Spatial variation in soil resistance to flowing water erosion along a regional transect in the Loess Plateau

The factors influencing soil erosion may vary with scale. It remains unclear whether the spatial variation in soil erosion resistance is controlled by regional variables (e.g. precipitation, temperature, and vegetation zone) or by local specific variables (e.g. soil properties, root traits, land use, and farming operations) when the study area enlarges from a hillslope or catchment to the regional scale. This study was performed to quantify the spatial variations in soil erosion resistance to flowing water under three typical land uses along a regional transect on the Loess Plateau and to identify whether regional or local specific variables are responsible for these changes. The results indicated that the measured soil detachment capacities (D_c) of cropland exhibited an irregular trend along the regional transect. The D_c of grassland increased with mean annual precipitation, except for two sites (Yijun and Erdos). The measured D_c of woodland displayed an inverted ‘U’ shape. The changes in rill erodibility (K_r) of three land uses were similar to D_c, whereas no distinguishable trend was found for critical shear stress (τ_c). No significant correlation was detected between D_c, K_r and τ_c, and the regional variables. The spatial variation in soil erosion resistance could be explained reasonably by changes in soil properties, root traits, land use, and farming operations, rather than regional variables. The adjustment coefficient of K_r for grassland and woodland could be well simulated by soil cohesion and root mass density (R² = 0.70, P < 0.01), and the adjustment coefficient of critical shear stress could be estimated with aggregate stability (R² = 0.57, P < 0.01). The results are helpful for quantifying the spatial variation in soil detachment processes by overland flow and to develop process‐based erosion model at a regional scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Predicting physical equations of soil detachment by simulated concentrated flow in Ultisols (subtropical China)

Soil detachment in concentrated flow is due to the dislodging of soil particles from the soil matrix by surface runoff. Both aggregate stability and shear strength of the topsoil reflect the erosion resistance of soil to concentrated runoff, and are important input parameters in predicting soil detachment models. This study was conducted to develop a formula to predict soil detachment rate in concentrated flow by using the aggregate stability index (As), root density (R_d) and saturated soil strength (σ_s) in the subtropical Ultisols region of China. The detachment rates of undisturbed topsoil samples collected from eight cultivated soil plots were measured in a 3.8 m long, 0.2 m wide hydraulic flume under five different flow shear stresses (τ = 4.54, 9.38, 15.01, 17.49 and 22.54 Pa). The results indicated that the stability index (As) was well related with soil detachment rate, particularly for results obtained with high flow shear stress (22.54 Pa), and the stability index (As) has a good linear relationship with concentrated flow erodibility factors (K_c). There was a positive linear relationship between saturated soil strength (σ_s) and critical flow shear stress (τ_c) for different soils. A significant negative exponential relationship between erodibility factors (K_c) and root density (R_d) was detected. This study yielded two prediction equations that allowed comparison of their efficiency in assessing soil detachment rate in concentrated flow. The equation including the root density (R_d) may have a better correlation coefficient (R² = 0.95). It was concluded that the formula based on the stability index (As), saturated soil strength (σ_s) and root density (R_d) has the potential to improve methodology for assessing soil detachment rate in concentrated flow for the subtropical Chinese Ultisols. Copyright © 2012 John Wiley & Sons, Ltd.     

Soil erosion resistance effects on rill and gully initiation points and dimensions

Recent research has indicated the large spatial and temporal variation in soil erosion resistance against concentrated flow (SER). This study analyzes this variability in relation to rill and gully initiation locations on slopes and the downslope eroded volumes. The soil erodibility (Kc) and critical flow shear stress (τ_cr), were estimated from topsoil properties and correlated to eroded rill and gully volumes and their initiation points on slopes in the Belgian loess belt. Therefore, concentrated flow paths and topsoil properties were measured in their vicinity. The results show that rill and gully initiation points, and hence the lengths of concentrated flow paths, depend on τ_cr, which is controlled by soil surface conditions and can be predicted from saturated soil shear strength. Soil erosion control measures that increase soil shear strength (e.g. thalweg compaction), can therefore decrease rill and gully lengths. Once a rill or an ephemeral gully is initiated, its cross‐section was found to depend on Kc, which can be estimated from the soil water content, dry bulk density, and the dry density of roots and crop residues incorporated in the topsoil. 74% of the variation in the channel cross‐sectional area measured in the study area could be predicted from the combined effect of flow intensity and these three soil properties, whereas flow intensity alone could only account for 31% of the variation. Soil conservation measures affecting one of the soil properties that control Kc (e.g. double drilling of the thalweg, conservation tillage) can therefore decrease the cross‐sections of the concentrated flow paths. These findings also indicate that rill and gully initiation points are not only topographically controlled but also depend on the SER, which in turn determines the dimensions of these concentrated flow paths. Hence, knowledge of the variability in SER is indispensable. Copyright © 2009 John Wiley & Sons, Ltd.     

Soil erodibility as impacted by vegetation restoration strategies on the Loess Plateau of China

Near soil surface characteristics change significantly with vegetation restoration, and thus, restoration strategies likely affect soil erodibility. However, few studies have been conducted to quantify the effects of vegetation restoration strategies on soil erodibility in regions experiencing rapid vegetation restoration. This study was conducted to evaluate the effects of vegetation restoration strategies on soil erodibility, reflected by soil cohesion (Coh), penetration resistance (PR), saturated conductivity (K_s), number of drop impacts (NDI), mean weight diameter of soil aggregates (MWD), and soil erodibility K factor on the Loess Plateau. One slope farmland and five 25-year-restored lands covered by old world bluestem, korshinsk peashrub, shrub sophora, sea-buckthorn, and black locust were selected as test sites. The old world bluestem was restored via natural succession, while the other four lands were restored by artificial planting. A comprehensive soil erodibility index (CSEI) was produced by a weighted summation method to quantify the effects of vegetation restoration strategies on soil erodibility completely. The results showed that Coh, K_s, NDI, and MWD of the five restored lands were greater than those of the slope farmland. However, the PR and K of the five restored lands were less than those of the slope farmland. CSEI varied greatly under different restoration strategies, from 1 to 0.214. Compared with the control, these indices decreased on average by 68.2%, 78.6%, 72.7%, 75.8%, and 62.8% for old world bluestem, korshinsk peashrub, shrub sophora, sea-buckthorn, and black locust, respectively. The variation in soil erodibility was significantly influenced by biological crust thickness, bulk density, organic matter content, plant litter density, and root mass density. Shrub-lands via artificial planting, especially korshinsk peashrub, were considered the most effective restoration strategies to reduce soil erodibility on the Loess Plateau. The results are helpful for selecting vegetation restoration strategies and asking their benefits in controlling soil erosion. © 2018 John Wiley & Sons, Ltd.     

Variation in soil saturated hydraulic conductivity along the hillslope of collapsing granite gullies

Saturated hydraulic conductivity (K_s) affects the soil hydrological process and is influenced by many factors that exhibit strong spatial variations. To accurately measure K_s and its scale, spatial variability and relationship with collapsing gullies, we analysed four double-ring infiltrometer diameters in three soil layers during in situ experiments designed to measure K_s in two typical collapsing gullies (three slope sites) in Tongcheng County of China. The results showed that K_s increased with increasing inner ring diameter, but no significant difference existed between inner diameters of 30 and 40 cm. The K_s in red soil layers was higher than that in sandy soil layers, the transition layers had the lowest values. K_s also varied with slope position, gradually decreasing with distance from the gully head. The suggestion is that the spatial variation in K_s is affected not only by the intrinsic soil properties but also by the interaction with the collapsing gully.     

Comparing water use patterns of sand-binding species in an alpine semi-arid desert

The processes of species obtaining water resources are crucial to evaluate the adaptability of plantations, which can affect the establishment and survival of restored ecosystems and functions of water-limited ecosystems. However, there are still limitations in understanding water use patterns of different four plantations (Populus sylvestris, Salix cheliophe, Hippophae rhamnoides and Artemisia ordosica) in the Qinghai Lake basin. Stable isotopes of xylem water of four individual species were analysed at different dunes located at the eastern shore of Qinghai Lake in 2018 and 2019. The purpose of this study was to explore potential water sources used by different life form plants, to identify whether the soil moisture content and root distribution determined the plants water use patterns. Results from the Bayesian mixing model MixSIAR showed that all species mainly uses three levels of soil water, but they extracted soil water from different layers in different growing seasons among species. All species primarily depended on water from upper soil layers at the early growing stage, but P. sylvestris and S. cheliophe also absorbed much proportion middle and deep soil layers' water in dry year of 2019. All four species shift to use middle or deep soil layer water rather than shallow water in July with the most rainfall and soil water content (SWC). However, it was only used by P. sylvestris and H. rhamnoides in 2018. In all, seasonal water use pattern of different species was affected by SWC, soil texture and root distribution. Compared with moratorium plantations such as S. cheliophe, mixed afforestation of P. sylvestris and H. rhamnoides maximized of soil water sources absorbed by the plants. The study can shed light on plant–water relationships to facilitate the woody species for afforestation and desertification restoration management in the semi-arid desert ecosystem.     

Synergistic effect of vegetation and air temperature changes on soil water content in alpine frost meadow soil in the permafrost region of Qinghai‐Tibet

Seasonal changes over 2 years (2004–2006) in soil moisture content (θ_v) of frozen alpine frost meadow soils of the Qinghai‐Tibet plateau permafrost region under three different levels of vegetation cover were investigated. Vegetation cover and air temperature changes had significant effects (synergistic effect) on θ_v and its distribution in the soil profile. During periods of soil freezing or thawing, the less the vegetation cover, the quicker the temperature drop or rise of soil water, and the shorter the duration of the soil water freeze–thaw response in the active soil layer. Under 30% and 65% vegetation cover the amplitude of variation in θ_v during the freezing period was 20–26% greater than that under 93% cover, while during the thawing period, it was 1·5‐ to 40·5‐fold greater. The freezing temperature of the surface soil layer, ^fT_s, was 1·6 °C lower under 30% vegetation cover than under 93% vegetation cover. Changes in vegetation cover of the alpine frost meadow affected θ_v and its distribution, as well as the relationship between θ_v and soil temperature (T_s). As vegetation cover decreased, soil water circulation in the active layer increased, and the response to temperature of the water distribution across the soil profile was heightened. The quantity of transitional soil phase water at different depths significantly increased as vegetation cover decreased. The influence of vegetation cover and soil temperature distribution led to a relatively dry soil layer in the middle of the profile (0·70–0·80 m) under high vegetation cover. Alpine meadow θ_v and its pattern of distribution in the permafrost region were the result of the synergistic effect of air temperature and vegetation cover. Copyright © 2007 John Wiley & Sons, Ltd.     

The impact of natural vegetation restoration on surface soil moisture of secondary forests and shrubs in the karst region of southwest China

Soil moisture is crucial to vegetation restoration in karst areas, and climate factors and vegetation restoration are key factors affecting changes in soil moisture. However, there is still much controversy over the long-term changes in soil moisture during vegetation restoration. In order to reveal the changes in soil moisture during vegetation restoration, we conducted long-term positioning monitoring of soil moisture at 0–10 and 10–20 cm on secondary forests sample plot (SF, tree land) and shrubs sample plot (SH, shrub land) in karst areas from 2013 to 2020. The results showed that the aboveground biomass of SF and SH increased by 50% and 240%, respectively, and the soil moisture of the SF and SH showed an increasing trend. When shrubs are restored to trees in karst areas, the soil moisture becomes more stable. However, the correlation coefficients (R²) between the annual rainfall and the annual average soil moisture of SF and SH are 0.84 and 0.55, respectively, indicating that soil moistures in tree land are more affected by rainfall. The soil moisture of shrubs and trees are relatively low during the months of alternating rainy and dry seasons. Rainfall has a very significant impact on the soil moisture of tree land, while air temperature and wind speed have a significant impact on the soil moisture of tree land, but the soil moistures of shrub land are very significantly affected by rainfall and relative humidity. Therefore, during the process of vegetation restoration from shrubs to trees, the main meteorological factors that affect soil moisture changes will change. The results are important for understanding the hydrological processes in the ecological restoration process of different vegetation types in karst areas.     

Vertical variations and transport mechanism of soil moisture in response to vegetation restoration on the Loess Plateau of China

Soil moisture is essential for vegetation restoration in arid and semi-arid regions. Ascertaining the vertical distribution and transportation of soil moisture under different vegetation types has a profound effect on the ecological construction. In this study, the soil moisture at a depth of 500 cm for four typical vegetation types, including Robinia pseudoacacia, Caragana korshinskii, Stipa bungeana, and corn, were investigated and compared in the Zhifanggou watershed of the Loess plateau. Additionally, hydrogen and oxygen stable isotopes were detected to identify the transport mechanism of soil moisture. The results showed vertical distribution and transportation of soil moisture were different under different vegetation types. Depth-averaged soil moisture under S. bungeana and corn generally increased along the profile, while C. korshinskii and R. pseudoacacia showed weakly increasing and relatively stable after an obvious decreasing trend (0–40 cm). The soil moisture under R. pseudoacacia was lower than that under other vegetation types, especially in deep layer. However, the effect of R. pseudoacacia on soil moisture in the topsoil (< 30 cm) could be positive. For R. pseudoacacia (160–500 cm), C. korshinskii (0–500 cm), and S. bungeana (0–100 cm), the soil moisture declined with increased in vegetation age. Planting arbor species such as R. pseudoacacia intensified the decline of soil moisture on the Loess Plateau. The capacity of evaporation fractionation of soil moisture followed the sequence: corn > S. bungeana > R. pseudoacacia > C. korshinskii. The δ¹⁸O values in soil water fluctuated across the profile. The δ¹⁸O values changed sharply in upper layer and generally remained stable in deep layer. However, in middle layer, the vertical distribution characteristics of the δ¹⁸O values were different under different vegetation types. We estimated that piston flow was the main mode of precipitation infiltration, and the occurrence of preferential flow was related to vegetation types. These results were helpful to improve the understanding of the response of deep soil moisture to vegetation restoration and inform practices for sustainable water management.     

Gully and soil and water conservation structure densities in semi‐arid northern Ethiopia over the last 80 years

Gullies have been a common phenomenon in semi‐arid northern Ethiopia for the last centuries. However, soil and water conservation (SWC) structures have been implemented for a long time to curb soil erosion. Though, like most of the affected areas worldwide, density and distribution of gullies and SWC structures, their causes and interrelations are poorly understood. The aims of this study were to develop a technique for mapping these densities of gullies and SWC structures, to explain their spatial distribution and to analyze changes over the period 1935–2014. Aerial photographs from 1935 to 1936 and Google Earth images from 2014 of the 5142 km² Geba catchment were used. Transect lines were established to count gullies and SWC structures in order to calculate densities. On average, a gully density of 1.14 km km^?2 was measured in 1935–1936 of which the larger portion (75%) were vegetated, indicating they were not very active. Over 80 years, gully density has significantly increased to 1.59 km km^?2 with less vegetation growing in their channel, but 66% of these gullies were treated with check dams. There was c. 3 km km^?2 of indigenous SWC structures (daget or lynchets) in 1935–1936 whereas a high density (20 km km^?2) of introduced SWC structures (mainly stone bunds and terraces) were observed in 2014. The density of gullies is positively correlated with slope gradient and shrubland cover and negatively with cropland cover, whereas the density of SWC structures significantly increased with increasing cropland cover. Density maps of gullies and SWC structures indicate sensitive areas to gully formation and priority areas for the implementation of SWC structures in Geba catchment. The obtained results illustrate the feasibility of the methods applied to map the density of gullies and SWC structures in mountainous areas. Copyright © 2018 John Wiley & Sons, Ltd.     

Relationships between physical‐geochemical soil properties and erodibility of streambanks among different physiographic provinces of Tennessee,USA

Erosion of cohesive soils in fluvial environments is dependent on physical, geochemical and biological properties, which govern inter‐particle attraction forces and control detachment rates from stream beds and banks. Most erosion rate models are based on the excess shear stress equation where the soil erodibility coefficient (k_d) is multiplied by the difference between the boundary hydraulic shear stress (τ_b) and the soil critical shear stress (τ_c). Both k_d and τ_c are a function of soil properties and must be obtained through in situ field or laboratory testing. Many studies have generated predictive relationships for k_d and τ_c derived from various soil properties. These studies typically were conducted in watersheds within a single physiographic region with a common surficial geology and/or investigated a limited number of soil properties, particularly geochemical properties. With widely reported differences in relationships between τ_c and soil properties, this study investigated differences in predictive relationships for τ_c among different physiographic provinces in Tennessee, USA. Erodibility parameters were determined in the field using a mini‐jet test device. Among these provinces, statistically four unique clusters were identified from a dataset of 128 observations and these data clusters were used to develop predictive models for τ_c to identify dominant properties governing erosion. In these clusters, 16 significant physical and geochemical soil properties were identified for τ_c prediction. Among these soil properties, water content and passing #200 sieve (percentage soil less than 75 μm) were the dominant controlling parameters to predict τ_c in addition to clay percentage (< 2 μm), bulk density, and soil pore water chemistry. This study suggests that unique relationships exist for physiographic provinces that are likely due to soil physical‐geochemical processes associated with surficial geology that determine minerology of the cohesive soil. Copyright © 2017 John Wiley & Sons, Ltd.     

Rainfall‐induced consolidation and sealing effects on soil erodibility during concentrated runoff for loess‐derived topsoils

Flume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples (0·36 × 0·09 × 0·09 m³) to measure the effect of rainfall‐induced soil consolidation and soil surface sealing on soil erosion by concentrated flow for loess‐derived soils and to establish a relationship between soil erodibility and soil bulk density. Soil consolidation and sealing were simulated by successive simulated rainfall events (0–600 mm of cumulative rainfall) alternated by periods of drying. Soil detachment measurements were repeated for four different soil moisture contents (0·04, 0·14, 0·20 and 0·31 g g^?1). Whereas no effect of soil consolidation and sealing is observed for critical flow shear stress (τ_cr), soil erodibility (Kc) decreases exponentially with increasing cumulative rainfall depth. The erosion‐reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil. After about 100 mm of rainfall, Kc attains its minimum value for all moisture conditions, corresponding to a reduction of about 70% compared with the initial Kc value for the moist soil samples and only a 10% reduction for the driest soil samples. The relationship estimating relative Kc values from soil moisture content and cumulative rainfall depth predicts Kc values measured on a gradually consolidating cropland field in the Belgian Loess Belt reasonably well (MEF = 0·54). Kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments, suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling. Copyright © 2007 John Wiley & Sons, Ltd.     

Identifying effects of local and nonlocal factors of soil water storage using cyclical correlation analysis

There are various factors governing the spatial and temporal variability of soil water storage including soil properties, topography and vegetation. Some factors act locally, whereas others act nonlocally, which means that a factor measured at one location has effect on soil water storage at another location. The objective of this study was to examine the effects of local and nonlocal controls of soil water storage in a hummocky landscape using cyclical correlation analysis. Soil water storage, soil properties and terrain indices were measured along a 128‐point transect of 576 m long from the semiarid, hummocky, prairie pothole region of North America. There are large coefficients of determination (r²) between soil water storage and sand content (r² = 0.32–0.53), organic carbon content (r² = 0.22–0.56), depth to carbonate layer (r² = 0.13–0.63), wetness index (r² = 0.25–0.45) and other variables at the measurement scale at different times, indicating strong local effects from these variables. The correlation coefficients were also calculated by physically shifting the spatial series of soil water storage with respect to that of controlling factors. The shifting improves the correlation between the spatial series, and the length of shifting indicated the difference in the response of soil water to its controlling factors. For example, the value of r² increased more than eightfold (r² = 0.47–0.64) after shifting the spatial series of soil water storage by 54 m, almost equal to the average length of existing slope, compared with the very weak correlation (r² = 0.02–0.08) at the measurement scale. This indicated the nonlocal effect from the relative elevation. The identification of nonlocal effects from factors improves the prediction of soil water storage. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of microbiotic crusts under cropland in temperate environments on soil erodibility during concentrated flow

Several studies illustrate the wind and water erosion‐reducing potential of semi‐permanent microbiotic soil crusts in arid and semi‐arid desert environments. In contrast, little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter. This study attempts to fill the research gap through (a) a field survey assessing the occurrence of biological soil crusts on loess‐derived soils in central Belgium in space and time and (b) laboratory flume (2 m long) experiments simulating concentrated runoff on undisturbed topsoil samples (0.4 × 0.1 m²) quantifying the microbiotic crust effect on soil erosion rates. Three stages of microbiotic crust development on cropland soils are distinguished: (1) development of a non‐biological surface seal by raindrop impact, (2) colonization of the soil by algae and gradual development of a continuous algal mat and (3) establishment of a well‐developed microbiotic crust with moss plants as the dominant life‐form. As the silt loam soils in the study area seal quickly after tillage, microbiotic soil crusts are more or less present during a large part of the year under maize, sugar beet and wheat, representing the main cropland area. On average, the early‐successional algae‐dominated crusts of stage 2 reduce soil detachment rates by 37%, whereas the well‐developed moss mat of stage 3 causes an average reduction of 79%. Relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover (b = 0·024 for moss‐dominated and b = 0·006 for algae‐dominated crusts). In addition to ground surface cover by vegetation and crop residues, microbiotic crust occurrence can therefore not be neglected when modelling small‐scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments. Copyright © 2007 John Wiley & Sons, Ltd.     

Water use characteristics of coexisting sand-binding vegetation in two typical soil habitats in the desert steppe of China

Understanding the water use characteristics and water relationship of coexisting vegetation in a mixed-species plantation of trees and shrubs is crucial for the sustainable restoration of degraded arid areas. This study investigated the water use characteristic of coexisting sand-binding vegetation combinations in the sierozem habitat (Populus przewalskii Maxim namely Populus-S and Caragana liouana) and aeolian sandy soil habitats (Populus przewalskii Maxim namely Populus-A and Salix psammophila) of the desert steppe. By analysing the δ²H and δ¹⁸O isotopes in xylem, soil water, groundwater and precipitation, a Bayesian MixSIAR model was employed to quantitatively assess the water utilization characteristics of plants. Throughout the growing season, in the sierozem habitat, C. liouana exhibits the highest efficiency in utilizing soil moisture above 60 cm (53.45%) and displays adaptable water use strategies. In contrast, Populus-A predominantly relies on deep soil moisture below 60 cm plus groundwater (63.89%). In the aeolian sandy soil habitat, both Populus-A and S. psammophila similarly favour deep soil moisture below the 60 cm soil plus groundwater (66.77% and 67.60%, respectively). During the transition period from the dry to the wet seasons, although both Populus-A and S. psammophila in the aeolian sandy soil habitat shifted their water sources from deeper to shallower ones, there was considerable overlap in the water sources used by Populus-A and S. psammophila. This overlap led to competition for water resources and exacerbated the depletion of deep soil moisture in both seasons. Conversely, in the sierozem habitat, the partitioning of water sources between Populus-S and C. liouana facilitated the allocation and utilization of water resources between the two species. The findings highlight the need for species-specific consideration in water resource allocation within mixed-species plantations of trees and shrubs, which is crucial for sustainable vegetation restoration in sand-binding ecosystems.     

Carbon dioxide fluxes and DOC concentrations of eroding blanket peat gullies

UK peatlands are affected by severe gully erosion with consequent impacts on ecosystem services from these areas. Incision into the peat can damage the vegetation and hydrology and lead to increases in carbon loss and sediment transfer downstream. Gullies represent then a conduit for and a hotspot of carbon loss but the relatively high water tables of gullies have meant that they have been identified as areas with a high restoration potential because of easily restored peat‐forming conditions. This study uses a series of gully sites, subject to different restoration interventions, to investigate differences in carbon pathways (DOC, CO₂) and hydrology between restoration strategies and gully position. The results show that the position within the gully (interfluve, gully side, or gully floor) does not significantly affect water quality but that it plays a significant role in CO₂ exchange. Gully floors are areas of high photosynthesis and ecosystem respiration, though net ecosystem exchange is not significantly different across the gully. While gully position plays a role in the cycling of some carbon species, this study highlights the importance of vegetation as a key control on carbon cycling. Copyright © 2012 John Wiley & Sons, Ltd.     

Differences in soil moisture in two Middle Eastern oak forests: Comparing the effects of trees and soil composition

The incidence of large rain events in Mediterranean ecosystems vary among years. Summer aridity is interpreted as a resetting event, eliminating previous soil‐moisture dynamics. The dynamics of soil moisture and retention are critical to tree survival, particularly in dry regions. This study examines the long‐term soil water content (θ_V) dynamics in two distinct locations within the forest, under the canopy and forest clearing, within two diverse oak forests: subhumid mixed oak forests (MG) and semiarid monospecific oak woodlands (YE). Plots were established at small‐scale catchments and soil water contents were measured during 2010–2013, at three depths in the two different locations. Cumulative rainfall was used as an independent proxy for θ_V analysis. A novel bell‐bilogistic mathematical model of wetting, saturation, and drying arms was developed. We aimed to study the θ_V distribution differences between soil profiles giving the large climatic gradient between the two forested sub basins, the differences in vegetation traits along with soil attributes. We further aimed at determining the role of an individual tree in regulating soil‐moisture dynamics. We hypothesized the occurrence of distinct responses between sites in all soil‐moisture indices with higher θ_V at the wetter site. We tested the hypothesis that seasonal cumulative rainfall dictates the variations in soil‐moisture regimes throughout contiguous years. Annual rainfall was higher than long‐term average throughout the study. Soil profiles under the canopies at both sites were consistently wetter. Infiltration and depletion constants were higher at MG whereas maximum soil moisture was higher at YE. Homogenous recharge patterns were seen at MG although YE evinced more variation. Oaks had no effect on recharge at MG compared with the forest clearing. Soil properties primarily affected the wetting arm whereas vegetation composition regulated the drying arm. Mixed‐stands characterized by ever‐green and deciduous species may maintain favourable soil‐moisture conditions, in comparison with other mixed stand morphologies. The increasing role of slacking forces in infiltration process may alter the interaction between trees and herbaceous vegetation.     

Estimation of initiation thresholds and soil loss from gully erosion on unpaved roads on China's Loess Plateau

Gullies form easily on unpaved road surfaces during heavy rainstorms on China's Loess Plateau. The integrated effect of rainfall, topography, vegetation, land use, and other factors determines where and when gully erosion occurs; however, the mechanisms driving gully erosion on unpaved road surfaces need to be further understood. Repeated gully erosion on some roads during the storm season provides a good opportunity to better understand the mechanisms behind gully erosion. This article aimed to quantify the integrated threshold conditions required for gully initiation in terms of topography, event rainfall, and upslope land use, and to propose an event-based model to predict the position and magnitude of road gully erosion. Rill and gully erosion on unpaved roads were investigated after an extreme rainstorm of 212.2 mm in 2017 and a regular rainstorm of 83.8 mm in 2018. A digital surface model (DSM) derived from unmanned aerial vehicle (UAV) images was used to analyze the road gradient (S) and upslope area (A). The runoff (Q_RS) of each road segment (RS) was estimated by the runoff curve number method. The results showed the following: (1) The mean and total gully volumes under the regular rainstorm were only 26.3% and 8.1% of those under the extreme rainstorm, respectively; (2) Gully formation on the surveyed roads under both the regular and the extreme rainstorm could be explained by the threshold relationship (S − 0.056)Q_RS^3.363 ≥ 72.444; and (3) A non-linear relationship between gully erosion in road segments and event runoff (Q_RS) and road gradient (S) was found, and was subsequently used to predict road gully erosion on an event basis.     

Sensitivity of soil moisture field evolution to rainfall forcing

In this paper the temporal behaviour of soil moisture is modelled and statistically characterized by use of the zero‐dimensional model for soil moisture dynamics and the rectangular pulses Poisson process model for rainfall forcing. The mean, covariance and spectral density function of soil moisture (both instantaneous and locally averaged cases) are analytically derived to evaluate its sensitivity to the model parameters. Finally, the probability density function of soil moisture is derived to evaluate the effect of rainfall forcing. All the model parameters used have been tuned to the Monsoon '90 data. Results can be summarized as follows. (1) Only the soil moisture model parameters (η and nZ_r) are found to affect the autocorrelation function in a distinguishable manner. On the other hand, both the rainfall model parameter (θ) and the effective soil depth (nZ_r) are found to be of impact to the soil moisture spectrum. However, as the smoothing (or damping) effect of soil is so dominant, about ±20% variation of one parameter seems not to affect significantly the second‐order statistics of soil moisture. (2) More difference can be found by applying a longer averaging time, which is found to obviously decrease the variance but increase the correlation even though no overlapping between neighbouring soil moisture data was allowed. (3) Among rainfall model parameters, the arrival rate (λ) was found to be most important for the soil moisture evolution. When increasing the arrival rate of rainfall, the histogram of soil moisture shifts its peak to a certain value as well as becomes more concentrated around the peak. However, by decreasing the arrival rate of rainfall, a much smaller (almost to zero) mean value of soil moisture was estimated, even though the total volume of rainfall remained constant. This indicates that desertification may take place without decreasing the total volume of rainfall. Copyright © 2005 John Wiley & Sons, Ltd.