing plot "edge effects" (Childs, 1943). Two plots, each ~ 2 5 0 0 m 2 in area, were installed and instrumented in summer 1977 (Fig. 3). One remained undrained, while the other was moledrained along the ridge and furrows (at 45 cm depth and 2 m spacing). The upslope boundary to the plots was defined by a tile drain backfilled with gravel. Total flow was collected at the downslope end of each plot by a tile
209
drain backfilled with gravel (90 cm deep drain for the drained pl ot and 30 cm deep for the undrained plot). Each drain was led to a thin plate V-notch weir with 1:1 stage r e c or der and discharged into an existing ditch. Discharge was calculated using a standard formula (British Standards, 1965) which was verified by manual volumetric checks at low flows. In the first year of oper a t i on rainfall records were used from a m et eorological station run by the Institute of H ydrol ogy, 1.5 km to the west. In the following year a tipping-bucket raingauge was installed and a water-level sensor (Strangeways and Templeman, 1974) was added to each weir box, supplementing the chart recorders already in use. The water-level sensors and raingauge were c o n n e c t e d to a single data logger with synchronized recording at 5-min. intervals. Measurement of rainfall and flow at the site was restricted to t he winter m ont hs (October--April) since it has been claimed (e.g. Smith and Trafford, 1976) t h a t artificial drains rarely flowed in summer, and winter had been suggested as the main period of potential risk of increased flooding (Trafford, 1973). Breaks in the flow records occurred as a result of freezing of the weir boxes on occasions. A m ore serious problem limited to the first winter of 1 9 7 7 / 1 9 7 8 was backing up of water in the outfall ditch above the level of the plot weirs. A crest gauge installed in the ditch indicated that this effect was n o t always readily apparent in the recorded stage levels. This problem was, however, eliminated (in summer 1978) when the ditch was regraded.
SOIL MOISTURE M E A S U R E M E N T S
Soil moisture status on the drained and undrained plots was assessed using t e n s i o m e t e r and n e u t r o n pr obe measurements. The m e r c u r y m a n o m e t e r tensiometers were located in three groups: one consisting of four profiles on a ridge to f u rr ow slope in the drained site at depths of 5, 15, 25, 50 and 100 cm; one on a similar slope central to the undrained site with tensiometers at 15 and 100 cm; and one group of 100-cm tensiometers in a line parallel to the mole drains on the drained site. F o u r n e u t r o n probe access tubes were installed close to the banks of tensiometers on the drained site, and t w o more on the undrained site (Fig. 3). It was in ten de d to use the tensiometers at 1 0 0 c m depth in place of piezometers to indicate the dept h of the water table. Since tensiometers require only a small water flow to indicate a change in water potential, t h e y would be e x p e c t e d to give a b e t t e r indication of changes in the level o f saturation in this clay soil (see, e.g., Bouma et al., 1980). In fact, t he t e n s i o m e t e r measurements were of limited utility in this study. The f r e q u e n t occurrence o f frost in winter required r e peat ed purging of air from the system despite the use of protective insulation. Following purging it was shown that at depths below the r o o t zone, up to 48 hr. were necessary for potentials to equilibrate (Beven, 1980). In summer, potentials were generally
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below the measurement range o f the tensiometer cups ( ~ -- 80 kPa). The potential measurements that were obtained showed that on the drained site, the water table level indicated by the nearest tensiometer showing a positive pressure, generally rose over the winter period, but rarely penetrated the topsoil (Fig. 4a) and that there was a d o w n s l o p e gradient o f the water table towards the tile drain (Fig. 4b). These measurements
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must, however, be interpreted with care. Closer examination of the individual patterns of hydraulic head on each day suggested complex flow patterns with irregular (and sometimes reversed) hydraulic gradients both above and below the water table, even in winter with negligible evapotranspiration. It is felt that the tensiometers did not give a good indication of the flow field in this heavy clay soft. While the tensiometers may have given a good indication of local intra-pedal potentials, other evidence suggests that, certainly under storm conditions, the major flow pathways were interpedal fissures (see also Bouma et al., 1980). This point will be discussed further in a later section. The neutron probe measurements showed that the ridge sites were always drier than the furrow sites on both plots, and that the drained sites tended to be drier than the equivalent undrained sites (Fig. 5). The differences were greatest in the upper 30 cm of soil, with the clay below 50 cm on both sites tending towards constant moisture content throughout the winter. The annual range in total moisture content over the 10--95 cm depth range of the measurements was of the order of 150 mm of water (Fig. 5). The most obvious effect of the mole drains on soil moisture was in the observed patterns of surface saturation. This is shown, for example, in Fig. 6, a red-filtered aerial photograph of the plots taken on March 7, 1978. Areas of surface saturation and standing water show as dark tones and reveal the ridge and furrow topography quite clearly. The drained site and the mole
212
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213 surface. The difference between the plots was confirmed over a complete winter period by crest stage gauge measurements of m a x i m u m depth of surface water on both plots. No surface water was detected on the drained plot, while depths of up to 4 cm were measured in the furrows of the undrained plot. Surface flow detectors suggested that the occurrence of surface flow was related to complete saturation of the soil rather than to an "infiltration excess" mechanism (Beven, 1980). Surface flow was rare on the drained site.
WATER BALANCE DATA Breaks in the discharge data prevented a comparison of the overall water balance for the two plots, but balances could be obtained for periods beginning and ending with similar flow rates, and examples of such calculations are given in Table I. Data from the Institute of Hydrology's automatic weather station (Fig. 1) were used to derive Penman--Monteith estimates of potential evaporation. Actual evapotranspiration was then estimated using a simple moisture extraction model based on the root constant concept (Penman, 1949; Grindley, 1970). The root constant specifies a certain a m o u n t of soil moisture that can be extracted by plants w i t h o u t difficulty, so t h a t evapotranspiration is assumed to occur at the potential rate. Further moisture can only be extracted with increasing difficulty, and actual evapotranspiration is assumed to decrease with moisture deficit as a linear proportion of the potential rate until a specified m a x i m u m water capacity has been exhausted. Values of 50 mm r o o t constant and 125 mm available water capacity were used as reco m m e n d e d in Gardner (1981). Soil moisture values were obtained by neutron probe measurements in the plots. The indicated errors in the water balances are reasonably small given that t h e y are derived from measurements or estimates of four components, each subject to error. This supports the assumption that boundary errors were small and the measured plot discharges were representative of the hydrological response of the plots. Total volume of outflow from the drained plot was generally greater than from the undrained plot, primarily due to higher recession discharges. Seasonal changes in the relative water yields were apparent: in a u t u m n the {wetter) undrained plot at first gave a higher yield, but shortly after soil moisture deficits (SMD) had become zero, the drained plot had a higher yield (Fig. 7).
STORM HYDROGRAPHS Outflow hydrographs from the two plots are shown for a n u m b e r of storms in Fig. 8. There was no consistent relation between the peak discharges of the plots, with sometimes one plot yielding the higher storm
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Fig. 7. Cumulative outflows from the drained (dashed line) and undrained (solid line) plots for: (a) autumn 1980; and (b)spring 1981.
peak and sometimes the other. Peak storm discharges for 39 storms are plotted in Fig. 9 and show a wide degree of scatter, although, there is an apparent t e n d e n c y for the undrained plot to give the higher peak for the smaller events, and the drained plot the higher peak for larger events. The variable pattern of peak storm r u n o f f from the plots indicates the complex pattern of processes operating, and the need perhaps to consider factors such as storm rainfall and ground state in attempting to interpret the observed behaviour. Rainfall volume and peak intensity were, however, found to have little correlation with the observed scatter of data points. To obtain a comparable index of soil moisture status for all storms (the neutron probe measurements were only available from 1977--79), soil moisture deficit (SMD) prior to each storm was calculated from the measured rainfall and estimated actual evapotranspiration. Notionally, SMD is the depth of water required to bring a soil to "field capacity". The SMD concept is clearly simplistic (see, e.g., the discussion in Gardner, 1981) and we recognize that moisture and actual evapotranspiration conditions will not be identical on the two plots. However, it serves here merely as a useful index of moisture status. A limitation of the SMD calculation scheme adopted here is that it assumes that excess water when SMD ~ 0 will drain away within a day. As a consequence small non-zero values of SMD can persist during rainless periods in winter. To avoid this problem an arbitrary figure of 5 mm was used to distinguish between storms on wet and dry ground. In fact it was found that storms could broadly be divided into those on ground at or above field capacity (SMD ~ 5 m m ) , when the undrained plot gave the higher storm discharge peak, and those on dry ground ( S M D ) 5 mm), when peak outflows from
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the moled plot were greater (Fig. 9). Exceptions included small events (generally with peak flows < 0.2 mm hr.- l ) for which the (wetter) undrained plot gave the higher maximum discharge irrespective of the SMD, together with two storms on ground estimated to be at field capacity, when the drained plot gave the higher peak. The first resulted from very low intensity rainfall (Fig. 8a) and closely followed a small storm for which the undrained plot peak was higher, and the second was a storm (Fig. 8d) that occurred when the SMD had reached zero only two days before.
W A T E R M O V E M E N T IN C L A Y S O I L S
Although the drained plot is drier than the undrained (based on the evidence of its higher yield, the tensiometer and neutron probe readings, and the absence of standing water), it can still yield comparable and even higher discharges and this provokes speculation on the nature of the movement of water in clay soils. In this respect the storm shown in Fig. 8d is of particular interest since it represents an extreme departure from the general broad similarity of response. Very intense rainfall (15 mm hr.- 1 ) resulted in a rapid response from
218 the drained plot, but n o t the undrained. The lack of r u n o f f from the undrained plot argues against overland flow as the main mechanism on the drained plot, and suggests t h a t rapid subsurface flow was occuring. This could n o t take place through the soil matrix, due to its low permeability, and could only have occurred by means of rapid flow through cracks and fissures in the soil. Evidence for such flow through clay soils has come primarily from the use of dyes to trace actual flow paths (e.g., Ritchie et al., 1972; Bouma and Dekker, 1978), but there is ample evidence for the importance of such macropores in water movements in field soils (see Beven and Germann, 1982). A dye tracing experiment was carried out on the drained plot in May 1979. Several doses of Rhodamine® dye were sprinkled onto the soil surface. A pit was dug to observe the path of the dye and it was noted there was general infiltration in the top 5 cm of the soil, but below this all movement of dye was confined to cracks between soil peds and earthworm channels. Such macropores were f o u n d to extend into the compact subsoil at over 50 cm. On one occasion dye was observed in a mole drain only 100 s after application at the surface, although previous applications might have established a network of water and dye-filled fissures from which water could have been displaced into the mole. Such a network of natural and artificial macropores could account for the very rapid response of the drained plot observed in Fig. 8d. The intense rain may have generated flow in the macropores down to the mole drains. On the undrained plot the absence of mole drains to provide a rapid link between the fissures and the lower collecting drain precluded such a rapid response of appreciable amounts of water. The higher moisture levels on the undrained plot would also tend to lead to earlier closing up of existing cracks than on the drained plot where the moles drain the fissures between storms and may help maintain the pedal soil structure. Most flow in the undrained plot presumably occurred as lateral flow in the upper, more permeable soil horizons (e.g., Trafford and Rycroft, 1973). Some water in the undrained plot may have found its way directly to the collecting tile drain, enabling its discharge to increase rapidly, but it was very limited in quantity (Fig. 8d). The limited extent of such a flow independent of mole drains helps explain the persistence of standing water on the undrained plot in winter. The crack cut by the tine of the mole plough remained visible t h r o u g h o u t the following winter and may have helped account for the higher flow from the drained plot in wet conditions shown in Fig. 8a. The importance of mechanically induced fissures in promoting a fast flow response to rainfall in mole drains has been demonstrated by Leeds--Harrison et al. (1982). It is suggested t h a t the following description of water movement is applicable to these plots. When the soil is dry (high SMD) both plots have significant storage available. On the drained plot, dependent on rainfall intensities and durations, water m a y be transmitted rapidly through macropores to the mole drains and consequently produce a very rapid response at the outflow.
219
The undrained plot would behave similarly, or by more general lateral flow in the topsoil layer, but the zone of influence of the collection system is limited by the absence of mole drains. However, w i t h o u t significant generation of macropore flows (small storms or very high deficits), the generally greater storage available in the drained plot may result in lower peak flows (see the wetting sequence of Fig. 8b). The dependence of peak flows on antecedent storage and rainfall intensities is also demonstrated by Fig. 8c and d. As the soil reaches "field capacity" and later water is standing in the furrows of the undrained plot, the clay will tend to swell and the cracks will start to close. They may not close completely on the drained plot, even after prolonged wetting, due to the action of the drains in removing excess water. However, there is a tendency for the discharge from the undrained plot to become less peaky. The average lag between the centroid of the rainfall and peak discharge for the drained plot was 1.5 hr. longer under wet conditions (with SMD ~< 5) than for SMD > 5. The major mechanisms of flow in the undrained plot under wet conditions will be lateral subsurface flow in the topsoil layer and surface flow in the saturated furrows. The result is generally higher peak discharges from the undrained plot when SMD ~< 5 (Figs. 8e, 8f and 9). Some data on the hydraulic characteristics of the soil at the site have been reported by Germann and Beven (1981). Their results suggest that macroporosity (small fissures and earthworm channels) may have a very important effect on flow rate even in a wetted sample. The apparent hydraulic conductivity of a large (25 cm diameter) core taken within the A--G0 horizon transition (20--35 cm depth) decreased from 1.3" 10 -4 cm s -1 at saturation, to 3 . 0 " 1 0 -s c m s -1 at a tension of ~ 0 . 5 k P a (drained porosity 0.045 cm 3 cm -3) to 4 . 0 " 1 0 -7 c m s -1 at a tension of 5 k P a (drained porosity 0 . 0 6 c m 3 cm-3 ).
D E T E R I O R A T I O N OF MOLE DRAINS
Mole drains are known to deteriorate over time, and the preceding interpretation of the plot data assumes the mole drains continued to function adequately over the 5-yr. study period. The life expectancy of mole drains ranges from a couple of years to over 10, and depends not only on soil type, but also on ground wetness at the time the moles are drawn, an SMD of at least 50 mm being recommended for successful mole drainage (Smith and Trafford, 1976). The performance of the mole drains was assessed by reference to a number of observations. Firstly, the difference in recession flows has already been noted as the most consistent difference between the plots. Storm discharge recessions from the drained plot were compared and there was no t e n d e n c y
220
for any change in rate of decline over the period. Secondly, the speed of response of storm flows was examined and there was found to be a small increase in "lag", defined as the time from the centroid of total rainfall to the hydrograph peak. The mean increase for the drained plot was ~ 0.57 hr. between 1977--1979 and 1980--1982, which was only slightly higher than the 0.45 hr. increase for the same storms on the undrained plot, and which was attributed largely to difference in storm profiles and antecedent conditions. Aerial photography in 1981 indicated that the drained plot did not have the standing water observed on adjacent undrained land. From the available evidence it would therefore appear the mole drains continued to function adequately over the study period, and although there may have been a small a m o u n t of deterioration, it was very limited. Evesham soils generally respond well to mole drainage, and properly formed mole drains in calcareous clays have a life expectancy of ~ 7 -10 yr. (Jarvis, 1973).
O B S E R V A T I O N S IN R E L A T I O N TO O T H E R S T U D I E S
It has been shown that the addition of mole drains above pipes in a clay soil gives higher peak rates of r u n o f f than from pipe drains alone (e.g., Trafford and Rycroft, 1973). Burke et al. (1974) compared r u n o f f from mole drained and undrained plots at Ballinamore, Ireland, and concluded that the hydrographs were very variable, though peak flows were in general higher from the moled plot. Interestingly, mole drainage appeared to delay the time of peak r u n o f f in that particular experiment. R y c r o f t and Massey (1975) analysed a number of events from Burke et al.'s study and while agreeing that the moled plot gave a later peak, paradoxically reached the opposite conclusion about peak flows, claiming the data indicated that they were lower from the mole drained plot. This disparity was noted by Bailey and Bree (1981) although t h e y could offer no explanation. The results from Grendon Underwood offer a possible reconciliation of these differing interpretations, since the hydrographs shown in Burke et al. (1974) were from summer events, while those examined by R y c r o f t and Massey (1975) were specifically confined to winter months, following Trafford's argument that winter months were the time of greatest risk of increased flooding. Cracking of the clays could account for higher mole drainage peaks in summer storms (Burke et al., 1974) while for winter storms, when structural cracks closed, the undrained plot could give higher peaks ( R y c r o f t and Massey, 1975). The data collected by Burke et al. have not been reworked in detail, but the soil at Ballinamore is known to be subject to some swelling and shrinkage, and it does appear that the mole flow peaks did exceed the undrained plot surface flow peaks when heavy rainfall occurred on dry ground in summer, and that the opposite occurred on wet ground in winter (W. Burke, pers. commun., 1982).
221 IMPLICATIONS OF FIELD DRAINAGE ON RIVER FLOWS Most drainage schemes are small in area --- 75% o f individual schemes in England and Wales being under 8 ha (Armstrong, 1978). Since only a small part of a given catchment would be drained in a particular year, and the installation of new drainage schemes would in part be balanced by the decay of old ones, Bailey and Bree (1981) have suggested that any effect on flood peaks would be unlikely. This is not necessarily correct, however, since it only means that it m a y be difficult to detect changes from year to year. The proportion of a catchment with artificial drainage may still be an important c a t c h m e n t parameter when, for example, comparing mean annual floods between catchments having differing proportions of drained land. Additionally, drainage work does n o t proceed at a constant rate, with large annual variations resulting from changes in factors such as the level of government grant available, and weather conditions (the rate of drainage work usually increases after a succession of very wet winters). This could give rise to large variations in the age distribution of the drainage schemes within an individual catchment. It may also take several years before the hydraulic properties of the soil stabilize after drainage (e.g., Nemec, 1976). The dependence of the differences in the hydrological response of the two plots on antecedent conditions that has been demonstrated in this study will make the detection and modelling of long-term changes due to drainage very difficult. Daffy rainfall and streamflow data for the years 1964--1975 were used to develop and test a lumped conceptual model for the River Ray (Eeles, 1978) and no evidence of a change in streamflow pattern due to drainage was found during that period (Eeles, pers. commun., 1980). Similar conclusions were reached by Beven (1980) from a study of m o n t h l y water balances and r u n o f f coefficients, and winter storm peaks, timing and r u n o f f coefficients based on hourly data.
CONCLUSIONS (1) Mole drainage of swelling clay soils may have a variable effect on hydrological response of an area depending on antecedent moisture status and, to a lesser extent, rainfall intensity. (2) If it is true that, as suggested here, discharge from the drained plots is critically dependent on the generation of macropore and fissure flows, and on the time-dependent hydraulic characteristics of those fissures, then modelling the effect of field drainage on river flows will be very difficult. Simple moisture accounting models that make no allowance for the bypassing of water through macropores in soils that are well below "field capacity", cannot be expected to model the demonstrated behaviour of the experimental plots.
222
(3) Mole drainage may be expected to reduce the magnitude of winter flood peaks, but may increase the magnitude of summer and autumn flood peaks before the soil has wetted up and the hydraulic efficiency of the macropores has been decreased by swelling of the clay.
ACKNOWLEDGEMENTS
Special thanks are due to William and John George who allowed the use of their land for the experiments. Thanks are also due to Liz Morris, Ken Blyth, Peter Germann and Rod Furnell who helped with the experimental work. This paper is published with the permission of the Director of the Institute of Hydrology.
REFERENCES Armstrong, A.C., 1978. A digest of drainage statistics. U.K. Min. Agric. Fish. Food, Field Drain Exp. Unit, Tech. Rep. 78/7. Avery, B.W., 1973. Soil classification in the Soil Survey of England and Wales. J. Soil Sci., 24: 324--338. Bailey Denton, J., 1862. On the discharge from under-drainage and its effects on the arterial channels and outfalls of the country. Proc. Inst. Civ. Eng., 21: 48--130. Bailey, A.D. and Bree, T., 1981. Effect of improved land drainage on river flows. In: Flood Studies Report -- five years on. Proc. Conf., Inst. Civ. Eng., Manchester, July 1980. Thomas Telford, London, 159 pp. Beven, K.J., 1980. The Grendon Underwood field drainage experiment. Inst. Hydrol., Wallingford, Rep. No. 65. Beven, K.J. and Germann, P., 1982. Macropores and water flow in soils. Water Resour. Res. (in press). Bouma, J. and Dekker, L.W., 1978. A case study on infiltration into dry clay soil, 1. -Morphological observations. Geoderma, 20: 27--40. Bouma, J., Dekker, L.W. and Haans, J.C.F.M., 1980. Measurement of depth to water table on a heavy clay soil. Soil Sci., 130(5): 264--270. British Standards, 1965. Measurement of liquid flow in open channels. Br. Stand., BS 3680, Part 4A. Burke, W., Mulqueen, J. and Butler, P., 1974. Aspects of the hydrology of a gley on a drumlin. Irish J. Agric. Res., pp. 215--228. Childs, E.C., 1943. Studies in mole-draining -- interim report on an experimental drainage field. J. Agric. Sci., 33: 136--146. Eeles, C.W.O., 1978. A conceptual model of the estimation of historic flows. Inst. Hydrol., Wallingford, Rep. No. 55. Gardner, C.M.K. (Editor), 1981. The MORECS discussion meeting. Inst. Hydrol., Wallingford, Rep. No. 78. Germann, P. and Beven, K., 1981. Water flow in soil macropores, I. An experimental approach. J. Soil Sci., 32: 1--13. Green, F.H.W., 1975. Ridge and furrow, mole and tile. Geogr. J., 141 : 88--93.
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Green, F.H.W., 1979. Field drainage in Europe: a quantitative survey. Inst. Hydrol., Wallingford, Rep. No. 57. Green, F.H.W., 1980. Current field drainage in northern and western Europe. J. Environ. Manage., 10: 149--53. Grindley, J., 1970. Estimation and mapping of evaporation. Symp. on World Water Balance, Int. Assoc. Hydrol. Sci., Publ. No. 92, pp. 200--213. Howe, G.M., Slaymaker, H.O. and Harding, D.M., 1966. Some aspects of the flood hydrology of the upper catchments of the Severn and Wye. Trans. Inst. Br. Geogr., 41: 33--58. Jarvis, M.G., 1973. Soils of the Wantage and Abingdon district. Mem. Soil Surv., G.B., Harpenden, 200 pp. Kendall, R.G., 1950. Land Drainage. Faber and Faber, London, 133 pp. M.A.F. (Ministry of Agriculture and Fisheries), 1951. Land drainage in England and Wales. Rep. Land Drain. Legislation Sub-Comm., H.M.S.O., London. Leeds-Harrison, P., Spoor, G. and Godwin, R.J., 1982. Water flow to mole drains. J. Agric. Eng. Res., 27: 81--91. Nemec, J., 1976. Time variability of hydraulic conductivity in heavy clay soils. Proc. Bratislava Symp. on Water in Heavy Clay Soils, 2: 81--92. Nicholson, H.H., 1953. The Principles of Land Drainage. Cambridge University Press, London, 165 pp. Penman, H.L., 1949. The dependence of transpiration on weather and soil conditions. J. Soil Sci., 1 : 74--89. Ritchie, J.T., Kissel, D.E. and Burnett, E., 1972. Water movement in undisturbed swelling clay soils. Soil Sci. Soc. Am. Proc., 36: 874--879. Robson, J.D. and Thomasson, A.J., 1977. Soil water regimes. Soil Surv. England--Wales, Harpenden, Tech. Monogr. No. 11. Rycroft, D.W. and Massey, W., 1975. The effect of field drainage on river flow. Ministry of Agriculture, Fisheries and Food. Field Drain. Exp. Unit, Tech. Bull. 75/9. Smith, L.P and Trafford, B.D., 1976. Climate and drainage. U.K. Min. Agric., Fish. Food, H.M.S.O., London, Tech. Bull. 3 4 , 1 1 9 pp. Strangeways. I.C. and Templeman, R.F., 1974. Logging river level on magnetic tape. Water Serv., 78: 57--60. Trafford, B.D., 1973. The relationship between field drainage and arterial drainage -theoretical aspects. U.K. Min. Agric. Fish. Food, Field Drain. Exp. Unit, Tech. Bull. 73/10. Trafford, B.D. and Rycroft, D.W., 1973. Observations on the soil water regimes in a drained clay soil J. Soil. Sci., 24: 380--391. Ward, R.C., 1975. Principles of Hydrology. McGraw-Hill, London, 2nd ed., 367 pp.