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1.
Riparian vegetation is known to exert a number of mechanical and hydrologic controls on bank stability. In particular, plant roots provide mechanical reinforcement to a soil matrix due to the different responses of soils and roots to stress. Root reinforcement is largely a function of the strength of the roots crossing potential shear planes, and the number and diameter of such roots. However, previous bank stability models have been constrained by limited field data pertaining to the spatial and temporal variability of root networks within stream banks. In this paper, a method is developed to use root‐architecture data to derive parameters required for modeling temporal and spatial changes in root reinforcement. Changes in root numbers over time were assumed to follow a sigmoidal curve, which commonly represents the growth rates of organisms. Regressions for numbers of roots crossing potential shear planes over time showed small variations between species during the juvenile growth phase, but extrapolation led to large variations in root numbers by the time the senescent phase of the sigmoidal growth curve had been reached. In light of potential variability in the field data, the mean number of roots crossing a potential shear plane at each year of tree growth was also calculated using data from all species and an additional sigmoidal regression was run. After 30 years the mean number of roots predicted to cross a 1 m shear plane was 484, compared with species‐specific curves whose values ranged from 240 roots for black willow trees to 890 roots for western cottonwood trees. In addition, the effect of spatial variations in rooting density with depth on stream‐bank stability was modeled using the bank stability and toe erosion model (BSTEM). Three root distributions, all approximating the same average root reinforcement (5 kPa) over the top 1 m of the bank profile, were modeled, but with differing vertical distributions (concentrated near surface, non‐linear decline with depth, uniform over top meter). It was found that stream‐bank FS varied the most when the proportion of the failure plane length to the depth of the rooting zone was greatest. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

2.
Plants interact with and modify the processes of riverbank erosion by altering bank hydrology, flow hydraulics and bank geotechnical properties. The physically based slope stability model GWEDGEM was used to assess how changes in bank geotechnical properties due to the roots of native Australian riparian trees affected the stability of bank sections surveyed along the Latrobe River. Modelling bank stability against mass failure with and without the reinforcing effects of River Red Gum (Eucalyptus camaldulensis) or Swamp Paperbark (Melaleuca ericifolia) indicates that root reinforcement of the bank substrate provides high levels of bank protection. The model indicates that the addition of root reinforcement to an otherwise unstable bank section can raise the factor of safety (F s) from F s = 1·0 up to about F s = 1·6. The addition of roots to riverbanks improves stability even under worst‐case hydrological conditions and is apparent over a range of bank geometries, varying with tree position. Trees growing close to potential failure plane locations, either low on the bank or on the floodplain, realize the greatest bank reinforcement. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

3.
In steep soil‐mantled landscapes, the initiation of shallow landslides is strongly controlled by the distribution of vegetation, whose roots reinforce the soil. The magnitude of root reinforcement depends on the number, diameter distribution, orientation and the mechanical properties of roots that cross potential failure planes. Understanding how these properties vary in space and time in forests remains a significant challenge. Here we test the hypothesis that spatio‐temporal variations in root reinforcement along a hillslope occur as a function of topographic soil moisture gradients. To test this hypothesis we compared root reinforcement measurements from relatively dry, divergent noses to relatively wet, convergent hollows in the southern Appalachian Mountains, North Carolina, USA. Our initial results showed that root reinforcement decreased in areas of higher soil moisture because the tensile strength of roots decreased. A post hoc laboratory experiment further demonstrated that root tensile strength decreased as root moisture content increased. This effect is consistent with other experiments on stem woods showing that increased water content in the cell wall decreases tensile strength. Our experimental data demonstrated that roots can adjust to changes in the external root moisture conditions within hours, suggesting that root moisture content will change over the timescale of large storm events (hours–days). We assessed the effects of the dynamic changes in root tensile strength to the magnitude of apparent cohesion within the infinite slope stability model. Slopes can be considerably less stable when precipitation‐driven increases in saturated soil depth both increase pore pressures and decrease root reinforcement. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

4.
Water flow in the soil–root–stem system was studied in a flooded riparian hardwood forest in the upper Rhine floodplain. The study was undertaken to identify the vertical distribution of water uptake by trees in a system where the groundwater is at a depth of less than 1 m. The three dominant ligneous species (Quercus robur, Fraxinus excelsior and Populus alba) were investigated for root structure (vertical extension of root systems), leaf and soil water potential (Ψm), isotopic signal (18O) of soil water and xylem sap. The root density of oak and poplar was maximal at a depth of 20 to 60 cm, whereas the roots of the ash explored the surface horizon between 0 and 30 cm, which suggests a complementary tree root distribution in the hardwood forest. The flow density of oak and poplar was much lower than that of the ash. However, in the three cases the depth of soil explored by the roots reached 1·2 m, i.e. just above a bed of gravel. The oak roots had a large lateral distribution up to a distance of 15 m from the trunk. The water potential of the soil measured at 1 m from the trunk showed a zone of strong water potential between 20 and 60 cm deep. The vertical profile of soil water content varied from 0·40 to 0·50 cm3 cm?3 close to the water table, and 0·20 to 0·30 cm3 cm?3 in the rooting zone. The isotopic signal of stem water was constant over the whole 24‐h cycle, which suggested that the uptake of water by trees occurred at a relatively constant depth. By comparing the isotopic composition of water between soil and plant, it was concluded that the water uptake occurred at a depth of 20 to 60 cm, which was in good agreement with the root and soil water potential distributions. The riparian forest therefore did not take water directly from the water table but from the unsaturated zone through the effect of capillarity. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

5.
Tree roots contribute to the resistance of riparian sediments to physical deformation and disintegration. Understanding reinforcement by roots requires information on root distributions within riparian soils and sediments. Continuous‐depth models or curves have been proposed to describe vertical root density variations, providing useful indicators of the types of function that may be appropriate to riparian trees, but have generally been estimated for terrestrial species or broad vegetation types rather than riparian species or environments. We investigated vertical distributions of roots >0.1 mm diameter of a single riparian tree species (Populus nigra L.) along the middle reaches of a single river (Tagliamento River, Italy), where Populus nigra dominates the riparian woodland. Root density (hundreds m?2) and root area ratio (RAR in cm2 m?2) were measured within 10 cm depth increments of 24 excavated bank profiles across nine sites. Sediment samples, extracted from distinct strata within the profiles, were analysed for moisture content, organic matter content and particle size. Statistical analyses identified two groups of wetter and drier profiles and five sediment types. Following loge‐transformation of root density and RAR, linear regression analysis explored their variation with depth and, using dummy variables, any additional influence of moisture and sediment type. Significant linear regression relationships were estimated between both root density and RAR and depth which explained only 15% and 8% of the variance in the data. Incorporating moisture and then sediment characteristics into the analysis increased the variance explained in root density to 29% and 36% and in RAR to 14% and 26%. We conclude that riparian tree root density and RAR are highly spatially variable and are poorly explained by depth alone. Complex riparian sedimentary structures and moisture conditions are important influences on root distributions and so need to be incorporated into assessments of the contribution of roots to river bank reinforcement. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

6.
Rapid changes in the composition of hillslope vegetation due to a combination of changing climate and land use make estimating slope stability a significant challenge. The dynamics of root growth on any individual hillslope result in a wide range of root distributions and strengths that are reflected as up to an order of magnitude variability in root cohesion. Hence the challenge of predicting the magnitude of root reinforcement for hillslopes requires both an understanding of the magnitude and variability of root distributions and material properties (e.g. tensile strength, elasticity). Here I develop a model for estimating the reinforcement provided by plant roots based on the distribution of biomass measured at the biome level and a compilation of root tensile strength measurements measured across a range of vegetation types. The model modifies the Wu/Waldron method of calculating root cohesion to calculate the average lateral root cohesion and its variability with depth using the Monte Carlo method. The model was validated in two ways, the first against the predicted depth‐reinforcement characteristics of Appalachian soils and the second using a global dataset of landslides. Model results suggest that the order of magnitude difference in root cohesions measured on individual hillslopes can be captured by the Monte Carlo approach and provide a simple tool to estimate root reinforcement for data‐poor areas. The model also suggests that future hotspots of slope instability will occur in areas where land use and climate convert forest to grassland, rather than changes between different forest structures or forest and shrubland. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

7.
Several mechanisms contribute to streambank failure including fluvial toe undercutting, reduced soil shear strength by increased soil pore‐water pressure, and seepage erosion. Recent research has suggested that seepage erosion of noncohesive soil layers undercutting the banks may play an equivalent role in streambank failure to increased soil pore‐water pressure. However, this past research has primarily been limited to laboratory studies of non‐vegetated banks. The objective of this research was to utilize the Bank Stability and Toe Erosion Model (BSTEM) in order to determine the importance of seepage undercutting relative to bank shear strength, bank angle, soil pore‐water pressure, and root reinforcement. The BSTEM simulated two streambanks: Little Topashaw Creek and Goodwin Creek in northern Mississippi. Simulations included three bank angles (70° to 90°), four pore‐water pressure distributions (unsaturated, two partially saturated cases, and fully saturated), six distances of undercutting (0 to 40 cm), and 13 different vegetation conditions (root cohesions from 0·0 to 15·0 kPa). A relative sensitivity analysis suggested that BSTEM was approximately three to four times more sensitive to water table position than root cohesion or depth of seepage undercutting. Seepage undercutting becomes a prominent bank failure mechanism on unsaturated to partially saturated streambanks with root reinforcement, even with undercutting distances as small as 20 cm. Consideration of seepage undercutting is less important under conditions of partially to fully saturated soil pore‐water conditions. The distance at which instability by undercutting became equivalent to instability by increased soil pore‐water pressure decreased as root reinforcement increased, with values typically ranging between 20 and 40 cm at Little Topashaw Creek and between 20 and 55 cm at Goodwin Creek. This research depicts the baseline conditions at which seepage undercutting of vegetated streambanks needs to be considered for bank stability analyses. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

8.
Tree roots provide surface erosion protection and improve slope stability through highly complex interactions with the soil due to the nature of root systems. Root reinforcement estimation is usually performed by in situ pullout tests, in which roots are pulled out of the soil to reliably estimate the root strength of compact soils. However, this test is not suitable for the scenario where a soil progressively fails in a series of slump blocks – for example, in unsupported soils near streambanks and road cuts where the soil has no compressive resistance at the base of the hillslope. The scenario where a soil is unsupported on its downslope extent and progressively deforms at a slow strain rate has received little attention, and we are unaware of any study on root reinforcement that estimates the additional strength provided by roots in this situation. We therefore designed two complementary laboratory experiments to compare the force required to pull the root out. The results indicate that the force required to pull out roots is reduced by up to 50% when the soil fails as slump blocks compared to pullout tests. We also found that, for slump block failure, roots had a higher tendency to slip than to break, showing the importance of active earth pressure on root reinforcement behaviour, which contributes to reduced friction between soil and roots. These results were then scaled up to a full tree and tree stand using the root bundle and field-measured spatial distributions of root density. Although effects on the force mobilized in small roots can be relevant, small roots have virtually no effect on root reinforcement at the tree or stand scale on hillslopes. When root distribution has a wide range of diameters, the root reinforcement results are controlled by large roots, which hold much more force than small roots. © 2019 John Wiley & Sons, Ltd.  相似文献   

9.
Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D25 of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D70 and larger (88–155 mm), while particles in the range D30–D70 (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
Numerous processes may instigate bank retreat and the consequent collection of failed cohesive materials at the bank toe. Cohesion between the failed material and the substrate can provide additional strength to resist direct fluvial entrainment. Failed, cohesive material can act as a form of natural bank‐toe protection by consuming and diverting flow energy that may otherwise be used to further scour the basal zone of incising channels. Investigations in Goodwin Creek, Mississippi, have revealed the existence of apparent cohesion between failed, cohesive blocks and their underlying surface. The method used to assess this cohesion involved a pulley system mounted on a tripod and supporting a load cell. Mean and maximum apparent‐cohesion values of 1·08 kPa and 2·65 kPa, respectively, were measured in this way, identifying a source that bonds blocks to the underlying surface. Cohesion values and types vary spatially and temporally. Tensiometric tests beneath blocks suggest that cohesion resulting from matric suction alone may be as much as 3·5 kPa in summer and 1·8 kPa in winter. Apparent cohesion is believed to have been sufficient to help prevent removal of the largest blocks by a peak flow of 66·4m3/s on 23 September 1997. Maximum excess shear stress required to entrain a D75 block can be augmented by as much as 97% by the presence of apparent cohesion at the block–substrate interface when compared with a condition with zero apparent cohesion at the block underside. Given these findings, it is no longer sufficient to estimate block entrainment in the basal area from block size or bed roughness alone, as in a Shields‐type approach. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

11.
Modelling increased soil cohesion due to roots with EUROSEM   总被引：3，自引：0，他引：3  
As organic root exudates cause soil particles to adhere firmly to root surfaces, roots significantly increase soil strength and therefore also increase the resistance of the topsoil to erosion by concentrated flow. This paper aims at contributing to a better prediction of the root effects on soil erosion rates in the EUROSEM model, as the input values accounting for roots, presented in the user manual, do not account for differences in root density or root architecture. Recent research indicates that small changes in root density or differences in root architecture considerably influence soil erosion rates during concentrated flow. The approach for incorporating the root effects into this model is based on a comparison of measured soil detachment rates for bare and for root‐permeated topsoil samples with predicted erosion rates under the same flow conditions using the erosion equation of EUROSEM. Through backwards calculation, transport capacity efficiencies and corresponding soil cohesion values can be assessed for bare and root‐permeated topsoils respectively. The results are promising and present soil cohesion values that are in accordance with reported values in the literature for the same soil type (silt loam). The results show that grass roots provide a larger increase in soil cohesion as compared with tap‐rooted species and that the increase in soil cohesion is not significantly different under wet and dry soil conditions, either for fibrous root systems or for tap root systems. Power and exponential relationships are established between measured root density values and the corresponding calculated soil cohesion values, reflecting the effects of roots on the resistance of the topsoil to concentrated flow incision. These relationships enable one to incorporate the root effect into the soil erosion model EUROSEM, through adapting the soil cohesion input value. A scenario analysis shows that the contribution of roots to soil cohesion is very important for preventing soil loss and reducing runoff volume. The increase in soil shear strength due to the binding effect of roots on soil particles is two orders of magnitude lower as compared with soil reinforcement achieved when roots mobilize their tensile strength during soil shearing and root breakage. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

12.
The effects of vegetation root distribution on near‐surface water partitioning can be two‐fold. On the one hand, the roots facilitate deep percolation by root‐induced macropore flow; on the other hand, they reduce the potential for deep percolation by root‐water‐uptake processes. Whether the roots impede or facilitate deep percolation depends on various conditions, including climate, soil, and vegetation characteristics. This paper examines the effects of root distribution on deep percolation into the underlying permeable bedrock for a given soil profile and climate condition using HYDRUS modelling. The simulations were based on previously field experiments on a semiarid ponderosa pine (Pinus ponderosa) hillslope. An equivalent single continuum model for simulating root macropore flow on hillslopes is presented, with root macropore hydraulic parameterization estimated based on observed root distribution. The sensitivity analysis results indicate that the root macropore effect dominates saturated soil water flow in low conductivity soils (Kmatrix below 10?7 m/s), while it is insignificant in soils with a Kmatrix larger than 10?5 m/s, consistent with observations in this and other studies. At the ponderosa pine site, the model with simple root‐macropore parameterization reasonably well reproduces soil moisture distribution and some major runoff events. The results indicate that the clay‐rich soil layer without root‐induced macropores acts as an impeding layer for potential groundwater recharge. This impeding layer results in a bedrock percolation of less than 1% of the annual precipitation. Without this impeding layer, percolation into the underlying permeable bedrock could be as much as 20% of the annual precipitation. This suggests that at a surface with low‐permeability soil overlying permeable bedrock, the root penetration depth in the soil is critical condition for whether or not significant percolation occurs. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

13.
The effects of root systems on soil detachment by overland flow are closely related to vegetation types. The objective of this study was to quantify the effects of two gramineous roots (Paspalum mandiocanum with shallow roots and Pennisetum giganteum with deep roots) on soil detachment capacity, rill erodibility, and critical shear stress on alluvial fans of benggang in south-east China. A 4-m-long and 0.12-m-wide flume was used. Slope steepness ranged from 9% to 27%, and unit flow discharge ranged from 1.39 × 10−3 to 4.19 × 10−3 m2 s−1. The mean detachment capacities of P. mandiocanum and P. giganteum lands were 18% and 38% lower than that of bare land, respectively, and the effects of root on reducing soil detachment were mainly reflected in the 0- to 5-cm soil layer. The most important factors in characterizing soil detachment capacity were root length density and soil cohesion, and soil detachment capacity of the two grass lands could be estimated using flow shear stress, soil cohesion, and root length density (NSE = 0.90). With the increase in soil depth, rill erodibility increased, whereas shear stress decreased. The mean rill erodibilities of P. mandiocanum and P. giganteum lands were 81% and 61% as much as that of bare land, respectively. Additionally, rill erodibilities of the two grass lands could be estimated as an exponential function by root length density and soil cohesion (NSE = 0.88). The mean critical shear stress of P. mandiocanum and P. giganteum lands was 1.29 and 1.39 times that of bare land, respectively, and it could be estimated with a linear function by root length density (NSE = 0.76). This study demonstrated that planting of the two grasses P. mandiocanum and P. giganteum could effectively reduce soil detachment and enhance soil resistance to erosion on alluvial fans, with the deep roots of P. giganteum being more effective than the shallow roots of P. mandiocanum. The results are helpful for understanding the influencing mechanism of root systems on soil detachment process.  相似文献   

14.
A review of present modelling approaches for root reinforcement in vegetated steep hillslopes reveals critical gaps in consideration of plant–soil interactions at various scales of interest for shallow landslide prediction. A new framework is proposed for systematic quantification of root reinforcement at scales ranging from single root to tree root system, to a stand of trees. In addition to standard basal reinforcement considered in most approaches, the critical role of roots in stabilizing slopes through lateral reinforcement is highlighted. Primary geometrical and mechanical properties of root systems and their function in stabilizing the soil mass are reviewed. Stress–strain relationships are considered for a bundle of roots using the formalism of the fiber bundle model (FBM) that offers a natural means for upscaling mechanical behavior of root systems. An extension of the FBM is proposed, considering key root and soil parameters such as root diameter distribution, tortuosity, soil type, soil moisture and friction between soil and root surface. The spatial distribution of root mechanical reinforcement around a single tree is computed from root diameter and density distributions based on easy to measure properties. The distribution of root reinforcement for a stand of trees was obtained from spatial and mechanical superposition of individual tree values with regard to their positions on a hillslope. Potential applications of the proposed approach are illustrated in a numerical experiment of spatial strength distribution in a hypothetical slope with 1000 trees randomly distributed. The analyses result in spatial distribution of weak and strong zones within the soil where landslide triggering is expected in large and continuous zones with low reinforcement values. Mapping such zones would enhance the quality of landslide susceptibility maps and optimization of silvicultural measures in protection forests. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

15.
Vegetation evapotranspiration (ET) induced soil water suction reduces hydraulic conductivity and increases shear strength of slopes. Several field studies have been conducted to investigate suction distribution in vegetated slopes. However, these studies were conducted on natural slopes, which are prone to heterogeneity in vegetation and soil conditions. Moreover, studies quantifying the effect of different vegetation species, root characteristics (root depth and root area index) and transpiration reduction function (Trf) on suction in slopes under natural variation are rare. This study investigated the suction distribution and root characteristics in recompacted slopes vegetated with two different species, i.e. Cynodon dactylon (Bermuda grass) and Schefflera heptaphylla (ivy tree). Bare slope served as a control. Suction distributions during different seasons and rainfall events were monitored. It is found that during the dry season, slope vegetated with young Schefflera heptaphylla seedlings have substantially higher suction within the root zone compared with bare slope and slope vegetated with Cynodon dactylon. This is because Schefflera heptaphylla has a higher root biomass, Trf and ET than Cynodon dactylon. It was also found that suctions within root zones of vegetated slopes and bare slope were completely destroyed under rainfall events corresponding to 2 years and 20 years return period. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

16.
The important role of vegetation in adding cohesion and stabilizing streambanks has been widely recognized in several aspects of fluvial geomorphology, including stream restoration and studies of long‐term channel change. Changes in planform between braided, meandering, and anabranching forms have been attributed to the impacts of vegetation on hydraulic roughness and bank stability. However, these studies focus either on flume studies where analog vegetation is used, or case studies featuring one species, which is commonly invasive. We present functional differences of bank‐stabilizing root characteristics and added cohesion, with vegetation categorized as woody and non‐woody and by the vegetation groups of trees, shrubs, graminoids, and forbs. We analyzed root morphology and tensile strength of 14 species common to riparian areas in the southern Rocky Mountains, in field sites along streambanks in the montane and subalpine zones of the Colorado Front Range. Using the vegetation root component (RipRoot) of a physically‐based bank stability model (BSTEM), we estimated the added cohesion for various sediment textures with the addition of each of the 14 species. Significant differences exist between woody and non‐woody vegetation and between the four vegetation categories with respect to the coefficient of the root tensile strength curve, lateral root extent, and maximum root diameter. Woody vegetation (trees and shrubs) have higher values of all three parameters than non‐woody species. Tree roots add significantly more cohesion to streambanks than forb roots. Additionally, rhizomes may play an important role in determining the reach‐scale effects of roots on bank stabilization. Differences in root characteristics and added cohesion among vegetation categories have several important implications, including determining the likelihood of planform change, developing guidelines for the use of bank‐stabilizing vegetation, and linking the effect of vegetation to geomorphic structure that can benefit ecosystem functioning. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

17.
The strength and architecture of roots and other below-ground organs of riparian and aquatic plants affect plant resistance to uprooting and contribute to reinforcing river bank, bar and bed materials. Therefore, root properties are an important element in models for estimating river bank stability and such models may focus on the role of plants by using root strength–diameter relationships for the particular plant species that are present. Here we explore the degree to which there appear to be significant differences in strength–diameter relationships between and within species-specific data sets obtained for two riparian tree/shrub (Populus nigra, Salix alba) and two emergent aquatic macrophyte (Sparganium erectum, Phalaris arundinacea) species in different European river environments. While the analysed data sets were not specifically collected to answer these research questions, the results are sufficiently compelling to make the case for the collection of a more comprehensive data set and its rigorous analysis. This would allow recommendations to be made on the degree to which (i) species-specific or more general relationships between root/rhizome strength and diameter are appropriate, (ii) such relationships are applicable within and between rivers in different geographical regions and subject to different local environmental conditions, and (iii) further (minimalist) field observations are needed to calibrate such relationships for investigations of new locales or species. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

18.
This paper presents an assessment of the relationship between near-surface soil moisture (SM) and SM at other depths in the root zone under three different land uses: irrigated corn, rainfed corn and grass. This research addresses the question whether or not near-surface SM can be used reliably to predict plant available root zone SM and SM at other depths. For this study, a realistic soil-water energy balance process model is applied to three locations in Nebraska representing an east-to-west hydroclimatic gradient in the Great Plains. The applications were completed from 1982 through to 1999 at a daily time scale. The simulated SM climatologies are developed for the root zone as a whole and for the five layers of the soil profile to a depth of 1·2 m. Over all, the relationship between near-surface SM (0–2·5 cm) and plant available root zone SM is not strong. This applies to all land uses and for all locations. For example, r estimates range from 0·02 to 0·33 for this relationship. Results for near-surface SM and SM of several depths suggest improvement in r estimates. For example, these estimates range from − 0·19 to 0·69 for all land uses and locations. It was clear that r estimates are the highest (0·49–0·69) between near-surface and the second layer (2·5–30·5 cm) of the root zone. The strength of this type of relationship rapidly declines for deeper depths. Cross-correlation estimates also suggest that at various time-lags the strength of the relationship between near-surface SM and plant available SM is not strong. The strength of the relationship between SM modulation of the near surface and second layer over various time-lags slightly improves over no lags. The results suggest that use of near-surface SM for estimating SM at 2·5–30 cm is most promising. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

19.
Increased bank stability by riparian vegetation can have profound impacts on channel morphology and dynamics in low‐energy systems, but the effects are less clear in high‐energy environments. Here we investigate the role of vegetation in active, aggrading braided systems at Mount Pinatubo, Philippines, and compare results with numerical modeling results. Gradual reductions in post‐eruption sediment loads have reduced bed reworking rates, allowing vegetation to finally persist year‐round on the Pasig‐Potrero and Sacobia Rivers. From 2009–2011 we collected data detailing vegetation extent, type, density, and root strength. Incorporating these data into the RipRoot model and BSTEM (Bank Stability and Toe Erosion Model) shows cohesion due to roots increases from zero in unvegetated conditions to > 10·2 kPa in densely‐growing grasses. Field‐based parameters were incorporated into a cellular model comparing vegetation strength and sediment mobility effects on braided channel dynamics. The model shows both low sediment mobility and high vegetation strength lead to less active systems, reflecting trends observed in the field. The competing influence of vegetation strength versus channel dynamics is a concept encapsulated in a dimensionless ratio between timescales for vegetation growth and channel reworking known as T*. An estimated T* between 1·5 and 2·3 for the Pasig‐Potrero River suggests channels are still very mobile and likely to remain braided until aggradation rates decline further. Vegetation does have an important effect on channel dynamics, however, by focusing flow and thus aggradation into the unvegetated fraction of braidplain, leading to an aggradational imbalance and transition to a more avulsive state. The future trajectory of channel–vegetation interactions as sedimentation rates decline is complicated by strong seasonal variability in precipitation and sediment loads, driving incision and armoring in the dry season. By 2011, incision during the dry season was substantial enough to lower the water‐table, weaken existing vegetation, and allow for vegetation removal in future avulsions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

20.
We present a geotechnical stability analysis for the planar failure of riverbanks, which incorporates the effects of root reinforcement and surcharge for mature stands of woody riparian vegetation. The analysis relies on a new method of representing the root distribution in the soil, which evaluates the effects of the vegetation's position on the bank. The model is used in a series of sensitivity analyses performed for a wide range of bank morphological (bank slope and height) and sedimentological (bank cohesion and friction angle) conditions, enabling discrimination of the types of bank environment for which vegetation has an effect on bank stability. The results indicate that woody vegetation elements have a maximal impact on bank stability when they are located at the ends of the incipient failure plane (i.e. at the bank toe or at the intersection of the failure plane with the floodplain) and that vegetation has a greater effect on net bank stability when it is growing on low, shallow, banks comprised of weakly cohesive sediments. However, the magnitude of these effects is limited, with vegetation typically inducing changes (relative to non‐vegetated banks) in simulated factors of safety of less than 5%. If correct, this suggests that the well documented effects of vegetation on channel morphology must be related to alternative process mechanisms (such as the interaction of vegetation with river flows) rather than the mechanical effects of vegetation on bank failure, except in special cases where the equivalent non‐vegetated bank has a highly marginal stability status. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

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