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We developed a catchment model based on a hydrogeomorphic concept that simulates discharge from channel-riparian complexes, zero-order basins (ZOB, basins ZB and FA), and hillslopes. Multitank models simulate ZOB and hillslope hydrological response, while kinematic wave models predict saturation overland runoff from riparian zones and route inputs from ZOB and riparian corridors through the channel. The model was parameterized and tested in the Hitachi Ohta Experiment Watershed, Japan. Tank models were parameterized for a 6 month period from May to October 1992, and these models were then tested for the same 6 month period in 1993. In ZB, with relatively shallower soils, total outflow for the 6 month period in 1993 was underpredicted by 25%. Better predictions were obtained for outflow from FA (deeper soils; -17%) and the entire catchment (-5%). Total runoff from the channel and riparian area depends on the ratio of this area to the total catchment area because this corridor is assumed to be saturated at all times. Stormflow response from ZOB was limited during relatively dry conditions and increased substantially during wetter conditions, especially in ZB, which has shallower soils (1.4 m of average); such effects were diminished in FA (deeper soils) and hillslopes. Outflow from ZB had the highest proportion of rapid flow, while slower flow dominates outflow from FA and hillslopes; these different responses appear to be mainly associated with soil depth and topography. Groundwater recharge, estimated by leakage from the lowermost tank in the models, was as high as 61 mm week-1 from ZB, with lesser recharge from other geomorphic components (18-21 mm week-1). These spatially explicit simulations provide a simpler approach to the greater data demands of distributed hydrologic models without compromising process function. [ABSTRACT FROM AUTHOR]

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Slope stability models traditionally use simple indicators of root system structure and strength when vegetation is included as a factor. However, additional root system traits should be considered when managing vegetated slopes to avoid shallow substrate mass movement. Traits including root distribution, length, orientation and diameter are recognized as influencing soil fixation, but do not consider the spatial and temporal dimensions of roots within a system. Thick roots act like soil nails on slopes and the spatial position of these thick roots determines the arrangement of the associated thin roots. Thin roots act in tension during failure on slopes and if they traverse the potential shear zone, provide a major contribution in protecting against landslides. We discuss how root traits change depending on ontogeny and climate, how traits are affected by the local soil environment and the types of plastic responses expressed by the plant. How a landslide engineer can use this information when considering slope stability and management strategies is discussed, along with perspectives for future research. This review encompasses many ideas, data and concepts presented at the Second International Conference ‘Ground Bio- and Eco-engineering: The Use of Vegetation to Improve Slope Stability—ICGBE2’ held at Beijing, China, 14–18 July 2008. Several papers from this conference are published in this edition of Plant and Soil. [ABSTRACT FROM AUTHOR]

Abstract: An unlikely combination of relatively low rainfall intensity and gently sloping topography produced numerous landslides and debris flows during a prolonged storm in July 2006 around Okaya City, Nagano Prefecture, Japan, and indicated the existence of other factors contributing to the landslide initiation. We focused our detailed investigation on a relatively large landslide/debris flow near Okaya City where the highly variable soil characteristics in the landslide scarp led us to examine the variability of physical and hydrological properties throughout the soil profile and within the area above the landslide. The soil in our study area ranges from gravelly loamy sand to loamy silt and is the result of ≈10,000years of erosion processes which have accumulated soil in this geomorphic hollow above a thin residual soil. A zone of black soil found at depths ranging from 20 to 280cm upslope of the landslide scar is indicative of sequential forest burning practiced during Jomon age hunting and gathering. Previous C-14 dates indicate these anthropogenic practices occurred between 10,200 and 4510years BP at this site. Based on the average thickness and age of this anthropic soil sequence, the average infilling rate of this geomorphic hollow (primarily by surface erosion) was about 0.45Mg ha−1 year−1 during this ≈5700year period. Saturated hydraulic conductivity (Ks) tended to be higher, but more variable in the upper meter of the soil profile where soils were affected by terracing and agriculture practices of the past 200years. At depths below 1.4m, Ks was a bit lower and less variable, especially in the black soil layers (minimum 1.11×10-5 and maximum 4.03×10-5 m s-1). Because this black anthropic soil is located near the middle of the profile, it may significantly affect the hydrologic response to rainfall inputs and the resulting slope stability by intercepting infiltrating rain water and developing a temporary perched water table or by absorbing and accumulating large quantities of water and delaying the further deep percolation in subsequent storm events. A detailed auger survey immediately above the landslide scarp revealed that the black layer is quite variable in depth but is thickest near the longitudinal axis of the concave hollow and then thins out and disappears closer to the ridges bounding the hollow. Total soil depth was also variable (1.8 to 4.0m) and exhibited a similar pattern of distribution within the hollow — i.e., deepest along the concave axis. The variable distribution of the black layer and its association with soil depth may contribute to the location of the landslide. Thus, both the vertical variability and spatial propagation of soil properties appear to suggest the specific site of landslide occurrence. Understanding the genesis of the black layer and the magnitude of anthropogenic effects in this area underline the need to assess historic land use patterns over thousands of years rather than relying only on the effects of recent human practices related to slope stability. [Copyright &y& Elsevier]

Summary: Overland flow generation was monitored in large plots (8×25m) on four hillslopes in a 4.9-ha catchment in Mie Prefecture, Japan. Three Japanese cypress (hinoki, Chamaecyparis obtusa) treatments (including three different understory conditions) and one deciduous forest treatment were studied. For all plots, including deciduous hillslopes, we observed overland flow even for small storm events (<10mm in total precipitation). The mean runoff coefficients in dense Japanese cypress plots with sparse understory were highest (13.0%) followed by dense Japanese cypress with fern ground cover (6.7%), and coefficients in managed cypress and deciduous forest were 3.6% and 1.2%, respectively. The runoff coefficients tended to be higher during storms that were preceded by dry conditions. High soil water repellency initially occurred in Japanese cypress forests between the litter and mineral soil horizon and might have been partly responsible for overland flow generation. During storms with total precipitation >180mm, runoff from Japanese cypress plots with dense fern understory exhibited a delayed and higher peak associated with return flow. The dominance of hillslope-scale flow contribution to catchment runoff was also affected by changes in the dominance of overland flow and return flow. Understory vegetation cover and the availability of a litter layer altered the amount of overland flow, which was mediated by soil water repellency and soil moisture. Observations at the hillslope scale are essential for conceptualization of runoff mechanisms and pathways in forested headwaters. [Copyright &y& Elsevier]

This research combines field, laboratory and numerical investigations to estimate the development of a wetting front within a 1.2 m residual soil mantle on a steep forested slope during rainfall events. The field-monitored variations in matric suction due to rain-water infiltration during various events revealed that the maximum infiltration rate was much higher when the wetting front resided in the upper 20 cm of soil compared to the case when the wetting front advanced to depths > 20 cm. Laboratory investigations on soil hydraulic properties (i.e., soil water characteristic curve, and hydraulic conductivity) were useful to establish the parameters of a multilayer finite-element model for one-dimensional vertical infiltration. These parameters were subsequently calibrated by matching the predicted and field measured transient pore water pressure responses during actual rainstorms with irregular rainfall patterns. The calibrated simulation model was used to assess the migration of the wetting front under uniform rainfall with different intensities. Based on the numerical results, a hyperbolic equation was developed to predict the duration of uniform rainfall required for the propagation of wetting front to a certain depth for a given rainfall intensity. The proposed equation was subsequently tested against field-monitored advancements of the wetting front during real rainstorms with variable rainfall intensity. [ABSTRACT FROM AUTHOR]

The article discusses the study on persistence of road runoff generation in a logged catchment in Peninsular Malaysia. In this study, the measurements of saturated hydraulic conductivity and diagnostic model simulation show that all types of logging roads/trails in the Bukit Tarek Experimental Catchment 3 (BTEC3) generate substantial Horton overland flow (HOF) during most storms, regardless of design and level of trafficking. It discusses that the most feasible strategy for reducing long-term road-related impacts in BTEC3 is limiting the depth of excavation and designing a more efficient road network.

Root cohesion is an important parameter governing slope stability in steep forested terrain. Forest harvesting impacts root cohesion, and although the temporal effects have been noted, this dynamic parameter is often assumed to be spatially uniform. A model was developed to simulate the variation in root cohesion on a hillslope resulting from various forest management treatments. The model combines physical data on the horizontal rooting distribution of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) together with a temporal relation of root cohesion decay. Harvesting methods examined include clear-cutting, single-tree selection cutting, and strip-cutting. Model outputs are analysed qualitatively for regions of root cohesion minima and quantitatively for the average root cohesion within the simulated hillslope. A selection cutting simulation maintained the highest average root cohesion value, decreasing to only 81% of the preharvest condition. In contrast, the minimum root cohesion following clear-cutting declined to 38% of the preharvest value. Selection and strip-cutting scenarios resulted in smaller areas of reduced root cohesion that were adjacent to areas with high root cohesion. Such partial cutting methods shorten the period of reduced root cohesion following timber harvesting compared with clear-cutting. [ABSTRACT FROM AUTHOR]

Resource managers require objective methodologies to optimize decisions related to forest road deactivation and other aspects of road management, especially in steep terrain, where road-related slope failures inflict extensive environmental damage. Decision analysis represents a systematic framework that clearly identifies real options and critical decision points. This framework links current decisions with expected future outcomes and provides advantages such as a common currency to systematically explore the liability consequences of limited budget expenditures to road deactivation and other road-related activities. Furthermore, the decision framework prevents the analysis from becoming hopelessly entangled by the vast number of possibilities generated by the alternative occurrences, magnitudes, and consequences of landslide/debris flow events and provides the information required for the first step of an adaptive management process. Here, a structured analysis of potential environmental risks for a road deactivation project in coastal British Columbia, Canada is presented. The application of decision analysis generates a ranking of the expected benefits of proposed deactivation activities on various road sections. The ranking distinguishes between road sections that offer high expected benefit from those that offer moderate to low expected benefit. Seventeen of 171, 100–m road segments accounted for 18% of the cumulative cost and 98% of the cumulative expected net benefits from road deactivation. Furthermore, the cost of deactivating a section of road is related to the expected benefit from such deactivation, thus providing the basis for more effective resource allocation and budgeting decisions. [ABSTRACT FROM AUTHOR]