2CEffects of strip thinning on suspended sediment yields using paired-catchment analysis○Sooyoun Nam, Takashi Gomi, Dung X Bui, Marino Hiraoka (Tokyo University of Agriculture and Technology)
Yuichi Onda, Hiroaki Kato (University of Tsukuba)
1. INTRODUCTION
Paired-catchment approach has been widely applied for detecting the effects of forest management with hydrology (Hewlett,1971). Changes in the suspended sediment yields (SSY) due to forest harvesting were combined impacts of changes inhydrological responses and sources of SSY (Asselman, 1999). Dung et al., (2013) showed that catchment runoff increased 227
to 667 mm after 50% of forest thinning depending on the size of drainage areas. However, changes in SSY was never been
examined using paired catchment approaches, particularly for 50% of commercial thinning. We examined (1) changes insuspended sediment yield using paired catchment experiment after forest thinning; (2) compared to the order of magnitude ofchances compared to the other studies.

2. STUDY SITE AND METHODThis study is conducted in two small
headwater catchments (K2- 0.17 km2, K30.02 km2) covered by 20-50 yrs Japanesecypress (Chamaecyparis obtusa) and cedar
plantations (Cryptomeria japonica) at Field
Museum (FM) Karasawayama in Tochigi
prefecture (E 36.36 N 139.60) (Fig. 1).
Mean precipitation is 1,239 mm and meanannual temperature is 13.9°. The altitude
ranges from 90 to 290 m above the sea level.
Dominant hillslope gradient ranges from 20to 45°. The underlined geology is consisted ofFig. 1 Location of study catchment areas
sedimentary rock.
Discharge and turbidity of catchment K2and K3 was monitored from September,2010 to February, 2012. Because 50%thinning of Japanese cypress and cedar of K2
was removed by machine with skid trail
installation (total length (626 m) and area(0.003 km2)), we defined the period before
thinning effects was pre-thinning, while postthinning period was after thinning effects
Fig. 2 Schematic illustration for paired-catchment analysis based on (a) preincluding thinning.
treatment calibration and (b) pre- and post-thinning treatments (Hewlett, 1982)
Paired-catchment approach was used on anassumption of SSY in control and treatedcatchments in pre- and post-thinning periods. This analysis can separate the changes in vegetation conditions from climate
effects (Bosch and Hewlett, 1982). The calibration equation involved development of a regression of SSY between pairedcatchments (Fig. 2a). We then calculated the SSY of the post-thinning period in K2 using pre-thinning calibration. Treatment
effects was estimated from observed SSY and predicted ones in K2 (Fig. 2b)

3. RESULTS AND DISCUSSDION
The pre-treatment regression was significant at a 90% confidence level (P = 0.09) (Fig.3a). Treatment effects of pre- andpost-thinning periods also significantly differed (P = 0.04) (Fig.3b).
Greatest treatment effects were occurred storm events with large and small precipitation during thinning. In post-thinning㪄㩷㪘㪄㪉㪊㪇㩷㪄
period, estimated SSY was 1.0 t km-2 yr-1, while observed SSY was in 2.0 t km-2 yr-1. Therefore, we found that 200% of greater
increases in SSY after thinning.
Dung et al. (2013) showed thatrunoff increases of treated catchment(1,931 mm yr-1) were 2.6 times greaterthan one of the control catchment (735mm yr-1) in post-thinning period.
Therefore, our SSY changes afterthinning was similar at the order ofmagnitude of changes in hydrologicalresponses.
We summarized the previousFig.3 (a) Comparison on SSY in control and treated catchments; Straight and dotted line
publications for changes in SSY after
were calculated using regression analysis in pre- and post-thinning periods, separately; (b)thinning and clear cut . Changes in
Differences between observed and estimated SSY in pre- and post-thinning periods
SSY in these studies was ranged from
2 and 53 times compared to theTable 1. Summary on effects of thinning and clear cut on SSY
previous studies (Table 1). Most of theArea
SSY (t km-2 yr-1) Increase inprevious studies were also reported
Thinning Treatment
Reference
SSY (times)
PrePost(km )that timber harvesting including
Selective thinning (25% of the
10303
Kasran & Nik (1994)
0.28
stocking and 60-45 cm DBH)
cutting plus skid trial installation
Selective thinning (cutting regimes :
0.13
1427
2
Kasran (1988)
increased SSY. Because, skid trail
only 60 cm)
Selective thinning (cutting regimes:
0.307122
Kasran (1988)
installing increased slope gradient on
60-90 cm DBH )Foresttothinning(40%extraction
cut and fill slopes in mountainous
0.13
1427
2
Zulkifli et al. (1990)
unsupervisd)
areas, hence skdi trial provide SS to
10.00
Selective thinning (Steepland)
54282653
Lai (1992)
0.5-1.0 Selective thinning
60660
11Dougla et al. (1992)
stream. Our SSY in the post- thinning
1.37
Slective thinning (20% of sub-basin)
018Aderson and Potts (1987)
period (1.0 t km-2 yr-1) tended to be
0.89
Clear cut (20%of the catchment)
2444
2
Stott et al. (2001)
lower than most of the previous
3.08
Clear cut2457
2
Leeks and Roberts (1987)
studies. Beacase, our SSY based on
0.84
Clear cut55394
7Freguson et al (1991)
Clear cut (progressive felling 50% of
the paried-catchment analysis was
6.85
19116
6Johnson (1993)
catchment)
influenced by sptial specific results for
0.75
Clear cut (82%)
8182
Beschta (1980)
minizing time effects, while SSY insingle catchment analysis (Table 1)
0.96
Clear cut (100%)
14170
12Grant and Wolff (1991)
1.01
Selective thinning (25% patch cut)
150
260
2
Grant and Wolff (1992)
was impacted form both of individual
n/aLogging
n/an/a45
Megahan (1972)
spatial and temporal specific results.
n/a : Not available
Therefore, we can expect that pairedcatchment analysis would be likely toapply for effects on strip thinning on SSY because of maximizing treatment effects.
24. SUMMARY
Our summaries are (1) forest thinning increased 2.0 t km-2 yr-1, as observed SSY, and 1.0 t km-2 yr-1, as estimated SSY by paired
catchment analysis; (2) estimated SSY be more reliable than observed SSY for indentifying treatment effects due to maximizing
spatial specific, while minimizing temporal specific; and (3) from previous studies and this study, increases in SSY after strip
thinning were greater than careful management for minimizing soil compaction. We further need to analyze the sources of SSY
using the nested catchment monitoring in K2.

REFERENCES
Dung BX, Gomi T, Onda Y, Kata H, Hiraoka (2013): Conference on Japan Society of Erosion Control Engineering, Shizuoka, JapanU
Hewlett JD (1971): Water Resources Bulletin 7: 376-381.
Stednick (1996) : Journal of Hydrology 176: 79-95.

㪄㩷㪘㪄㪉㪊㪈㩷㪄