Overland Flow Processes Research Articles

A hydrologic signature approach to analysing wildfire impacts on overland flow

AbstractPost‐fire flooding and debris flows are often triggered by increased overland flow resulting from wildfire impacts on soil infiltration capacity and surface roughness. Increasing wildfire activity and intensification of precipitation with climate change make improving understanding of post‐fire overland flow a particularly pertinent task. Hydrologic signatures, which are metrics that summarize the hydrologic regime of watersheds using rainfall and runoff time series, can be calculated for large samples of watersheds relatively easily to understand post‐fire hydrologic processes. We demonstrate that signatures designed specifically for overland flow reflect changes to overland flow processes with wildfire that align with previous case studies on burned watersheds. For example, signatures suggest increases in infiltration‐excess overland flow and decrease in saturation‐excess overland flow in the first and second years after wildfire in the majority of watersheds examined. We show that climate, watershed and wildfire attributes can predict either post‐fire signatures of overland flow or changes in signature values with wildfire using machine learning. Normalized difference vegetation index (NDVI), air temperature, amount of developed/undeveloped land, soil thickness and clay content were the most used predictors by well‐performing machine learning models. Signatures of overland flow provide a streamlined approach for characterizing and understanding post‐fire overland flow, which is beneficial for watershed managers who must rapidly assess and mitigate the risk of post‐fire hydrologic hazards after wildfire occurs.

Remote sensing evapotranspiration in ensemble-based framework to enhance cascade routing and re-infiltration concept in integrated hydrological model applied to support decision making

Integrated hydrological models (IHMs) help characterize the complexity of surface–groundwater interactions. The cascade routing and re-infiltration (CRR) concept, recently applied to a MODFLOW 6 IHM, improved conceptualization and simulation of overland flow processes. The CRR controls the transfer of rejected infiltration and groundwater exfiltration from upslope areas to adjacent downslope areas where that water can be evaporated, re-infiltrated back to subsurface, or discharged to streams as direct runoff. The partitioning between these three components is controlled by uncertain parameters that must be estimated. Thus, by quantifying and reducing those uncertainties, next to uncertainties of the other model parameters (e.g. hydraulic and storage parameters), the reliability of the CRR is improved and the IHM is better suited for decision support modelling, the two key objectives of this work. To this end, the remotely sensed MODIS-ET product was incorporated into the calibration process for complementing traditional hydraulic head and streamflow observations. A total of approximately 150,000 observations guided the calibration of a 13-year MODFLOW 6 IHM simulation of the Sardon catchment (Spain) with daily stress periods. The model input uncertainty was represented by grid-cell-scale parameterization, yielding approximately 500,000 unknown input parameters to be conditioned. The calibration was carried out through an iterative ensemble smoother. Incorporating the MODIS-ET data improved the CRR implementation, and reduced uncertainties associated with other model parameters. Additionally, it significantly reduced the uncertainty associated with net recharge, a critical flux for water management that cannot be directly measured and rather is commonly estimated by IHM simulations.

Infiltration-Based Variability of Soil Erodibility Parameters Evaluated with the Jet Erosion Test

Soil erosion by water on agricultural hillslopes leads to numerous environmental problems including reservoir sedimentation, loss of agricultural land, declines in drinking water quality, and requires deep understanding of underlying physical processes for better mitigation. It is imperative to accurately predict soil erosion caused by overland flow processes so that soil conservation efforts can be undertaken proactively before large-scale sedimentation problems arise. Soil detachment is often described by the excess shear stress equation that contains two physical soil erodibility parameters, erodibility coefficient, and critical shear stress. These parameters are normally assumed to be constant but can change across varying soil texture classes as well as during surface runoff events due to changes in soil cohesion and potential dependency on soil moisture content. These changes may significantly affect soil erosion rates at the field and watershed scale. In this study, the erodibility parameters of three soil types (sandy loam, clay loam, and silty clay loam) were analyzed using a laboratory mini-Jet Erosion Test (JET) to determine the effect of soil sample infiltration and moisture condition. Results from the experiments depicted a dynamic relationship between the soil erodibility parameters and amount of infiltrated mass of water. Data analysis displayed that for soils of different texture critical shear stress exhibited local minimum with higher values for very dry and saturated soils, while erodibility coefficient tended to increase with the increase of mass of soil water. Utilizing these dynamic soil erodibility parameters did not result in a significant difference in soil erosion rates when compared to using the averaged soil erodibility parameters taken from the experiment but the range of potential erosion rates increases with the increase of applied sheer stress to soil surface. The erosion rates with the experiment-based coefficients were found to be higher than with the baseline WEPP-based coefficients. These results highlight the importance of evaluating the effect of intrastorm dependent factors during surface runoff events, such as antecedent soil moisture content, time to peak from the start of runoff, soil cohesion, etc., on soil erodibility parameters to accurately calculate erosion rates, especially for initially dry soils or during earlier stages of surface runoff when critical shear stresses were highly affected. Further assessment of such factors with JET or other laboratory and field tests is recommended.

An urban hydrological model for flood simulation in piedmont cities: Case study of Jinan City, China

Urbanization has great potential to adversely affect the local hydrological cycle, as well as result in severe ecological and environmental problems. Piedmont cities have a special topography and complicated overland flow processes, thus creating challenges for the simulation of urban flooding/waterlogging processes. The TVGM-USWM model, a new distributed urban hydrological model that considers the flow routing in urban drainage systems and complicated overland flow processes, was developed in this study. We applied this model to the Huangtaiqiao drainage basin of Jinan City, China and set up two simulation scenarios. The spatial heterogeneity of the proportion of directly connected impervious area (DCIA) and disconnected impervious area (DIA) in sub-catchments was considered in scenario B, but not in scenario A. We conducted sensitivity analysis on model parameters by using the Sobol method. Results showed that: (1) model parameters have a certain degree of uncertainty, and the parameter characterizing the proportion of the DCIA and DIA showed a relatively high sensitivity under rainstorms of different levels and different accuracy assessment standards; (2) The simulation of scenario A was poor (with a mean Nash coefficient of 0.59), which ignored the different flow paths followed by runoff from DCIA and DIA; and (3) Considering the spatial differences in the overland flow process, the prediction accuracy of scenario B significantly increased (with a mean Nash coefficient of 0.82). This indicated the importance of the disconnected impervious area and the lateral water exchange across the impervious-pervious interface on the flood simulation in a piedmont city. Compared to most urban hydrological models, the TVGM-USWM model can represent complex rainfall-runoff mechanisms in urban areas with varied terrain. The findings of this study can provide scientific decision-making for the planning and construction of a sponge city in Jinan City.

A Sink Screening Approach for 1D Surface Network Simplification in Urban Flood Modelling

Sinks configure the surface networks for overland flow processes representations during 1D hydrodynamic modelling. The excessive number of sinks detected from high-resolution DEMs can boost 1D computational costs significantly. To pursue optimal sink numbers and their optimal spatial distribution, a Volume Ratio Sink Screening (VRSS) method was developed to screen for computationally important sinks, while compensating for volume losses from removed (unimportant) sinks, such that 1D hydrodynamic modelling yields faster computing times without significant loss of accuracy. In comparison with an existing geometry-based sink screening method, we validated this method by conducting sensitivity analyses for the proposed screening criteria in three Danish case areas of distinct topographies. Two iterative procedures were programmed to assess and compare their sink screening performances in terms of sink number reductions and volume loss reductions, and a volume loss solver was developed to quantify catchment-wide volume losses in the 1D surface network. Compared to a geometry-based sink screening method, the VRSS method performs more robustly and produces more efficient reductions in the number of sinks, as well as efficient reductions in volume losses.

Modelling the Event-Based Hydrological Response of Mediterranean Forests to Prescribed Fire and Soil Mulching with Fern Using the Curve Number, Horton and USLE-Family (Universal Soil Loss Equation) Models

The SCS-CN, Horton, and USLE-family models are widely used to predict and control runoff and erosion in forest ecosystems. However, in the literature there is no evidence of their use in Mediterranean forests subjected to prescribed fire and soil mulching. To fill this gap, this study evaluates the prediction capability for runoff and soil loss of the SCS-CN, Horton, MUSLE, and USLE-M models in three forests (pine, chestnut, and oak) in Southern Italy. The investigation was carried out at plot and event scales throughout one year, after a prescribed fire and post-fire soil mulching with fern. The SCS-CN and USLE-M models were accurate in predicting runoff volume and soil loss, respectively. In contrast, poor predictions of the modelled hydrological variables were provided by the models in unburned plots, and by the Horton and MUSLE models for all soil conditions. This inaccuracy may have been due to the fact that the runoff and erosion generation mechanisms were saturation-excess and rainsplash, while the Horton and MUSLE models better simulate infiltration-excess and overland flow processes, respectively. For the SCS-CN and USLE-M models, calibration was needed to obtain accurate predictions of surface runoff and soil loss; furthermore, different CNs and C factors must be input throughout the year to simulate the variability of the hydrological response of soil after fire. After calibration, two sets of CNs and C-factor values were suggested for applications of the SCS-CN and USLE-M models, after prescribed fire and fern mulching in Mediterranean forests. Once validated in a wider range of environmental contexts, these models may support land managers in controlling the hydrology of Mediterranean forests that are prone to wildfire risks.

Bi-directional coupling of an open-source unstructured triangular meshes-based integrated hydrodynamic model for heterogeneous feature-based urban flood simulation

Urban pluvial flash floods have become a matter of widespread concern, as they severely impact people’s lives in urban areas. Hydrological and hydraulic models have been widely used for urban flood management and urban planning. Traditionally, to reduce the complexity of urban flood modelling and simulations, simplification or generalization methods have been used; for example, some models focus on the simulation of overland water flow, and some models focus on the simulation of the water flow in sewer systems. However, the water flow of urban floods includes both overland flow and sewer system flow. The overland flow processes are impacted by many different geographical features in what is an extremely spatially heterogeneous environment. Therefore, this article is based on two widely used models (SWMM and ANUGA) that are coupled to develop a bi-directional method of simulating water flow processes in urban areas. The open source overland flow model uses the unstructured triangular as the spatial discretization scheme. The unstructured triangular-based hydraulic model can be better used to capture the spatial heterogeneity of the urban surfaces. So, the unstructured triangular-based model is an essential condition for heterogeneous feature-based urban flood simulation. The experiments indicate that the proposed coupled model in this article can accurately depict surface waterlogged areas and that the heterogeneous feature-based urban flood model can be used to determine different types of urban flow processes.

Effects of Different Aboveground Structural Parts of Grass Strips on the Sediment-Trapping Process

Grass strips can decrease erosion, trap sediment in silt-laden water flowing downhill, and control nonpoint source pollution. Determining the effects of different parts of grass strips on silt-laden overland flow will improve our understanding of sediment trapping by grass strips with different structures. Sediment trapping by grass strips was studied using a 5° slope, 30 L min−1 m−1 flow rate, 120 g L−1 sediment concentration, and different aboveground components of grass strips (complete grass, removed green grass, and removed green and withered grass). The whole overland flow process was monitored. Meanwhile, the runoff sediment samples at the outlet were collected and measured. Sediment trapping by aboveground grass parts was quantified at different stages. Of the soil bed surface, green grass, and withered grass, the soil bed surface dominated sediment trapping in the initial stage of the sediment-trapping process, contributing about 90% of total sediment deposition in the first 5 min. As the sediment-trapping process continued, the effect of the soil bed surface weakened, and the green grass played a major role at the later stage of sediment trapping. The ratio of the soil bed surface, green grass, and withered grass contributions to total sediment deposition at the stable stage of the experiments was approximately 3:5:2. The results will help assess the effects of vegetation restoration on sediment transport in entire watersheds.

Soil Erosion and Controls in the Slope-Gully System of the Loess Plateau of China: A Review

The Loess Plateau has long been suffering from serious soil erosion of which erosion from the slope-gully system is now dominant. The slope-gully system is characterized with distinctive erosion distribution zones consisting of inner and inter gully areas wherein erosion patterns spatially vary, acting as both sediment source and the dominant sediment and water transport mechanism. In this paper, a substantial body of research is reviewed concentrating on the soil erosion processes and control practices in the slope-gully system. The inner gully area is identified as the main sediment source while runoff and sediment from the inter-gully upland is found to significantly affect down slope erosion processes. Correspondingly, the protective vegetation pattern and coverage should be strategically designed for different erosion zones with an emphasis on the critical vegetation cover and pattern to reduce sediment yield of the whole slope-gully system. Check-dam could change the base level of erosion and reduce the slope length of the gully side, which will further decrease the possibility and magnitude of gravity erosion. We concluded that understanding the erosion processes and implementing erosion practices for the slope-gully system are of importance and require more research efforts that emphasize: 1) the influence of upland runoff on erosion processes at downslope; 2) the relationship between hydraulic characteristics of overland flow and erosion process at a slope-gully system scale; 3) physical mechanisms of different vegetation patterns on the slope-gully erosion process.

Comparing overland flow processes between semiarid and humid regions: Does saturation overland flow take place in semiarid regions?

Two main processes of runoff are commonly thought to take place in a drainage basin: infiltration-excess overland flow, known also as Hortonian overland flow (HOF), and saturation overland flow (SOF). While HOF mainly takes place in arid and semiarid regions once the infiltration capacity of the upper skin of the soil (usually ~1 mm thick) is surpassed, SOF may takes place in wet humid and subhumid climates. SOF takes place once the groundwater table reaches the surface (mainly close to the stream channel), or due to an impermeable subsurface soil layer. Despite the low precipitation in semiarid regions, a number of publications advocated the occurrence of SOF also in semiarid regions, subsequently underplaying the centrality of biocrusts to effectively generate runoff. These publications are mainly based on moisture measurements that were conducted with dielectric-based sensors. In the present work, it will be argued that: (a) the amount and duration of the rain cannot possibly result in SOF, (b) the location of the alleged SOF, as reported by some publications (at the up- or mid-slope), is unlikely to facilitate SOF, (c) the likelihood that the electrical pulses produced by the dielectric-based sensors will distinguish between completely saturated and nearly saturated conditions is low, (d) in certain occasions even when the moisture content, as detected by these sensors, was slightly below saturation, the authors tended to assume that the soil was saturated, (e) while claiming that the subsurface soil layer reached saturation, the upper layer of the soil (where overland flow takes place) was occasionally reported to remain below saturation, (f) no supportive evidence for the occurrence of SOF or even subsurface flow was provided, such as visible wet belts with dense vegetation. It is therefore concluded that the occurrence of SOF in semiarid regions should be cautiously considered and re-examined.

The effect of storm movement on infiltration, runoff and soil erosion in a semi‐arid catchment

AbstractRainfall characteristics are key factors influencing infiltration and runoff generation in catchment hydrology, particularly for arid and semiarid catchments. Although the effect of storm movement on rainfall‐runoff processes has been evaluated and emphasized since the 1960s, the effect on the infiltration process has barely been considered. In this study, a physically based distributed hydrological model (InHM) was applied to a typical semi‐arid catchment (Shejiagou, 4.26 km2) located in the Loess Plateau, China, to investigate the effect of storm movement on infiltration, runoff and soil erosion at the catchment scale. Simulations of 84 scenarios of storm movement were conducted, including storms moving across the catchment in both the upstream and downstream directions along the main channel, while in each direction considering four storm moving speeds, three rainfall depths and two storm ranges. The simulation results showed that, on both the hillslopes facing downstream (facing south) and in the main channel, the duration of the overland flow process under the upstream‐moving storms was longer than that under the downstream‐moving storms. Thus, the duration and volume of infiltration under upstream‐moving storms were larger in these areas. For the Shejiagou catchment, as there are more hillslopes facing downstream, more infiltration occurred under the upstream‐moving storms than the downstream‐moving storms. Therefore, downstream‐moving storms generated up to 69% larger total runoff and up to 351% more soil loss in the catchment than upstream‐moving storms. The difference in infiltration between the storms moving upstream and downstream decreased as the storm moving speed increased. The relative difference in total runoff and sediment yield between the storms moving upstream and downstream decreased with increasing rainfall depth and storm speed. The results of this study revealed that the infiltration differences under moving storms largely influenced the total runoff and sediment yield at the catchment scale, which is of importance in runoff prediction and flood management. The infiltration differences may be a potential factor leading to different groundwater, vegetation cover and ecology conditions for the different sides of the hillslopes.

Overland flow velocities measured using a high-resolution particle image velocimetry system

Accurate measurements of the overland flow velocity along the flow depth are critical to understanding hydraulic and soil erosion processes on hillslopes; however, this topic has not been investigated in detail to date. This study used a high-resolution particle image velocimetry method to capture the flow fields of overland flow on a fixed smooth bed. Experiments were conducted under eight shallow flow conditions at flow depths ranging from 0.49 to 1.1 × 10−2 m. A fully developed open-channel flow condition was the control group. The Reynolds stress values indicated the presence of two regions, namely the inner and outer regions. The inner region was affected by viscous stress and Reynolds stress, whereas the outer region was affected only by Reynolds stress. The boundary between the two regions was variable, whereas the open-channel flow had a fixed boundary. Three subregions were observed in the inner region, and two subregions were detected in the outer region. The free surface was primarily affected by surface tension, which weakened as the flow depth H and the Weber number We increased. At We > 6, the surface tension effect was buffered, and no free surface region was observed in the velocity profile. Our results indicated that the division of the velocity region for overland flow differed from that of the open-channel flow, and the expressions of the subregions were also different in terms of the empirical constant A and integration constant C. Overland flow occurred when the dimensionless flow depth (H+=Hu∗/ν) was less than 550. This study provides an approach to improve the measurement accuracy and theoretical understanding of overland flow processes and hydrodynamic mechanisms on hillslopes.

Are submarine and subaerial drainages morphologically distinct?

AbstractThe qualitative resemblance between terrestrial and submarine branched valley networks has led to speculation that common underlying processes control their formation. However, quantitative comparisons have been impeded by methodological limitations and coarse resolution in marine systems. We analyze channel concavity and steepness indices of 23 terrestrial and 29 submarine catchments to determine whether their profile morphologies are distinct. Statistical comparisons of these quantities demonstrate that concavity indices in submarine systems are, in general, lower than in subaerial systems, and that submarine tributaries are steeper than their associated mainstem. These differences may reflect distinct drainage formation mechanisms and dynamics of submarine sediment gravity flows as compared to overland flow processes.

Construction of a cellular automata-based model for rainfall-runoff and NPS pollution simulation in an urban catchment

Overland routing processes are critical for understanding the water environment impact of the urbanization-induced land-cover change and guiding the optimized landscape planning. However, current models generally tend to simplify the overland flow process by a semi-distributed routing approach. In this study, the cellular automata-based hydrology and NPS pollution model (CA-HNPSM) is established by integrating the CA-based overland runoff routing algorithm and the NPS pollution module. Three-layer spaces were also constructed by considering the hydrological connection among urban roofs, land surfaces and artificial drainage networks. Simplified sewer routing modules were introduced to improve the applicability of the CA algorithm in urban NPS pollution studies with insufficient drainage data. Then, the CA-HNPSM is tested in a real case study on an urban catchment in Beijing, China. Based on the results, the CA-HNPSM shows good performance in both the hydrological and NPS simulations in terms of the Nash-Sutcliffe efficiency coefficient and coefficient of determination. Detailed overland routing processes and hydrological and pollution results are provided. In addition, landscape configurations along the overland flow routs could also be identified. These overland results can be used as powerful reference for optimized landscape planning and the allocation of management measures.

Effects of rainfall, overland flow and their interactions on peatland interrill erosion processes

AbstractInterrill erosion processes on gentle slopes are affected by mechanisms of raindrop impact, overland flow and their interaction. However, limited experimental work has been conducted to understand how important each of the mechanisms are and how they interact, in particular for peat soil. Laboratory simulation experiments were conducted on peat blocks under two slopes (2.5° and 7.5°) and three treatments: Rainfall, where rainfall with an intensity of 12 mm h−1 was simulated; Inflow, where upslope overland flow at a rate of 12 mm h−1 was applied; and Rainfall + Inflow which combined both Rainfall and Inflow. Overland flow, sediment loss and overland flow velocity data were collected and splash cups were used to measure the mass of sediment detached by raindrops. Raindrop impact was found to reduce overland flow by 10 to 13%, due to increased infiltration, and reduce erosion by 47% on average for both slope gradients. Raindrop impact also reduced flow velocity (80–92%) and increased roughness (72–78%). The interaction between rainfall and flow was found to significantly reduce sediment concentrations (73–85%). Slope gradient had only a minor effect on overland flow and sediment yield. Significantly higher flow velocities and sediment yields were observed under the Rainfall + Inflow treatment compared to the Rainfall treatment. On average, upslope inflow was found to increase erosion by 36%. These results indicate that overland flow and erosion processes on peat hillslopes are affected by upslope inflow. There was no significant relationship between interrill erosion and overland flow, whereas stream power had a strong relationship with erosion. These findings help improve our understanding of the importance of interrill erosion processes on peat. Copyright © 2017 John Wiley & Sons, Ltd.

The relative importance of different grass components in controlling runoff and erosion on a hillslope under simulated rainfall

The effects of vegetation cover on overland flow and erosion processes on hillslopes vary with vegetation type and spatial distribution and the different vegetation components, including the above- and below-ground biomass. However, few attempts have been made to quantify how these factors affect erosion processes. Field experimental plots (5 m × 2 m) with a slope of approximately 25° were constructed and simulated rainfall (60 mm hr−1) (Rainfall) and simulated rainfall combined with upslope overland flow (20 L min−1) (Rainfall + Flow) were applied. Three grass species were planted, specifically Astragalus adsurgens (A. adsurgens), Medicago sativa (M. sativa) and Cosmos bipinnatus (C. bipinnatus). To isolate and quantify the relative contributions of the above-ground grass parts (stems, litter cover and leaves) and the roots to reducing surface runoff and erosion, each of the three grass species was subjected to three treatments: intact grass control (IG), no litter or leaves (only the grass stems and roots were reserved) (NLL), and only roots remaining (OR). The results showed that planting grass significantly reduced overland flow rate and velocity and sediment yield, and the mean reductions were 21.8%, 29.1% and 67.1%, respectively. M. sativa performed the best in controlling water and soil losses due to its thick canopy and dense, fine roots. Grasses reduced soil erosion mainly during the early stage of overland flow generation. The above-ground grass parts primarily contributed to reducing overland flow rate and velocity, with mean relative contributions of 64% and 86%, respectively. The roots played a predominant role in reducing soil erosion, with mean contribution of 84%. Due to the impact of upslope inflow, overland flow rate and velocity and sediment yield increased under the Rainfall + Flow conditions. The results suggest that grass species on downslope parts of semi-arid hillslopes performed better in reducing water and soil losses. This study is beneficial for forage selection, allocation and management practices, such as forage harvesting, when implementing restoration strategies to control soil and water losses.

Experimental Study on Anisotropic Attributes of Surface Roughness in Watersheds

AbstractThe spatial variability of surface roughness can affect the overland flow processes. To accurately simulate the overland flow processes of watersheds, many theories have been developed whic...

Adapting HYDRUS‐1D to Simulate Overland Flow and Reactive Transport during Sheet Flow Deviations

Core Ideas HYDRUS‐1D code was adapted to solve the diffusion wave equation for overland flow. The modified code can simulate transport and fate of many different kinds of solute. The model can simulate physically nonequilibrium overland flow and transport processes. Surface runoff is commonly described in numerical models using either the diffusion wave or kinematic wave equations, which assume that surface runoff occurs as sheet flow with a uniform depth and velocity across the slope. In reality, overland water flow and transport processes are rarely uniform. Local soil topography, vegetation, and spatial soil heterogeneity control directions and magnitudes of water fluxes. These spatially varying surface characteristics can generate deviations from sheet flow such as physical nonequilibrium flow and transport processes that occur only on a limited fraction of the soil surface. In this study, we first adapted the HYDRUS‐1D model to solve the diffusion wave equation for overland flow at the soil surface. The numerical results obtained by the new model produced an excellent agreement with an analytical solution for the kinematic wave equation. Additional model tests further demonstrated the applicability of the adapted model to simulate the transport and fate of many different solutes (non‐adsorbing tracers, nutrients, pesticides, and microbes) that undergo equilibrium and/or kinetic sorption and desorption and first‐ or zero‐order reactions. HYDRUS‐1D includes a hierarchical series of models of increasing complexity to account for both uniform and physical nonequilibrium flow and transport, e.g., dual‐porosity and dual‐permeability models, up to a dual‐permeability model with immobile water. This same conceptualization was adapted to simulate physical nonequilibrium overland flow and transport at the soil surface. The developed model improves our ability to describe nonequilibrium overland flow and transport processes and our understanding of factors that cause this behavior.

Effect of tree thinning and skidding trails on hydrological connectivity in two Japanese forest catchments

Land use composition and patterns influence the hydrological response in mountainous and forest catchments. In plantation forest, management operations (FMO) modify the spatial and temporal dynamics of overland flow processes. However, we found a gap in the literature focussed on modelling hydrological connectivity (HC) in plantation forest under different FMO. In this study, we simulated HC in two steep paired forest subcatchments (K2 and K3, 33.2ha), composed of Japanese cypress (Chamaecyparis obtusa Endl.) and Japanese cedar (Cryptomeria japonica D. Don) plantations (59% of the total area) against a tree thinning intensity of 50% at different time. Additionally, construction of new skidding trails and vegetation recovery was simulated on five thinning-based scenarios that covered a 40-month test period (July 2010 – October 2013). As a future scenario, six check-dams located in the main streams were proposed to reduce sediment and radionuclide delivery. An updated version of Borselli's index of runoff and sediment connectivity was run, using the D-infinity flow accumulation algorithm and exploiting three 0.5-m resolution digital elevation models. On the basis of the pre-FMO scenario, HC increased at catchment scale owing to tree thinning and the new skidding trails. This change was more noticeable within the area affected by the FMO, where HC increased by 11.4% and 10.5% in the cypress and cedar plantations in K2 respectively and by 8.8% in the cedar plantation in K3. At hillslope plot and stream scales, the evolution in the values of HC was less evident, except the increment (by 5.4%) observed in the streams at K2 after the FMO. Progressive vegetation recovery after the FMO triggered a slight reduction of connectivity in all compartments of both subcatchments. Forest roads and especially skidding trails presented the highest values of HC, appearing as the most efficient features connecting the different vegetation patches with the stream network. The spatial and temporal evolution of HC over the five past scenarios correlated well with the observed changes in runoff yield, as well as with the available values of rainfall interception and throughfall before, during, and after the FMO. The simulation of the proposed scenario recommends the construction of check-dams as effective landscape features to somewhat reduce HC and thus to decrease the sediment and radionuclide delivery rates from the two subcatchments.

Post-eruptive sediment transport and surface processes on unvegetated volcanic hillslopes – A case study of Black Tank scoria cone, Cima Volcanic Field, California

Conical volcanic edifices that are made up from lapilli to block/bomb pyroclastic successions, such as scoria cones, are widespread in terrestrial and extraterrestrial settings. Eruptive processes responsible for establishing the final facies architecture of a scoria cone are not well linked to numerical simulations of their post-eruptive sediment transport. Using sedimentological, geomorphic and 2D fragment morphology data from a 15-ky-old scoria cone from the Cima Volcanic Field, California, this study provides field evidence of the various post-eruptive sediment transport and degradation processes of scoria cones located in arid to semi-arid environments. This study has revealed that pyroclast morphologies vary downslope due to syn-eruptive granular flows, along with post-eruptive modification by rolling, bouncing and sliding of individual particles down a slope, and overland flow processes. The variability of sediment transport rates on hillslopes are not directly controlled by local slope angle variability and the flank length but rather by grain size, and morphological characteristics of particles, such as shape irregularity of pyroclast fragments and block/lapilli ratio. Due to the abundance of hillslopes degrading in unvegetated regions, such as those found in the Southwestern USA, granulometric influences should be accounted for in the formulation of sediment transport laws for geomorphic modification of volcanic terrains over long geologic time.