P-137
Effects of forest thinning on plot and catchment runoff responses
in Japanese headwater basinsTokyo University of Agriculture and Technology
Disaster Prevention Research Institute, Kyoto University
Graduate School of Agriculture, Kyoto University
Graduate School of Life and Environmental Sciences, University of Tsukuba○Bui Xuan Dung
Takashi Gomi
Shusuke Miyata
Ken’ichirou Kosugi
Yuichi Onda

1. Introduction
Forest thinning is an important part of the silvicultural treatments which is the preferred harvesting
method in water producing watersheds. Thinning is the only currently used practice to maximize
productivity of water by potential increasing water availability (Dung et al., 2010). However, effects ofthinning on runoff responses in the previous studies were not consistent. A few studies showed
increases of runoff, whereas other results showed decreases or no effect. Most of the previous studies
originated only in the catchment scale, whereas the changes in net precipitation and infiltration afterthinning occur in hillslope scales. For examining the effects of forest thinning on hydrological process,we evaluated; (1) Overland flow runoff response in hillslope scale and (2) Storm and base flow runoffresponse in catchment scale after thinning.
2. Study site
This study was conducted in two small
catchments (0.19ha: catchment M4 and 0.35ha:catchment M5) in Mie Prefecture (34°21′ N,136°25′ E), south-central Japan (Fig. 1a). The
catchment area of M5 was covered by
Japanese cypress with 3500 stems/ha in
pre-thinning. Mean annual precipitation and airtemperature was 2000 mm and 14°C,respectively. Dominant hillslope gradient
ranges from 350 to 450. The soil depth ranged
from 0.6 to 1.8 m. Bedrock is consisted byCambisols.

Hillslope plot 13. Method
Figure 1. (a) Location of study site; (b) location andWe established hillslope plots within two
topography of study catchments and hillslope plots; (c)small catchments for monitoring overland
Treatment characteristics of hillslope plot 1 and catchment
flow generation and catchment runoff.
M5Catchment M5 (including hillslope plot 1) wasa treatment catchment in which 58.3% of stems (corresponding to 43.2% of basal area) were removed,
whereas the catchment M4 (including hillslope plot 2) remained untreated as a control (Fig. 1b and 1c).
Overland flow generation and catchment runoff were monitored over 2 years pre- and 2 year
post-thinning periods. Estimation of quick and delayed runoff using hydrograph separation analysis
adopted from Hewlett and Hibbert (1967).To examine the effect of forest thinning on monthly runoff
responses, we applied statistical analysis for paired catchment data. We established a pre-logging
calibration equation between catchments M5 and M4 using Generalized Linear Model (GLM). Then
we predicted post-thinning runoff using the calibration model. Finally, we conducted t-test for value ofresiduals (observed – predicted values). The statistical analysis was conducted using R package
4. Results and discussion
Catchment and hillslope runoff from M5 and M4 were significantly correlated when developing thecalibration equations (Fig.2). Significant differences appeared in total monthly catchment runoff anddelayed runoff between the pre-thinning and post-thinning periods. Treatment effects were consistently
positive in both catchment and plot runoff in the post-thinning period (Fig. 2). However, no significant- 600 -Catchment M5 (mm)Hillslope plot 1 (mm)differences appeared in the monthly quick runoff of catchment M5 and monthly overland flow fromhillslope plot 1.
(a) Monthly catchment runoff(b) Monthly overland flow
Paired-catchment analysis of monthly data
100100also revealed significant increases incatchment runoff, but hillslope plot runoff
1010
did not change significantly. Estimatedincrease in mean annual runoff was 240.7
11mm of the post-thinning period. This increase
Pre-thinning
Post-thinningof runoff was mostly associated with the0.10.1
increase of the delayed runoff.
0.1110100
0.1110100The duration and magnitude of daily
Hillslope plot 2 (mm)Catchment M4 (mm)precipitation were similar during pre- andpost-thinning periods (Fig. 3a). Mean daily

Figure 2. (a) Monthly runoff from treated catchment M5 andcontrol catchment M4; and (b) monthly hillslope plot runoff
from treated plot 1 and control plot 2.

Overland flow (mm day-1)

Catchment runoff (mm day-1)

Precipitation (mm day-1)

catchment runoff during the pre-thinning(a)
1st year
100Pre-thinning
2nd yearperiod was 1.8 mm for M5 and 1.1 mm for1st yearM4. During the post-thinning period, mean
10
Post-thinning
2nd yeardaily runoff from M5 increased to 3.1 mm, but1
remained relatively constant from M4 (mean:0.9mm) (Fig. 3b). Mean overland was similar
0.1
between pre- and post-thinning periods in
Catchment M4
Catchment M5(b)100hillslope plots 1 and 2 in M5 and M4 (Fig. 3c).
At M5, mean daily quick runoff during the10pre- and post-thinning periods was 0.9 and 0.7
1mm, respectively, while mean daily delayed
runoff during the pre- and post-thinning
0.1
periods was 0.9 and 2.4 mm, respectively.
(c)Hillslope plot 2
Hillslope plot 1
100
Mean daily flow duration of the ephemeralchannel in catchment M5 was 207 days10(58.6% of the year) during the pre-thinningperiod. The duration of catchment runoff in
1
M5 was 287 days (78.6% of the year) in the0.1
first year of the post-thinning period and 249
020406080
100 020406080
100
Percentage of day (%)
days (68.2% of the year) in the second year ofthe post-thinning period. In M4, the duration ofFigure 3. Percentage of days of (a) precipitation; (b)runoff in the ephemeral channel ranged fromcatchment runoff responses; and (c) overland flow responses
130–202 days (35.6–55.3% of 365 days) during during the two years pre- and two year post-thinningthe first and second years of the post-thinning
periods.
period. Changes in daily flow duration between the first and second year after thinning suggested that
canopy closer after thinning and increased evapotranspiration may occur from the first to second year.
5. Conclusion
Based on our field observations and analysis, we concluded the following issues for effect ofthinning on runoff generation: (1) Forest thinning increased annual runoff by 240.7 mm; (2) Forest
thinning increased flow duration of an ephemeral channel from 56.7–73.3%; (3) Increases in plot
runoff were not significant and did not directly contribute to the increases in catchment runoff, and (4)Increases in runoff response were associated with increases in base flow components, which wererelated to changes in net precipitation, soil water availability, and evapotranspiration.
References
Dung BX, Miyata S, and Gomi T., 2010. Effect of forest thinning on overland flow generation on hillslopes covered byJapanese cypress. Ecohydrology. Doi: 10.1002/eco.135
Hewlett JD, Hibbert AR. 1967. Factors affecting the response of small watersheds to precipitation in humid areas, in:Sopper, W.E., Lull, H.W. (Eds.), Intl. Symp. For. Hydrol. Pergamon, New York, pp. 275-290.
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