IN-02
Hillslope-channel sediment production in a headwater catchment of Tanzawa Mountain1

Rose TIRTALISTYANI 1, Marino HIRAOKA 2, Takashi GOMI 1, Yoshimi UCHIYAMA 3International Environmental and Agricultural Science, Tokyo University of Agriculture and Technology
2
Faculty of Life and Environmental Science, University of Tsukuba
3Natural Environment Conservation Center, Kanagawa

INTRODUCTION
Sediment production in headwater catchments is an
important factor for the dynamics of sediment transportand routing from up to downstream (Gomi et al., 2002).
Sediment supplied from hillslopes is often directly linkedto channel especially in steep mountainous catchments(Benda et al., 1998). For instance, Lehre (1982) reported
sediment production during 4 years observation period in
1.74 km2 Small Coast Ranges Drainage Basin, California
ranges from 148 to 1575 t/km2/yr, which mainly causedby landslide, sheet, and gully erosion.
Various methodologies have been used to estimate
sediment production on hillslopes. In general, closed plot
(e.g., 1 x 1 m plots) is commonly used for estimating
sediment production (i.e., Ghahramani et al, 2011).
Hence, such closed plots could not consider continuous
soil movement from upper to lower parts of hillslopes
because upper boundary prevents additional soil
movement from the outside of plots. Furthermore, soil
properties often change inside the closed plots byselective movement of soil. Therefore, the efforts still
remains for estimating soil erosion from hillslope tostreams which are important for developing catchment
sediment budgets (Slaymaker, 1993).
For conquering these problems, we developed open
plots for capturing the sediment on steep hillslopes.
Various reseaches such as Imaizumi et al. (2015) usedopen plots (no upper boundary) for using the sedimenttransport based on the unit length of movement (e.g.,weight/width of plot/period). Based on the topographic
configuration, we applied open plot methods for
estimatingthesedimentproduction
rate(weight/area/period). Open plots also possible for
estimating the sediment entering to the channels via up
slope. Thus, the primal objective of this study is toestimates the amount of hillslope to stream sedimentproduction in forested headwater catchments. Based on
the field investigation, we discussed factors for temporaland spatial variability of sediment production in forested
hillslopes.
STUDY SITE AND METHODOLOGYThis research conducted in two forested headwater
catchments at the Oborasawa Watershed (35o28’N,139o12’E) in eastern of Tanzawa Mountains, Kanagawa,Japan (Fig.1). The headwater catchments are further
referred as catchment No.3 and No.4 with drainage area
7 ha and 5 ha, respectively. The climate is humid andmild with annual precipitation around 3000 mm. Mean
hillslope gradient of the catchments are 36o, steep
hillslope >40o is distributed along the stream (Gomi et al.,2013 in Hiraoka et al., 2015).
The sediment traps were placed in four different
locations with gradients ranged from 41o to 53o. The
sediment traps were designed to capture delivered
sediment from hillslopes to the channels by installing a
low-cost geotextile fences in the foot of the hillslopes.
The sediment traps have 1 m width and located 4 to 15 m

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from the stream surface. Understory vegetation barely
covered on hillslope. Distribution of leaf litter varied
depending on availability of litter and micro-topography
including tree stands, litter, and small tree branches
found on forest floor. All of hillslope plots wereconsidered to be planar shape.
Further, sediment and litter inside the sediment traps
were collected every one to two month and brought to thelaboratory for further analysis. Sediment and litter wereseparated inside a bucket by washing of the remaining
sediment on the litter with water. The remaining waterand sediment were stored in the undisturbed storage to beleft settled for 24 hours. The settled sediment in thebucket then roughly drained to reduce the water. The
sediment and litter were oven dried with temperature
105o for 24 hours and weight to measure the dry mass.
RESULTS
Forty-four sediment samples from various
observation periods were captured in continues
monitoring from 2010 to 2015. The annual precipitation
during the observation period was ranged from 3222 mm
in 2014 to 2739 mm in 2013, with the mean annual
precipitation of 2992 mm/yr and maximum 24 hours of57 mm/hr in 2012. Total sediment during 6 year
observation period was 0.4 tons, where 42% of thesediment is produced from sediment trap of P3-2. The
lowest amount of sediment, consisted only 8%, found in
P4-2 plot.
The mean annual sediment production rate of fourplots varied from 0.2 mm/yr in 2013 to 1.2 mm/yr in 2012
(Fig.2). Mean six years sediment production rate was 0.6± 0.6 mm/yr. Highest sediment production rates occurredin plot P3-2 with mean sediment production rates 1.0 ±0.4 mm/yr. Order of magnitude of sediment production
rate in P3-1 (0.5 ± 0.2 mm/yr) and P4-1 (0.1 ± 0.05mm/yr) was similar, while the standard deviation
substantially greater in P3-1.
Runoff litter occupied at least 7% of the totalmaterial collected from the traps. The mean annual litter
runoff during observation periods was 60.9 ± 49.7

susceptible to disturbance such as raindrop or freezethaw process (Bryan, 2000). Mix overstory vegetationsuch as Japanese cedar and broadleaf trees tends to createa stable litter cover that protect the soil surface from theimpact of rainfall (Hiraoka et al., 2013).
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headwater catchment, forested catchment- 381 -