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Groundwater zone formation in the soil layers of a headwater catchment is an important factor that controls volumetric and chemical changes in streamflow; it also induces shallow landslides. Previous studies have suggested that the groundwater zone in soil layers generally forms transiently atop low-permeability layers in response to rainfall. This study focused on an unchanneled hollow in a serpentine headwater catchment, where a semi-perennial to perennial groundwater zone was observed in thin organic soil layers (OSLs) overlying thick clay mineral soil layers (CMLs), even during dry periods. We conducted detailed observations in this catchment to clarify the formation processes of the semi-perennial to perennial groundwater zone. The results showed that water is supplied from the CMLs to the OSLs as unsaturated upward flow in areas where the OSLs are dry. This water then accumulates in the downslope hollow, which sustains the groundwater zone in the OSLs during dry periods. The frequent and long-term occurrence of upward flow can be attributed to differences in the hydraulic properties of OSLs and CMLs. This process prevents the OSLs in the hollow from drying, presumably causes volumetric and chemical changes in streamflow, and reduces the stability of OSLs.

In snow-covered regions, a large number of landslides are induced by infiltration of snowmelt water. Although it is very important to early find signs of increase in landslide activity such as cracks or bulges for preventing or mitigating snowmelt-induced landslide disasters, thick snow cover often makes it difficult to find them. In such cases, frequent patrols of slopes prone to landslides during periods with high risk can be effective. In Japan, snowmelt advisories are issued by the Japan Meteorological Agency while snowmelt-induced disasters (e.g., flood and landslides) are predicted based on meteorological conditions. Although it seems that snowmelt advisories can be used for judging whether patrols are required, it has been reported that snowmelt advisories are not issued for some days with high risk of snowmelt-induced landslides (Irasawa et al, 2011). Focused exclusively on landslides, Nakaya et al (2008) and Touhei et al (2016) proposed methods for capturing 70% of landslides by setting a critical level using reservoir inflow and river water level and flow rate as hydrological indices. These methods, however, are difficult to apply for areas affected by human impacts including irrigation and water intake and drainage of power stations. In this study, based on the antecedent precipitation index, reported as a hydrological index showing a good correlation with slow-moving landslide velocity (e.g., Enokida et al, 2002), we propose an extensively applicable method for setting snowmelt-induced landslides warning periods. The target areas are three 5-km meshes in Joetsu and Myoko Cities, Niigata Prefecture, central Japan, where heavy snowfall in winter and the underlying Tertiary sedimentary rocks cause many snowmelt-induced landslides every year. We used for analyses 285 landslide cases that occurred from December to May in 1979 to 2020 reported in data set on landslides compiled by the Niigata Prefectural government. We used (meltwater and/or rainwater), which is the total amount of water reaching the ground surface, instead of precipitation, for calculating the antecedent precipitation index. The amount of snowmelt was estimated based on the heat balance method using the Japan Meteorological Agency observation data alone (Matsunaga, 2019) for the center of each mesh with an average elevation within the mesh. and the antecedent index with a various half-life were calculated hourly. Using the standard score, calculated by normalizing the antecedent index, we determined the critical standard score capturing 70% of the target landslides in each mesh and the half-life minimizing the landslides warning periods (i.e., periods during which the standard score exceeds the critical standard score). These procedures resulted in the average landslides warning periods per year of 36 to 50 days with 36 to 318 hours of the half-life for all meshes. On the other hand, snowmelt advisories were issued for 30 days per year in average from 2013 to 2020, capturing only 36% of the target landslides. Thus, the method proposed in this study shows more than 30% higher landslide capture ratio and therefore is better than snowmelt advisories for setting snowmelt-induced landslides warning periods.

The 2018 Hokkaido Eastern Iburi Earthquake triggered numerous shallow landslides on slopes covered with thick pyroclastic fall deposits. These shallow landslides tended to occur on concave rather than convex slopes and their slip surfaces were very wet, indicating that water played an important role in landslide initiation. As a first step toward clarifying the role of water in these landslides, we used tensiometers to monitor pressure head dynamics on a natural hillside covered with thick pyroclastic deposits that remained in place throughout the earthquake. We found that on concave slopes, the lower part of the pyroclastic fall deposits throughout the weathered basement complex (sedimentary rock) were always wet. Notably, the interface between the pyroclastic fall deposits and weathered basement complex, which forms a potential slip surface for earthquake-induced landslides, was always at or near saturation. On convex slopes, the weathered basement complex was never saturated and showed greater pressure head fluctuation. We infer that the pyroclastic fall deposits over the basement complex tend to weather more easily and are more susceptible to intense ground motion on concave than on convex slopes and the landslide slip surface was saturated at the timing of the earthquake on concave slopes. We conclude that these factors contributed to the larger number of shallow landslides initiated on concave slopes.

Rainfall-runoff processes in a headwater catchment have been typically explained by water flow in permeable soil layers (comprised of organic soil layers and mineral soil layers produced by weathering of bedrock) overlying less permeable layers (i.e., bedrock). In a catchment where mineral soils are characterized by clayey materials (e.g., mudstone, slate, and serpentine catchment), it is possible that mineral soil layers function substantially as less permeable layers because of a low permeability of clayey materials. However, roles of clay layers in rainfall-runoff processes in such a headwater catchment are not fully understood. In this study, we conducted detailed hydrological, hydrochemical, and thermal observations in a serpentinite headwater catchment (2.12 ha) in Hokkaido, Northern Japan, where mineral soil layers consisting of thick clay layers (thickness: approximately 1.5 m) produced by weathering of the serpentinite bedrock underlies organic soil layers (thickness: approximately 0.4 m). Saturated hydraulic conductivity (Ks) and water retention curve of these two layers were also measured in a laboratory. The observation results demonstrated that groundwater was formed perennially in the organic soil layers and clay layers. The groundwater level within the organic soil layers and specific discharge of the catchment showed rapid and flashy change in response to rainfall. In contrast, the groundwater level within the clay layers showed slow and small change. Temperature of the groundwater and stream water suggested that water from the depth of the organic soil layers contributed to streamflow. The electric conductivity (EC) of the groundwater in the clay layers was very high, ranging from 321 to 380 µS cmˉ¹. On the other hand, the EC of soil water (unsaturated water stored in the organic soil layers) was relatively low, ranging from 98 to 214 µS cmˉ¹. Hydrograph separation using EC showed that contribution of water emerging from the clay layers to the total streamflow ranged from 31 to 76% in low to high flow periods. Temporal variation in the total head, measured using tensiometers installed at four depths at the ridge of the catchment, indicated that in wet periods when the groundwater level in the organic soil layers was high, water flow from the organic soil layers to the clay layers occurred, whereas, in dry periods, water flow from the clay layers into the organic soil layers occurred. The laboratory measurements showed that the organic soil layers had high Ks (10ˉ² cm sˉ¹) and low water-holding capacity, whereas the clay layers had low Ks (10ˉ⁴ cm sˉ¹) and high water-holding capacity. It can be concluded from these results that clay layers play two roles: (1) forming perched groundwater table and lateral flow on the clay layers (the role of less permeable layers) and (2) supplying water into the organic soil layers in the dry periods (the role of water supplier).

The heat balance method has often been used for calculating the snowmelt for the purpose of estimating watershed water resources in the form of snow in winter and analyzing snowmelt runoff. Because the method requires many weather elements, some of which are not observed frequently (e.g., longwave radiation), methods of estimating such less frequently observed weather elements from more frequently observed ones have also been proposed. However, few previous studies have developed a method of estimating the hourly snowmelt based on the heat balance method using the frequently observed weather elements alone and applied for analyzing the hourly groundwater level fluctuation in a landslide site in snow-covered area. In this study, we developed a model of estimating the hourly snowmelt based on the heat balance method using the Japan Meteorological Agency observation data, the most commonly available weather data in Japan, alone, (i.e., temperature, precipitation, wind speed, sunshine duration, atmospheric pressure, and vapor pressure), and applied the model to a past landslide site with deep sliding surface (approximately 20 m) in snow-covered area in Hokkaido, Northern Japan. Moreover, we applied the functional models based on the antecedent precipitation index calculated using (the meltwater and/or rainwater) instead of the rainfall to reproduce the hourly groundwater level fluctuation observed in the site. The results showed good agreement between the observed and calculated snowmelt and groundwater level. The models proposed and used in this study are useful for estimating the hourly snowmelt and analyzing groundwater level fluctuation in a landslide sites in snow-covered area, and should be tested for other landslide sites to further verify the applicability.

Summary The controls on the rapid mobilization and runoff of pre-event water are an important issue in hillslope hydrology. One of the mechanisms involved is groundwater ridging, based on the capillary fringe effect. The typical conditions for groundwater ridging to occur are as follows: (1) the slope consists of fine- to medium-textured materials with a large extent of the tension-saturated zone, (2) the slope gradient is low, and (3) the initial groundwater level is high. Where these three conditions are met, a further condition, (4) a small amount of rainwater, would then be enough to trigger groundwater ridging. In this study, we detail groundwater ridge formation in the bedrock layers of Akakabe Watershed (Japan) as recorded by detailed field observations using tensiometers and bedrock boreholes under conditions violating the above. The study site consisted of materials showing almost no tension-saturated zone and had a relatively high gradient (22°). Moreover, the initially high groundwater level did not always generate a groundwater ridge, and a large total rainfall (>160 mm) was required to trigger groundwater ridging. Hence the conventional mechanism cannot explain the groundwater ridging at the field site studied here. It seems that the smaller distance from the groundwater table to the ground surface and the higher hydraulic conductivity in the downslope areas compared to the upslope areas caused a more rapid and larger groundwater level rise in the downslope areas, which accounted for the observed groundwater ridging. This study demonstrated that a groundwater ridge can be formed in the absence of the capillary fringe effect and that bedrock groundwater can contribute to storm runoff generation in headwater catchments under the influence of groundwater ridging.

Learning the lessons to be taught by large earthquakes of the past is one key to solving the problems of sediment-related disasters of the future, including slope failures, deep-seated landslides, and landslide dam (natural barriers formed by landslides). Our case subject is the Iwate-Miyagi Nairiku Earthquake in 2008 and the emergency response to disasters of Japan’s central government and other organizations. The earthquake occurred on 14th June 2008 and had a JMA (Japan Meteorological Agency) magnitude of 7.2 and a maximum seismic intensity of 6 upper on the JMA seismic intensity scale. The hypocenter in a mountainous area underlain by thick volcanic ejecta triggered over 3,000 slope failures, deep-seated landslides, and debris flows. The earthquake created 15 landslide dams which were expected to cause serious damage downstream if dams collapsed. Emergency measures taken included channel excavation and pumping of landslide dams. Moreover, emergency checking of potential danger sites immediately after the earthquake found 20 sites requiring emergency measures. The relationship between seismic intensity and sites of slope failure and deep-seated landslide showed that seismic intensity exceeding 5 upper caused such disasters and required emergency checking.

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.

As a first step toward describing water flow processes in bedrock, a coil-type time domain reflectometry (TDR) probe capable of measuring volumetric water content, θ, in weathered bedrock at three depths was prepared. Because the coil-type TDR probe is large in diameter (19 mm), it can be installed even in highly weathered bedrock more easily and appropriately than conventional TDR probes that consists of two or three rods of small diameter (5-8 mm). The probe calibrations suggest that the values measured by the probe are very sensitive to changes in θ. Using the calibrated probe together with commercially available profile soil moisture sensors, the θ profile was monitored for 1 year. Even rainfall events with relatively small cumulative rainfall of 15 mm increased the bedrock θ, and the increments were comparable to those in the soil. After the end of the rainfall events, the bedrock θ displayed a more rapid drop than the soil, and varied little during the period of no rainfall. The water storage showed similar tendencies. These observations suggest that the bedrock θ is controlled by clearly distinguishable macropores and micropores within the bedrock. It is concluded that the coil-type TDR probe is very effective in determining θ in weathered bedrock, and that bedrock, conventionally defined by conducting cone penetration tests and treated as impermeable, does conduct and hold substantial amounts of water, and therefore contribute greatly to hydrological processes in headwater catchments. Copyright © 2007 John Wiley & Sons, Ltd.

As a first step toward describing water flow processes in bedrock, we determined the hydraulic properties of three trimmed samples of weathered granitic bedrock (referred to as Samples A, B, and C, in order of size) in the laboratory. Silicone rubber was used to fill the space between each sample and the surrounding cylinder wall, ensuring accurate measurement of hydraulic properties of the samples. All samples showed similar saturated hydraulic conductivity values of 1 × 10 −4 cm s −1 , with the saturated water flow in all samples obeying Darcy9s Law. Unsaturated hydraulic conductivity and water retention functions of Sample A were determined by means of a multistep outflow experiment. Parameters in both functions were optimized by comparing observed and computed cumulative outflow rates. The resulting computed cumulative outflow rates using the optimized parameters showed a good match to the observed cumulative outflow data. Moreover, the derived water retention function agreed closely with the function measured by the pressure plate method. We conclude that the methods proposed in this study are effective for determining the hydraulic properties of weathered bedrock. The bedrock water retention curve exhibited small changes in volumetric water content throughout the measurement range where the pressure head, ψ, was greater than −200 cm. The bedrock hydraulic conductivity function showed a small decrease in hydraulic conductivity in the very wet range of ψ greater than −30 cm, and then declined gradually with decreasing ψ.

Electrical resistivity imaging (ERI) as an effective method to evaluate water flow processes through bedrock in a hillslope in a headwater catchment was validated by invasive hydrometric observations. Distributions of increases and decreases in electrical resistivities ρ relative to a reference ρ profile (Δρ) corresponded well with the increases and decreases in volumetric water content θ (Δθ) calculated from the directly observed pressure head ψ using tensiometers and borehole wells. This demonstrates the applicability of time-lapse ERI measurement for qualitatively evaluating the spatial and temporal variations in θ (i.e., wetting and drying processes) for not only soil mantles but also for bedrock in a natural hillslope. There was a reasonable correlation ( R 2 = 0.69 to 0.77) between each average θ and ρ in regions assumed to have different degrees of weathering, indicating the potential of ERI for quantitatively evaluating moisture conditions within an entire natural hillslope, including bedrock, based on field-scale calibrations with invasive methods. Fluctuations in groundwater tables in boreholes within bedrock along the survey line and discharge from two differently sized catchments including the study slope were both successfully reflected in the temporal variation in mean ρ in the regions located just above and below the groundwater tables. This indicates the potential of ERI for estimating groundwater levels and runoff from a watershed based on temporal ρ monitoring within an entire slope, including the bedrock; such estimations may be more difficult to achieve with invasive methods in many mountain slopes.

[1] Understanding a discharge hydrograph is one of the leading interests in catchment hydrology. Recent research has provided credible information on the importance of bedrock groundwater on discharge hydrographs from headwater catchments. However, intensive monitoring of bedrock groundwater is rare in mountains with steep topography. Hence, how bedrock groundwater controls discharge from a steep headwater catchment is in dispute. In this study, we conducted long-term hydrological observations using densely located bedrock wells in a headwater catchment underlain by granitic bedrock. The catchment has steep topography affected by diastrophic activities. Results showed a fairly regionalized distribution of bedrock aquifers within a scale of tens of meters, consisting of upper, middle, and lower aquifers, instead of a gradual and continuous decline in water level from ridge to valley bottom. This was presumably attributable to the unique bedrock structure; fault lines developed in the watershed worked to form divides between the bedrock aquifers. Spatial expanse of each aquifer and the interaction among aquifers were key factors to explain gentle and considerable variations in the base flow discharge and triple-peak discharge responses of the observed hydrograph. A simple model was developed to simulate the discharge hydrograph, which computed each of the contributions from the soil mantle groundwater, from the lower aquifer, and from the middle aquifer to the discharge. The modeling results generally succeeded in reproducing the observed hydrograph. Thus, this study demonstrated that understanding regionalized bedrock aquifer distribution is pivotal for explaining discharge hydrograph from headwater catchments that have been affected by diastrophic activities.

[1] The formation of groundwater in the soil mantle has a dominant effect on rainwater discharge and shallow landslide occurrence in headwater catchments. Here, we report 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 physical hydrology models based on a geographic information system (GIS). The other was a semi-perennial-type response, rarely reported in previous studies, which cannot be explained by the existing physical models. 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 shallow landslides. Moreover, peaks in the semi-perennial groundwater lagged considerably behind rainstorm events, which probably affected base flow discharge by forming a delayed peak. Geochemical and geothermal observations indicated that the source of the semi-perennial groundwater was deep bedrock groundwater, demonstrating the considerable effects of bedrock groundwater on surface hydrological processes.

[1] Recent studies have suggested that bedrock groundwater can exert considerable influence on runoff generation, water chemistry, and the occurrence of landslides in headwater catchments. To clarify water infiltration and redistribution processes between soil and shallow bedrock and their effect on storm and base flow discharge processes in a small headwater catchment underlain by weathered granite, we conducted hydrometric observations using soil and bedrock tensiometers combined with hydrochemical measurements and water budget analyses at three different spatial scales. Results showed that in an unchanneled 0.024-ha headwater catchment, saturated and unsaturated infiltration from soil to bedrock is a dominant hydrological process at the soil-bedrock interface. Annual bedrock infiltration ranged from 35 to 55% of annual precipitation and increased as precipitation increased, suggesting a high level of potential bedrock infiltration, partly explained by the high buffering capacity of the soil layer overlying the bedrock. This physical property of the soil layer was an important factor in controlling the generation of bedrock infiltration and saturated lateral flow over the bedrock. In a 0.086-ha watershed including the unchanneled headwater catchment, exfiltration from the bedrock toward the soil layer composed more than half the annual discharge.