Ken'ichirou Kosugi

Kyoto University, Kioto, Kyōto, Japan

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Publications (43)107.5 Total impact

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    Wei‐Li Liang, Ken'ichirou Kosugi, Takahisa Mizuyama
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    ABSTRACT: Although many studies have investigated the ecological or hydrological influences of stemflow on soil water dynamics in the wetting process, the process by which stemflow affects soil water redistribution in the drying process has rarely been discussed. To address how soil water depletion occurs around a tree and the effects of stemflow on soil water depletion, we used a 2-year data set of soil water content to compare soil water depletion in the drying process before (SF period) and after (no-SF period) the interception of stemflow around a tree on a hillslope. The results revealed a dramatic soil water depletion at locations in the downslope area near to the tree, which was attributed to stemflow and root-induced pathways. The effect of stemflow on soil water depletion was significant in the short period (0.5 h) after the end of rainfall events, which increased the initial soil water content at the start of drying. The effect of root-induced pathways enhanced drainage during the drying process and resulted in irregular vertical distributions of changes in soil water content. Although soil water depletion at locations near to a tree would be expected to be greater than in locations farther away, due to root uptake in the drying process, most soil water depletion in the vicinity of a tree occurred in the SF period rather than the no-SF period. This was mainly attributed to greater instantaneous soil water depletion due to rainwater drainage rather than evapotranspiration, which occurred in the short time period between rainfall events. This study furthers our understanding of the separate involvement of stemflow and root channels in the drying process in a warm temperate climate. We also suggest that the double-funneling effect of a tree is valid not only for the wetting process but also for the drying process, and can result in dramatic variations in soil water dynamics within the stemflow- and root-influenced area of a forested stand. This article is protected by copyright. All rights reserved.
    Ecohydrology 12/2014; DOI:10.1002/eco.1589 · 2.63 Impact Factor
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    ABSTRACT: Heterogeneous hydrological properties in a foot slope area of mountainous hillslopes should be assessed to understand hydrological phenomena and their effects on discharge and sediment transport. In this study, we analyzed the high-resolution and three-dimensional water movement data to clarify the hydrological process, including heterogeneous phenomena, in detail. We continuously monitored the soil matric pressure head, psi, using 111 tensiometers installed at grid intervals of approximately 1 meter within the soil mantle at the study hillslope. Under a no-rainfall condition, the existence of perennial groundwater seepage flow was detected by exfiltration flux and temporal psi waveforms, which showed delayed responses, only to heavy storm events, and gradual recession limbs. The seepage water spread in the downslope direction and supplied water constantly to the lower section of the slope. At some points in the center of the slope, a perched saturated area was detected in the middle of soil layer, while psi exhibited negative values above the bedrock surface. These phenomena could be inferred partly from the bedrock topography and the distribution of soil hydraulic conductivity assumed from the result of penetration test. At the peak of a rainfall event, on the other hand, continuous high pressure zones (i.e., psi > 50 cmH2O) were generated in the right and left sections of the slope. Both of these high pressure zones converged at the lower region, showing a sharp psi spike up to 100 cmH2O. Along the high pressure zones, flux vectors showed large values and water exfiltration, indicating the occurrence of preferential flow. Moreover, the preferential flow occurred within the area beneath the perched water, indicating the existence of a weathered bedrock layer. This layer had low permeability, which prevented the vertical infiltration of water in the upper part of the layer, but had high permeability as a result of the fractures distributed heterogeneously inside the layer. These fractures acted as a preferential flow channel and flushed the water derived from lateral flow accumulated from the upslope area during the rainfall event. These phenomena occurring at the peak of rainfall event could not be inferred from the parameters derived from the penetration test.
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    ABSTRACT: Electrical resistivity imaging (ERI) as a method for effectively evaluating soil water content distribution on natural hill slopes was validated in site by combining ERI technique with the invasive measurement of volumetric water content (?) using a newly developed combined penetrometer-moisture probe (CPMP) in two head-water catchments underlain by weathered granite and weathered granite porphyry. The moisture sensor of a CPMP adopts time-domain reflectometry (TDR) and the probe, which is attached at the tip of the soil penetrometer, consists of two stainless steel wires coiled along grooves in acrylic pipe. The CPMP is a highly maneuverable technique and could provide simultaneous measurements of the penetration resistance and water content of soil layers. There was some reasonable correlation between ? and ? within each slope, indicating the potential of ERI for at least qualitatively evaluating moisture conditions within soil layers of natural hill slopes without directly measuring ? using any invasive method. These ? - ? datasets of two catchments with different geological condition were both roughly consistent with fitted functional models (Archie's equation), indicating the possibility of quantitatively evaluating ? of soil layer on natural hill slopes using ERI based on field-scale calibrations with invasive methods. The difference of the fitted functional models between the two catchments seems attributable to a difference in geological and soil conditions. Inconsistencies between ? and ? within each dataset of the two catchments may be significantly attributable to not only limitations on spatial resolution of ERI technique related to the issue of representative volumes of the technique and inversion analysis to obtain ? profiles but also the assumption that soil properties and pore-water resistivity of the entire slope are homogeneous. Using a CPMP as invasive method, detecting heterogeneous ? distribution more accurately than ERI technique, together with ERI is one of the most reasonable ways of effectively complementing the spatial resolutions of ERI as well as quantifying soil water content distribution on natural hill slopes.
  • Suimon mizu shigen gakkaishi Journal of Japan society of hydrology & water resources 01/2013; 26(2):85-98. DOI:10.3178/jjshwr.26.85
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    ABSTRACT: The usefulness of electrical resistivity imaging (ERI) as a highly accurate method for determining the soil thickness distribution on hillslopes was validated by combining intensive measurements using invasive methods, i.e., cone penetration testing and boreholes, with ERI in three granitic watersheds. Areas of high electrical resistivity (ρ) contrast reflecting soil–bedrock interfaces were found in all three study watersheds. However, ρ values of soil and weathered granite just below the soil mantle varied over a relatively wide range at each site, as well as considerably from site to site. The patterns of low–high contrast in ρ profiles, reflecting the soil–bedrock interface, also differed from site to site despite similarly dry conditions. Differences in the water retention characteristics of soil and weathered granitic bedrock, as found by a previous study of bedrock hydrological properties, may have been a major factor in the observed subsurface ρ variations. The ERI method, with electrode spacing of 0.5 to 2.0 m, was successful in determining soil thickness distributions ranging from about 0.5 to 3 m depth based on its ability to detect high contrast in ρ in the subsurface zone. Closer electrode spacings are expected to more sensitively reveal the distribution of ground material properties and thus more accurately replicate the soil–bedrock interface. ERI failed to clearly identify the soil–bedrock interface at some points along our measurement lines because of local intermediate materials with different properties such as unconsolidated soil and clayey intermediation just below the soil–bedrock interface. Two types of seismic survey (SS) techniques were also used, combining seismic refraction (SR) and the surface wave method (SWM) with the ERI method in a granitic watershed to compare ERI with other geophysical methods. The profile of S-wave velocity (Vs) by SWM also reasonably duplicated the soil–bedrock interface; the Vs profile showed larger variation in lateral direction and corresponded to the soil thickness distribution better than the P-wave velocity (Vp) profile by SR. The combined use of ERI and SWM may be more effective in detecting the soil–bedrock interface because each method compensates for the deficiencies of the other method.
    Geomorphology 04/2012; s 145–146:56–69. DOI:10.1016/j.geomorph.2011.12.035 · 2.58 Impact Factor
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    ABSTRACT: Understanding the effects of severe human induced forest disturbances with soil loss on rainfall‐runoff responses is important for future forest management. However, few studies have addressed this issue, which is methodologically difficult compared with the hydrological assessments of the effects of logging. In this study, several small catchments in Japan with different soil and geological conditions were compared using the runoff model HYCYMODEL to reveal their runoff characteristics. The results were then examined on the basis of runoff mechanisms to demonstrate the possible ranges of the effects derived from human disturbances for each geological type. For granite mountains, bare land can be considered the severest case of disturbances leading to high stormflow peaks, although a large baseflow remains because of the water storage fluctuation in weathered bedrock. For sedimentary rock mountains, the severest case may be a forest on the clayey soil without brown forest soil producing flashy runoff characteristics including a large stormflow volume with a sensitive response to the antecedent dryness and a low baseflow rate. Copyright © 2011 John Wiley & Sons, Ltd.
    Hydrological Processes 03/2012; 26(6-6):809-826. DOI:10.1002/hyp.8295 · 2.70 Impact Factor
  • Vadose Zone Journal 01/2012; 11(1). DOI:10.2136/vzj2011.0029 · 2.41 Impact Factor
  • Vadose Zone Journal 01/2012; 11(2). DOI:10.2136/vzj2011.0064 · 2.41 Impact Factor
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    ABSTRACT: Processes of multi-peaked discharge hydrograph formationDensely nested bedrock wells excavated in a mountain with steep topographyFinding localized bedrock aquifer distribution
    Water Resources Research 07/2011; 47(7). DOI:10.1029/2010WR009884 · 3.71 Impact Factor
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    ABSTRACT: Both evergreen and deciduous forests (Efs and Dfs) are widely distributed under similar climatic conditions in tropical monsoon regions. To clarify the hydraulic properties of the soil matrix in different forest types and their effects on soil water storage capacity, the soil pore characteristics (SPC) were investigated in Ef and Df stands in three provinces in Cambodia. Soils in the Ef group were characterized in common by large amounts of coarse pores with moderate pore size distribution and the absence of an extremely low Ks at shallow depths, compared to Df group soils. The mean available water capacity of the soil matrix (AWCsm) for all horizons of the Ef and Df group soils was 0·107 and 0·146 m3 m−3, respectively. The mean coarse pore volume of the soil matrix (CPVsm) in the Ef and Df groups was 0·231 and 0·115 m3 m−3, respectively. A water flow simulation using a lognormal distribution model for rain events in the early dry season indicated that variation in SPC resulted in a larger increase in available soil water in Ef soils than in Df soils. Further study on deeper soil layers in Ef and each soil type in Df is necessary for the deeper understanding of the environmental conditions and the hydrological modelling of each forest ecosystem. Copyright © 2010 John Wiley & Sons, Ltd.
    Hydrological Processes 02/2011; 25(5):714 - 726. DOI:10.1002/hyp.7859 · 2.70 Impact Factor
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    Wei-Li Liang, Ken'ichirou Kosugi, Takahisa Mizuyama
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    ABSTRACT: A tree can partition rainfall into throughfall and stemflow (SF), causing water to be funneled around the tree base, and can preferentially divert rainwater in soil layers, causing water to be funneled around tree roots. To determine the effects of each on soil water dynamics, we compared soil water dynamics around a tree on a hillslope on the basis of 2 years of field observations before (SF period) and after (non-SF period) intercepting the stemflow of the tree. Additionally, two sprinkling experiments were conducted using different dye tracers to separately indentify infiltration pathways derived from throughfall and stemflow. The observation results in the SF period showed irregular variations in soil water content, high soil water storage, and significant saturated zone development in the downslope region from the tree, which were attributed to stemflow concentrated on the downslope side of the tree. Although dramatic variations in soil water dynamics disappeared in the non-SF period, asymmetrical soil water response patterns were also observed, which were mainly attributed to root-induced bypass flow. Focusing on the downslope region in the SF and non-SF periods, the frequency of saturated zone generation at the soil-bedrock interface decreased from 58% to 16%, but the frequency of bypass flow occurrence varied little. Saturated zone generation at the soil-bedrock interface underneath the tree in both the SF and non-SF periods suggests that trees are key locations for rainfall infiltration and that tree-induced saturated zone generation should be considered carefully, even in conditions without stemflow supply.
    Water Resources Research 02/2011; 47(2). DOI:10.1029/2010WR009856 · 3.71 Impact Factor
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    ABSTRACT: Overland flow, which occurs when the rainfall intensity exceeds the infiltration rate, is an important factor in hillslope hydrological processes. Recent studies have suggested that a cause of Hortonian overland flow on forested hillslopes is the water repellency of surface soils. However, few studies have addressed the contribution of overland flow on storm runoff in such catchments. The overland flow generated on hillslopes needs to reach stream channels in order to contribute to storm runoff from a catchment. Therefore, the spatial pattern of the infiltration rate in a hillslope is essential for understanding the contribution of overland flow on storm runoff. To clarify the spatial pattern of infiltration for a hillslope, and its effect on overland flow generation and storm runoff from a small catchment with water repellent surface soil, we conducted artificial rainfall experiments along a hillslope transect (49 m) on 15 occasions over 20 months and measured the overland flow at a hillslope plot (8 × 20 m), stream flow at an outlet of a small catchment (0·43 ha), and the matric potential head along the hillslope transect. The replicated measurements suggest that the relationship between the infiltration rate and soil moisture was positive due to the impact of soil water repellency, and that the infiltration rate in the lower part of the hillslope was significantly higher than that of the upper and middle parts. Overland flow for individual storms measured at the plot scale was generally greater during dry periods than wet periods, suggesting that water repellency reduced the infiltration rate in dry periods as noted at the experiment plot scale. In contrast, stormflow for all events during wet and dry periods showed the opposite trend, indicating that the impact of soil water repellency during dry periods was not sufficient or continuous enough to cause measurable increases in stormflow. For a storm event < 100 mm in total precipitation, the comparison of hydrological responses during storm events between the dry and wet periods suggests that subsurface flow rather than Hortonian overland flow contributed to the storm runoff even though a substantial amount of overland flow was generated in the hillslope plot. Copyright © 2010 John Wiley & Sons, Ltd.
    Hydrological Processes 02/2010; 24(5):535 - 549. DOI:10.1002/hyp.7549 · 2.70 Impact Factor
  • 01/2010; 3(1):20-26. DOI:10.13101/ijece.3.20
  • Vadose Zone Journal 01/2010; 9(3). DOI:10.2136/vzj2010.0012 · 2.41 Impact Factor
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    ABSTRACT: Recent studies have emphasized the importance of bedrock in hydrologic processes occurring in headwater catchments. To understand water flow processes through variously weathered bedrock, we measured the saturated hydraulic conductivity, K(s), and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples. On the basis of these core-scale properties, along with the core shape and in situ K(s) measurements, we defined two groups of bedrock: C(M) class (weakly weathered) and C(L) to D(L) class (moderately to highly weathered). The C(M) class bedrock cores had almost no effective porosity (i.e., the amount of porosity that effectively contributes to water flow) and therefore extremely small core-scale K(s), indicating that the matrix could be regarded as essentially impermeable. The in situ K(s) was much larger than the core-scale values, however, and the core shape showed apparent fractures, suggesting that water did flow preferentially through the fractures. The volumetric water content of the C(L)-to D(L)-class bedrock water retention curves changed little in the dry range but changed gradually in the wet range, resulting in a moderate core-scale K(s) of 10(-5) to 10(-3) cm s(-1). The core-scale K(s) values were well explained by the parameters characterizing the water retention curve. The similarity of the in situ K(s) to the core-scale values, and the lack of fractures in the core shape, suggested that water flow could be characterized as matrix flow. The hydraulic properties of weathered granite at other sites followed the trends observed at our sites, implying wide applicability of the findings in this study to various types of weathered granite.
    Vadose Zone Journal 08/2009; 8(3). DOI:10.2136/vzj2008.0142 · 2.41 Impact Factor
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    ABSTRACT: We examined the effects of forest floor coverage on overland flow generation and soil erosion in mature Japanese cypress plantations with different coverage conditions: sparse understory and litter (uncovered plots), dense fern understory and litter (covered plots), and experimental removal of vegetative floor coverage on the covered plots (removal plots). We measured soil hydraulic properties and monitored overland flow and soil erosion in three replicated plots (approximately 1 × 2 m each) representing uncovered, covered, and removal conditions. Because of the strong water repellency of the surface soil, a substantial amount of overland flow occurred, even in covered plots. Nevertheless, the annual overland flow in covered plots was 37% of that in uncovered plots. Annual soil erosion in uncovered plots was 3.7 times greater than that in covered plots. Although overland flow in removal plots was similar to that in uncovered plots, soil erosion in the former was significantly greater than in the latter. These results suggest that differences in soil erodibility between the plots were essential determining factors of erosion and were no less important than floor coverage. We quantified the effects of floor coverage and soil erodibility independently and examined the relationship between coverage and erosion by applying an erosion model. In covered plots, floor coverage prevented 95% of soil detachment by raindrops, which was the dominant mechanism in reducing soil erosion, as compared with it inhibiting overland flow and resisting sediment transport. The soil erodibility of plots with ground cover was 4.5 times higher than that of uncovered plots. This implies that simply comparing plots with different coverage conditions is not sufficient for examining the effects of vegetation coverage on soil erosion.
    Water Resources Research 06/2009; 45(6). DOI:10.1029/2008WR007270 · 3.71 Impact Factor
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    ABSTRACT: Information on spatial distributions of soil water content and mechanical strength is fundamental to hydrogeomorpho-logical studies in mountainous watersheds. For simultaneous measurements of soil water content, theta, and penetration resistance, N(c), of soil mantles on natural hillslopes, we developed a new type of combined penetrometer-moisture probe (CPMP). The new CPMP has a robust configuration to reduce the frequency of moisture probe breakdowns during penetration into gravelly and rocky natural soils, and it has a penetration depth of up to 552 cm, which is about four to 14times greater than the maximum measurable depth of any previous CPMP developed for agricultural soils. Laboratory calibrations and field validations showed that the CPMP succeeded in producing vertical distributions for theta and N(c) similar to those measured with conventional methods. The CPMP provided less time-consuming and less destructive measurements of theta profiles than conventional methods that require excavation of deep trenches. Because theta profiles consider the stratified characteristics of soil mantles as well as the depths of groundwater tables, the CPMP was more effective than the conventional cone penetrometer for surveying the hydrogeomorphological structure of soil mantles. The CPMP was successful in determining the spatial distributions of theta and N(c) in a headwater basin underlain by weathered granitic bedrock. From a direct comparison between two. and N(c) values measured on two different days at nearby points and the same depth, it was shown that N(c) tended to decrease when theta increased by more than 0.15.
    Vadose Zone Journal 02/2009; 8(1). DOI:10.2136/vzj2008.0033 · 2.41 Impact Factor
  • Yuki Hayashi, Ken'ichirou Kosugi, Takahisa Mizuyama
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    ABSTRACT: Heterogeneous water flow is known to be an important factor of hydrological processes in a natural forested hillslope. To model heterogeneous water flow, the characterization of spatial variability in water retention curve (WRC) is required, The scaling technique introduced by Miller and Miller is effective to characterize it for conditions of constant standard deviation (STD) in pore-size distribution and porosity, but this is not necessarily appropriate for forested hillslopes. We tested the conventional scaling method and the two proposed methods that presume that field soils do not exhibit constant STD and porosity. The observed WRCs were fitted using a model, which assumes a lognormal pore-radius distribution and contains three parameters: the matric pressure head related to the median pore radius, psi(m); the STD of the log-transformed pore radius distribution, sigma; and the effective porosity, theta(e). In Method 1, which corresponds, to the conventional scaling method, psi(m) was optimized for each soil, whereas the values of sigma and theta(e) were common for the whole data set. In Method 2, sigma was optimized for each soil, and in Method 3, 0, was optimized for each soil, whereas the values of the remaining parameters were common for the whole data set. Method 3 produced the best description of spatial variability in the WRCs. This result indicates that in the natural forested hillslope, variability in the pore-size distribution is characterized by variability in effective porosity. In practical aspect, we suggested an alternative simpler method to Method 3. In this method, the theta(e) for each location was estimated from soil penetration resistances measurable in situ. This method explained 59.3% of the spatial variability in WRCs on the Studied natural forested hillslope.
    Soil Science Society of America Journal 01/2009; 73(1). DOI:10.2136/sssaj2007.0235 · 2.00 Impact Factor
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    ABSTRACT: Understanding the seasonal and regional patterns of nitrogen (N) components in the headwaters of forested watersheds is important for forests management. Here, we investigated the NO3-N concentration and its seasonal variations in the baseflow headwaters in Japanese coniferous forests with different N saturation status in five regions (i.e., Nagano, Tokyo, Aichi, Kochi and Mie). We found that the ratios of NO3-N to DON were significantly higher in the N-saturated regions including Nagano (14.1 ± 2.2) and Tokyo (16.5 ± 4.1), compared with the regions, which didn't experience N saturation including Aichi (1.4 ± 0.2), Kochi (5.1 ± 1.8), and Mie (2.2 ± 0.5). In comparison with the regions without experiencing N saturation, the relatively higher NO3-N concentration, as well as its contrastive variation between growing season and dormant season, was also characteristic of the seasonal NO3-N concentration in the N-saturated regions. Our findings have indicated that NO3-N concentration, its seasonal variations, and the ratio of NO3-N to DON, are the better and applicable indicators for evaluating N saturation status in Japanese coniferous forests.
    Catena 12/2008; 76(1):63-69. DOI:10.1016/j.catena.2008.09.007 · 2.48 Impact Factor
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    ABSTRACT: 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.
    Science of The Total Environment 09/2008; 402(1):113-22. DOI:10.1016/j.scitotenv.2008.04.045 · 3.16 Impact Factor