Gully Head Research Articles

The allocation and erosive effects of on-wall flow on loess gully heads

On-wall flow and jet flow converted from upstream flow at the gully brink play important but differential roles in gully head erosion. However, on-wall flow allocation at gully brinks and its erosive effects on vegetation-covered loess gully heads remain unclear. Simulated flow scouring experiments were conducted on grass-covered gully heads under different inflow rates (3.0–7.2 m3/h) and upstream slope gradients (1–7°) to investigate the on-wall flow proportion and relationships with the hydraulic parameters of upstream flow, as well as erosion and sediment yield characteristics. The results showed that the on-wall flow rates linearly increased with increasing inflow rate (P < 0.01) but decreased as the slope gradient increased, whereas the jet flow rate increased as the inflow rate or slope gradient increased. The on-wall flow proportions ranged from 24.6 % to 58.7 % and decreased with increasing inflow rate or slope gradient, whereas the jet flow proportions exhibited the reverse trend. The on-wall and jet flow proportions had the most significant correlations with the Froude number, following a negative power function and positive logarithmic function (P < 0.01), respectively. The breadth and depth of the scour hole that developed on the gully headwall increased linearly with the on-wall flow rate (P < 0.01). Jet flow had a limited effect of accumulation on scour hole development, with average increases of 13.5 % and 8.6 % in the breadth and depth, respectively. On-wall flow generally contributed 17.6 %–48.4 % of the sediment driven by on-wall and jet flows but played a dominant role under 1° upstream slope gradient and 3.6 m3/h inflow rate in which the contribution reached 64.9 %. On-wall flow might have dominated the runoff volume and played a dominant role in the sediment yield under low upstream slope gradients and inflow rates and was essentially responsible for scour hole development, which could destabilize and retreat the gully head.

Quantifying the contributions of precipitation, topography and human activity and their coupling to the development of permanent gully

Gully erosion poses greater threat to ecological and food security globally. The gully development rate and the contributions of primary influencing factors have not been thoroughly understood, primarily due to the complicated environmental conditions characterized by climate, soil, topography, and human activities. Therefore, this study was conducted to explore the gully development and contributions of its driving factors from 2013 to 2021 based on historical Google Earth images and field investigation in a typical rolling hilly watershed of northeast China. The climatic factors reflected by precipitation, topographical factors related to gully, and anthropological factors reflected by tillage ridge orientation and linear anthropological factors (LAF) including roads and shelterbelts were obtained. The tillage effect index (TEI), LAF effect index (LEI) and the distance from gully to LAF (DL) were calculated to analyze the influence of anthropological factors on gully development. The results showed that the linear, areal and volumetric gully development rates (RL, RA and RV) showed great variabilities in different periods with the average of 5.69 m year−1, 137.37 m2 year−1 and 428.54 m3 year−1, respectively. The accumulated RL, RA and RV increased exponentially or linearly with accumulated precipitation amount (P), erosive rainfall duration (RD), erosive rainfall (ER), maximum 30-min rainfall intensity (I30) and rainfall erosivity (RE) increased (P < 0.001). The slope of the gully head (HS) was identified as the primary topographic factor influencing RL. Both RA and RV showed positive power relationships with the drainage area of the gully (CA) and the upstream gully head (HA). The human activity reflected by tillage ridge orientation, unpaved road and shelterbelts had different effects on RL and RA. The increased orientation angle between gully and tillage ridge (ROA) mitigated headcut retreat for normal precipitation year but it would aggravate headcut retreat in the periods of continuous heavy rainfall. Gully bank with the ROA of 30−90° was more prone to areal development. The TEI, LEI and DL had significantly positive linear, power or exponential relations with gully development rates. The PLSR and VPA analysis indicated that topography, the interaction between topography and human activity, and the coupled effects of precipitation, topography and human activity were the main contributor of RL, RA and RV, respectively, and accounted for 34 %, 50 %, and 40 % of total variability of RL, RA and RA, respectively. This study can provide guidance for the control of gully erosion and the optimization of land use structure in the gully catchment.

Modeling of driving factors and headcut rates of ephemeral gullies in the loess plateau of China using high-resolution remote sensing images

ABSTRACT Ephemeral gully headcut erosion contributes significantly to global land degradation and increased sediment yields, but the underlying driving factors and prediction models remain poorly understood. We conduct a comprehensive quantitative analysis of ephemeral gully headcut erosion in the Loess Plateau using an optimal parameters-based geographical detector (OPGD) model, leveraging high-resolution remote sensing images. Our findings reveal a varied ephemeral gully head advance rate spanning 0.04–5.54 m yr−1 between 2009 and 2021 (average 1.37 m yr−1), with over 58% of the erosion rates falling between 0.50 and 2.00 m yr−1. Catchment area emerges as the primary driving factor, with an explanatory power of 61%. Moreover, the interactions between catchment area and slope degree, rainfall erosivity, and fractional vegetation coverage (FVC) had explanatory powers exceeding 80%. Furthermore, we developed a robust prediction model for ephemeral gully head advance rates based on the results from the OPGD model, incorporating the FVC factor. The validation of our model yielded a high coefficient of determination (R2 = 0.92 m yr−1) and low root mean square error (RMSE = 0.31 m yr−1). Our study offers new insights into ephemeral gully headcut erosion control in the Loess Plateau and serves as a valuable reference for loess regions worldwide.

Gully regulates snowmelt runoff, sediment and nutrient loss processes in Mollisols region of Northeast China

Gully is a prominent indicator of land degradation in agroecosystems, functioning as a crucial pathway connecting upslopes to downstream channels. However, little is known about how gully regulates runoff, sediment, and nutrient loss processes in the catchment during snowmelt. In this study, we monitored these processes in situ at both the gully head (the upslope accumulated catchment of the gully head, CGH) and outlet of two representative and typical gully-dominated catchments (F1 and F2) during snowmelt in Mollisols region of Northeast China. Our results showed that runoff discharge of CGH and outlet exhibited a multi-peak trend during snowmelt, driven by the transition from snow melting to soil thawing. This transition resulted in distinct runoff patterns in both CGH and outlet, with significant differences in their response to air temperature. The total runoff yield of CGH accounted for 57.8 % in F1 and 40.6 % in F2 of the total runoff yield of the outlet. Notably, the peak sediment concentration displayed a marked lag compared to the peak runoff discharge, primarily dominated by the increased sensitivity of gully erosion after the thawing of gully slopes. Gully erosion was the main source of sediment yield in the catchment, contributing 98.2 % in F1 and 96.6 % in F2. Furthermore, nutrient concentrations exhibited a decreasing trend during snowmelt. The comparison of high nutrient concentrations in CGH and relatively low nutrient concentrations in outlet highlighted the gully's role in intercepting and diluting runoff nutrients. Hysteresis analysis confirmed the differential contribution of CGH and gully to nutrient sources. CGH accounting for 50.9 % and 93.3 % of runoff TN and runoff TP loss, while contributing only 8.3 % and 5.8 % to sediment TN and sediment TP loss, respectively. These findings offer valuable insights for effective erosion control and nonpoint source pollution management in gully-dominated agroecosystems during snowmelt.

Long-term monitoring (1953–2019) of geomorphologically active sections of Little Ice Age lateral moraines in the context of changing meteorological conditions

Abstract. We show a long-term erosion monitoring of several geomorphologically active gully systems on Little Ice Age lateral moraines in the European Central–Eastern Alps, covering a total time period from 1953 to 2019 and including several survey periods in order to identify corresponding morphodynamic trends. For the implementation, DEM (digital elevation model) of Differences (DoDs) were calculated, based on multitemporal high-resolution digital elevation models from historical aerial images (generated by structure from motion photogrammetry with multi-view stereo) and light detection and ranging from airborne platforms. Two approaches were implemented to achieve the corresponding objectives. First, by calculating linear regression models using the accumulated sediment yield and the corresponding catchment area (on a log–log scale), the range of the variability in the spatial distribution of erosion values within the sites. Second, we use volume calculations to determine the total and the mean sediment yield (as well as erosion rates) of the entire sites. Subsequently, both the sites and the different time periods of both approaches are compared. Based on the slopes of the calculated regression lines, it can be shown that the highest variability in the sediment yield at the sites occurs in the first time period (mainly 1950s to 1970s). This can be attributed to the fact that within some sites the sediment yield per square metre increases clearly more strongly (regression lines with slopes up to 1.5). In contrast, in the later time periods (1970s to mid-2000s and mid-2000s to 2017/2019), there is generally a decrease in 10 out of 12 cases (regression lines with slopes around 1). However, even at sites with an increase in the variability in the sediment yield over time, the earlier high variabilities are no longer reached. This means that the spatial pattern of erosion in the gully heads changes over time as it becomes more uniform. Furthermore, using sediment volume calculations and corresponding erosion rates, we show a generally decreasing trend in geomorphic activity (amount of sediment yield) between the different time periods in 10 out of 12 sites, while 2 sites show an opposite trend, where morphodynamics increase and remain at the same level. Finally, we summarise the results of long-term changes in the morphodynamics of geomorphologically active areas on lateral moraines by presenting the “sediment activity concept”, which, in contrast to theoretical models, is based on actually calculated erosion. The level of geomorphic activity depends strongly on the characteristics of the sites, such as size, slope length, and slope gradient, some of which are associated with deeply incised gullies. It is noticeable that especially areas with influence of dead ice over decades in the lower slope area show high geomorphic activity. Furthermore, we show that system internal factors, as well as the general paraglacial adjustment process, have a greater influence on long-term morphodynamics than changing external weather and climate conditions, which, however, had a slight impact mainly in the last, i.e. most recent, time period (mid-2000s to 2017/2019) and may have led to an increase in erosion at the sites.

Seasonal change of tensile crack morphology and its spatial distribution along gully bank and gully slope in the Mollisols region of Northeast China

Gully bank expansion is a critical process of gully development, with tensile cracks development being a necessary condition for gully bank instability and collapse. However, there has been a lack of research on the morphological characteristics of tensile cracks and their distribution along gully bank and slope, which hinders our understanding of gully expansion and its underlying mechanisms. This study aims to address this knowledge gap by investigating the temporal changes in the morphology and spatial distribution of tensile cracks in the Mollisols region of Northeast China. We conducted field investigations at three times: 2020–10 (before freeze–thaw), 2021–05 (after freeze–thaw), and 2021–10 (after rainy season). Morphological and topographical indicators, as well as the locations of each crack, were taken into determination, and an unmanned aerial vehicle (UAV) was employed to capture images of the gullies for clarifying the distribution of tensile cracks. A total of 284 cracks were found with 66.9 % of them on the gully bank and 33.1 % on the gully slope, with their average length, width, and depth of 4.69 m, 8.88 cm, and 20.55 cm, respectively. The length of cracks after freeze–thaw increased by 12.8 %, while the width and depth decreased by 31.2 % and 32.5 %, respectively, and the width of cracks increased by 16.5 % but their length and width decreased by 14.8 % and 24.8 % after rainy season, respectively. The cracks in the rainy season were mainly extinction, and the total length of the cracks is 86.3 % higher than that in the freeze–thaw period, while the freeze–thaw period is dominated by the new formation of cracks with the length of the newly formed cracks being 28.3 % higher than that in the rainy season. Most cracks were distributed in the middle and lower part of gully, and the cracks were closer to the gully head and the gully channel centerline after freeze–thaw. The length, width, and depth of cracks showed a significantly negative correlation with upstream slope, and they were significantly and positively related with gully slope and gully-slope length. Our results provide critical insights into the nature and severity of cracks, lending vital support to the study of gully bank expansion.

Harnessing the Power of Remote Sensing and Unmanned Aerial Vehicles: A Comparative Analysis for Soil Loss Estimation on the Loess Plateau

This study explored the innovative use of multiple remote sensing satellites and unmanned aerial vehicles to calculate soil losses in the Loess Plateau of Iran. This finding emphasized the importance of using advanced technologies to develop accurate and efficient soil erosion assessment techniques. Accordingly, this study developed an approach to compare sinkholes and gully heads in hilly regions on the Loess Plateau of northeast Iran using convolutional neural network (CNN or ConvNet). This method involved coupling data from UAV, Sentinel-2, and SPOT-6 satellite data. The soil erosion computed using UAV data showed AUC values of 0.9247 and 0.9189 for the gully head and the sinkhole, respectively. The use of SPOT-6 data in gully head and sinkhole computations showed AUC values of 0.9105 and 0.9123, respectively. The AUC values were 0.8978 and 0.9001 for the gully head and the sinkhole using Sentinel-2, respectively. Comparison of the results from the calculated UAV, SPOT-6, and Sentinel-2 data showed that the UAV had the highest accuracy for calculating sinkhole and gully head soil features, although Sentinel-2 and SPOT-6 showed good results. Overall, the combination of multiple remote sensing satellites and UAVs offers improved accuracy, timeliness, cost effectiveness, accessibility, and long-term monitoring capabilities, making it a powerful approach for calculating soil loss in the Loess Plateau of Iran.

Dynamics of run‐off hydraulic characteristics during gully headcut erosion driven by head height

AbstractIn the granitic red soil hills region of South China, gully headcut erosion is considered one of the main processes in the development of gullies. However, the dynamics of run‐off hydraulic characteristics such as hydrodynamics and energy consumption during erosion have not been clarified. We investigated the effects of five headcut heights (25 to 125 cm) on hydrodynamic and energy consumption characteristics using field in situ water discharge scouring experiments. The results demonstrated that the flow velocity (V), Froude number (Fr), Reynolds number (Re), run‐off shear (τ) and run‐off power (ω) all indicate the upstream area (UA) &gt; gully bed (GB). Except for V and Fr, several other hydraulic parameters exhibited a trend of sandy soil layer &gt; red soil layer, and a fluctuating increase with increasing headcut height was observed in the GB. The jet velocity at the edge of the headcut (Vb), jet shear stress (τj), potential energy conversion (∆EP) and bottoming instantaneous kinetic energy (EKbottom) all increased exponentially with increasing headcut height by factors of 1.10 to 1.37, 1.16 to 2.00, 1.07 to 2.06 and 1.22 to 1.86, respectively. Except for ∆EP, all others performed a trend of sandy soil layer &gt; red soil layer. The energy consumption at the gully head (∆EH) and its contribution tended to significantly increase with increasing headcut height, and the ∆EH of sandy soil layer was 13.20% to 49.89% higher than that of red soil layer. When the headcut height was greater than 100 cm, the contribution of energy consumption exceeded 50%. In addition, the sediment deposition process throughout the system was significantly influenced by the hydraulic and jet characteristics, and energy consumption was positively correlated with soil loss. The study results contribute to a profound understanding of gully headcut erosion and its hydrodynamic mechanisms.

The study on morphological evolution process of gully headcut erosion in granite red soil hilly area based on an in situ scouring experiment

Gully headcut erosion (GHE) is a key process of gully erosion. However, there is a lack of effective monitoring of the spatiotemporal changes in gully head morphological characteristics that occur during the continuous GHE process. Through a continuous scouring experiment in the field, GHE morphological evolution characteristics were examined for different exposed soil layers (laterite layer and sandy layer), headcut heights (25, 50, 75, 100 and 125 cm) and flow discharge levels (60 and 120 L/min) in typical granite red soil hilly area. The results indicated that the width–depth ratio (WDR) of the erosion gullies decreased in the slope direction and that erosion gullies violently developed on the downslope when stable runoff was generated across the upstream surface. The GHE could be divided into three periods, namely, the gully formation period, active headcut period and stable erosion period, which were mainly reflected in the gully head (GH) and plunge pool erosion (PPE) below the gully bed. The gully headcut retreat (GHR) distance and headwall collapse (HC) range increased with increasing headcut height and flow discharge. PPE exhibited a disordered downward cutting pattern, and the degree of development fluctuated. The GHE degree was jointly affected by GHR, HC and PPE, and these three processes interacted. The gully headcut erosion amount (GHEA) of the laterite layer was smaller than that in the sandy layer, but the average headcut erosion contribution rate (HECR) was 58.51 %, which was higher than that of the sandy layer (48.53 %). The GHE was the main erosion path in the two soil layers, and the HECR was not affected by a single erosion process. The sandy layer was easily eroded, and the GH morphology changed markedly during scouring, while the gully erosion amount (GEA) ranged from 0.02 m3 to 0.21 m3. This was mainly affected by GH and PPE. In this study, niche-like caves may form in the sandy layer to intensify gully erosion. When the headcut height and flow discharge reached a certain degree, the GEA of the laterite layer with strong anti-erodibility will also reach a higher level, and the GEA varied between 0.01 and 0.17 m3. GHR and HC were the dominant gully erosion factors. GHE prevention should focus on the laterite layer, as an effective protective layer in granite red soil areas.

Effects of gully land consolidation on soil organic carbon, soil enzymes and its influences on maize yield

The change characteristics of 5 indicators of soil organic carbon (SOC) content, dehydrogenase activity, protease activity, catalase activity, and maize yield were analyzed at 0a before and 3, 4, and 8 years after remediation in the study area. Results showed that the coefficient of variation (CV) of each index in the study area was generally small, the variation degree of soil organic carbon content was the highest, and the variation degree of catalase activity and maize yield was the least. After the implementation of the gully land consolidation, the CV of each index in the gully showed a decreasing trend, and the distribution of each index in the gully tended to be uniform. With the increase of remediation years, SOC content, soil enzyme activity and maize first yield showed a trend of decrease and then increase, and the change trend of each index was slightly different at gully head, the middle of gully and the gully outlet, the change is significant. At the gully head, the SOC content did not return to the level of 0a after 8a of remediation, and the other indicators returned to the level of 0a or higher than that of 0a. At the gully head, at p &lt; 0.05 level, maize yield showed a very significant positive correlation with catalase activity, and a negative correlation with protease activity. In the middle of gully, there was a significant positive correlation with organic carbon content, soil protease and catalase activities, and it was negatively correlated with dehydrogenase activity but not significantly. At the gully outlet, there was a very significant positive correlation with protease activity. Bangladesh J. Bot. 52(2): 459-466, 2023 (June) (Special)

Effects of gully land consolidation on soil organic carbon, soil enzymes and its influences on maize yield

The change characteristics of 5 indicators of soil organic carbon (SOC) content, dehydrogenase activity, protease activity, catalase activity, and maize yield were analyzed at 0a before and 3, 4, and 8 years after remediation in the study area. Results showed that the coefficient of variation (CV) of each index in the study area was generally small, the variation degree of soil organic carbon content was the highest, and the variation degree of catalase activity and maize yield was the least. After the implementation of the gully land consolidation, the CV of each index in the gully showed a decreasing trend, and the distribution of each index in the gully tended to be uniform. With the increase of remediation years, SOC content, soil enzyme activity and maize first yield showed a trend of decrease and then increase, and the change trend of each index was slightly different at gully head, the middle of gully and the gully outlet, the change is significant. At the gully head, the SOC content did not return to the level of 0a after 8a of remediation, and the other indicators returned to the level of 0a or higher than that of 0a. At the gully head, at p &lt; 0.05 level, maize yield showed a very significant positive correlation with catalase activity, and a negative correlation with protease activity. In the middle of gully, there was a significant positive correlation with organic carbon content, soil protease and catalase activities, and it was negatively correlated with dehydrogenase activity but not significantly. At the gully outlet, there was a very significant positive correlation with protease activity. Bangladesh J. Bot. 52(2): 459-466, 2023 (June) (Special)

Assessment of the Implementation Effect of a Gully Consolidation and Highland Protection (GCHP) Project Based on the GeoWEPP Model

The Gully Consolidation and Highland Protection (GCHP) project is an important governance measure for controlling source erosion and reducing soil erosion in the Loess Plateau, which has been explored and developed continuously in recent decades. However, there is no international precedent for research on the implementation effect of the GCHP project, and it is still relatively weak. In order to quantify the erosion of a small watershed under the construction of a gully head landfill, this study selected Yangjiagou (YJG) as the research area. The spatial analysis function of ArcGIS was used to process DEM and soil type data, the GeoWEPP model was used to simulate soil erosion, and the changes of runoff and sediment yield before and after gully head landfill were analyzed. The results showed that compared with the simulated original soil erosion amount, the annual runoff decreased by 13.13%, and the sediment yield decreased by 37.61% after gully head landfill, indicating that the GCHP project positively influenced soil erosion control. After the gully head landfill measures are taken, the flow path becomes shorter, so the flow scour capacity is weakened. Soil and water control is very effective in the short term, but if long-term maintenance is not carried out, the intensity of soil and water loss is likely to be aggravated. This study provides an effective verification method for the feasibility of a soil loss control scheme on the Loess Plateau and provides a reference for promoting ecological priority and efficient management in the Loess gully area. Ultimately, it will serve the ecological protection and high-quality development of the Yellow River Basin.

Nutrient assessment and land reclamation in the Loess hills and Gulch region in the context of gully control

Abstract Soil quality is the key to the success or failure of the gully land reclamation project. Studying the variation characteristics of soil nutrients and evaluating the nutrient classes in the gully after the implementation of gully land reclamation project in the loess hilly and gully region can provide a basis for adjusting the spatial layout of gully crops, rational planning and utilization of land resources, and soil nutrient management. In this study, a typical gully in Yan’an City, Shaanxi Province, China was taken as the research object. Based on the data of soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) before 0 year (0a) and after 2, 4, and 8 years (2a, 4a, and 8a) gully remediation. From the spatial distribution characteristics of soil nutrients and its levels, the classical statistical analysis, geostatistical analysis, and matter-element model were used to reveal the variation characteristics of soil nutrients in the gully before and after the gully land reclamation project. The results showed that (1) before remediation, except for AK which reached the level of 1–3, the rest of the nutrients content was generally low and the degree of variation was large, after remediation the degree of variation decreased and the coefficient of variation was between 0.23–0.54, the distribution tended to be uniform. (2) SOM content showed a “decrease–increase” change process, reaching the highest after remediation (8a); TN and AP increased first and then decreased; AK showed a “downward–upward–downward” change process. (3) The soil nutrient grade belongs to the appropriate level. After the implementation of the project, only the gully head is reduced to the poor level. After 4a of remediation, the soil nutrient grade is restored and has the potential to continue to improve. Compared with the slope land remediation, the soil nutrient recovery time of gully land creation is shorter, the spatial layout of crops can be adjusted according to the soil nutrient characteristics of different locations in gully, and different levels and methods of management measures can be implemented to increase food production.

Infiltration and shear strength characteristics of gully heads soil of typical vegetation on the gullied Loess Plateau, Northwest China.

Gully head is the main active part of gully erosion, which seriously affects the occurrence of gully headcut erosion. To investigate root distribution and soil physical and mechanical characteristics of typical vegetation gully head, we analyzed the infiltration, root distribution, physical and mechanical properties of soil-root complex of soil in different layers (0-1 m) in natural restoration gully head and artificial restoration gully head. The results showed that the variability of soil bulk and total porosity among different vegetation gully heads was low, with bulk density ranging from 1.10 to 1.37 g·cm-3 and total porosity ranging from 48.3% to 58.4%. Infiltration index of different vegetation gully heads generally decreased with increasing soil depth. The infiltration rate of different soil layers in natural restoration gully head tended to stabilize in 20-30 min, while that of artificial restoration gully head tended to stabilize in 40 min. The infiltration capacity and average infiltration rate of artificial restoration gully head were generally higher than those of natural restoration gully head in all soil layers. Root length density, root surface area density, and average diameter all tended to decrease with increasing soil depth. Except for the 20-40 cm soil layer, root length density, root surface area density and average diameter of natural restoration gully head were all lower than those of artificial restoration gully head. Root system of both vegetation gully heads mainly consisted of 0-0.5 mm roots, accounting for 84.2%-93.6% of the total root length. In the vertical depth, with the increases of water content, the cohesion force decreased linearly with the deepening of soil layer, ranging from 0.42 to 22.67 kPa. The average cohesion force of artificial restoration gully head was higher than natural restoration gully head at each level of water content. The study revealed the effects of vegetation on the gully head cut erosion, which could provide scientific basis for the effective prevention and control of soil erosion in the region.

Response of soil erodibility of permanent gully heads to revegetation along a vegetation zone gradient in the loess-table and gully region of the Chinese Loess Plateau

Revegetation has been proven to significantly affect soil erodibility of gully heads, and climate conditions are expected to affect soil erodibility by determining the vegetation characteristic. However, there are crucial scientific/knowledge gaps regarding the change in the response of soil erodibility of gully heads to revegetation along a vegetation zone gradient. Therefore, we selected the gully heads with different restoration years along a vegetation zone gradient encompassing the steppe zone (SZ), forest-steppe zone (FSZ), and forest zone (FZ) on the Chinese Loess Plateau to clarify the variation in soil erodibility of gully head and its response to soil and vegetation properties from SZ to FZ. Furtherly, we systematically and comprehensively reveal driving factors of changes in soil erodibility in three vegetation zones. Results showed that: (1) Vegetation and soil properties were affected positively by revegetation and differed significantly in three vegetation zones. (2) Soil erodibility of gully heads in SZ was significantly higher than in FSZ and FZ, by 3.3 % and 6.7 % on average, respectively, and it showed a significantly different decrease with restoration years in three vegetation zones. (3) Standardized major axis analysis proved that the sensitivity of response soil erodibility to vegetation characteristics and soil characteristics presented a significant difference as the revegetation proceeded. Vegetation roots were the primary driver in SZ, but soil organic matter content dominated the change in soil erodibility in FSZ and FZ. (4) Structural equation modeling indicated that climate conditions played an indirect role in regulating soil erodibility of gully heads by mediating vegetation characteristics. This study offers essential insights for assessing the ecological functions of revegetation in the gully heads of the Chinese Loess Plateau under different climatic scenarios.

Effects of vegetation stem coverage on the hydraulic characteristics and on-wall and jet flow proportions at the gully head

Vegetation in the upstream area played a critical role in influencing runoff processes and subsequent headcut erosion at the gully head. However, it remains unclear how the vegetation stem coverage affects the hydraulic parameters and the on-wall and jet flow characteristics at the gully head. A simulation flow scouring experiment was performed to explore the influences of the vegetation stem coverage (5 levels) on the hydraulic parameters and on-wall and jet flow proportions at the gully head under inflow discharges ranging from 3.6–9.6 m3 h−1. The results indicated that the runoff in the upstream area occurred in the supercritical turbulent flow regime, and its velocity decreased while the Manning roughness coefficient and Darcy–Weisbach coefficient increased with increasing vegetation stem diameter. The runoff velocity at the gully brink decreased with increasing vegetation stem diameter and increased as a linear function of the inflow discharge (R2 = 0.849–0.997). With increasing vegetation stem diameter, the proportion of on-wall flow increased, and the proportion of jet flow decreased. The on-wall and jet flow proportions ranged from 7% to 92% and 8% to 93%, respectively, under inflow discharges from 3.6–9.6 m3 h−1, thereby attaining the highest correlation with the runoff velocity, Reynolds number, and runoff kinetic energy and following a significant logarithmic function. The vegetation stem influences the hydraulic parameters of the upstream area and gully head and thus influences the on-wall/jet flow proportion. This study could help elucidate the effect of vegetation on the hydraulic mechanism of headcut erosion and provide a theoretical foundation for improving gully headcut erosion models.

Revegetation-induced changes in vegetation diversity improve soil properties of gully heads

Revegetation is among the most efficient methods to improve gully headcut erosion. However, the influencing mechanism of revegetation on the soil properties of the gully head (GHSP) is still unclear. Thus, this study hypothesized that the variations in the GHSP were influenced by vegetation diversity during nature revegetation, and the influence pathways were mainly root traits, aboveground dry biomass (ADB), and vegetation coverage (VC). We studied six grassland communities of the gully head with different natural revegetation ages. The findings showed that the GHSP were improved during 22-year revegetation. The interaction effect of vegetation diversity, roots, aboveground dry biomass, and vegetation coverage on the GHSP was 43 %. In addition, vegetation diversity significantly explained >70.3 % of the changes in the root traits, ADB, and VC of the gully head (P < 0.05). Therefore, we combined vegetation diversity, roots, ADB, and VC to establish the path model to explain the GHSP changes, and the goodness of fit of the model was 82.3 %. The results showed that the model explained 96.1 % of the variation in the GHSP, and the vegetation diversity of the gully head affected the GHSP through roots, ADB, and VC. Therefore, during nature revegetation, vegetation diversity dominates the improvement of the GHSP, which has important significance for designing an optimal vegetation restoration strategy to control gully erosion.

Quantifying the effect of freeze–thaw on the soil erodibility of gully heads of typical gullies in the Mollisols region of Northeast China

The soil erodibility of gully headwalls is a critical factor influencing gully erosion. In some cold regions, the soil of gully headwalls is subjected to a bidirectional freeze–thaw effect (FT), while the quantification of the influence of FT on the soil erodibility of gully heads is unclear. Therefore, five typical and representative gullies developed in farmlands (GF1 and GF2), along unpaved road (GR), in the woodland (GW) and in stabilized gully bed (GS) were selected to quantify the influence of FT on the soil erodibility of gully heads reflected by five soil erodibility parameters including the K factor (K), soil disintegration rate (SDR), mean weight diameter (MWD), soil shear strength (SS) and saturated soil hydraulic conductivity (SHC). A comprehensive soil erodibility index (CSEI) was constructed by combining the above five soil erodibility parameters. This study revealed that FT significantly affected the five soil erodibility parameters and CSEI, of which the K and SDR increased by 22.7 % and 52.3 %, respectively, but the MWD, SHC and SS decreased by 29.3 %, 30.4 % and 26.1 %, respectively. The change in soil erodibility of the gully heads was closely related to the soil and root properties, of which the contribution of roots to the change in soil erodibility of gully heads was increased by 78.5 % after FT on average. Furthermore, FT significantly increased the CSEI at depths of 0–50 cm in GF1, GF2 and GR, and at depths of 0–80 cm in Gw and GS. FT increased the CSEIs of the five gully heads by 20.1 %-37.3 %, with an average of 26.1 %. Evidently, the organic matter content (OMC), soil texture and temperature dominated the change in the CSEI, of which the extremely low temperature and temperature rangeability during freeze–thaw period exhibited the greatest impact on the soil erodibility of the gully heads.

Topographic threshold of gully erosion on the Badlands ridges in Lower Chambal Valley, India

Badlands along the lower Chambal valley has been recognised as one of India's worst forms of soil erosion. Theories of their origin vary from surface warping because of Himalayan orogeny to climate change (an increased rainfall in the past). Despite the wide recognition of the Chambal badlands, studies on gully morphology in the badlands have been scarce. The objectives of this study are to investigate the erosion processes governing gully erosion using the topographic threshold concept of the SA (Slope and contributing Area) relationship. As per this concept, the combination of S and A at the gully head expresses critical flow conditions that lead to the formation of the gully head, and mathematically it can be expressed as S = aAb. To study the same, 29 gullies were measured within the badlands in the Lower Chambal Valley. Considering the whole data, the relationship was found to be S = 0.041A−0.086 with a wide scatter. Interestingly, on classifying the data based on the width-depth ratio (WDR), two different and contrasting relationships were found, i.e., S = 0.032A-0.125 (WDR > 1), and S = 0.18A0.048 (WDR < 1). This may indicate that classification based on WDR can differentiate between overland and seepage flow. The weak negative SA relationship further indicates a dominant role played by soil piping and mass movement in the formation and evolution of gully within the badlands in the LCV. Moreover, strong positive relationships were found between average width and average depth, indicating the same processes were involved in shaping the depth and width of studied gullies.

A data driven gully head susceptibility map of Africa at 30 m resolution

Predicting gully erosion at the continental scale is challenging with current generation models. Moreover, datasets reflecting gully erosion processes are still rather scarce, especially in Africa. This study aims to bridge this gap by collecting an extensive dataset and developing a robust, empirical model that predicts gully head density at high resolution for the African continent. We developed a logistic probability model at 30 m resolution that predicts the likelihood of gully head occurrence using currently available GIS data sources. To calibrate and validate this model, we used a new database of 31,531 gully heads, mapped over 1216 sites across Africa. The exact location of all gully heads was manually mapped by trained experts using high-resolution imagery available from Google Earth. This allowed the extraction of detailed information at the gully head scale, such as the local soil surface slope. Variables included in our empirical model are topography, climate, vegetation, soil characteristics and tectonic context. They are consistent with our current process-based understanding of gully formation and evolution. The model shows that gully occurrences mainly depend on slope steepness, soil texture and vegetation cover and to a lesser extent on rainfall intensity and tectonic activity. The combination of these factors allows for robust and fairly reliable predictions of gully head occurrences, with Areas Under the Curve for validation around 0.8. Based on these results, we present the first gully head susceptibility map for Africa at a 30 m resolution.