Changes in catchment runoff following drainage and afforestation

Journal of Hydrology, 86 (1986) 71~84 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

71

[3] CHANGES IN CATCHMENT RUNOFF FOLLOWING DRAINAGE AND AFFORESTATION

M. ROBINSON Institute of Hydrology, Crowmarsh Gifford, Wallingford, Oxon OXIO 8BB (U.K.)

(Received February 14, 1986; revised and accepted March 21, 1986)

ABSTRACT Robinson, M., 1986. Changes in catchment runoff following drainage and afforestation. J. Hydrol., 86: 7144. In many parts of N. Europe artificial drainage is necessary to establish plantation forestry. Data from a small research catchment indicates this drainage altered both the timing and the total quantity of flow. Annual runoff increased due to the augmentation of flows between storms (baseflows were doubled). In storm periods the open drains helped to quickly remove surface layer flows, leading to shorter flow response times and higher peaks. The trees are not yet mature and these results refer mainly to the effects of the drains but there is evidence that the magnitude of these changes decreases over time due to tree growth and as vegetation colonisation of the drains impairs their hydraulic efficiency.

INTRODUCTION A c o n s i d e r a b l e l i t e r a t u r e has been built up r e g a r d i n g the effects of forests on runoff, b o t h by c a t c h m e n t studies, often i n v o l v i n g c l e a r felling (Hibbert, 1967; Rodda, 1976; B l a c k i e et al., 1980), and m o r e r e c e n t l y by process studies of e v a p o r a t i o n and t r a n s p i r a t i o n ( R u t t e r et al., 1971; Calder, 1982). F o r e s t s h a v e g e n e r a l l y been held responsible for r e d u c t i o n s both in the w a t e r yield due to h i g h e r e v a p o r a t i o n losses, a n d in the flood p e a k s due to r e d u c e d s t o r m flows. These studies h a v e u s u a l l y comprised e i t h e r small densely i n s t r u m e n t e d plots or else large-scale c a t c h m e n t s in a r e a s t h a t h a v e n o t been artificially drained. In n o r t h e r n E u r o p e p l a n t a t i o n forests are often on soils r e q u i r i n g artificial drainage, and the influence of these p l o u g h d r a i n s on r u n o f f has received c o m p a r a t i v e l y little a t t e n t i o n . In B r i t a i n a l m o s t all the new land for f o r e s t r y is on u p l a n d soils w h i c h h a v e first to be p l o u g h e d (Binns, 1979). I n the e a r l y stages of a f f o r e s t a t i o n it is t h e s e d r a i n s r a t h e r t h a n the y o u n g saplings t h a t m a y be expected to exert the d o m i n a n t h y d r o l o g i c a l effect. It has been suggested t h a t a f u r t h e r 1.8 × 10~ha c o u l d be p l a n t e d in B r i t a i n ( F o r e s t r y Commission, 1977). This w o u l d be c o n c e n t r a t e d in u p l a n d areas o v e r the next h a l f 0022-1694/86/$03.50

© 1986 Elsevier Science Publishers B.V.

72

century and increase the percentage of the uplands under forest from the present 15% to nearly 50% (Centre for Agriculture Strategy, 1980). With cropping cycles of 50-60 yrs duration for managed forests in the UK this early establishment phase would comprise an appreciable part of the cycle, and be a significant land use in its own right. It has been claimed that such plantation forestry drainage increases peak flows in the early stages of the cropping cycle before the trees reach maturity (Howe et al., 1967; Graesser, 1979) but few quantitative data are available. This paper details the effects of pre-planting drainage in a specially instrumented research catchment to provide a scientific basis to this debate. The data cover the period from the establishment of the catchment in 1966 through its drainage in 1972, and up to the present time. The trees have not yet reached canopy closure, and their effect on evaporation losses and soil moisture is still small, so the hydrological changes described are predominantly a result of the artificial drainage.

STUDY CATCHMENT AND INSTRUMENTATION

The Coalburn catchment is situated in northern England (55 ° 0'N, 2 ° 37'W, Fig. 1). It is a 1.5 km 2 upland area of fairly smooth topography, varying in

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Fig. 1. Map of the Coalburn catchment and location of the instrumentation.

73 altitude from 270 to 330 m a.s.1, and with a main channel gradient of about 2 5 m k m -1. The average annual rainfall (1941-70) of about 1200ram is distributed fairly evenly through the year. The area is covered by glacial boulder clay deposits which are up to 5 m thick and much of the catchment has a surface layer of blanket peat. This peat is 0.5-3 m deep and comprises a thin surface layer of partly decomposed plant roots, above more decomposed Sphagnum and Eriophorum fibres in an amorphous matrix. At the start of the study the land was used as rough grazing for sheep, and the vegetation comprised Molinia grassland, and peat bog species (including Eriophorum, Sphagnum, Juncus and Plantago). The ground was saturated for long periods in winter, but the catchment did not contain any lakes. Given the difficulty of defining the watershed of a small catchment on gentle topography an artificial boundary ditch was cut at the start of the project. This enabled the area of moorland draining to the weir to be known, and ensured t h a t it was not altered later by the cutting of the drains for forestry. The ditch followed the natural topographic divide as closely as possible, with allowances for any existing ditches such as t h a t on the edge of a plantation immediately to the north east of the catchment. The weir was sited where the bedrock outcropped near the surface, ensuring sound foundations and minimising the possibility of flow under the weir. A compound Crump weir design (e.g. Ackers et al., 1978) was adopted for ease of construction at this remote site. It was built to standard specifications (British Standards, 1969), and contains all flow up to 5.75m3s -1 (13.6mmh-1). The theoretical stage~lischarge relation, checked by current metering, has been used for the whole period of study. A large apron was constructed upstream to reduce afliux at high flows and encourage sediment deposition away from the weir. A recorder hut contains the stilling well and a number of water level recorders. To determine the spatial variation of rainfall across the catchment storage gauges were sited at 14 locations, and read at two-week intervals. The number of raingauges had to be reduced prior to afforestation owing to problems of interference with the forestry operations, and the need for an open clearing around each gauge. A preliminary study of about six months data (Rodda, 1970) found only small differences between the catches of the gauges, and concluded t h a t one or two gauges would be adequate for the basin mean rainfall. Further analyses of four years data has confirmed the uniform precipitation, and the raingauge network was reduced to four gauges. In common with many upland sites it is, however, difficult to know how accurately the storage gauges measure snowfall. Recording raingauges sited by two of the storage gauges provide detailed information on storm profiles. Since 1971 an automatic weather station (Strangeways, 1972) near the centre of the catchment has provided measurements of solar radiation, ambient temperature and wet bulb depression, wind run and direction, thus enabling the calculation of potential evaporation. About 5½yrs of data were collected before the Forestry Commission drained the catchment in summer 1972 by cutting open drains (ditches) using a drainage plough pulled by crawler tractors (see plate 1). The purposes of drainage

74

PLATE 1 The drainage plough cuts a single drain and deposits the excavated soil to either side in ridges (Forestry Commissionphoto B6601). include the regulation of water movement and the improvement of aeration, the mobilisation of nutrients, and the reduction of competition from natural vegetation. Furrow ditches approximately 0.5 m deep and at about 4.5 m spacing were generally aligned with the ground slope and provided turf ridges 2-2.5 m apart as elevated drier sites for planting. Deeper drains cut close to the contours collect water from these plough furrows. The density of this artificial network is about 200 km km 2 (60 times greater t h a n the original stream network). Large amounts of sediment were released by the ploughing operations, and this has been the subject of a separate study (Robinson and Blyth, 1982). Sediment deposition in the approach channel and damage to the weir by scour resulted in an incomplete record of flows from October 1972 to June 1973 inclusive. Following the common forestry practice of leaving the soil for up to a year to dry after drainage, Sitka spruce (Picea sitchensis) were planted in 1973. They were planted at about 2 m spacing along the turf ridges. Due to the severe site conditions, their growth has been relatively slow: after 5 yrs their height was typically 1 m, and after 12 yrs, about 2.5-3 m.

75 WATER BALANCE AND FLOW REGIME A n n u a l precipitation and runoff depths, 1967-83 are shown in Fig. 2. For completeness, the periods of missing flow data in 1972 and 1973 were estimated using the IH conceptual model (Blackie and Eeles, 1985), but have not been included in the subsequent analyses. It is apparent that for equivalent annual rainfall totals, flows were higher after drainage than before. The data are summarized in Table 1, divided into three 5-yr periods. Before drainage, whilst the catchment was still used as rough grazing, flow accounted for about twothirds of the precipitation and evaporation for about one third. In the first period after drainage (1974-78) the annual rainfalls and their seasonal distribution were similar to those before drainage, but the annual flows were about 60 mm higher. An increase in annual flows following artificial drainage has been noted in a number of studies (e.g. Green, 1970; Seuna, 1980). In the second period after drainage (1979-83), rainfall was about 20% higher, resulting in much higher flows and an annual percentage runoff averaging 80%. The higher runoff was also partly due to a shift in the seasonal occurrence of rainfall from summer (when potential evaporation is greatest) to winter months. An increase in the annual rainfalls and a tendency to drier summers and wetter winters over this period has been noted elsewhere in Britain. Since 1977 the annual 1300-

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Fig. 2. Annual precipitation and discharges before (o) and after (e) drainagefor the calendar years 1967~83. The numbers refer to the year. An adjustmentof 150mm was made between 1981 and 1982 for snow lying at the year end.

76 TABLE 1

Water balance of the Coalburn Catchment. Values are average annual depths (mm) Period

Precipitation

Discharge

Losses

(1)

(2)

(1) - (2)

°/o runoff

% annual rain in Winter (Oct-March)

Before drainage: 1967-71

1230

825

405

67

52

1245 1505

890 1205

355 300

71 80

54 60

After drainage: 1974-78 1979-83

rainfall over England and Wales has been about 10% higher than average, and flows 10-20% higher (Marsh and Lees, 1986). The effects upon the flow regime of both the artificial drainage and the change in rainfall are shown by daily flow duration curves (Fig. 3). In the first period after drainage (1974-78) there was an increase in the magnitude of low and moderate flows. The 90 percentile daily flow was double that before drainage, and the values for individual years were significantly greater than those before drainage at the a = 0.05 level (Mann Whitney Rank Test, e.g. Pearson and Hartley, 1972). There was, however, no corresponding difference between the distribution of daily rainfalls in these two periods. In the second period after drainage (1979-83), the much higher rainfall resulted in larger discharges 1,0-

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) drainage.

i 99

77

for the whole flow range. The 90 percentile flow was double t h a t in the first 5 yrs after drainage. LOW FLOWS

Comparison of the flow duration curves suggests t h a t forestry drainage increased the catchment runoff through an increase in low and moderate flows. The flow duration curves were, however, clearly sensitive to weather variations and to confirm the effect of land use changes on runoff a more stable measure was required. In a recent study of the low flows of rivers in the United Kingdom a baseflow index (BFI) was developed which was found to give consistent values for a given catchment, and did not vary with factors such as the rainfall in the period of record (Institute of Hydrology, 1980). The BFI is defined as the proportion of the annual discharge of a catchment which occurs as "baseflow" and is calculated from daily mean flow data by first taking the minima of successive five day periods and then identifying turning points to obtain the separated hydrograph. The average annual value of BFI for Coalburn doubled from 0.099 prior to drainage to 0.202 afterwards (1974-83), confirming the difference noted in the flow duration curves of an increase apparent in the magnitude of low flows. The BFI was also calculated for three nearby catchments known to have little or no forestry drainage taking place, using data from the UK national flow archive (Anon, 1985). Annual values of BFI for 10 yrs following the drainage of Coalburn were standardised by their average value prior to 1972 (Fig. 4). For the three control catchments the BFI values were generally with + 15% of their

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78 pre-1972 mean value, whilst those for Coalburn ranged from + 155 to + 260% and were highly significantly different at the ~ = 0.005 level (Mann Whitney Rank Test). Unlike the flow d u r a t i o n curves t here was no evidence of sensitivity to the h ig h er rainfall after 1979. The effect of moorland drainage on minimum flows has been the source of much controversy with claims t h a t it causes streams to dry up sooner in dry w eath er periods (Graesser, 1979). The evidence from this study is t h a t baseflows were in fact increased by drainage. Possible reasons for the higher flows include dewatering of the peat, and a reduction in evaporation losses due to a lowering of the w a t e r table. Any effect of the former would have been small, however, since the runoff data used in this study excluded the first 18 months after drainage when the release of water would have been greatest, and because measurements at nearby sites showed no lowering of the ground surface after drainage (D.A. Thompson, Forestry Commission, pers. commun., 1979). Any reduction in ev apor a t i on due to a lowering of the water table also appears to have been small because of the limited n a t u r e of the lowering. A transect of dip wells was installed at right angles to the ground slope between two furrow drains in an area without trees. The wells were covered to prevent the entry of rainfall, and water levels were manually read at approximately 2-week intervals over th r ee years and have been summarised as the levels exceeded on 25 and 75% of the readings (Fig. 5). For long periods in the winter the water remained close to the surface within less t han 1 m of the drain. The restricted lateral effect of drains in peaty soils in upland Britain (Hudson and Roberts, 1982; Robinson and Newson, 1986) contrasts with the findings of some overseas studies (Rayment and Cooper, 1968). In the past, the British Forestry Commission used deep drains to try to dry peat soils, but it found t hat their drainage effect was small and has adopted a combination of shallow furrow drains for t u r f provision, with deeper cross drains (Pyatt and Low, 1985). Thus whilst the DRAIN

TURF RIDGE

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Fig. 5. Water levels adjacent to a forestry drain, which were exceededfor 25 and 75% of the time 1983~5. Note how the turf ridge provides an elevated drier site for tree establishment.

79 artificial d r a i n a g e n e t w o r k a t C o a l b u r n is m u c h m o r e e x t e n s i v e t h a n t h e o r i g i n a l s t r e a m n e t w o r k (60-fold increase), it is of a s i m i l a r depth. A s u r v e y of t h e s t r e a m c h a n n e l s before d r a i n a g e f o u n d a m e a n d e p t h of 47 cm w h i c h is a l m o s t i d e n t i c a l to t h a t of the f o r e s t r y drains. G i v e n t h e s h a l l o w n a t u r e of t h e d r a i n s a n d t h e limited d r a w d o w n a r o u n d t h e m it a p p e a r s t h a t the o b s e r v e d i n c r e a s e in low flows w a s p r i m a r i l y due to t h e m o r e e x t e n s i v e n e t w o r k of d r a i n s c a r r y i n g w a t e r f r o m a r e a s far f r o m t h e o r i g i n a l n e t w o r k , w h i c h h a d p r e v i o u s l y r a r e l y c o n t r i b u t e d to r u n o f f e x c e p t in times of s t o r m rainfall. FLOOD FREQUENCY P e a k flows in t h e five y e a r s p r i o r to d r a i n a g e (1967-71) w e r e c o m p a r e d w i t h t h o s e in t h e t e n y e a r s a f t e r w a r d s (1974-83). O w i n g to t h e r e l a t i v e l y s h o r t periods a v a i l a b l e for study, a p a r t i a l d u r a t i o n series of p e a k s a b o v e a t h r e s h o l d , w a s c h o s e n r a t h e r t h a n the a n n u a l m a x i m a . To e n s u r e t h a t t h e flow m a x i m a w e r e in f a c t i n d e p e n d e n t e v e n t s a n u m b e r of a r b i t r a r y b u t c o n s i s t e n t r u l e s w e r e applied (NERC, 1975). T h e c r i t e r i a a d o p t e d for this c a t c h m e n t w e r e t h a t successive p e a k s m u s t be >1 12 h a p a r t a n d t h a t t h e m i n i m u m flow b e t w e e n two p e a k s m u s t be ~< 66% of t h e first peak. T h e n u m b e r of i n d e p e n d e n t p e a k s was t h e n c o u n t e d for e a c h y e a r a n d for a r a n g e of t h r e s h o l d s from 0.9 up to 1.5 m 3 s (Table 2). T h e r e w a s a n a p p a r e n t i n c r e a s e in t h e n u m b e r of flood peaks, a l t h o u g h this i n c r e a s e w a s less for h i g h e r flow t h r e s h o l d s . I f it is a s s u m e d t h a t the n u m b e r of e x c e e d a n c e s is a P o i s s o n process, t h e n t h e s t a t i s t i c a l s i g n i f i c a n c e of t h e difference in t h e r a t e s of o c c u r r e n c e b e t w e e n t h e two periods m a y be tested (Cox a n d Lewis, 1966). F o r t h r e s h o l d s up to 1.2 m 3s i t h e n u m b e r of p e a k s in the two periods w e r e different a t at l e a s t the ~ = 0.10 level b u t for h i g h e r t h r e s h o l d s t h e r e w a s no s i g n i f i c a n t difference. B i n n s (1979) h a s a r g u e d t h a t an i n c r e a s e in p e a k flows following f o r e s t r y d r a i n a g e r e s u l t s from the r a i n w h i c h falls into t h e d r a i n s a n d so c o n t r i b u t e s d i r e c t l y to s t o r m runoff. S u c h a contrib u t i o n to flow w o u l d give a r e l a t i v e l y l a r g e i n c r e a s e in p e a k s for small events, b e c o m i n g p r o g r e s s i v e l y less i m p o r t a n t for b i g g e r floods. This could a c c o u n t for the s m a l l e r difference b e t w e e n t h e n u m b e r of p e a k s at h i g h e r t h r e s h o l d s , but TABLE 2 Average annual number of storm peaks above specified flow thresholds, before and after drainage Periods

1967-71 197~83 Significance levela

Mean annual flood (m3s 1)

Threshold flow (m3s 1) 0.9

1.0

1.2

1.5

4.6 6.9

3.0 5.7

2.4 3.8

1.6 1.7

2.0 2.3

5%

2%

10%

N.S

N.S

aN.S. indicates the difference between the periods was not significant at 10% level.

80

a number of factors argue against this mechanism being the cause. Even the lowest threshold in Table 2, giving five or six events per year excludes about another 50 smaller runoff events. The drains at Coalburn account for 10% of the catchment area, which is much smaller t h a n the runoff coefficient of 50-80% for the larger floods. Furthermore, rain falling directly into the drains will lead to a proportionally larger increase in peak flows in summer (when the ground is driest and catchment losses are highest), than in winter; yet both before and after drainage 70% of the threshold peaks occurred in winter months (October-March). An alternative explanation for the difference in the significance levels given in Table 2 is t h a t for the larger storms the pre-storm soil moisture has less effect on runoff, so drained and undrained land behave in a similar fashion. However in the case of Coalburn the effect of the drains on soil moisture was limited, whilst the 60-fold increase in drainage density considerably reduced overland travel distances leading to faster, more intense runoff. It appears more likely t h a t in the smaller sample of higher threshold events the individual storm rainfall characteristics (depth, intensity) had the dominant effect on peak discharges.

STORM RUNOFF TIMING

To confirm t h a t the artificial drainage had altered the pattern of storm discharge response about three dozen events giving high discharge peaks were chosen from those identified in the previous section. After excluding events with a snowmelt component, 30 events were used. The magnitude of the peak flows were similar before and after drainage, and in both groups averaged about 1.4 m 3s- 1 (sd = 0.4). Unit hydrographs were derived using a rainfall loss rate separation and matrix inversion (NERC, 1975). For each storm the shape of the unit hydrograph was summarised by its peak value, rise time and a measure of its duration. Average parameter values are given in Table 3. The data after drainage have been subdivided into two periods to look for changes in response over time due to vegetation growth. No seasonal differences were apparent between the parameter values from winter and summer storms, nor TABLE 3 Average values of the parameters of the half-hour unit hydrographs. Standard deviations are given in parentheses Before drainage 1967 71

Peak flow ordinate (% total quick runoff) Time to peak (h) Width at half peak (h) Number of storms

After drainage 1974-77

1981-83

12.0 (1.3)

16.5 (2.3)

14.3 (2.2)

2.1 (0.2) 3.4 (0.6) 14

1.6 (0.2) 2.5 (0.3) 10

1.7 (0.3) 3.1 (0.3) 6

81 was there a correlation in any period between the parameter values and storm magnitude. In the period 1974-77 the peak of the half hour unit hydrograph was about 40~/o greater t h a n the original moorland response whilst the time to peak and width at half peak were both shortened by about 250/0. These differences were statistically significant at the ~ = 0.001 level (Mann Whitney Rank Test). This change to a more flashy pattern of storm response resulted from the greater density of drainage channels considerably reducing surface flow travel distances, whilst having only a small effect on soil moisture conditions. There was a tendency for these changes to reduce over time. In the second period after drainage the average time for which flows equalled or exceeded half the peak value had lengthened to within 10% of the pre-drainage value (and was no longer significantly different). The peak value and rise time were within 20% of their average pre-drainage values (but still significantly different). A regression analysis was carried out between these parameters and the time elapsed since the drainage for the 16 storms, 1974-84. The slopes of the regression lines were significantly different from zero for both the width at half peak (~ = 0.001, r 2 = 53%) and the peak ordinate (~ = 0.005, r 2 = 40%). Time to peak did not alter over the period. A diminution of the effects of the drainage over time accords with the observation t h a t many of the ditches have been colonised by vegetation which would reduce their hydraulic efficiency.

STORM RUNOFF HYDROGRAPHS The magnitude of a storm hydrograph peak depends upon both the volume of storm runoff and its time distribution. For the storm events used in the unit hydrograph study there was no significant difference between the observed percentage runoffs from winter storms in the periods before and after drainage (both averaging about 70%). A regression analysis of the percentage runoff with the storm depth and measures of catchment wetness (SMD, API and pre-storm flow) indicated t h a t in both periods runoff amounts were only weakly dependent on ground wetness. In contrast there was a significant change in the observed storm hydrograph rise times after drainage. The average lag time (defined as the time from the centroid of storm rainfall to the flow peak) was reduced from 3 h before drainage to 2 h afterwards. These findings confirm t h a t the effect of the drainage on soil storage capacity and runoff quantities was relatively small. Its main role was to remove overland and surface water more quickly from the catchment. A similar conclusion was reached in an earlier study of moorland draining for pasture improvement (Robinson, 1985). The 40% increase in the unit hydrograph peak applies only to flows resulting from a half-hour rainstorm. It cannot be directly applied to flow hydrograph peaks from longer storms since the unit hydrograph must be convoluted with successive ordinates of the storm rainfall. The magnitude of the increase in flow peaks will depend upon both the duration and shape of the rainfall profile. In

82 the UK a standard profile shape is often used for engineering design, and is a symmetrical bell shaped profile which is peakier than 75% of the winter storms observed (NERC, 1975). Using this design profile for a 6h duration storm (equivalent to the time base of the unit hydrographs) indicated that for a given volume of storm runoff, the artificial drainage increased peak flows by about 20% in the first five years, reducing to about 10~o after ten years.

CONCLUSIONSAND SUMMARY Data were collected from a small catchment to study the hydrological effects of a change in land use from rough moorland to plantation forestry. The forest is not yet mature, so the results to date mainly relate to the forestry drainage. The drainage resulted in a small increase in the annual water yields, largely through an increase in low flows. This increase was of a similar magnitude to that resulting from year to year weather variability. Drainage also altered the pattern of storm response due to the reduced surface flow travel distances to a channel. The time to maximum flow was reduced by about one third. Peak flows were increased by the order of 20% and although this decreaed with time as the plantation forest grew and vegetation colonised the drains, 10 yrs after drainage peak flows were still 10°/0 higher than before. Dip well data demonstrated the limited effect of the drains on soil moisture, with little increase in the soil water storage available to "buffer" storm rainfall. Data collection is continuing at Coalburn to investigate the effect of the growing tree crop on flows. This will include the effect of forestry on water yield to test the prediction (Calder, 1979) that evaporation losses under mature forest at Coalburn will be almost double those from the original moorland. Further data will also help resolve whether the level of peak flows under mature forest is higher (Robinson and Newson, 1986) or lower (Binns, 1979) than that from the original moorland.

ACKNOWLEDGEMENTS The Coalburn research catchment is a long-term co-operative project into the hydrological effects of upland afforestation by the Institute of Hydrology, Forestry Commission and North West Water Authority. Many individuals have been involved since its inception, but particular thanks are due to W.O. Binns, T.J. Johnstone, D.G. P y a t t (F.C.), P.D. Walsh (N.W.W.A), R.K. Furnell and S.W. Smith (I.H.). This paper is a contribution to a study of Flow Regimes from Experimental and Network Data (F.R.E.N.D.) which is part of I.H.P. III, Project 6.1.

83 REFERENCES Ackers, P., White, W.R., Perkins, J.A. and Harrison, A.J.M., 1978. Weirs and Flumes in Flow Measurement. Wiley, Chichester, 327 pp. Anon., 1985. The 1982 Yearbook. Hydrological Data: UK Series. Inst. of Hydrology, Wallingford, 165 pp. Binns, W.O., 1979. The hydrological impact of afforestation in Great Britain. In: G.E. Hollis (Editor), Man's Impact on the Hydrological Cycle in the United Kingdom. Geo Books, Norwich, pp. 55~9. Blackie, J.R. and Eeles, C.W.O., 1985. Lumped catchment models. In: M.G. Anderson and T.P. Butt (Editors), Hydrological Forecasting. Wiley, Chichester, pp. 311-345. Blackie, J.R., Ford, E.D., Home, J.E.M., Kinsman, D.J.J., Last, F.T. and Moorhouse, P., 1980. Environmental effects of deforestation, an annotated bibliography. Freshwater Biological Association, Occas. Pap. No. 10, 173 pp. British Standards, 1969. Measurement of liquid flow in open channels. British Standard 3680, Part 4B. Calder, I.R., 1979. Do trees use more water than grass? Water Serv., 83: 11-14. Calder, I.R., 1982. Forest evaporation. Canadian Hydrology Symp., National Research Council of Canada, Ottawa, Ont., pp. 173-193. Centre for Agriculture Strategy (CAS), 1980. Strategy for the UK forest industry. Centre for Agricultural Strategy Rep. No. 6. Univ. of Reading, Reading, 347 pp. Cox, D.R. and Lewis, P.A.W., 1966. The statistical analysis of series of events. Methuen, London, 285 pp. Forestry Commission, 1977. The wood production outlook in Britain: A review. Edinburgh, 111 pp. Graesser, N.W., 1979. Effect on salmon fisheries of afforestation, land drainage and road making in river catchment areas. Salmon Net, 12: 38-45. Green, M.J., 1970. Calibration of the Brenig Catchment and the initial effects of afforestation. Int. Assoc. Hydrol. Sci. Publ., 96: 329-345. Hibbert, A.R., 1967. Forest treatment effects on water yield. In: W.E. Sopper and H.W. Lull (Editors), Forest Hydrology. Pergamon, Oxford, pp. 725-736. Howe, G.M., Slaymaker, H.O. and Harding, D.M., 1967. Some aspects of the flood hydrology of the upper catchments of the Severn and Wye. Trans. Inst. Br. Geogr., 41: 33-58. Hudson, J.A. and Roberts, G.A., 1982. The effect of a tile drain on the soil moisture content of peat. J. Agric. Eng. Res., 27: 495-500. Institute of Hydrology, 1980. Low Flow Study Report. Vol. 3, Section 3, pp. 13-19. Marsh, T.J. and Lees, M.L., 1986. The 1984 drought: A historical review. Hydrological Data: UK Ser., Institute of Hydrology, 81 pp. NERC, 1975. Flood Studies Report, Vol. I. Hydrological Studies. Natural Environment Research Council, London. Pearson, E.S. and Hartley, H.O., 1972. Biometrika Tables for Statisticians. The University Press, Cambridge. Pyatt, D.G. and Low, A.J., 1985. Forest drainage. Res. Inf. Note, Forestry Commission, Edinburgh. Rayment, A.F. and Cooper, D.J., 1968. Drainage of Newfoundland peat soils for agricultural purposes. 3rd Int. Peat Congress, Ottawa, pp. 345-349. Robinson, M., 1985. The hydrological effects of moorland gripping: a re-appraisal of the Moor House research. J. Environ. Manage., 21: 205-11. Robinson, M. and Blyth, K., 1982. The effect of forest drainage operations on upland sediment yields: a case study. Earth Surf. Process. Landforms, 7: 85-90. Robinson, M. and Newson, M.D., 1986. Comparison of forest and moorland hydrology in an upland area with peat soils. Int. Peat J., 1: 49-68. Rodda, J.C., 1970. On the questions of rainfall measurement and representativeness. Int. Assoc. Sci. Hydrol., 92: 173-186. Rodda, J.C., 1976. Basin Studies. In: J.C. Rodda (Editor), Facets in Hydrology. Wiley, Chichester, pp. 257-297.

84 Rutter, A.J., Kershaw, K.A., Robins, P.C. and Morton, A.J., 1971. A predictive model of rainfall interception in forests. I: Derivation of the model from observations in a plantation of Corsican Pine. Agric. Meteorol., 9: 367-384. Seuna, P., 1980. Long term influence of forestry drainage on the hydrology of an open bog in Finland. IAHS Publ., 130: 141-149. Strangeways, I.C., 1972. Automatic weather stations for network operations. Weather, 10: 403408.