Suspended solid yield from forest harvesting on upland blanket peat

HYDROLOGICAL PROCESSESHydrol. Process. 25, 207–216 (2011)Published online 7 September 2010 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.7836

Suspended solid yield from forest harvesting on uplandblanket peat

Michael Rodgers,1 Mark O’Connor,1 Mark Robinson,2 Markus Muller,1 Russell Poole3

and Liwen Xiao1*1 Department of Civil Engineering, The National University of Ireland, Galway, Ireland

2 Centre for Ecology and Hydrology, Wallingford, Oxfordshire, UK3 Marine Institute, Newport, County Mayo, Ireland

Abstract:

Forest harvesting activities, if not carefully carried out, can disturb the forest soils and can cause significant suspended solidconcentration increases in receiving water. This study examined how harvesting, following forestry guidelines, influencedsuspended solid concentrations and loads in the receiving water of a blanket peat salmonid catchment. The study sitecomprised of two forest coupes of 34-year-old conifers drained by a first-order stream. The upper coupe was not felledand acted as a baseline ‘control’ catchment; the downstream coupe was completely harvested in summer 2005 and served asthe ‘experimental’ catchment. Good management practices such as the proper use of brash mats and harvesting only in dryweather were implemented to minimize soil surface disturbance and streambank erosion. Stream flow and suspended solidmeasurements at an upstream station (US) and a downstream station (DS) in the study stream commenced over a year beforefelling took place. The suspended solid concentrations, yields and release patterns at US and DS were compared before andafter harvesting. These showed that post-guideline harvesting of upland blanket peat forest did not significantly increase thesuspended solid concentrations in the receiving water and the aquatic zone need not be adversely affected by soil releasesfrom sites without a buffer strip. Copyright 2010 John Wiley & Sons, Ltd.

KEY WORDS suspended solid; solid rating curve; forest clear felling and harvesting; blanket peat; best management practice;salmonid catchment

Received 31 March 2010; Accepted 13 July 2010

INTRODUCTION

Soil erosion is a two-phase process consisting of thedetachment of individual soil particles from the soilmass and their subsequent transport by erosive agentssuch as runoff (Rose, 1993). Erosion of upland blanketpeat is widespread in Britain and Ireland (Bradshaw andMcGEE, 1988; Evans and Warburton, 2007). In a sur-vey of erosion across the upland of England and Wales,McHugh et al. (2007) found that peat soils in the uplandsare the most severely eroded soil class. In Ireland, Brad-shaw and McGEE (1988) carried out a survey of blanketpeat in five mountain areas and reported extensive erosionin all areas. The erosion of peatlands is the greatest fromvery wet peatlands and from areas where there are lay-ers of mud or highly decomposed peat (Paavilainen andPaivanen, 1995). Peatland erosion results in the increaseof suspended solid concentrations in the receiving water,which could cause damage to the water ecology. Solidorganic matter silts up watercourses more than inorganicmatter. Moreover, organic matter is biologically active,thus consuming the oxygen resources of watercoursesas it is decomposed (Paavilainen and Paivanen, 1995).In a review, Greig et al. (2007) suggested that organic

* Correspondence to: Liwen Xiao, Department of Civil Engineering, TheNational University of Ireland, Galway, Ireland.E-mail: [email protected]

material deposited in gravels had a greater deleteriouseffect on salmon spawning grounds because of its oxy-gen demand. The causes of peat erosion have includedhuman disturbance and changes in the mechanical stabil-ity of the peat mass through time (Evans and Warburton,2007).

Most forest operations including forest harvestingcould create some mechanical disturbance on the groundsurface that can lead to the release of soil to riversystems (Robinson and Blyth, 1982; Everest et al., 1987).Erosion from timber harvesting and reforesting operationscan be significant in the absence of good managementpractice (Swank et al., 2001). In a catchment study inArkansas and Oklahoma, Scoles et al. (1996) found thatwhere no specific erosion control measures were applied,annual soil losses in the first year were statisticallysignificantly greater on clear felled and harvested sitesthan on selectively harvested and control sites. In astudy by Ahtiainen et al. (1988), a combination of clear-cutting, ditching, soil preparation in peatland catchmentincreased the amount of annual suspended solids from4 to 1010 kg/ha at its highest. Five to eight yearslater, the amount of annual suspended solids was stillapproximately 60 kg/ha.

Since the 1950s, large areas of upland peat wereafforested in northern European countries. In the UnitedKingdom, between the 1950s and 1980s, forests were

Copyright 2010 John Wiley & Sons, Ltd.

208 M. RODGERS ET AL.

planted on about 500 000 ha of peatland (Hargreaveset al., 2003). In Ireland, it was estimated that in 1990about 200 000 ha of forest was on peatland (Farrell, 1990)and between 1990 and 2000, about 98 000 ha of peatsoils was afforested (European Environmental Agency’sSpatial Analysis Group, 2004). Before the 1980s, mostof the Irish peatland forests were planted without riparianbuffer strips in upland areas that contain the headwatersof rivers, many of them salmonid. These forests are nowreaching harvestable age. Peatland is defined as a forestsoil type which has a greater risk of erosion in the IrishForest and Water Quality Guidelines (Forest Service,2000a). Due to the sensitivity of the upland water andblanket peat to the disturbance, concerns have been raisedabout the possible impacts of harvesting these forestsand associated activities on the receiving aquatic systems(Coillte Teo, 2007).

In order to minimize the amount of suspended solidsentering watercourses, good management practices wereintroduced in the United Kingdom (Forestry Commis-sion, 1988) and in Ireland (Forest Service, 2000a,b,c).These practices targeted the process of soil erosion, andincluded proper harvesting methods and the use of thickbrash mats to limit surface disturbance. The findings ofearlier harvesting studies in the United Kingdom andIreland were not relevant for the impact assessment offorestry operations carried out under the new forest andwater guidelines (Stott et al., 2001). To date, few studieshave focused on the impact of post-guideline harvest-ing on suspended solid yields (Nisbet, 2001; Stott et al.,2001).

In this study, an assessment of the impact of post-guideline harvesting on the suspended solid releasewas carried out in an upland blanket peat catchmentthat had been afforested in the 1970s without bufferstrips—typical of most Irish forests now approaching

harvestable age. It comprised of a control area upstreamof an experimental area. The experimental area washarvested in summer 2005. The measurement of sus-pended solid concentrations and stream flows, upstreamand downstream of the experimental area, commencedover a year earlier in summer 2004 and the suspendedsolid concentrations were intensively monitored. By pre-harvesting monitoring and comparing the experimentaldata from the experimental and upstream control areas,the impact of harvesting on suspended solid increasecould be accurately estimated (Ferguson, 1987).

The bedload in the flashy study stream was sand,stones and rocks. The average annual bedload mass of<50 kg, which accumulated and was cleared from themeasuring flume just downstream of the experimentalarea, was several orders smaller than the suspendedsolid mass released from the whole study area. In thisstudy, suspended solids refer to organic and inorganicmatter that occurs in water in suspended or particle form(Paavilainen and Paivanen, 1995). More than 80% ofthe suspended solids weight was lost after ignition at550 °C (APHA, 1995), indicating that the main contentof the suspended solids was organic matter, which wasconsidered to be the main threat to fauna in this salmonidcatchment.

The objectives of this study were to examine the impactof post-guideline harvesting of the blanket peat forest onthe suspended solid concentrations and yields.

STUDY SITE DESCRIPTION

The Burrishoole catchment, located in County Mayo,in the west of Ireland, consists of important salmonidproductive rivers and lakes (Figure 1). About 18% ofthe catchment is covered by forests that were plantedin the 1970s and which are now being, or are about

Figure 1. (a) Location of the Burrishoole catchment; (b) location of the study stream

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 25, 207–216 (2011)

SUSPENDED SOLID YIELD FROM FOREST HARVESTING ON A BLANKET PEAT 209

to be, harvested. The study site (9°550W, 35°550N),which is a sub-catchment of the Burrishoole catchmentdrained by a small first-order stream (Figure 1a), wasplanted with Lodgepole pine (Pinus contorta) betweenJanuary and April 1971. The stream is equipped withtwo flow monitoring stations at stable channel sections,an upstream station (US) and a downstream station (DS)of the experimental area (Figure 1b). The US measuresflows from the control area (area A in Figure 1b) of7Ð2 ha and the DS covers the control coupe and theexperimental coupe (coupes B in Figure 1b) with a totalcombined area of 17Ð7 ha. Before the start of this study,road drainage into the channel near the US gauge wasdiverted into an adjoining sub-catchment. In August2005, a wind-blown tree blocked one of the collectordrains, resulting in an increase of the upstream forestcontrol area (coupe D), to about 10Ð8 ha (coupes A and Din Figure 1b). Meanwhile the downstream harvested areaincreased to about 14Ð5 ha due to the blockage of a drainby brash mat during the harvesting, incorporating anotherpart of the total harvested area (coupe C). Fortunately, inboth cases the additional area had the same characteristicsof vegetation and soils, and the relative sizes of theUS and DS remained unchanged—US increasing onlymarginally from 41% of the total area to DS beforeharvesting and 43% after harvesting. All unit area depthsin this article have been calculated using these values.The blanket upland peat soil in all four areas from Ato D had been double mould board ploughed by a Fiattractor on tracks creating furrows and ribbons (overturnedturf ridges) with a 2-m spacing, aligned down the mainslope, together with several collector drains aligned closeto the contour. The trees were planted on the ribbonsat 1Ð5 m intervals, giving an approximate soil area of3 m2 per tree. The initial stand density was about 2800trees per ha but was reduced to about half by thinningand natural die-off before harvesting. The catchment hadan average peat depth of more than 2 m above thebedrock of quartzite, schist and volcanic rock, and thepeat typically had a gravimetric water content of morethan 80%. Close to the DS there is a deep incision,where the depth of the peat is about 0Ð5 m and rocksare found in the bed of the study stream. Compared tothe study catchment, this incision section is very small,with the area of less than 0Ð1 ha. In the catchments,the mean annual rainfall is more than 2000 mm and themean air temperature is about 11 °C. Hillslope gradientsin areas B and C average 8° and range between 0° and16°. Bole-only harvesting was conducted in areas B andC from 25 July to 22 September 2005. The timber washarvested using a Valmet 941 harvester, and the residues(i.e. needles, twigs and branches) were left on the soilsurface and collected together to form windrows. Duringharvesting, the boles were stacked beside the windrow forcollection. A Valmet 840 forwarder delivered the boles tothe truck collection points beside the forest service road.To minimize soil damage, the clear felling and harvestingwere conducted only in dry weather conditions from Julyto September 2005. This time period is recommended

for harvesting in the Irish Forest Harvesting and theEnvironment Guidelines since the ground conditionstend to be drier (Forest Service, 2000a). Mechanizedoperations were suspended during and immediately afterperiods of particularly heavy rainfall. Another importantgood management practice used during the harvestingoperation was the proper use of brash mats for machinetravelling. Tree residues (i.e. needles, twigs and branches)were collected together to form brash mats on which theharvesting machines travelled, thus protecting the soilsurface, and reducing erosion. In the lowest part of thesite where the stream is deeply incised, the trees werecut with a chain saw and left behind. The non-harvestedupstream areas of A and D was used as a control area inthis study as it had the same type and age of trees, similarsoil, hydrologic characteristics and size, as the harvestedexperimental areas of B and C. In the experimental area,the furrows and windrows/brash mats—formed from theharvest residues—are, in general, parallel with the studystream, which is at right angles to the contours. Thesurface water flow along the furrows, is collected bycollector drains (arrows in Figure 1c) and joins the studystream.

SAMPLING, MEASUREMENT

From April 2004 to March 2005, continuous water lev-els in the study stream were recorded at both the USand DS, and converted to flows by a rating equationbased on dilution gauging and current meter mea-surements. In April 2005, H-flume flow gauges wereinstalled at the sites for flow measurement. At USand DS, water samples were taken; (i) manually every20 min from April 2004 to March 2005 during floodevents, (ii) hourly from April 2005 to March 2006using ISCO automatic water samplers and (iii) manuallyin baseflow conditions through the study period. Sus-pended solid concentrations of the water samples weremeasured at the Marine Institute in Newport, CountyMayo in accordance with the standard methods (APHA,1995) using Whatman GF/C (pore size 1Ð2 µm) filterpapers.

THE POSSIBLE LONGEVITY OF THE IMPACT

Harvesting activities could immediately increase solidyield (Cornish and Binns, 1987). The longevity of theimpact of harvesting on suspended solid concentrationsdepends on the recovery of the catchments from soil dis-turbance, which could depend on: (i) weather conditions,(ii) soil properties and ground slopes and (iii) the growthof vegetation. Previous studies reported that the impact ofharvesting on solid concentrations could last from a fewmonths to a few years (Macdonald et al., 2003; Stott,2005). Figure 2a and b show the daily mean and peaksuspended solid concentrations at the US and DS dur-ing the study period. The daily mean suspended solidconcentration was calculated based on Ferguson (1987).



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Figure (a) Daily average suspended solid concentrations at US and DS stations beforeand after harvesting

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Figure (c) Monthly rainfall before and after harvesting

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Figure (b) Daily peak suspended solid concentrations at US and DS stations beforeand after harvesting

Clearfelling

Figure 2. (a) Daily average suspended solid concentrations at the US and DS before and after harvesting; (b) daily peak suspended solid concentrationsat the US and DS before and after harvesting; (c) monthly rainfall before and after harvesting

Harvesting did not result in an obvious increase in thedaily suspended solid mean concentrations at the DS.However, daily peak suspended solid concentrations atthe DS increased after harvesting and lasted for about7 months (September 2005–March 2006) before return-ing to the US levels. Compared to the previous andfollowing periods, no increase in monthly rainfall in theperiod from September 2005 to March 2006 was found(Figure 2c). The 7-month increase in peak concentrations

after harvesting could be due to the flushing out of loosematerial exposed by the felling activities. Short-term ele-vation in suspended solid concentrations could damagethe water ecology and result in the reduction of survivalrates of salmonid eggs and newly hatched alevins. Thisarticle focused on the assessment of the impact of har-vesting on the suspended solid concentrations in the first7 months post-harvesting. Intensively monitoring the sus-pended solid concentrations in this period would allow



us to detect any possible increases in soil release afterharvesting.

ANALYSIS METHODS

To determine the harvesting effect on soil release, a cal-ibration equation was established between US and DSsuspended solid concentration data for the pre-harvestingperiod. The dependent variable was the suspended solidat the DS and the independent variable was the suspendedsolid at the US. After harvesting, the same equation wasused to estimate ‘no-felling’ suspended solid concentra-tions at DS for the observed values of suspended solidat US. This should allow for the effect of weather con-ditions, so that the impact of the harvesting on the soilrelease can be established from (i) comparing the mea-sured and estimated suspended solid concentrations and(ii) determining the statistical significance of any concen-tration differences by, for example, using a t-test

The characteristics of solid yields were examined byusing the solid yield rating curve. In this study, the solidyield rating curve is defined as a simple power function(Hotta et al., 2007) and used for suspended solid yieldestimation

QS D ˛Qˇ �1�

where QS represents the solid yield, Q is the waterdischarge, and ˛ and ˇ are obtained by the least squaresmethod using observed solid yield and water dischargedata. The values of ˛ and ˇ were calculated and comparedbefore and after harvesting for the study and controlcatchments. Monthly suspended solid yields in storm

events at DS and US before and after harvesting werecalculated using Equation (1).

RESULTS

Suspended solid concentrations before and afterharvesting

During baseflow conditions, suspended solid concen-trations at the US and DS were generally low before andafter harvesting and ranged from 0Ð1 to 5 mg/l. Streamsuspended solids are usually episodic—most solids arecarried in high flows—so this study focused on thestorm events. Table I lists the studied storm events beforeand after harvesting. A rainfall event was defined as ablock of rainfall that was preceded and followed by atleast 12 h of no rainfall (Hotta et al., 2007). A totalof 23 events were studied in this paper: 8 before and15 after harvesting. A total of 114 and 394 water sam-ples were collected at both stations before and afterharvesting, respectively. Figure 3 shows the suspendedsolid concentrations and flows in some storm eventsbefore and after the harvesting period. As expected,variations in suspended solid concentration roughly cor-relate with the temporal profile of water discharge,and bigger storm events generally result in higher sus-pended solid concentrations. The biggest storm eventin the pre-harvesting period occurred on 22 June 2004with 86Ð8 mm rainfall, having a maximum intensity of2Ð2 mm/5 min and duration of about 32 h (Table I).The highest suspended solid concentrations during thisstorm were 37Ð8 mg/l at the US and 65 mg/l at theDS, respectively, which were the maximum suspended

Table I. Rainfall events during which samples were taken

Stormevents

Rainfall duration(dd/mm/yyyy)

Totalrainfall (mm)

Maximum runoffrate at the DS (l/s)

Maximum rainfallintensity (mm/h)

1 17/04/2004 to 18/04/2004 13Ð2 41Ð6 62 22/06/2004 to 23/06/2004 86Ð8 100Ð3 5Ð83 20/07/2004 to 20/07/2004 22Ð2 48 5Ð64 19/08/2004 to 20/08/2004 16Ð2 12Ð6 5Ð85 27/11/2004 to 28/11/2004 17Ð8 100Ð3 8Ð06 09/12/2004 to 10/12/2004 14Ð4 91Ð9 3Ð27 14/03/2005 to 15/03/2005 35Ð8 87Ð8 5Ð08 05/05/2005 to 05/05/2005 9Ð8 14Ð5 4Ð0Clear felling and harvesting (July to early September 2005)9 21/09/2005 to 22/09/2005 7Ð8 8Ð8 7Ð010 29/09/2005 to 01/10/2005 22Ð8 20Ð8 2Ð811 07/10/2005 to 10/10/2005 56Ð8 32Ð1 7Ð412 28/10/2005 to 30/10/2005 18Ð6 43Ð6 6Ð213 01/11/2005 to 04/11/2005 67Ð2 >158 8Ð814 07/11/2005 to 10/11/2005 56Ð8 93Ð6 7Ð415 10/11/2005 to 13/11/2005 24Ð4 34Ð9 4Ð016 30/11/2005 to 01/12/2005 8Ð4 22Ð3 1Ð617 07/12/2005 to 07/12/2005 23Ð2 107Ð1 518 22/12/2005 to 23/12/2005 15Ð4 86Ð1 2Ð619 09/01/2006 to 11/01/2006 35Ð6 59Ð8 7Ð020 17/01/2006 to 19/01/2006 17Ð4 88Ð4 4Ð421 13/02/2006 to 14/02/2006 38Ð8 152Ð1 9Ð022 06/03/2006 to 08/03/2006 21Ð8 69Ð3 3Ð023 13/03/2006 to 14/03/2006 29Ð4 154Ð5 6Ð4



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Figure (c) Post-harvesting (1-4/11/2005) (The flume capacity was about 158 l/s)

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Figure 3. Suspended solid concentrations (SS) in the storms at US and DS before and after harvesting. (a) Pre-harvesting (22 June 2004);(b) pre-harvesting (14 March 2005); (c) post-harvesting (1–4 November 2005) (The flume capacity was about 158 l/s) and (d) post-harvesting

(7 December 2005)



solid concentrations observed during the pre-harvestingperiod. During the post-harvesting study period, thebiggest storm event occurred on 1 November 2005 witha total rainfall of 67Ð2 mm, maximum rainfall intensityof 3Ð2 mm/5 min, and duration of 82 h (Table I). Sus-pended solid concentrations at the US increased from0Ð1 to 25Ð8 mg/l and then dropped back to 0Ð5 mg/l. Atthe DS, suspended solid concentrations increased from0Ð3 mg/l to a peak of 97Ð5 mg/l towards the begin-ning of the flood event as the flow rate increased fromabout 4Ð5 to 12Ð5 l/s, which was the highest suspendedsolid concentration observed during the post-harvestingstudy period (Figure 3c). Three water discharge peaks of(i) 140 l/s, (ii) >150 l/s and (iii) 57 l/s occurred in thisstorm event, with the three corresponding sediment con-centration peaks of 97Ð5, 44 and 15 mg/l, respectively.Much higher solid concentrations were observed in thefirst peak, although the second peak had a much higherwater discharge, indicative of a lack of available erodedsource material during the following flow peak. In mostof the studied storms, suspended solid increased quicklyat the beginning of the water discharge and reached themaximum prior to the water discharge peak, which couldbe due to the build-up of the soil fraction available forrelease and erosion prior to rainfall. Similar phenomenawere also observed by Drewry et al. (2008) and Baca(2002).

Figure 4a and b show the relationships between sus-pended solid concentrations of the US and DS before andafter harvesting, respectively. Larger scatter was foundin the correlation of US and DS suspended solid con-centrations after harvesting. Almost all of the highestpost-harvesting concentrations occurred in storm event

y = 1.3477x1.0043

R2 = 0.7813

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Figure (a) Pre-harvesting

y = 1.9836x0.8073

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Figure (b) Post-harvesting.

Figure 4. The relationship between the suspended solid concentrations atUS and DS stations (a) before and (b) after harvesting

on 2 November 2005—the first high storm event afterharvesting—and its following storm event on 11 Novem-ber 2005, which could be due to the flushing out of loosematerial exposed by the felling. In most of the stormevents, the peak flows passed the US earlier than theDS with the time difference of <30 min. Simple powerequations were used to describe the solid relationshipsbetween the two stations

CDS D a.CUSb �2�

where CDS and CUS are the suspended solid concentra-tions at DS and US stations, and a and b were obtainedby the least squares method.

Parameter a increased from about 1Ð35 before harvest-ing to about 1Ð98 after harvesting and b decreased from1Ð01 to 0Ð81. In Equation (2), an increase in b may resultin more significant increases in the solid at the DS thanan increase in a. In order to examine the impact of theharvesting activities on the sediment release, the solid atthe DS was estimated as the dependent variable by usingthe pre-harvesting power function equation (a D 1Ð35 andb D 1Ð0) and the observed post-harvesting solid at the USas the independent variable. The estimated and measuredsolid concentrations at DS were compared using a pairedsample t-test at the 95% significance level (P D 0Ð05)(http://www.spss.com), which indicated that there was nostatistically significant difference between the estimatedand measured concentrations.

USy = 8.1043x1.0818

R2 = 0.7101

DSy = 11.946x1.1073

R2 = 0.8298

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USy = 5.936x1.0103

R2 = 0.556

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Figure (b) Post- harvesting

Figure 5. The relationship between water discharge and solid loadscalculated using suspended solid concentrations and water discharge data

in the (a) pre- and (b) post-harvesting study periods



Pre- and post-harvesting solid rating curves

Figure 5a and b show the relationship between solidloads and water discharge calculated using suspendedsolid concentrations and water discharge data duringthe pre- and post-harvesting study periods, respectively,which reveal no detectable post harvest increase. Com-bining all the storms, before and after harvesting, ˛ andˇ in Equation (1) were obtained by applying the leastsquares method (Table II). At the US, ˛ and ˇ decreasedfrom 8Ð1 and 1Ð08 before harvesting to 5Ð94 and 1Ð01after harvesting, respectively. At the DS, ˛ decreasedfrom 11Ð95 before harvesting to 6Ð0 after harvesting andˇ slightly increased from 1Ð11 to 1Ð17.

Figures 6 and 7 show the relationship of monthly waterdischarges and solid yields, respectively. The solid yieldin storm events was calculated by placing the waterdischarge and the values of ˛ and ˇ in Table II intoEquation (1). The monthly solid yield was achieved byaccumulating the solid yields in all the storm events inthe month. As shown in Figure 6, the water discharge perunit area increased after harvesting, which was probablymainly due to lower interception losses (Calder, 1986;Robinson and Dupeyrat, 2005). The water discharge atthe DS and US had a very good linear relationshipduring the pre- and post- harvesting periods. The linearfactors were similar and close to 1 during the pre-and post- harvesting periods. A linear relationship wasalso found between the solid yields at the DS and USas shown in Figure 7. The slopes for pre-harvestingand post- harvesting were similar. Solid yields slightlyincreased after harvesting (Figure 7), which could beattributed to the increase in runoff. In order to examinethe impact of the harvesting activities on the solid yield,the sediment at DS was estimated as the dependentvariable by using the pre-harvesting linear regressionequation and the observed post-harvesting sediment yieldat US as the independent variable. The estimated andmeasured sediment yield at the DS was compared usinga paired sample t-test at the 95% significance level(P D 0Ð05) (http://www.spss.com), which indicated thatthere was no significant difference between the estimatedand measured sediment yield.

DISCUSSION

The pair of parameters ˛ and ˇ in the solid rating curvein Equation (1) represent the erosion characteristics ofthe catchment. The two parameters fluctuate from stormto storm (Morehead et al., 2003). The values of these

Table II. Pre- and post-harvest ˛ and ˇ in the control and studycatchments for all storms

Time Catchment ˛ ˇ r2

Pre-harvesting Control (US) 8Ð1 1Ð08 0Ð71Study (DS) 11Ð95 1Ð11 0Ð83

Post-harvesting Control (US) 5Ð94 1Ð01 0Ð56Study (DS) 6 1Ð17 0Ð76

Pre-harvestingy = 0.9919x - 13.717

R2 = 0.9439

Post-harvestingy = 0.9458x + 0.9268

R2 = 0.9425

0

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140

160

0 20 40 60 80 100 120 140 160

Water discharge from control site (mm/month)

Wat

er d

isch

arge

from

stu

dyca

tchm

ent (

mm

/mon

th) Post-harvesting

Pre-harvesting

Figure 6. The relationship of monthly water discharge of US and DSpre- and post-harvesting (Pre-harvesting: April 2004 to June 2005,except January 2005, March 2005 and April 2005 due to lack of data;

post-harvesting: October 2005 to March 2006)

Post-harvestingy = 1.7585x + 0.2189

R2 = 0.9489

Pre-harvestingy = 1.7043x - 2.4293

R2 = 0.9378

0

5

10

15

20

25

0 10 15

Estimated US load (kg/ha.month)

Est

imat

ed D

S lo

ad(k

g/ha

.mon

th)

Pre-harvesting

Post-harvesting

5

Figure 7. The relationship of calculated monthly solid loads of US andDS pre- and post-harvesting (Pre-harvesting: April 2004 to June 2005,except January 2005, March 2005 and April 2005; post-harvesting:

October 2005 to March 2006)

parameters in each of the studied storms are presented inFigure 8a and b for before and after harvesting, respec-tively, which indicated that the erosion characteristics ofthe study site were the same as the control site and didnot change significantly after harvesting.

0

0.5

1

1.5

2

1 10 100

Rating coefficient (α)

Rat

ing

coef

ficie

nt (

β)

USDS


Rating coefficient (α)

Rat

ing

coef

ficie

nt (

β)

00.5

11.5

2

2.53

0.01 0.1 1 10 100

US

DS

Figure (b) Post-harvesting

Figure 8. The relationships between the solid load rating curve parame-ters for individual storms (a) before and (b) after harvesting



Although higher daily peak suspended solid concen-trations were observed, there was no significant sus-pended solid concentrations increase after harvesting inthis study. Hotta et al. (2007) indicated that if appropriatemeasures are undertaken to prevent surface disturbance,there may not be an increase in the sediment concen-trations during and after harvesting; they used skylineharvesting treatment and found there were no sedimentconcentration or yield increases after harvesting. In thisBurrishoole study, the soil disturbance and streambankerosion during the harvesting operation were minimizedas much as possible by applying best management prac-tices (Forest Service, 2000a): harvesting was conductedonly in dry weather conditions; brash mats were prop-erly used and maintained; the harvester had a 10 m reachwhich minimized the soil disturbance within 10 m of thestudy stream; and hand cutting was used on steep slopesand the felled tree boles were left behind. No streambankerosion, due to the forest activities, was observed in thisstudy site. In their post-guideline harvesting study, Stottet al. (2001) emphasized the importance of the timingof harvesting work and recommended that the forestryguidelines should also include the hydrological and mete-orological conditions under which work can be under-taken near watercourses. A preliminary study carriedout by the authors—using laboratory flume technology(Rose, 1993) to monitor the effect of the harvest machinedisturbance—indicated that suspended solid concentra-tions (data not shown) could increase by two orders ofmagnitude from dry to wet conditions. Owende et al.(2002) investigated the progression of ground disturbanceon a peat site during forwarder extraction on a brash mat,and found that when maintenance of the brash mat wasconducted on an on-going basis, the deterioration of weakareas in the brash mat was prevented and, as a conse-quence, deep disturbance and rutting was minimized.

The hill slopes and streambanks are considered to bethe main solid sources in a catchment (Egashira andAshida, 1981; Hotta et al., 2007). Soil erosion generallydoes not occur in an undisturbed forest because duringmost rainfall events runoff only flows within the humiclayer. Smith (2008) reported that channels dominate solidsupply in sub-catchments. Smith and Dragovich (2008)investigated the solid sources by using radionuclides andfound that 81% of the solid flux was from the channel andgully wall in an upland catchment. In their study, Hottaet al. (2007) concluded that the streambanks/riparianzone, rather than the forest area, were the solid sourcesin their catchment. When a stream serves as the solidsource area, the solid-released patterns differ dependingon whether the water discharge is in the rising orfalling stage. As the water level rises, most of theerodible material on the surface of a streambank canbe readily transported by flushing, creating a suspendedsolid concentration peak in the early stage of rising. Inthis study, higher suspended solid concentrations werealso always in the rising stage in most of the stormsat the control and study site before and after harvesting(Figure 2). Therefore, the stream, drain and furrow banks

were considered to be the most likely main pre- and post-harvesting solid sources.

The solid yield was determined from the suspendedsolid concentrations and water discharge data. Anincrease in either or both could result in an increase ofsolid yield. Good management practice could prevent thesuspended solid concentration increase by minimizing thedisturbance of the soil, but cannot prevent the increase ofwater discharge after harvesting, due to the lower evapo-ration from the harvested area. This is especially the casein temperate maritime climates such as Britain and Ire-land where frequent light rainfall means tree canopies areoften wet and the interception losses are high (Robinsonand Dupeyrat, 2005). In this study, the slight increase insolid yields after harvesting could be due to the increasein water discharge, since no significant suspended solidconcentration increases were observed.

CONCLUSIONS

The results of this study indicated that post-guidelineharvesting did not have a long-term impact on thesuspended solid concentrations and did not change theerosion characteristics of the catchment. Solid yieldsslightly increased after harvesting could be due to theincrease in water discharge from the experimental area.The stream, drain and furrow banks were considered tobe the principal solid sources before and after harvesting.The study indicated that it is possible to prevent thesolid concentration increase after harvesting if goodmanagement practices are strictly followed.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the funding fromIreland EPA, COFORD, Coillte, National Parks andWildlife Service and the Marine Institute. They alsoacknowledge the assistance of their colleagues: Elvira deEyto and Liz Ryder.

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Suspended solid yield from forest harvesting on upland blanket peat