The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon catchments, mid-Wales
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The use of flow variability analysis to assess theimpact of landcatchments, m
David R. Archer
JB d Scie
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 catchment
afforested catchments at Plynlimon, mid-Wales to assess catchment differences in
pulse numbers and average duration above threshold discharges between catchmentsallows the effect of climate variability to be taken into account. Results demonstrate a
forest maturation and felling, and the seasonality of trends are also identified. Analysis of
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-
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
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Journal of Hydrology (2007) 347, 487496
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elsannual maximum rise and fall in discharge over short durations (
488 D.R. Archerdeforestation (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
Figure 1 The Plynlimon experimental catchments showunit 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,
ing the forested area in the upper Severn catchment.
Dupeyrat, 2004). The main species is Sitka spruce (Picea
The use of flow variability analysis to assess the impact of land use change on the paired Plynlimon 489using four 23 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 tositchensis). 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 occurs13 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).
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.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 (19712000) 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: 193738, 194850 and 196364 (Robinson and
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-
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).
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 19722004 8.7Wye Cefn Brwyn 055008 19692004 10.55
490 D.R. Archerfected and analysis here is limited to the period from 1975.
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 inspectedFigure 2 Definition diagram for pulse numbers and pulseduration.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. 47.
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.
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 consistentFigure 3 Average annual number of pulses for the Severn andWye catchments over the period 19722004.
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.
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 Plynlimon
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...