Tomomi Terajima

Experimental studies on the effects of pipeflow on throughflow partitioning

Abstract Recent research has revealed that natural soil pipes provide important pathways for sub-surface movement of water and solutes, as well as contributing to landslide initiation. A benchscale experiment (in a sloping box 1 m in length) was conducted with a uniform sand to evaluate the effect of pipeflow on the overall hydrologic regime. A single drainage pipe (13 mm inside diameter (ID)) composed of five 20 cm segments, each with a different roughness coefficient (Mannings n), was placed 5 cm above the base of the 12.8° sloping box. Roughness elements were arranged in four different spatial combinations during hydraulic experiments. Piezometric levels were highest and pipeflow was lowest when the high-roughness (n = 0.325) portion of the pipe was located at the downslope end of the box. Measured values of pipeflow for different hydraulic gradients in each experiment were related to piezometric head above the pipe raised to the 0.32–0.42 power. Pipeflow was proportional to matrix flow in the soil above the pipe raised to the power of 0.4–0.6. These findings may be useful in estimating pipeflow in uniform soils and in validating two-domain models involving preferential flow in soils.

Energy balance above a boreal coniferous forest: a difference in turbulent fluxes between snow-covered and snow-free canopies

To evaluate the interactive effects of snow and forest on turbulent fluxes between the forest surface and the atmosphere, the surface energy balance above a forest was measured by the eddy correlation method during the winter of 1995–1996. The forest was a young coniferous plantation comprised of spruce and fir. The study site, in Sapporo, northern Japan, had heavy and frequent snowfalls and the canopy was frequently covered with snow during the study period. A comparison of the observed energy balance above the forest for periods with and without a snow-covered canopy and an analysis using a single-source model gave the following results: during daytime when the canopy was covered with snow, the upward latent heat flux was large, about 80% of the net radiation, and the sensible heat flux was positive but small. On the other hand, during daytime when the canopy was dry and free from snow, the sensible heat flux was dominant and the latent heat flux was minor, about 10% of the net radiation. To explain this difference of energy partition between snow-covered and snow-free conditions, not only differences in temperature but also differences in the bulk transfer coefficients for latent heat flux were necessary in the model. Therefore, the high evaporation rate from the snow-covered canopy can be attributed largely to the high moisture availability of the canopy surface. Evaporation from the forest during a 60-day period in midwinter was estimated on a daily basis as net radiation minus sensible heat flux. The overall average evaporation during the 60-day period was 0·6 mm day−1, which is larger than that from open snow fields. Copyright

Morphology, structure and flow phases in soil pipes developing in forested hillslopes underlain by a Quaternary sand-gravel formation, Hokkaido, northern main island in Japan.

Preferential flow pathways, such as soil pipes, are usually present in the soil of slopes. Subsurface flow through the soil pipes affects the subsurface drainage system and is responsible for sediment removal from slopes. However, a record of the inner structure of soil pipes has rarely been reported for slopes. A fibrescope examination of the morphology and flow phases in soil pipes in hillslopes underlain by a Quaternary sand–gravel formation provided the following information: the main pores of the soil pipes ran mostly parallel with the slope gradient; the cross-sections of the soil pipes were approximately circular; and occurred on a few occasions; with some triple junctions being present. In addition, both full flow and partly full-depth conditions occurred simultaneously in the soil pipe. The full flow condition has long been used in hydrological studies to model the pipe flow mechanism. Both the full flow condition and the partly full-depth condition, however, must be examined closely in order to evaluate the subsurface hydrology in heterogeneous soil and the hydrogeomorphological processes of subsurface hydraulic erosion. Copyright

The effect of canopy-snow on the energy balance above a coniferous forest

To evaluate an interactive effect of snow on energy exchange between the forest and atmosphere, the energy balance above a forest was measured continuously between February and April of 1997 in the Hitsujigaoka Experimental Forest, Sapporo, northern Japan. The forest was a dense. 23-year-old, coniferous plantation. The study site had frequent snowfalls and the canopy was frequently covered with snow during the study period. Snow-cover on the canopy was monitored daily using a photo-camera mounted above the canopy, and the fraction of the snow-covered area on the canopy was determined as an index of the canopy-snow condition. Turbulent energy fluxes above the canopy were measured using the eddy and band-pass covariance method. The diurnal courses of the measurements showed that turbulent fluxes were sensitive to the snow on the canopy. Latent heat fluxes dominated above the snow-covered canopy and sensible heat flux prevailed above the snowfree canopy. The effect of the canopy-snow on turbulent energy exchange was examined using the evaporative efficiency for the canopy-layer in an energy balance model composed of two sources of vapor fluxes. The evaporative efficiency changed dynamically according to the canopy-snow condition in the short terms and could be positively related to the extension of the snow-covered area on the canopy. A quantitative relationship was empirically determined between the evaporative efficiency and the fraction of the canopy-snow area. The snowcover season in the study site can be classified into the following two types of turbulent flux composition: (1) in mid-winter, when the canopy was snow-covered, latent heat flux dominated and the Bowen ratio was positive and low; (2) during the snowmelt season, sensible heat flux from the dry canopy prevailed and the Bowen ratio was much larger than in mid-winter.

Suspended sediment discharge in subsurface flow from the head hollow of a small forested watershed, northern Japan

Surface flow and suspended sediment discharge from the head hollow of the Jozankei Experimental Watershed in Hokkaido, northern Japan, were measured to clarify the implications of subsurface hydrology for soil movement. Subsurface discharges during the extremely large storms of 1993 to 1994 were measured in a V-notch weir installed at a natural spring near the bottom of the head hollow, and shallow groundwater levels were observed in the wells excavated in the hollow. Sediment samples whose particle size range from 0·001 to 0·1 mm were manually and automatically collected at 15 to 60 min intervals, by use of 1 or 21 polyethylene bottles. Maximum concentration and flux of suspended sediment during the storms preceded the peak discharge of subsurface flow by several hours. Neither the changes in concentration (mg l−1) nor flux (mg s−1) of suspended sediment coincided with those in subsurface discharge (l s−1). Furthermore, sediment concentration was poorly correlated with the rate of change in subsurface discharge (l s−2) during the rising limb of the hydrograph. Suspended sediment flux during the acceleratory limb, however, was closely correlated with the rate of change in subsurface discharge. The relationship between suspended sediment flux and rate of change in subsurface discharge were in inverse proportion to initial subsurface discharge before the storm runoff and they represented rare seasonal variation. Subsurface hydraulic erosion and transport of suspended sediment resulting from changes in rate of change in subsurface discharge actively occur during the acceleratory rising limb of the hydrograph. Accordingly, subsurface hydraulic erosion during the acceleratory rising limb of the hydrograph can be physically understood by analysing suspended sediment flux associated with rate of change in subsurface discharge and initial subsurface discharge.

Baseflow concentrations of nitrogen and phosphorus in forested headwaters in Japan

A comprehensive investigation on all dissolved nitrogen and phosphorus components at both local and regional scales in the headwaters from forested watersheds is valuable to improve our understanding of the factors controlling water quality. Here, we investigated the baseflow concentrations of dissolved nitrogen and phosphorus components, N:P ratio, and their associations with region and vegetation type in forested headwaters in fives regions of Japan. We found that inorganic nitrogen and phosphorus were the dominant components in the 26 temperate forested streams, rather than organic forms. There were significant positive correlations between the concentrations of N and P components. Furthermore, the regional patterns of the concentrations of nitrate, dissolved inorganic P (DIP), and dissolved total N (DTN) and P (DTP) were similar. Our results suggest that the regional patterns of the concentrations of N and P components should be related to the regional atmospheric deposition of both N and P nutrients. We also found that the nitrate and DTN concentrations were higher in man-made evergreen conifer (EC) than those in the natural deciduous broadleaf (DB). In contrast, the DIP and DTP concentrations in EC were lower than those in DB. The uniformly higher N:P ratio in EC- than in DB-forested streams for each region suggest that EC-forested streams could be more affected by P-limited than DB-forested streams when N inputs from atmospheric sources increased.

Comparison of the effects of rainfall and snowmelt on the carbon discharge of a small, steep, forested watershed in Hokkaido, northern Japan

We measured the carbon discharge from a small, forested watershed that is covered with deep snow in winter. Snowmelt and rainfall both cause high flows, and we compared their effect on carbon discharge characteristics. The particulate organic carbon (POC) concentration was stable (around 5 mgC/l) at times of low flow and increased rapidly during high flow periods. The maximum POC concentration exceeded 80 mgC/l during rainfall, but it did not exceed 20 mgC/l during snowmelt, even though the rate of water discharge was high for both events. The dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations fluctuated seasonally, and also temporarily during high flow events. The DIC concentration decreased from 4.0 to 1.4 mgC/l as the spring thaw progressed. It increased progressively to a high of 7 mg/l in August and September, and then gradually decreased again. The DOC increased rapidly from 2 to 4 mg/l at the beginning of the thaw, then plunged to 2 mg/l when the snowpack disappeared. It increased from July until October (> 5 mgC/l) and then decreased once more. The DOC and DIC concentrations were not clearly synchronized during the snowmelt. During rainfall, the DIC concentration decreased in rising limbs of the hydrograph and increased in falling limbs, whereas the reverse was the case for the DOC concentration. In late November, however, both the DOC and DIC concentrations decreased with increasing water discharge. Values for the annual transport of water, POC, DIC, and DOC were 925 mm, 21 kgC/ha, 19 kgC/ha, and 33 kgC/ha, respectively. The fractions transported during the snowmelt, which spanned 62 days, were 69% for water, 54% for POC, 58% for DIC, and 65% for DOC. Excluding the snowmelt period, the fractions transported in the peak 15 days were 38% for water, 63% for POC, 30% for DIC and 46% for DOC. We discuss the effects of snowpack and of respiration by roots and heterotrophs on the carbon discharge patterns of the watershed.

Subsurface discharge and suspended sediment yield interactions in a valley head of a small forested watershed

Sediment hield caused by hydraulic agents is believed to be one of the most important geomorphic processes in headwaters. Observations of shallow groundwater levels and suspended sediment yield were used to develop relationships between subsurface discharge and sediment movement in a valley head of a small forested watershed in Hokkaido, northern Japan. Drainage capacity of the soil was closely correlated with subsurface soil movement in the valley head. An increase in subsurface discharge associated with an increase in the drainage capacity of the soil depended heavily on the discharge of fine-grained particles originating from the soil. Clogging created by the fine-grained particles in the soil caused a decrease in subsurface discharge associated with the decline in the drainage capacity. In addition, neither the drainage capacity of the soil nor subsurface discharge changed because of the sediment unavailability within the subsurface flow system. Accordingly, as shown by the changes in the drainage capacity of the sedimentary soil, subsurface discharge and suspended sediment yield from the valley head showed strong interactions.

Evaporation of snow intercepted by a todo-fir forest (I) Water balance measurements

In spring 1972, serious needle cast damage caused by a hitherto unknown species of Lophodermium occurred on Japanese red pine seedlings in many nurseries in Iwate Prefecture, northern Honshu, Japan. From 1972 to 1974, 6 to 7 million 2-year-old seedlings were affected by this fungus. The causal fungus was named Lophodermium iwatense Sakuyama after etiological and biological studies. The first symptoms of the disease are yellow spots on the current year s needles which usually appear in November. The affected needles turn brown in April of the following year, and begin to drop in May to June. Spermogonia and spermatia of the fungus are formed on the diseased needles from May to June. Mature apothecia and ascospores of the fungus are abundant on the diseased needles from July to September. In the field, ascospores are discharged from early July to early October, with a vigorous dispersal period from late July to early September. No black zone lines are usually observed on diseased needles, but brown zone lines are formed frequently. L. iwatense showed strong pathogenicity to Japanese red pine seedlings in inoculation tests using ascospores. Ascospores germinate betweem 5 and 30~ with an optimum at 20~ The mycelia of the fungus grow between 10 and 25~ with an optimum at 20-25~ Three other Lophodermium species, L. conigenum, L. nitens and L. staleyi were recorded for the first time in Japan. A satisfactory method of disease control is to spray Maneb as 1/500 dilution at the rate of 300 ml/m 2, four times at 2-weeks intervals from late July to early September.