Ken'ichirou Kosugi

Kyoto University, Kioto, Kyōto, Japan

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Publications (71)113.79 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; · 2.78 Impact Factor
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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.
    04/2013;
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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.
    04/2013;
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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. · 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):809-826. · 2.50 Impact Factor
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    ABSTRACT: In mountainous watersheds, groundwater flowing contributes significantly to runoff generation and plays an important role in the occurrence of landslides. Understanding the hydrological processes within not only the soil mantle but also bedrock is essential for modeling runoff generation and predicting landslides, but it is limited by the physical difficulties of observations. In this study, we conducted intensive in-situ investigations including hydrometric observations using dense borehole well network drilled within soil mantle (<2 m depth) and bedrock layer (<30 m depth) and geological investigations based on borehole logs and electrical resistivity survey, in a piedmont watershed (1.4 ha) underlain by granitic bedrock located in the southern part of Hyogo Prefecture, central Japan. Groundwater levels in soil mantle showed large spatial and temporal variations in response to rainfall; time lag of peaks between right and left banks in the watershed and localized existences of confined groundwater aquifers. The groundwater movement within soil mantle could be significantly affected by soil mantle structure, i.e., water retention characteristics of soil and soil thickness distributions, as well as groundwater flowing within bedrock. Moreover, the groundwater movement within bedrock also varied considerably with location, which could be controlled by structural condition such as weathering of the bedrock and existence of faults.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Understanding the groundwater dynamics in the soil layer is one of the key issues for accurate landslide prediction in steep mountains. Recent studies have pointed that the water movement from bedrock layer to soil layer may trigger the landslide occurrence. In order to have deeper and precise understanding, we need to get the results from direct observations of bedrock groundwater. Our research group has focused on the dynamics of bedrock groundwater using densely nested bedrock wells in a steep catchment. In our catchment, we recognized an existence of shallow landslide (2×5m) which recently occurred at a ridge on the surface topography in this investigation and also found the springs from the landslide scar. This study could aim to elucidate the bedrock groundwater movement and its influence to the landslide based on the hydrological and hydrochemical observations of bedrock spring and groundwater. The results have shown that the bedrock groundwater was distributed in three different aquifers: upper, middle and lower aquifers. The discharge of bedrock spring had rapid response for rainfall, similar to the response of lower bedrock aquifer. The chemistries of bedrock spring were close to that of bedrock groundwater located in the upper point rather than other groundwater of the lower aquifer. The groundwater in the middle aquifer moved to the lower aquifer across the topographic catchment boundary located on the upper areas of landslide scar. These results indicated that the bedrock groundwater flowed from neighboring catchment and exfiltrated as bedrock spring at the scar of landslide. Our research demonstrated that the bedrock groundwater flowpath across the surface divide that was not able to detect from surface topography is important for predicting the shallow landslide in steep catchments.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: We examined the hydrological responses after forest thinning in hillslope plot and catchment scales based on field investigation and modeling approaches. This study was conducted in two small catchments in 0.19 and 0.35 ha of drainage areas. Monitoring was conducted from June 2004 to June 2009. 58.3% of the stems in the treated catchment (M5) were removed in February 2007, while the control catchment (M4) was remained untreated. Paired catchment analysis revealed that annual runoff increased 240.7 mm after thinning. Based on annual flow days, flow duration in the ephemeral channel in catchment M5 increased from 56.9% in the pre-thinning to 73.3% in the post-thinning period. Delayed runoff (base flow component) increased significantly, while quick runoff including volume and timing of peak flow followed similar patterns in the pre- and post-thinning periods. Despite the changes in hydrological responses at the catchment scale, hillslope plot runoff associated with overland flow was not differ between the pre- and post-treatment. Based on summarized information of 30 previous studies related to forest harvesting (including thinning), increases in annual runoff ranged from 8 to 650 mm in the 13 to 100% of treated areas. Our estimated increase in annual runoff was 240.7 mm of post harvesting which were within the ranges of the reported values. For examining the patterns of the changes in runoff, we also applied Soil and Water Assessment Tool (SWAT) model in our study sites (M5 and M4). Based on the parameter estimation of the pre-thinning condition using SWAT model, we applied 50% of thinning and estimated runoff responses. Our field observation and modeling approaches suggested that hydrological responses after thinning treatment was governed by soil matrix flow and preferential flow path which was affected by net-increases in throughfall and decreases in evapotranspiration.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Previous studies have shown that the formation of groundwater in the soil mantle greatly affects slope instability on steep landscapes. To predict landslides, mathematical models based on a geographic information system, which organize geographic data such as information on upslope contributing areas and the local slope gradient, have been developed and tested. Although such models can be used to calculate the topographically driven convergence of rainwater and groundwater table developments in the soil mantle, thus providing a spatially distributed prediction of landslide occurrences, the accuracy of these mathematical models is still limited, mainly because they ignore storm responses in underlying bedrock. Recent research has provided credible information on the importance of bedrock groundwater on surface hydrological processes in headwater catchments. To elucidate the effects of bedrock groundwater, the dynamics of bedrock groundwater should be measured directly. However, intensive monitoring of bedrock groundwater is rare in mountains with steep topography. Consequently, how bedrock groundwater controls landslides in a steep headwater catchment is in dispute. In this study, we conducted long-term hydrological observations using densely nested bedrock wells along with monitoring of discharge hydrograph and soil mantle groundwater in a steep headwater catchment underlain by granitic bedrock. Bedrock wells with depths of 7-78 m were drilled at 31 points within the 2.10-ha catchment. Results showed that a hollow of bedrock aquifer was located at a ridge in the surface topography, clearly indicating bedrock groundwater flow across topographic divides. Around a point where the bedrock groundwater exfiltrated, we found scars of landslides. Such landslides cannot be explained by mathematical hydrology models, which calculate the topographically driven convergence of rainwater in the soil mantle. Moreover, at a point along the main hollow of the watershed, we observed a confined bedrock aquifer. The aquifer gathered groundwater recharged in the upper and middle parts of the watershed, and produced the maximal water table height of 6.3 m above the ground surface. The excessive pressure increase in the aquifer induced slope instability around the point. In addition, we observed two completely different groundwater responses within a single well excavated into the soil mantle. One was an ephemeral-type response that is well described by mathematical hydrology models based on a geographic information system. The other was a semi-perennial-type response that is fed by exfiltration of deep bedrock groundwater. The semi-perennial groundwater caused considerably high antecedent groundwater tables between storms, leading to an increased peak in the groundwater level during later heavy storm events and a likely increase in the risk of landslides. Moreover, peaks in the semi-perennial groundwater lagged considerably behind rainstorm events, probably affecting occurrences of landslides delayed with rainfall peaks. Thus, this study demonstrated that grasping bedrock aquifer dynamics is essential for understanding landslide hazards in headwater catchments.
    AGU Fall Meeting Abstracts. 12/2011;
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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 01/2011; 47(7). · 3.15 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 01/2011; 47(2). · 3.15 Impact Factor
  • Y. Hayashi, K. Kosugi, T. Mizuyama
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    ABSTRACT: We evaluated the effects of the forest soil on flood and drought mitigation, and decrease of the danger of slope failures. Forest soils usually contain a primary textural pore system, which is determined by solid particle-size distribution and particle arrangements, accompanied by a secondary structural pore system formed by the effects of forest ecosystems. With this background, we investigated the effects of the pore structural development on the unsaturated hydraulic properties and rainwater dynamics on a forested hillslope. The undisturbed soil samples were collected from an entire forested hillslope. The undisturbed soils contain the primary and secondary pore systems, and were set to be as structural developed soils. The water retention curve and saturated hydraulic conductivity of each sample were measured. After that, each undisturbed soil was crushed to break up aggregate structure and packed into core sampler to prepare the structural undeveloped soil. We conducted same measurement of the structural undeveloped soils as those used for the structural developed soils. Generally, compared with the structural undeveloped soils, the structural developed soils had large median pore radius and width of pore size distribution, and large saturated hydraulic conductivity. We conducted 2-dimentional numerical simulations of hydraulic processes on a forested hillslope with the code programmed by Kosugi (2007), using the data sets of the structural developed and undeveloped soils. The peak flow of stream is formed by the direct discharge from the lower end of the slope, and the base flow is formed by the seepage into the bedrock (Kosugi, 2007). Furthermore, slope failure are caused by the positive pore water pressure in the soil layer. Therefore, for evaluating the effect of forest soil on hydraulic processes, we estimated the discharge rate of the lower end of the slope and the rate of the seepage into the bedrock from the soil layer, and the matric pressure head. As a result, the structural developed soils had smaller seepage into the bedrock than the structural undeveloped soils. That is, the soil structural development let the base flow more moderate. On the other hand, under the condition of the heavy rainfall, the discharge from the lower end of the slope was occurred and the hydrograph for the structural developed soils had a larger flood peak than that for the structural undeveloped soils. However, at the same time, the structural developed soils let the rainwater held in the slope release so quickly that the area showing high positive pore water pressure did not expand as widely as for the structural undeveloped soils. This resulted in the decrease of the danger of slope failure. Reference Kosugi, K. 2007. Journal of Japan Society of Hydrology & Water Resources 20:201-213.
    AGU Fall Meeting Abstracts. 12/2010;
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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 09/2010; 25(5):714 - 726. · 2.50 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. · 2.50 Impact Factor
  • Vadose Zone Journal 01/2010; 9(3). · 2.20 Impact Factor
  • Y. Hayashi, K. Kosugi, T. Mizuyama
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    ABSTRACT: Describing the infiltration of water in soils on a forested hillslope requires the information of spatial variability of water retention curve (WRC). By using a scaling technique, Hayashi et al. (2009), found that the porosity mostly characterizes the spatial variability of the WRCs on a forested hillslope. This scaling technique was based on a model, which assumes a lognormal pore size distribution and contains three parameters: the median of log-transformed pore radius, psim, the variance of log-transformed pore radius, sigma, and the effective porosity, thetae. Thus, in the scaling method proposed by Hayashi et al. (2009), thetae is a scaling factor, which should be determined for each individual soil, and that psim and sigma are reference parameter common for the whole data set. They examined this scaling method using thetae calculated as a difference between the observed saturated water content and water content observed at psi = -1000 cm for each sample and, psim and sigma derived from the whole data set of WRCs on the slope. Then it was showed that this scaling method could explain almost 90 % of the spatial variability in WRCs on the forested hillslope. However, this method requires the whole data set of WRCs for deriving the reference parameters (psim and sigma). For applying the scaling technique more practically, in this study, we tested a scaling method using the reference parameter derived from the WRCs at a small part of the slope. In order to examine the proposed scaling method, the WRCs for the 246 undisturbed forest soil samples, collected at 15 points distributed from downslope to upslope segments, were observed. In the proposed scaling method, we optimized the common psim and sigma to the WRCs for six soil samples, collected at one point on the middle-slope, and applied these parameters to a reference parameter for the whole data sets. The scaling method proposed by this study exhibited an increase of only 6 % in the residual sum of squares as compared with that of the method using the whole data set of WRCs. This result showed that on the natural forested hillslope, the scaling method proposed by Hayashi et al. (2009) can describe the spatial variability in WRCs well without the whole data sets of WRCs on the slope. Reference Hayashi, Y., K. Kosugi, and T. Mizuyama. 2009. Characterization of soil WRCs of a natural forested hillslope using a scaling technique based on a lognormal pore-size distribution.Soil Science Society of America Journal. 73:55-64
    AGU Fall Meeting Abstracts. 12/2009;
  • W. Liang, K. Kosugi, T. Mizuyama
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    ABSTRACT: In rainfall redistribution processes in forest stands, a part of rainfall is intercepted by the canopy, while other parts are partitioned into throughfall and stemflow through the canopy as diffuse and point inputs to the forest floor, respectively. Therefore, the rainwater amount reaching the forest floor is considerably heterogeneous. To clarify the effects of stemflow on soil water dynamics, we conducted detailed observations of soil water content and pore water pressure around a tree on a forested hillslope which were separated into two periods of with or without stemflow. Observations were conducted from in April 2007 through April 2009 on a hillslope at the Kamigamo experimental station of Kyoto University, located in southern Kyoto Prefecture, central Japan. The hillslope has a mean gradient of 28 degrees, which is predominantly covered with ``tall stewartia'' (Stewartia monadelpha). To monitor the soil water dynamics around a tree, we selected a tall stewartia and delineated a longitudinal observation line from upslope to downslope of this tree. We installed a capacitance meter (Sentek, EasyAG-5p) at each of 10 points upslope and downslope from the tree stem. Each capacitance meter consisted of five sensors to measure soil water content at depths of 10, 20, 30, 40, and 50 cm. Additionally, we installed tensiometers at the soil-bedrock interface at the same 10 points to measure pore water pressure. In the second year (April 2008-April 2009), we intercepted stemflow of the tree and maintained observations above. We used two tubes cut longitudinally and wrapped spirally around the upslope and downslope sides of the trunk to collect separately the stemflow along the upslope and downslope sides of the trunk of the tree, and measured those using tipping-bucket gauges. The results before intercepting stemflow showed that the soil water content increased rapidly and greatly in the region downslope from the tree stem, especially at points close to the tree stem. At these points, maximal soil water storage was more than 100 to 200% of the cumulative open-area rainfall, and occurrences of bypass flow were recognized. Moreover, the pore water pressure at the soil-bedrock interface increased rapidly and greatly in the region downslope of the tree stem, especially at the points close to the tree stem. Locally concentrated rainwater inputs attributable to stemflow on the downslope side of the tree trunk probably caused the large and rapid increases in water content and pore water pressure in the region downslope of the tree stem, resulting in the development of an asymmetric saturated zone around the tree. On the other hand, the results after intercepting stemflow showed no clear differences in soil water dynamics between the regions upslope and downslope of the tree stem, which were dominated by a slow expansion of the wetting front. The frequency of saturated zone occurrence observed in the without-stemflow period was much smaller than in the with-stemflow period. It indicated the characteristics of stemflow infiltration process, in which water flowed rapidly through the deeper layer, causing an irregular distribution of vertical soil water content changes and saturated zone occurrence on the soil-bedrock interface.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: Recent studies have suggested that bedrock groundwater can exert considerable influence on hydrological processes in headwater catchments. However, direct measurements of water flow within bedrock are still very limited. To clarify the hydrological connectivity between soil and bedrock, we have conducted innovative field investigations at two headwater catchments; NO (1.87 ha) and AK (0.086 ha), both of which are underlain by granitic bedrock. In the catchment NO, observations of rainwater discharge and soil mantle groundwater level were combined with observations of bedrock groundwater level using eight boreholes 20 to 70 m deep. The intensive monitoring of bedrock groundwater with the high spatial resolution is expected to reveal the effect of bedrock groundwater on the surface hydrological processes. In catchment AK, tensiometric measurements were conducted at 48 points in the soil mantle and the layer of weathered bedrock. Bedrock groundwater was also monitored in three boreholes 15 to 20 m deep. The nests of tensiometers are expected to clarify the hydrological interactions between the soil mantle and the bedrock, and recharging processes of the bedrock groundwater. Results showed that, in the catchment NO, water levels in the eight boreholes exhibited different trends which were categorized in the following three types: In the upslope region, water level showed quick responses to rainfall input, despite of their depths from the soil surface (i.e., 30 to 70 m deep). In the mid-slope region, the water level had the most delayed and gradual response. Water level did not respond to storm events, but exhibited a gentle seasonal trend. In the downslope region, quick responses of the water level to each storm event were combined with a gentle seasonal change. Thus, this study proved the existence of three bedrock aquifers within the small headwater catchment. Moreover, we found that each bedrock aquifer had a unique effect on the surface hydrological processes. In the mid-slope region of the catchment, perennial or semi-perennial soil mantle groundwater was fed by the mid-slope bedrock aquifer. Furthermore, the downslope bedrock aquifer contributed to the base-flow discharge from the catchment. Results obtained in catchment AK revealed that the vertical unsaturated infiltration within the bedrock is the dominant process to recharge the bedrock groundwater. In the unsaturated bedrock, the water flow direction was always vertical, regardless of the depths and the positions on the slope. Around the outlet of the catchment, soil thickness was small and saturated lateral flow was frequently formed over the soil-bedrock interface. This reduces the amount of water infiltration into the bedrock. In the middle and upslope regions, the large water-holding capacity of the soil layer, due to its thickness, reduces infiltration intensity at the soil-bedrock interface. Consequently, all the rainwater infiltrates into the bedrock. Thus, middle and upslope regions contribute to the recharge of the bedrock groundwater. We concluded that the physical property (e.g., thickness and hydraulic conductivity) of the soil mantle is one of the most important factors to control the hydrological connectivity between soil and bedrock.
    AGU Fall Meeting Abstracts. 01/2009;
  • Vadose Zone Journal 01/2009; 8(3). · 2.20 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 01/2009; 45(6). · 3.15 Impact Factor

Publication Stats

739 Citations
113.79 Total Impact Points

Institutions

  • 1997–2012
    • Kyoto University
      • Graduate School of Agriculture / Faculty of Agriculture
      Kioto, Kyōto, Japan
  • 2008
    • Beijing Normal University
      • State Key Laboratory of Earth Surface Processes and Resource Ecology
      Beijing, Beijing Shi, China