[Show abstract][Hide abstract] 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 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-funnelling 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-influenced and root-influenced areas of a forested stand. Copyright
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] ABSTRACT: This study evaluated the applicability of tank models calibrated to an extremely large storm event using ordinary storm events. Rainfall and runoff were observed at three granitic watersheds having different forest recovery conditions: we studied a poor-vegetation watershed, a vegetation-recovered watershed, and a forested watershed. Parameters of the tank models were calibrated using rainfall and runoff data observed for ordinary storm events having total precipitation of less than 150 mm. Then those models were applied to simulate hydrographs for an extremely large storm event (Tokai heavy rain) having total precipitation of 457 mm. In the poor-vegetation watershed and vegetation-recovered watershed,which were characterized by thin soil layers, contributions of surface runoff to the total discharge were large even under ordinary storm events. In such watersheds, the tank models calibrated using the ordinary storm events reproduced hydrographs of the extremely large storm event. However, for the forested watershed, where surface runoff was rarely observed because of thick soil layers, the tank model was inaccurate for the extremely large storm event. The simulated hydrograph produced smaller peaks and gentler responses than the observations. We concluded that a large contribution of surface runoff generated only during an extremely large storm event caused underestimation of peak flows for the forested watershed.
No preview · Article · Jan 2013 · Suimon mizu shigen gakkaishi Journal of Japan society of hydrology & water resources
[Show abstract][Hide abstract] ABSTRACT: To survey heterogeneous hydrological properties in a footslope area of a mountainous watershed, we applied a new type of combined penetrometer-moisture probe (CPMP) for simultaneous measurements of soil water content, theta, and penetration resistance, N-c, of soil mantles. We examined the usefulness of CPMP data for inferring hydrological properties in a footslope area by comparing the spatial distributions of theta and N-c with pressure head, psi, obtained by continuous monitoring using densely nested tensiometers. By comparing theta and the topographic index of the bedrock surface obtained by the CPMP, we could effectively detect the heterogeneous water distribution that exists independently of topographic flow convergence. Such anomalous water distribution patterns suggest the existence of a preferential pathway, a soil layer with low permeability, and bedrock groundwater exfiltration. The CPMP data corresponded well with the tensiometer-observed psi under no-rainfall conditions. The results show that we can estimate the complex shape of the water flow domain, hydrological baseline, and point of bedrock groundwater exfiltration using CPMP measurements, without measuring. by tensiometers and soil hydraulic conductivity by collecting soil samples. Moreover, complicated water movement phenomena that occur during storm events, such as preferential water flow and variations in the hydrological baseline, could be inferred from the CPMP data under no-rainfall conditions. The CPMP technique was proven to be effective for understanding the heterogeneous hydrological properties of a footslope area.
No preview · Article · May 2012 · Vadose Zone Journal
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: 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 rho relative to a reference rho profile (Delta rho) corresponded well with the increases and decreases in volumetric water content theta (Delta theta) calculated from the directly observed pressure head psi using tensiometers and borehole wells. This demonstrates the applicability of time-lapse ERI measurement for qualitatively evaluating the spatial and temporal variations in theta (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 theta and rho in regions assumed to have different degrees of weathering, indicating the potential of ERI for quantitatively evaluating moisture conditions within an en tire 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 rho 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 rho monitoring within an en tire slope, including the bedrock; such estimations may be more difficult to achieve with invasive methods in many mountain slopes.
No preview · Article · Feb 2012 · Vadose Zone Journal
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jul 2011 · Water Resources Research
[Show abstract][Hide abstract] 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.
Full-text · Article · Feb 2011 · Water Resources Research
[Show abstract][Hide abstract] ABSTRACT: A newly developed, combined penetrometer-moisture probe (CPMP), together with two conventional geophysical methods, electrical resistivity imaging (ERI) and ground-penetrating radar (GPR), was used to determine soil depth, soil type, and water distribution of a forested slope underlain by granitic bedrock. Profiles of volumetric water content (θ) measured using the CPMP accurately depicted alternations of sandy, loamy, and gravelly layers above the water table. In most cases soil-bedrock interfaces were successfully detected by ERI because of the high contrast between the electrical resistivity of dry surface soil and lightly weathered bedrock. In some areas, the soil-bedrock interface was obscured by the presence of pore water or by heavily weathered bedrock. Although a good match was obtained between θ, as directly measured using the CPMP, and θ estimated from electrical resistivity (ρ), as measured with ERI, the CPMP was more accurate than ERI for detecting heterogeneous θ distributions caused by alternations of sandy, loamy, and gravelly soil layers. Using the CPMP and ERI together is effective in quantifying soil depth, soil material, and water content distribution on a forested slope. Although soil thickness, distribution, and groundwater depth were not successfully detected using GPR on the forested slope studied, it may be possible to achieve quantitative estimates of the soil thickness and water table by calibrating GPR data with CPMP data obtained from the same locality at the same time.
No preview · Article · Aug 2010 · Vadose Zone Journal
[Show abstract][Hide abstract] ABSTRACT: In physical models that evaluate the stability of hillslope surface soil masses, soil depth contributes directly to slope stability due to its weight and volume, as well as control of groundwater movement. Electrical resistivity imaging (ERI), a noninvasive geophysical technique, is a recently introduced method used to detect geological structures in landslide-prone areas. However, this method has not been well studied as a tool to detect the depth of the surface soil layer. Questions remain about the reliability of ERI, especially in surface zones of mountainous areas. Here, we present a case study of the use of ERI to detect the surface soil thickness of hillslopes in granitic and slate watersheds in central Japan. Like invasive penetration tests and geotechnical surveys using boreholes, ERI appears to be suitable for detecting soil-bedrock interfaces, due to the high contrast of resistivity values between surface soil and bedrock layers that we found in all of the watersheds. However, ER subsurface values vary over a relatively wide range, as well as from site to site. ERI also failed to clearly identify the soil-bedrock interface at some points along our measurement line. By referring to hydrological properties of bedrocks observed in a previous study, we presume that differences in the water retention characteristics of weathered granitic bedrock are a major factor in the variation in bedrock ER values that we observed.
[Show abstract][Hide abstract] 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.
No preview · Article · Aug 2009 · Vadose Zone Journal
[Show abstract][Hide abstract] 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.
Full-text · Article · Jun 2009 · Water Resources Research
[Show abstract][Hide abstract] 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.
No preview · Article · Feb 2009 · Vadose Zone Journal
[Show abstract][Hide abstract] 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.
No preview · Article · Jan 2009 · Soil Science Society of America Journal
[Show abstract][Hide abstract] 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.