P5)

The relationship between river runoff and soil moisture content
in the Saromabetsu river basin, Hokkaido○Mohammad T. Islam., Kaz A. Chikita., Tomoyuki Wada., Takuma Yamaguchi
Laboratory of Physical Hydrology, Graduate School of Science, Hokkaido University,

1. Introduction
The importance of macropores as preferential
pathways of water, air and chemicals in the soil has been
widely recognized (Beven and Germann, 1982; Lin et al.,2005; Jarvis, 2007). Soil moisture is also widely
recognized as a key parameter in environmental
processes, including meteorology, hydrology, agriculture
and climate change. From a hydrologic point of view,soil moisture controls the partitioning of rainfall into
runoff and infiltration and therefore has an important
effect on the runoff behavior of catchments (Aubert et al.,year. The objective of this study is to clarify how

Fig. 1. Location of observation sites in the Saromabetsu
river basin, Hokkaido. site E, gauging station ofSaromabetsu River; site S, weather station; site H,rainfall station.

infiltrated rainwater or snowmelt water behaves tosurface. The basin geology is Cretaceous to Paleogenic

produce river runoffs by soil moisture monitoring andmarine sedimentary rocks. A continuous river stage

runoff analyses. In this study, simulations are performed

recorder (hourly river depth) was installed at site E. Afor both rainfall and snowfall runoffs by the tanks model

rating curve was established on the basis of successive(Sugawara, 1972).

measurements of discharge and gauge height taken at

2. Study area and methods

different stage during 2009. The hourly data of river2003). The Hokkaido island in the subarctic regions
experiences both rainfall and snowmelt runoffs every

The Saromabetsu river basin in Hokkaido is located atstage at site E and weather conditions at sites S and H

latitude of 43°49 to 44°04N and longitude of 143°04 to

were supplied from the Abashiri District Public Works

144°00 E (Fig. 1). The river basin has the area of 387.0

Management Office, Hokkaido. The river stage was

km2 with the maximum elevation of 811 m a.s.l. (mean,converted to discharge by the established rating curve.

203.3 m a.s.l.) and the mean basin slope angle of 9.73°.

Daily evapotranspiration rates were determined by theThe records by the AMeDAS at the Saroma town inPenman-Monteith equation. Hourly data of volumetric1980 - 2009 show the mean annual precipitation of 789

water content (cm3/cm3) were obtained by soil moisturemm (583mm in summer of May – November andprofilers, and pF values by tensiometers at forest,

206mm in winter of December – April) and the annual

grassland and field on the basin slope.
3. Result and discussion

mean air temperature of 5.3° C (12.4 °C in summer and 4.6 °C in winter). The land cover of the river basin is

Common to rainfall and snowmelt events, the whole

composed of 73.0 % forest, 26 % agricultural landlayer (especially, 0-20 cm depth) in forest can store(mainly, grassland), 1.3 % urban area, and 1.1 % water

rainwater or meltwater a few days after events, and then1

return to the previous moisture level by gradual drainage.
However, in grassland, the drainage after events is very
weak except for the surface layer of 0-10 cm depth.
Hence, in forest, a heavy rainfall during the gradualdrainage could produce more drainage to generate larger
river runoffs. At grassland, only the 0-10 cm layer may
produce quick flow to the river channel. The tank model(Sugawara, 1972; Yue and Hashino, 2000; Chikita et al.,2007). was applied to the simulation of river runoffs,
because many distributed models never gave satisfactory
results for snowmelt runoffs. Here, three serial tanks
wereadopted

corresponding

tosurface

runoff,intermediate runoff and baseflow. Hydrographs in thesummer of 2008 and the snowmelt season of 2009 wereFig.3. Temporal variations of volumetric water

simulated by the tank model. The simulated results arecontent from profilers at forest & grassland in thevery reasonable to the observed results with R2 =0.934

snowmelt season of 2009

and 0.973 and RMSE=0.902 and 0.961 m3/s (Fig. 2 & 3).
The surface runoff occupied 41% in 2008 and 34% in2009 for total runoffs. We calculated total surface runoff
and total throughflow at forest and grassland for each
event in 2008 summer and 2009 spring (Fig. 5). It isevident that the two variables are close to the 1:1
relationship. The nearly 1:1 relationship between total
surface runoff and throughflow indicates that the surface
runoff may be produced by unsaturated throughflow in
Fig. 4. Comparison between simulated and observed
discharges and simulated flow components forsnowmelt runoffs.

the surface soil layer.

Fig.2. Temporal variations of volumetric watercontent from profilers at forest & grassland in thesummer of 2008

2

Fig.5. Relationship between total surface runoff andthroughflow at forest (upper) and grassland (lower).