Journal of Hydrology (2007) 347, 487–496

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The use of flow variability analysis to assess theimpact of land use change on the paired Plynlimoncatchments, mid-Wales

David R. Archer

JBA Consulting Engineers and Scientists, South Barn, Broughton Hall, Skipton, North Yorkshire BD23 3AE, UK

Received 7 March 2007; received in revised form 20 August 2007; accepted 18 September 2007

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KEYWORDSPlynlimon;Land use effects;Afforestation;Flow variability;Seasonality;Trend

22-1694/$ - see front mattei:10.1016/j.jhydrol.2007.09

E-mail address: david.arch

r ª 200.036

er@jbac

Summary The impact of land use change on flood response is an issue of considerablepractical importance for rural land use management. However firm evidence of catchmentscale impacts have been hard to find, mainly because of the confounding influence of cli-mate variability. Methods based on flow variability analysis have been used to demon-strate the effects of land use change. This study uses the paired moorland andafforested catchments at Plynlimon, mid-Wales to assess catchment differences inresponse over a wide flow spectrum up to flood flows. Analysis using differences in annualpulse numbers and average duration above threshold discharges between catchmentsallows the effect of climate variability to be taken into account. Results demonstrate agreater level of flow variability, especially in summer, and shorter hydrograph duration,especially in winter, on the moorland Wye catchment. Trends and variations arising fromforest maturation and felling, and the seasonality of trends are also identified. Analysis ofannual maximum rise and fall in discharge over short durations (<2 h) are shown to bemore effective in distinguishing between catchment responses than peak flow.ª 2007 Elsevier B.V. All rights reserved.

Introduction

Following the work of Law (1956) who questioned the per-ceived notion that a forest cover was beneficial to water re-sources, the Plynlimon experiment was set up in mid-Walesto investigate the effects of land use on the water yield ofupland catchments (Kirby et al., 1991). Originally two rep-

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resentative catchments were chosen and instrumented,the 10.55 km2 grassland upper Wye catchment at CefnBrwyn, and the 8.70 km2 upper Severn catchment at Plynli-mon, which at the initiation of the experiment was 67% for-ested (Fig. 1). Further sub-catchments were later added tothe experiment but their data have not been used in thisstudy.

By virtue of its longevity, the Plynlimon experiment hasbeen used to test the impact of both afforestation and

.

Figure 1 The Plynlimon experimental catchments showing the forested area in the upper Severn catchment.

488 D.R. Archer

deforestation (which began in 1983) on the catchmentwater balance, low flows and flood flows and also as a basisfor understanding hydrological processes. Early studies con-vincingly demonstrated a larger water use by the forestedcatchment than the grassland one, with the conclusion thata completely forested catchment would lose an additional15% of runoff compared with the grassland catchment (Kirbyet al., 1991). With respect to low flows, although differ-ences between sub-catchments were identified, no ten-dency was found for forested and grassland catchments tohave different base-flow characteristics although, more re-cently, Robinson and Dupeyrat (2004) found clear evidencethat felling augmented low flows.

For flood flows, Kirby et al. (1991) found no statisticallysignificant difference in annual flood peak magnitudes per

unit area between the catchments. Similarly, comparisonsof individual peak flows for over 100 storms, before the har-vesting commenced. Robinson and Newson (1986) alsofound no apparent difference between the two catchmentsfor large flow events. However, for small storms, with flowswell below the long-term mean annual flood, peaks wereconsistently smaller from the forested than from the grass-land catchment. Flood hydrograph analysis of moderate tolarge flood hydrographs showed that volumes were greaterfrom the grassland catchment.

Robinson and Dupeyrat (2004) extended the analysis toconsider conditions after forest felling commenced. Theycompared several hundred peak flows between the Wyeand the Severn over a range between 10% and 240% of themean annual flood to look for evidence of a trend over time,

The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon 489

using four 2–3 year snapshots over three decades. This anal-ysis showed a statistically significant reduction in peak flowsof the Severn over the first decade from mid-1970s to early1980s. The reduction in peak flows continued into the initialfelling period in the late 1980s but there was then no furtherchange to the final period, when the cumulative felled areaattained 25% of the total catchment. This somewhat surpris-ing result (no increase in flood flows after felling) wasattributed by Robinson and Dupeyrat (2004) to the applica-tion of modern forest management guidelines (ForestryCommission, 1993) with care being taken during felling toreduce soil damage, and hence surface runoff, by the useof brash mats.

The use of limited duration intervals for analysis by Rob-inson and Dupeyrat (2004) did not permit a full time seriesof change to be examined. In addition, although a greaterthan normal range of flood peak magnitude was investi-gated, a full investigation of hydrograph properties wasnot carried out. The use of a particular type of flow variabil-ity analysis (Archer, 2000, 2004; Archer and Newson, 2002;Newson et al., 2002), has proven to be an effective wayto identify the effects of land use change. The method en-ables the full time series of hydrograph variability, ex-pressed as the annual number and duration of pulsesabove selected discharge thresholds, to be examined, andit provides the opportunity to consider change over a verywide flow spectrum from half the median flow to floodflows. Whilst flow variability (or flashiness) is related tothe frequency and magnitude of flood flow, it is an impor-tant property in its own right with respect to influence onsediment transport and channel morphology (Sear, 2004;Hassan et al., 2006) and on river ecology (Resh et al.,1988; Clausen and Biggs, 1997).

A key problem in assessing the impact of land use changeon flow regime is the confounding effect of climate variabil-ity. With respect to afforested catchments in the Penninemountains of northern England, it was found that the influ-ence of climate on pulse numbers and duration could effec-tively be distinguished by regression with simple annualrainfall statistics (Archer, 2000). In the case of the pairedPlynlimon catchments, this approach is unnecessary, ascomparison of the forested catchment with the adjacentcontrol grassland catchment enables the effects of climateto be screened out. In this study therefore, comparisons aremade on an annual and seasonal basis between the twocatchments in terms of pulse number and duration and anassessment is made of how the differences have changedover time. In addition, a further measure of flow variability,the amount of rise and fall in discharge over selected shortdurations has been analysed and compared betweencatchments.

The catchments and their land use changes

Numerous previous reports and research papers have de-scribed the physical background and climate of the Plynli-mon catchments (Kirby et al., 1991; Brandt et al., 2004).The headwater catchments of the Rivers Wye and Severnare contiguous and lie on the eastern slopes of the Plynli-mon upland massif in mid-Wales. Their topography, geologyand soils are similar; their underlying geology comprisesmudstones, grits, siltstones and slates that are thought to

ensure the basins are watertight. The elevation range forthe Wye is from 344 to 742 m OD and for the Severn is 328to 739 m OD. The longitudinal slope of the main channelof the Severn is steeper than that of the Wye but the Severncontains a significantly greater proportion of blanket peaton its unforested headwaters – a factor which might proveimportant to flow variability. Average annual catchmentrainfall (1971–2000) is 2570 mm for the Wye and 2478 forthe Severn. There is a distinct winter maximum but everymonth has an average rainfall greater than 125 mm.

The difference in land cover between the Wye and theSevern provides the justification for their use in assessingrunoff impacts. Until the late 1930s both catchments hada moorland vegetation which Newson (1976) subdivided intoheath, grassland and mire, with grassland occupying thegreatest proportion. Both catchments were used as roughpasture for sheep grazing.

The forest in the Severn catchment was planted in threephases: 1937–38, 1948–50 and 1963–64 (Robinson andDupeyrat, 2004). The main species is Sitka spruce (Piceasitchensis). At the commencement of the data analysis per-iod in 1972, 67% of the Severn catchment had been plantedand trees had an age range from 8 to 35 years. No furthertrees were planted until a second rotation crop followingharvesting which commenced in 1983. Harvesting has in-volved the removal of the main stems leaving much of thebrash from the side branches on the ground according to na-tional guidelines (Forestry Commission, 1993). Old drainagechannels were not cleaned out. Only small areas of forestwere cleared at a time but by 2001 about 48% of the for-ested area of the Severn catchment (c. 32% of the total ba-sin area) had been cut. However, replanting usually occurs1–3 years after felling, allowing time for the brash to rot,so that the total forested area remains approximately thesame but with a patchwork of trees of differing age range.

Whilst the Wye catchment has continued to be used asrough pasture for sheep grazing, there has been a reductionin stocking density following reforms to the Common Agri-cultural Policy in 1992 (APEM, 1998) which might conceiv-ably also have an impact on flood and flashiness potentialthrough the reduced potential for soil compaction (Carrollet al., 2004).

Data

Flow data at a 15-min interval were provided by the Centrefor Ecology and Hydrology (CEH) for the two principal sta-tions of the Plynlimon experiment Table 1).

The streamflow record has been extensively describedand reviewed (Smart, 1977; Kirby et al., 1991; Hudson andGilman, 1993). Given the initial purpose of measurementto assess the water balance, the standards of gauging sta-tion construction and calibration have been of a high stan-dard, although problems arising from sediment depositionin the approach channel may have increased the measure-ment error in the Severn record before 1975.

The data were further checked for completeness andconsistency. A few gaps of less than one day were infilledby interpolation. The only breaks in excess of one day werein 2001 at Plynlimon Flume (12 days) and at Cefn Brwyn (26days); these were left as missing data.

Table 1 Flow gauging stations on the Severn and Wye

River Station name Station number Period of record Catchment area (km2)

Severn Plynlimon flume 054022 1972–2004 8.7Wye Cefn Brwyn 055008 1969–2004 10.55

490 D.R. Archer

Although values are listed by 15-min intervals, in fact forthe period up to December 1974 the data are hourly, i.e. thesame value is listed for four consecutive readings. This re-sults in lower sensitivity in the analysis of both pulse numberand in pulse duration but since the same effect applies toboth catchments, the common record from 1972 has beenused for both catchments. Annual maximum changes in dis-charge for 15 min and 30 min intervals are more severely af-fected and analysis here is limited to the period from 1975.

Methods

The method of analysis of hydrological variability or distur-bance has been described by Archer and Newson (2002).Essentially it is based on the frequency and duration ofpulses above threshold flows, selected as multiples of themedian flow (Fig. 2). A pulse is an occurrence of a rise abovea given flow, and pulse duration (between arrows) is thetime from rising above the threshold to falling below thesame threshold. The 15-min digital flow record is analysedin yearly blocks. For each year the total number of pulsesis counted and the total duration above the threshold forthe year and the mean duration per pulse are computed.Incomplete pulses at the beginning and end of the yearare excluded. The full spectrum of disturbance is assessedby repeating for 18 selected multiples of median flow (M)for the full period, as 0.5 M, M, 2 M, 3 M, 4 M, 5 M, 6 M,7 M, 8 M, 10 M, 15 M, 20 M, 30 M, 40 M, 50 M, 60 M, 80 Mand 100 M.

The number and duration of pulses are then comparedbetween the Severn and Wye catchments as a basis forestablishing the influence of afforestation and deforestationon the Severn catchment, with the general assumption thatthe response of the two catchments was virtually identicalbefore afforestation occurred. Differences in pulse numbersover each threshold between the two catchments are thencomputed on an annual basis to give a time series of pulsenumber and duration differences which can be inspected

Figure 2 Definition diagram for pulse numbers and pulseduration.

for trend over the period of record. Since the two smallcatchments are adjacent and subject to the same climaticvariability at a seasonal and annual time scale, taking differ-ences (or ratios) accounts for the year to year influence ofclimate variability on pulse numbers. However, it is notedthat infrequent storm events affect one catchment butnot the other, notably the thunderstorm of August 1977 onthe Severn but not the Wye (Newson, 1976).

Analysis of differences is then repeated for seasonal var-iability, based on 3 four-month periods, as a basis for assess-ing differences in comparative behaviour between the threeseasons.

Trends are tested by linear regression between year andobservation, and correlation coefficients computed. Signifi-cance of test results are computed by permutation samplingwith 1000 resamples (Kundzewicz and Robson, 2004), amethod which does not require any assumption about theform of distribution from which the data are derived. Anal-ysis is carried out using the HYDROSPECT data analysis sys-tem (Radzeijewski and Kundzewicz, 2004). Correlationcoefficients (r) and significance of trends (Sig) are shownin Figs. 4–7.

A separate assessment of flow variability is based on an-nual maximum rise and fall in discharge over 15, 30, 60 and120 min intervals. Median annual maximum rise and fall arethen calculated and frequency distributions computed.Comparisons are then made for medians and distributionsbetween the two catchments.

Results

Comparison of annual pulse numbers and duration

The average annual number, total duration and averageduration of pulses was calculated for the Severn and Wyecatchments over a wide range of flow from half the medianflow (0.5 M) to 100 times the median (100 M). Results forpulse numbers are shown in Fig. 3. A clear and consistent

Figure 3 Average annual number of pulses for the Severn andWye catchments over the period 1972–2004.

Table 2 The ratio (%) of annual pulse number, total duration and average pulse duration between the Wye and Severncatchments over the period from 1972 to 2004 (Severn/Wye · 100)

0.5 M M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 10 M 15 M 20 M 30 M 40 M 50 M 60 M

Pulses 72 75 76 72 78 77 79 83 85 90 86 87 78 75 67 65Tot Dur 89 100 100 102 103 104 105 108 108 107 103 98 84 82 68 58Avg Dur 133 133 132 140 131 133 132 129 128 118 120 113 104 109 102 79

The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon 491

difference in annual pulse numbers between the two catch-ments is apparent. An average of nearly 20 pulses more peryear occurs on the grassland catchment than on the for-ested catchment at maximum variability around 3 M.

Ratios of pulse numbers and durations are shown inTable 2. This shows that the number of pulses is significantlylower on the forested catchment through a wide flow range.The ratio appears to increase in the range from 6 M to 20 Mand to decrease again at higher M, but differences at higherM may be affected by sampling error when the number ofpulses per year decreases to below 5 at 40 M. Differencesin the total duration of pulses is less marked than for pulsenumbers, with the Severn having a slightly higher durationfrom 3 M to 15 M but decreasing at higher flows. However,average durations of pulses are much higher for the Severnthan the Wye – over 30% higher in the flow range up to 6 Mbut gradually declining to near equality at 50 M.

Time series of pulse numbers and duration

Fig. 4 shows a comparison of time series of pulse numbersover 15 M and 20 M for Severn and Wye.

Whilst there is not much evidence of trend in pulse num-bers on the Severn for these or other thresholds (Fig. 4a),there is considerable variability. Some groups of lower pulsenumbers at low M occur in the early 1970s, the mid 1990sand 2003 which are related to low rainfall. Fig. 4b showsequivalent pulse numbers for the Wye at Cefn Brwyn. Thereis slight evidence of an increase in pulse numbers at severalflow levels. However, the principal feature is again the var-iability through time, with years of high and low pulse num-bers coinciding with the Severn. There is no obvious visualevidence of trend in the average duration statistics.

Direct inspection of the raw time series of pulse numberor duration is unlikely to yield significant evidence of trend,

Figure 4 Time series of pulse numbers over 15 an

owing to the overwhelming influence of annual climatevariability.

Time series of differences in annual pulse numbersand duration

Pulse numbers for each year and multiple of M for Plynlimonwere subtracted from the equivalent figures for the Wye togive time series of residuals that take into account the ef-fects of common climatic conditions in each year. Fig. 5shows pulses over the range from 5 M to 20 M.

There is a significant indication of trend in pulse numberdifferences over most of the flow range which persists fromthe early to the later part of the record, with the Wyeincreasing in pulse numbers in relation to the Severn. Thereis little evidence for the reversal of the trend with forestharvesting after 1983 although there is a suggestion of a lev-elling off or reduction in some parts of the flow range afterthe late 1990s.

Fig. 6 shows equivalent difference between the twocatchments for average pulse duration. In general, averagedurations are less on the Wye than on the Severn, hence giv-ing negative difference. In this case there is a significantlyincreasing difference over the range from up to 15 M butlimited evidence for change at 20 M and higher flows.Assuming the Wye response to climate has not changed,the trends imply that there is a relative increase in the aver-age pulse duration on the forested catchment, continuingthrough the period of harvesting.

Assuming that change in hydrological response is primar-ily on the Severn catchment the results are consistent with arelative decrease in pulse numbers for the Severn corre-sponding with a relative increase in average pulse duration.However, these changes show up clearly at different thresh-olds for the data sets. The beginning of harvesting in 1983

d 20 M thresholds for: (a) Severn and (b) Wye.

Figure 5 Time series of difference in pulse numbers between Wye and Severn for: (a) 5 M and 10 M and (b) 15 M and 20 M.

Figure 6 Time series of difference in average pulse duration between Wye and Severn for: (a) 5 M and 10 M and (b) 15 M and 20 M.

492 D.R. Archer

appears not to coincide with the onset of change in hydro-logical response; the limited effects are delayed for at leasta decade.

Assessment of seasonal differences and changes

The annual data set has been divided into three seasonalperiods, from January to April (‘spring’), May to August(‘summer’) and September to December (‘winter’) to assessthe extent to which the annual differences between catch-ments and trends can be attributed to each of these sea-sons. Ratios of seasonal pulse numbers and durations areshown in Table 3.

A number of observations may be made from Table 3:

1. Pulse numbers are lower on the Severn for all seasonsand flow than on the Wye (except above 50 M in summerwhere affected by individual extreme events). Spring andwinter ratios are very similar throughout the range butsummer ratios are rather lower.

2. Total pulse duration is marginally higher on the Severnthan on the Wye in spring and winter but reversed duringthe summer months.

3. Average duration is virtually always higher on the Severnthan the Wye. Again the ratios are very similar for Springand Winter but the summer ratios are much lower.

These results seem to imply that the effects of land useare most marked during the summer season, suggesting theinfluence of increased interception, evapotranspiration andstorage on the afforested catchment. Nevertheless, the

spring and winter months are also affected in spite of lim-ited evapotranspiration and soil moisture deficit duringthese months on these high rainfall catchments.

Given these results one might expect that trends in thedifferences between the two catchments would show amaximum in the summer months. However, plots of thetrend in the differences between catchments in summerpulse numbers show surprisingly little evidence of trend.In contrast, for the winter period there is evidence of weakbut positive trend through the full range of flow (Fig. 7a).For the spring period (Fig. 7b) trends show significant in-crease up to 15 M. Thus, trend in the annual data, shownin Fig. 5, results primarily from changes during the periodfrom September to April rather than from May to August.The consistency of behaviour between the spring and wintersuggests that the trend is real rather than a transient fea-ture of the data.

Similarly, although there are no clear trends in differ-ences in summer pulse duration, winter and spring showdownward trends in differences in average pulse duration(Wye–Severn) through the early part of the record in theflow range up to 20 M. A reversal in trend occurs in the early1990s. Assuming that the Wye catchment has remained un-changed, this implies that Severn average duration isincreasing up to the early 1990s and decreasing thereafter.

Comparison of short duration changes in discharge

Maximum annual changes in discharge (rise and fall) werecalculated for selected short durations, 15 min, 30 min,60 min and 2 h for both stations for the period from 1975

Figure 7 Trend in difference in pulse numbers between Wye and Severn for, top (a) Winter (January–April) and, bottom (b) Spring(September–December) for 5 and 10 times the median flow (left) and 15 and 20 times the median flow (right).

Table 3 The ratio (%) of seasonal pulse number, total duration and average pulse duration between the Severn and Wyecatchments over the period from 1972 to 2004 (Severn/Wye · 100)

0.5 M M 2 M 3 M 4 M 5 M 6 M 7 M 8 M 10 M 15 M 20 M 30 M 40 M 50 M 60 M

Pulse numbersSpring 62.3 64.6 72.9 72.0 78.4 78.5 81.5 85.8 87.7 92.6 87.8 88.5 81.4 66.0 65.2 50.0Summer 80.2 81.8 80.8 71.2 76.3 78.2 77.4 74.9 74.2 73.2 72.7 81.5 33.3 33.3 200. 200.Winter 62.0 78.6 76.0 73.4 77.6 74.8 76.6 82.4 85.2 92.0 87.6 86.4 82.3 85.5 62.5 62.5

Total durationSpring 91 99 106 110 112 112 113 114 114 112 110 108 91 84 77Summer 109 101 92 86 83 83 82 83 83 85 78 58 40 50 130Winter 60 99 100 103 104 105 107 110 111 109 102 97 85 89 62

Average durationSpring 171 168 144 149 138 140 133 129 126 121 131 124 112 122 112Summer 138 122 107 116 101 103 106 110 113 113 99 65 135 177 60Winter 74 131 133 140 131 136 133 131 128 111 113 113 104 95 100

The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon 493

to 2004. Median annual maximum rise and fall in dischargefor the selected intervals are shown in Table 4. These showthat on average the changes are much lower on the Severnthan on the Wye. The ratios are lower for falling than for ris-ing discharges and they are also lower than the ratio ofcatchment area (0.82) or of median discharge. There is amuch greater difference in maximum change statistics thanin the peak flow statistics.

The Winfap – FEH package was used to carry out fre-quency analysis of short duration changes in discharge forthe two stations using the Generalised Logistic distributionwhich is recommended as a default for UK flood frequency

estimation (Institute of Hydrology, 1999). In this case itgives a good graphical fit to both distributions. Results fora 60 min rise are shown in Table 5. The Wye has a distinctlymore rapid rise than the Severn for all except the most ex-treme return period. This is because of one exceptionalevent in 1977 which affected the Severn catchment butthe Wye catchment not at all (Newson, 1977). This analysissupports the view that annual maximum rise and fall statis-tics show differences between catchments to a much great-er extent than peak flow statistics. If this is so, then theeffects of land use should be more readily shown by statis-tics of short duration change in discharge.

Table 4 Median annual maximum rates of rise and fall indischarge (m3/s) for four selected durations for the Wye andSevern and their ratios

15 minm3/s

30 minm3/s

60 minm3/s

120 minm3/s

Rising dischargesSevern 1.91 3.37 5.43 8.22Wye 2.82 5.2 8.33 12.25Ratio 0.68 0.65 0.65 0.67

Falling dischargesSevern 1.09 1.94 3.44 5.84Wye 2.1 3.47 6.03 8.94Ratio 0.52 0.56 0.57 0.65

Table 5 Comparison of flood frequency statistics forannual maximum 60 min rate of rise in discharge (m3/s)

Returnperiod (yrs)

SevernPlynlimon(m3/s)

Wye CefnBrwyn (m3/s)

Ratio Severn:Wye

2 5.43 8.33 0.655 7.52 11.50 0.65

10 9.76 14.16 0.6925 14.27 18.49 0.7750 19.58 22.66 0.86

100 27.40 27.85 0.98500 63.1 45.57 1.38

494 D.R. Archer

Discussion

The viability of the Plynlimon experiment depends on theassumption that the hydrological response of the two catch-ments was closely similar before afforestation and there-fore that differences now observed result from the changein land use. This assumption cannot be proven, since affor-estation and the associated hydrological response had beenunder way for several decades before the experiment be-gan. Estimates of flood risk from catchment descriptorsusing Flood Estimation Handbook methods (Institute ofHydrology, 1999) suggest that the Severn, unaffected byafforestation may have had a 15% greater specific mean an-nual flood than the Wye. Kirby et al. (1991) found for dataup to 1985, that the order was reversed with the Wye 13%higher than the Severn although the difference was not sig-nificant statistically.

The present study demonstrates conclusively the greaterlevel of flow variability or flashiness on the Wye (Fig. 3 andTable 2) and the much greater annual maximum rise and fallin discharge (Tables 4 and 5) over the whole period of re-cord. While peak flow levels are of key human interest withrespect to flooding of land and property, flow variability mywell prove of greater relevance to river dwelling organisms(Resh et al., 1988) through influences on frequency andintensity of disturbance and the ability to mobilise sediment(Sear, 2004).

The examination of differences in pulse numbers be-tween the two catchments has effectively accounted for

the year to year climatic variability and enabled time seriesof pulses and pulse duration to be inspected for trend. Fig. 5suggests that annual pulse numbers were quite similar forthe two catchments in the early part of the flow record(1970s) but that the Severn has steadily decreased thereaf-ter in relation to the Wye at flow levels greater than 15 M.There was a concurrent relative increase in the averagepulse duration in the Severn (Fig. 6). Projecting these trendsbackward from the beginning of the flow record to thebeginning of afforestation suggests that the Severn mayhave had the greater variability under natural conditions.

A further possible influence on the trend in differencesbetween catchments includes effects of drainage on flashi-ness in the early part of the Severn record (Robinson and Ry-croft, 1999). In addition, upward trends in rainfall especiallyover the summer, noted by Hudson and Gilman (1993), alongwith a declining proportion as snow, may have had an un-equal impact on forested and grassland areas.

There is little evidence for the reversal of trends with for-est harvesting after 1983 although there is a suggestion of alevelling off or reduction in pulse number differences (in-crease on the Severn) in some parts of the flow range afterthe late 1990s. This concurs with the conclusion of Robinsonet al. (2003) that partial felling at Plynlimon produced only aweak tendency for higher peaks to increase. They note thatthe lack of response may be due to the application of moreenvironmentally sympathetic forestry felling methods andin particular to patchwork felling of areas and retention oftree brash on the surface. Differences in interception capac-ity between standing forest and that of brash on felled areasmaywell be limited until the brash rots. The presence of deb-ris dams in streams in the clear felled areas may also haveacted to attenuate surface runoff and hydrograph peaks.

Analysis of seasonal pulses indicates that the greatestdifference between catchments occur in summer (Table 3)with both the average annual number of pulses and the totalduration lower on the Severn than the Wye. This result isperhaps to be expected given the more marked effect ofevapotranspiration during this season. It is more surprisingthat there is no evidence of trend during the summermonths, either in the pre-felling or post-felling periods; thisseems to imply that hydrological changes which caused thelarge observed difference between Wye and Severn had al-ready occurred before the beginning of the common flow re-cord in 1972.

In contrast there are more distinct (and shared) trends inspring and winter time series of pulses and duration. Thegeneral trend in average duration difference is downward(Severn becoming greater relative to Wye) but with a sug-gestion of a reversal or levelling off in the early 1990s. Pulsenumber differences increase at most thresholds in both sea-sons (Severn becoming less relative to Wye). The directionof these trends between September and April is consistentwith a land use effect but are difficult to explain in termsof catchment processes during these months.

Conclusions

The impact of land use change on river flood flows has longbeen the subject of debate and speculation. Evidence fromthe plot scale suggests that land use and managementshould have an impact on runoff generation but effects have

The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon 495

been difficult to identify at the catchment scale especiallywith the confounding influence of climate variability. In par-ticular, O’Connell et al. (2005) note that analysis of peakrunoff records has so far produced very little firm evidenceof catchment scale impacts, resulting from rural land use orland management change. However, flow variability analy-sis has already been shown to be a viable means of detectingthe influence of land use change on catchments up to330 km2 (Archer, 2003) even without the benefit of pairedcatchments as at Plynlimon.

This method permits the forensic inspection of flowchanges at a sub-daily level over a range of flow thresholds.It is therefore more likely to reveal subtle influences of landuse change than alternative indices of flashiness such as theRichards–Baker Flashiness Index (Baker et al., 2004) or sev-eral of the Indicators of Hydrological Alteration (Richteret al., 1996), which usually yield a single annual or seasonalindex value based on daily mean flow.

In spite of uncertainties, policy-makers favour land usemanagement as one of the means of managing flood risk.It is for example included amongst the responses recom-mended in the UK Government’s Foresight Future Flooding(Office of Science and Technology, 2003). If policies for landuse management are to be established, methods must beavailable first to demonstrate that change has occurred,secondly the nature of the change and finally the sourceof the change, by establishing linkages between land usepractice and hydrological response.

The present study, focusing on the effects of afforesta-tion on flow variability has clearly demonstrated methodsto show that change has occurred and the nature of thechange. It is believed they have wider applicability inassessing the effects of rural land use or land managementchange. They do not address the source of the change, forwhich process based modelling will still be required. How-ever, the methods described here for paired catchmentsor, in Archer and Newson (2002) and Archer (2003), for sin-gle catchments, can be used as a rapid screening methodand as a means of targeting more intensive investigation.

Acknowledgements

Mark Robinson of Centre for Ecology and Hydrology, Walling-ford kindly provided the Plynlimon data. Stephen Rose ofJBA Consulting Engineers and Professor Malcolm Newson ofthe University of Newcastle upon Tyne made helpful com-ments on early drafts of this paper.

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