Short-term bank gully retreat rates in Mediterranean environments

In this study, short-term headcut retreat was monitored from 46 active bank gullies, selected in the Guadalentin and the Guadix basin in Southeast Spain. The measurements were carried out manually using an orthogonal reference system fixed by erosion pins around the gully heads, between April 1997 and April 1999 with a 1-year interval. The average volumetric retreat rate for all gullies was 4.0 m³ year⁻¹, corresponding with an average linear retreat rate of 0.1 m year⁻¹, but more erosion took place during the first monitored year (1997–1998) compared to the second (1998–1999). An interplay of spatial variations in rainfall distribution and tension crack activity is assumed to be responsible for the important difference in annual headcut retreat, compared to the small difference in annual rainfall amounts. Statistical analysis showed that the present drainage-basin area (A_p) was the most important topographical factor explaining average gully headcut retreat rate, both in terms of annual eroded volume (V_e) and annual linear retreat (R_l), and expressed by the power relationships V_e=0.04A_p^0.38 (R²=0.39) and R_l=0.01A_p^0.23 (R²=0.39). The V_e–A_p relationship was compared with the relationship between original drainage-basin area (A_o) and total eroded bank gully volume (Vol), i.e. Vol=1.71A_o^0.60 (R²=0.65). The importance of runoff generation from a drainage basin is shown by the positive correlation of linear headcut retreat and the runoff curve number (CN), representative for the conditions in the drainage basin. High CN values tend to coincide with higher annual eroded volumes in the relationship between present drainage-basin area (A_p) and annual eroded volume (V_e), but this effect was not observed in the relationship between original drainage-basin area (A_o) and total eroded bank gully volume (Vol). Stepwise multiple regression selected the relevant environmental parameters explaining annual eroded volume and linear retreat. In both equations, the present drainage-basin area explained the largest part of the variation. The CN was selected as another common parameter. Height of the headcut was the second most important variable explaining annual eroded volume, indicating the role of energy transfers and undercutting at the headcut. Linear retreat was further explained by the average slope of the present drainage-basin area, representing the effect of decreasing transmission losses and increasing flow velocity with steeper catchment slopes, and by the sand content, decreasing the cohesion of the soil material, promoting soil fall and headcut retreat. Spatial extrapolation of the measured volumetric retreat rate of 4.0 m³ year⁻¹ revealed that active bank gully heads contribute up to 6% of the sediment yearly filling up the Puentes reservoir. Estimated gully ages (i) based on the ergodic principle, and (ii) by linear extrapolation of actual gully retreat rates in the past, range between 63 years and 1539 and between 64 and 1720 years, respectively. The high correlation between the gully ages estimated by the two methods is attributed to the fact that most gullies have not reached the evolutionary stage of significantly declining retreat rates. Since medium-term gully retreat rates are more dependent on drainage-basin area compared to the short-term retreat rates obtained in this study, the estimated gully ages represent maximum values, assuming that present land-use and climate conditions prevailed over the last two millennia.     

A method for dendrochronological assessment of medium-term gully erosion rates

A method based on dendrochronology to estimate gully erosion rates was developed as an alternative of traditional methods for assessing medium-term gully retreat rates, such as field monitoring of headcuts or aerial-photo interpretation of gully retreat. The method makes use of trees or parts of a tree affected by gully erosion revealing information on the history of the erosion process by datable deviations of their normal growth pattern, hence defined as ‘datable objects’. These include roots exposed by erosion; browsing scars made by ungulates on exposed roots or on above-ground parts of fallen trees; exposed and dead root ends; root suckers; stems, branches and leading shoots of fallen trees; and a sequence of trees within a gully. The method is based on the differentiation between three main conditions depending on the relation between the dynamics of the datable object (part of the tree) and the development of the gully. The first condition implies that the datable object was created before erosion of the gully volume to be dated, e.g. exposed tree roots. According to the second condition, the datable object developed as an immediate consequence of the erosion event, e.g. growth reactions of a fallen tree. The third condition implies that the datable object was created some time after the erosion event took place, e.g. trees colonising the gully bed. Each principle has consequences for the accuracy and the correct interpretation of the estimated erosion rate, i.e. whether the true erosion rate is underestimated, exact or overestimated. In spite of methodological limitations and dendrochronological dating problems, the method was successfully applied in southeast Spain. Conservative estimations of gully-head retreat rate resulted in an average medium-term (3–46 years) value of 6 m³ year⁻¹ (n=9). For gully sidewall processes, the average minimum erosion rate per unit sidewall length amounted 0.1 m³ year⁻¹ m⁻¹ (n=9). A strong correlation was found between the headcut retreat rate (v_m(ortho), m³ year⁻¹) and the drainage–basin area (A, m²) of the gullies, expressed by v_m(ortho)=0.02A^0.57 (R²=0.93, n=9). Comparing the findings from this study with those obtained by short-term headcut retreat monitoring suggests a high reliability of the estimated retreat rates, supporting the applicability of the developed dendrochronological method.     

Gully development in the Moldavian Plateau of Romania

Gully erosion has been recognized as an important environmental threat in the Moldavian Plateau of Eastern Romania. The main objective of this study was to define the process-based gully development by providing quantitative information from long-term field measurements in small catchments.Three main areas of monitoring gullies were explored: aerial photographs of flights in 1960 and 1970, classical leveling and repeated survey through a particular close stakes grid after 1980. The Caesium-137 technique has been used effectively in the areas of deposition of gully sediments to obtain reliable information on dating specific levels of sediments and to provide chronological measures of gully development.Most of the discontinuous gullies exhibit both proper gullying, mainly if not exclusively by gully head advance, and aggradation of the gully basin floor. Results indicated that the mean gully head retreat was 0.92 m year^− 1 and the mean areal gully growth was 17.0 m² year^− 1. Both values indicate a slow erosion rate for this area. The average annual regime of gullying is pulsatory, one that is best described by great fluctuations. Conventional measurements on sedimentation over the period 1987–1997 indicate a higher rate of aggradation in the upper half of the gully floor. Information on the Caesium-137 depth profile was used to provide estimates of a mean sedimentation rate of 4.4 cm year^− 1 over the period 1963–1996 and 2.5 cm year^− 1 after 1986 for the short successive discontinuous gullies. A new classification of the discontinuous gullies based on two criteria, respectively, the field patterns and the rate of aggradation within the gully basin floor was established.For continuous gullies, linear gully head retreat, areal gully growth and eroded material rates were quantified for three periods (1961–1970, 1971–1980 and 1981–1990). Results indicate that gully erosion has decreased since 1960. This gullying decline is due to the rainfall distribution, and the increased influence of soil conservation practices. The mean gully head retreat of 12.5 m year^− 1 between 1961–1990 was accompanied by a mean areal gully growth of 366.8 m² year^− 1 and a mean erosion rate of 4168 t year^− 1. As with discontinuous gullies, continuous gullies showed pulsatory development.The critical period for gullying over 1981–1996 covers 4 months from Mid March to Mid July in an area with mean annual precipitation around 500 mm. Another main finding of this 16-year stationary monitoring was that 57% of the total gullying occurred during the cold season, with the remainder during the warm season. Of the total gully growth, 66% results from only four years (1981, 1988, 1991 and 1996), when a greater amount of precipitation fell.     

The validation of a gully headcut retreat model in short-term scale based on an in-situ experiment in dry-hot valley

Purpose

How to predict gully erosion rates was one of the most important issues at present. A model implemented in the Channel Hillslope Integrated Landscape Development (CHILD) model based on the shape factor (S_f) of the plunge pools, which successfully predicted the headcut retreat rates over a long timescale. But whether this model was also valid in the short-term scale? What factors affected the prediction effects of the model? These issues still need evaluation.

Material and method

Four gully head plots with consistent initial topography and soil types were constructed for conducting in-situ scouring experiments with different flow discharges. Eighteen tests were conducted in each plot to monitor the plunge pool growth and headcut retreat rates using an unmanned aerial vehicle (UAV) to create high-precision topographic data.

Results and discussion

The topographic parameters of the plunge pools generally increased with scouring duration and were strongly correlated with the total runoff energy consumption (0.55 < R² < 0.92, P < 0.01). In contrast, headcut retreat was an intermittent process that suddenly migrated in a short time after relatively long stable periods lasting from 20 to 130 min. The relationships between the headcut retreat rates and total energy consumption were weaker than those with the plunge pools.

Conclusion

As the timescale shortened, the contribution of mass failure due to gravity to headcut migration significantly increased, leading to the pool effects of the model to influence the prediction of the gully headcut retreat rates, which assumed that flow hydraulics were the key dynamics.

     

Topographical thresholds for ephemeral gully initiation in intensively cultivated areas of the Mediterranean

Ephemeral gullying is an important erosion process in Mediterranean areas suffering from land degradation because of increased drought and human impact. An ephemeral gully is formed when a hydraulic threshold for incision into a resistant soil surface is exceeded. The flow intensity at the soil surface of any landscape position is controlled by topographical parameters, such as local slope (S) and drainage-basin area (A). The objective of this study was to investigate to what extent one could use field measurements of S and A to predict the location of ephemeral gully initiation in intensively cultivated Mediterranean environments. Two relatively homogeneous study areas in Southeast Spain and Northeast Portugal with widespread ephemeral gullying were selected. S and A were measured by standard methods at the initiation point of 50 ephemeral gullies in each study area. The relationship between S and A was statistically investigated. On a double logarithmic scale, the S and A data showed considerable scatter. Orthogonal regression resulted in the threshold relationship S=aA^−b with b=0.133 for Spain, and b=0.226 for Portugal. The analysis also showed that S and A were weakly correlated for the Spanish dataset, and more strongly correlated for the Portuguese dataset. The correlation was considerably strengthened by including additional information on planform curvature and on tillage direction relative to the gully direction. Greater soil stoniness and the presence of a less permeable horizon generally increased the threshold level in both study areas. The threshold relationships resulting from this study were compared with literature data. The indices A_cS and ln(A_c/S) were applied to our study sites but neither relationship individually described the general trend between the data of S and A. Therefore, an alternative index, A_c^bS, with 0.133≤b≤0.226, according to the empirical trend between drainage basin area and local slope, was proposed to describe the topographical threshold conditions for ephemeral gully initiation in similar Mediterranean areas.     

甘肃省小陇山林区降雨型浅层滑坡汇水面积-坡度关系及其影响因素

沟头发生侵蚀的地形临界模型可有效预测侵蚀沟的形成条件,浅层滑坡失稳形成的洼地也是沟头形成的方式之一。为探究浅层滑坡临界起动模型的特点,以甘肃省天水市小陇山林区的降雨型浅层滑坡为研究对象,运用汇水面积-坡度关系,构建临界起动模型,与黄土高原典型侵蚀沟(浅沟、切沟)的临界起动模型进行对比分析,并探讨土地利用类型、植被类型和土壤质地对该模型的影响。结果表明:(1)浅层滑坡临界起动模型为S=3.50A_s^-0.34,其侵蚀阈值为3.50,大于黄土高原典型浅沟(0.96)和切沟(1.54)的侵蚀阈值。研究区浅层滑坡一般发生于土层较薄的陡坡地带,其平均坡度(S=1.26)大于浅沟(S=0.35)与切沟(S=0.46),单位汇水面积(A=89.08 m²/m)小于浅沟(A=920.93 m²/m)和切沟(A=1 129.82 m²/m)。(2)汇水面积与坡度平方的乘积(AS²)代表了沟头产生侵蚀的能量指标值。研究区浅层滑坡AS²值在269.1~5 703.2 m²,平均值为1 772.97 m²,黄土高原浅沟AS²值在4.74~892.66 m²,切沟在41~814 m²,启动能量值方面,浅沟<切沟<浅层滑坡。(3)土地利用类型、植被类型和土壤质地通过影响土壤的抗冲力、渗透性和黏粒含量,从而对浅层滑坡的起动难易程度产生影响。在不同的土地利用方式中,农地最易发生侵蚀,其次是林地。油松林附近浅层滑坡的抗侵蚀能力高于日本落叶松林。研究结果为探究浅层滑坡的起动条件提供理论依据。     

金沙江干热河谷冲沟形态发育特征及其侵蚀速率估算

冲沟侵蚀是金沙江干热河谷土壤流失的重要原因，造成严重的土地退化，威胁区域农业发展和粮食安全。探究冲沟的形态发育特征明确其侵蚀速率，对于冲沟侵蚀量化评估和土地保护等具有重要意义。为摸清冲沟发育演变规律，该研究选取元谋干热河谷金雷国家水土保持科技示范园区附近沟谷地貌流域为研究区，选择35条冲沟，通过无人机遥感影像提取冲沟形态参数，分析形态特征，并结合高分遥感影像计算冲沟侵蚀速率。结果表明：1）研究区冲沟直线长度、顶宽、底宽、深度的范围分别为25.21～180.43 、10.76～51.76、3.56～26.85、1.22～11.92 m；面积、体积的范围分别为257.50～8987.88 m²、142.64～19479.25 m³。冲沟形态参数差异性较大，长度在发育过程中受坡面限制小于其他地区。2）冲沟底部宽度和顶部宽度比值（BW/TW）的范围为0.24～0.59，冲沟主要为“V型”和“V～U型”；冲沟顶部宽度和深度比值（TW/D）的范围为2.56～26.29，均值为6.04，横向侵蚀速率远大于下切侵蚀速率。3）2015—2022年，35条冲沟中，19条长度明显增长，冲沟溯源侵蚀速率为0.02～1.10 m/a（均值0.34 m/a）；27条宽度明显增长，冲沟横向侵蚀速率为0.07～1.10m/a（均值0.36 m/a）；29条面积和体积明显增长，冲沟面积增长速率为1.64～105.40 m²/a（均值23.44 m²/a）；冲沟体积侵蚀速率为1.26～339.42 m³/a（均值51.21 m³/a）。冲沟横向侵蚀速率大于溯源侵蚀速率，在面积和体积增长中贡献较大。研究结果可为金沙江干热河谷地区大尺度冲沟的发育演变规律和侵蚀定量评估提供方法和依据。     

A non-linear technique based on fractal method for describing gully-head changes associated with land-use in an arid environment in China

Only few studies have focused on quantifying and/or modeling changes in gully-head shape. The present paper proposed a non-linear technique based on fractal method for describing gully-head changes quantitatively, and carried out a case study in the Yuanmou Basin in the Longchuan River Valley, Yunnan Province, southwest China. Seventy-seven gullies under 5 different land-uses were investigated in 1998 and again in 2002. For the study period, the gully-head retreat rate in different land-uses showed a decreasing order BL (bare land) > FL (farmland for only crops) > OC (mixture of orchards and crops) > FG (combination of forest and grass) > FSG (combination of forest, shrubs and grass). Correspondingly, the three proposed fractal characteristics (the fractal curvature P_τ, ΔP_τ = change in the fractal curvature P_τ, ΔD = change in the fractal dimension D) of the gully-heads followed the same order BL > FL > OC > FG > FSG, also. These findings suggested that the three parameters (P_τ, ΔP_τ, ΔD) proposed can sensitively reflect the gully-head changes associated with land-use, and may be used as feasible parameters for understanding and predicting gully development under land-use change and global change.     

Gully incision as a key factor in desertification in an arid environment, the Negev highlands, Israel

Gully incision has been eroding the alluvial sediments and loess soils deposited and developed along the valleys in the arid and semiarid regions of Israel. This phenomenon is critical in the arid regions of the Negev Highlands where the agricultural fields, the main floral biomass and the areas which have the highest grazing value, are limited to narrow valleys filled with redeposited loessial sediments. The headcut migration and gully development in the region were studied between 1990 and 2001 in three representative drainage basins (Zipporim, Revivim and Sekher). During flood events, the runoff penetrates the alluvial cover of the valleys, forming vertical headcuts, which gradually retreat up the valley. The runoff is channeled into narrow gullies, preventing the floodwater from spreading over the whole width of the valley. The change in irrigation efficiency along the valleys is reflected in a sharp estimated drop of 70–90% in the floral biomass, causing the reduction of the range value by 83–99%. During the monitoring time interval (1990–2001), the linear gully retreat in the study area ranged between 12.3 and 250 m an average rate of 1.12–22.7 m year^{− 1} for each gully head. The process is accompanied by erosion of soil, which has high agricultural and range value. The total soil losses in these sites ranged between 800 and 9000 m³ at an average rate of 81–818 m³ year^{− 1} for each gully head, which is equivalent to 121–1227 Mg year^{− 1}. During the monitoring period, approximately 0.11–0.87 ha of land lost its agricultural and range value in each basin under study, at an average rate of 0.01–0.079 ha year^{− 1}. Since the Byzantine period (1400 BP), approximately 6.5 ha, which is 10% of the land that had high agricultural and range value in the Zipporim valley, lost its value due to gully erosion. No recovery effects of the gully channels were found in the nearby region. The soil erosion is generated by a long-term natural dynamic change in the soil / rock ratio evolving within the drainage basins through time since the termination of the last glacial phase. The loessic sediments, originally deposited within the drainage basins during the late Pleistocene glacial stage, are being removed under the present Holocene climate in several erosion stages. This ongoing phenomenon is causing degradation of soil and biomass and is severely reducing the agricultural and range potential of the region. These parameters indicate that an ongoing process of desertification is active in the arid environment of the Negev Highlands, and is advancing in proportion to the headcut retreat rates in the region. The soil erosion and headcut retreat have been active in the Negev Highlands for the last few millennia. If these processes continue in the future, the Negev Highlands region will lose its agricultural potential within a few millennia. However, the fact that the ancient inhabitants of the region implemented successful long-term land conservation techniques already 3000 years ago, implies that a sustainable land management policy can be adapted to the Negev Highlands, as well as to other semi-arid regions in the Middle East.     

陕北黄土区退耕前（1976—1997）坡面切沟发育特征

以陕北黄土区坊塌流域内7条大切沟及其谷缘上的小切沟为研究对象,通过Arc GIS和MATLAB从基于1976、1997年1:10000比例尺地形图生成的数字高程模型(DEM)上提取沟沿线,进而获得1976、1997年大切沟的面积、周长和大切沟上小切沟沟头前进距离,并结合当时的土地利用图和植被覆盖图,定量研究了退耕前陕北黄土区切沟发育的速率,以及土地利用类型和植被覆盖度对切沟发育的影响.结果显示:在1976-1997年间,7条大切沟的面积、周长增长率分别为11.01%~180.46%和8.07%~86.75%,大切沟上小切沟沟头年均前进速率为0.26~0.84m;由溯源侵蚀导致的小切沟沟头前进是研究区内大切沟上的主要侵蚀方式,大切沟沟谷拓宽和形成新的小切沟分别是对大切沟的面积和周长增长具有较高贡献率的侵蚀方式;林草覆盖能控制切沟发育,集水区内覆盖度大于65%的植被能更加有效地控制大切沟内以沟谷拓宽为主的、多过程、多部位的综合切沟侵蚀,覆盖度大于45%的植被能更加有效地控制大切沟上小切沟沟头的溯源侵蚀.研究表明基于GIS技术和不同年份的地形图,可以确定切沟侵蚀的方式及速率,实现对切沟发育的动态监测.     

Short-term gully retreat rates over rolling hill areas in black soil of Northeast China

Soil loss has become one severe problem in black soil areas of Northeast China after several decades of cultivation. Gully erosion is one of its main components. In this study, short-term gully retreat was monitored from 5 active gullies selected in representative black soil area during April 2002 to June 2004, using differential global positioning system (GPS). With the support of geographic information system (GIS), multitemporal digital elevation models (DEM) were constructed from the data collected by GPS and then used for further analysis. This presents a new method to compute the retreat rate of gully heads and the rate of soil losses caused by gully erosion. The results indicate that the average volumetric retreat rate was 729.1 m³ year^− 1, corresponding with an average linear retreat rate of 6.2 m year^− 1 in gully head and planimetric changes of 323.6 m² year^− 1 during the two monitored years, but more erosion took place during the second and third monitored period compared to the first. The erosion by freeze thawing and snowmelt accounts for a large percent. And this will be emphasized when rainfall is added in spring. If only considering the third monitored period, the conservatively estimated retreat rate by freeze thawing and snowmelt (i.e. before rainy season) may even reach 8.6 m year^− 1 in gully head, with a volumetric rate of 120.9 m³ year^− 1 and planimetric changes of 173.6 m² year^− 1. These results reveal that gully erosion is a great threat in the study area and conservation measures are urgently needed. Based on the analysis of multi-temporal DEM, one conceptual model for gully developing in black soil of Northeast China is proposed, which is supported by the data.     

A method for estimating coefficients of soil organic matter dynamics based on long-term experiments

The one-compartment C model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t) is being long used to simulate soil organic C (SOC) stocks. C_t is the SOC stock at the time t; C₀, the initial SOC stock; k₂, the annual rate of SOC loss (mainly mineralization and erosion); k₁, the annual rate to which the added C is incorporated into SOC; and A, the annual C addition. The component C₀e^−k₂t expresses the decay of C₀ and, for a time t, corresponds to the remains of C₀ (C_0 remains). The component k₁A/k₂(1−e^−k₂t) refers, at time t, to the stock of SOC derived from C crops (C_crop). We herein propose a simple method to estimate k₁ and k₂ coefficients for tillage systems conducted in long-term experiments under several cropping systems with a wide range of annual C additions (A) and SOC stocks. We estimated k₁ and k₂ for conventional tillage (CT) and no-till (NT), which has been conducted under three cropping systems (oat/maize −O/M, vetch/maize −V/M and oat + vetch/maize + cowpea −OV/MC) and two N-urea rates (0 kg N ha⁻¹ −0 N and 180 kg N ha⁻¹ −180 N) in a long-term experiment established in a subtropical Acrisol with C₀ = 32.55 Mg C ha⁻¹ in the 0–17.5 cm layer. A linear equation (C_t = a + bA) between the SOC stocks measured at the 13th year (0–17.5 cm) and the mean annual C additions was fitted for CT and NT. This equation is equivalent to the equation of the model C_t=C₀e^−k₂t+k₁A/k₂(1−e^−k₂t), so that a=C₀e^−k₂t and bA=k₁A/k₂(1−e^−k₂t). Such equivalences thus allow the calculation of k₁ and k₂. NT soil had a lower rate of C loss (k₂ = 0.019 year⁻¹) than CT soil (k₂ = 0.040 year⁻¹), while k₁ was not affected by tillage (0.148 year⁻¹ under CT and 0.146 year⁻¹ under NT). Despite that only three treatments had lack of fit (LOFIT) value lower than the critical 5% F value, all treatments showed root mean square error (RMSE) lower than RMSE 95% indicating that simulated values fall within 95% confidence interval of the measurements. The estimated SOC stocks at steady state (C_e) in the 0–17.5 cm layer ranged from 15.65 Mg ha⁻¹ in CT O/M 0 N to 60.17 Mg ha⁻¹ in NT OV/MC 180 N. The SOC half-life (t_1/2 = ln 2/k₂) was 36 years in NT and 17 years in CT, reflecting the slower C turnover in NT. The effects of NT on the SOC stocks relates to the maintenance of the initial C stocks (higher C_0 remais), while increments in C_crop are imparted mainly by crop additions.     

Relationships between rainfall characteristics and ephemeral gully erosion in a cultivated catchment in Sicily (Italy)

Recent research has shown a lack of long-term monitoring for detailed analysis of gully erosion response to climate characteristics. Measures carried out from 1995 to 2007 in a wheat-cultivated area in Raddusa (Sicily, Italy), represent one of the longest series of field data on ephemeral gully, EG, erosion. The data set collected in a surface area of almost 80 ha, permits analysis of the influence of rainfall on EG formation and development. Ephemeral gullies formed in the study area were measured on a yearly scale with a Post-Processing Differential GPS for length and with a steel tape for the width and depth of transversal sections. Ephemeral gully formation was observed for 8 years out of 12, which corresponds to a return period of 1.5 years. The measurements show strong temporal variability in EG erosion, in agreement with the rainfall characteristics. The total eroded volumes ranged between 0 and ca. 800 m³ year⁻¹, with a mean of ca. 420 m³ year⁻¹, corresponding to ca. 0.6 kg m⁻² year⁻¹. Ephemeral gully erosion in the study area is directly and mainly controlled by rainfall events. An antecedent rainfall index, the maximum value of 3-days rainfall (H_max3_d), is the rain parameter which best accounts for EG erosion. This index is used here as a simple surrogate for soil water content. An H_max3_d threshold of 51 mm was observed for EG formation. The return period of the H_max3_d threshold is almost the same as the return period for EG formation. Although a mean of seven erosive rain events were recorded in a year, EG formation and development generally occur during a single erosive event, similarly to other semiarid environments. The most critical period is that comprised between October and January, when the soil is wetter and the vegetation cover is scarce. Empirical models for EG eroded volume estimation were obtained using the data set collected at this site. A simple power-type equation is proposed to estimate the eroded volumes using H_max3_d as an independent variable. This equation shows an R² equal to 0.67 and a standard error of estimation of 0.79.     

Estimating the rate and consequences of gully development,a case study of umbulo catchment in Southern Ethiopia

In southern Ethiopia wide and deep gullies are common features, significantly affecting farmers' livelihoods. Little action is being taken to address the problem despite farmers being knowledgeable about erosion and its control measures. A study was conduced to investigate the nature and rate of gully development in Umbulo catchment, southern Ethiopia, based on interviews with farmers and field measurements. The results indicated a rapid, down slope development of gullies over the last 30 years with average rate of soil loss from 11 to 30 t ha⁻¹ y⁻¹. The critical period of development of the gully system was estimated to be between 1974 and 1985, but since then the gully system has enlarged both in depth and width. Wide channels with deep, nearly vertical walls are typical. The trigger for gully initiation is believed to have been human, but the soil properties may have played an important role. Under the current situation agricultural production is unsustainable unless the gullies are controlled using integrated measures at the catchment scale. Copyright © 2008 John Wiley & Sons, Ltd.     

Ex-post evaluation of erosion control measures in southern Mali

As part of an impact study of a soil and water conservation (SWC) project in southern Mali, the effect of erosion control measures on soil erosion was evaluated. In one village, a baseline situation from 1988 was compared with the situation in 2003, after farmers had installed stone rows, live fences and grass strips, and had started cultivating across to the slope. This comparison showed a spectacular decrease in gully volume in cultivated fields of 87%, from 58 to 8 m³ ha⁻¹. Estimated annual soil loss decreased with 77% from 42 to 10 t ha⁻¹ year⁻¹. However, baseline data on erosion gullies were not available for other villages. In the absence of baseline data, a simple ‘with–without’ comparison does not allow a correct evaluation because farmers install erosion control measures especially in fields with more erosion. Two alternative methods were used: a reconstructed baseline and a virtual time series. Using the reconstructed baseline, looking not only at active gullies but also at (partly) reclaimed gullies, we concluded that line interventions, gully interventions, and a combination of both, reduced the proportion of active gullies by 48%, 47% and 70%, respectively. Using a virtual time series, comparing erosion in fields with erosion control measures installed in different years, we concluded that erosion gradually decreased by 50% during the 3 years after installation of erosion control measures. In a separate study, we found a positive effect of gully interventions and sowing across to the slope in reducing sheet erosion. A reduction of the slope of the sowing direction by 1% reduced the cover of runoff deposit of coarse sand with 8%. Although a documented baseline is preferred, both a reconstructed baseline and a virtual time series are useful tools and make ex-post evaluations more relevant than a simple with–without comparison.     

Soil salinity and its distribution determined by soil sampling and electromagnetic techniques

Abstract. Diagnosis of soil salinity and its spatial variability is required to establish control measures in irrigated agriculture. This article shows the usefulness of electromagnetic (EM) and soil sampling techniques to map salinity. We analysed the salinity of a 1‐ha plot of surface‐irrigated olive plantation in Aragon, NE Spain, by measuring the electrical conductivity of the saturation extract (EC_e) of soil samples taken at 22 points, and by reading the Geonics EM38 sensor at 141 points in the horizontal (EM_H) and vertical (EM_V) dipole positions. EM_H and EM_V values had asymmetrical bimodal distributions, with most readings in the non‐saline range and a sharp transition to relatively high readings. Most salinity profiles were uniform (i.e. EM_H=EM_V), except in areas with high salinity and concurrent shallow water tables, where the profiles were inverted as shown by EM_H > EM_V, and by EC_e being greater in shallow than in deeper layers. The regressions of EC_e on EM readings predicted EC_e with R² > 84% for the 0–100 to 0–150 cm soil depths. We then produced salinity contour maps from the 141 EC_e values estimated from the electromagnetic readings and the 22 measured values of EC_e. Owing to the high soil sampling density, the maps were similar (i.e. mean surface‐weighted EC_e values between 3.9 dS m^?1 and 4.2 dS m^?1), although the electromagnetically estimated EC_e improved the mapping of details. Whereas soil sampling is preferred for analysing the vertical distribution of soil salinity, the electromagnetic sensor is ideal for mapping the lateral variability of soil salinity.     

Historical evolution of permanent gullies in the Myjava Hill Land, Slovakia

The Myjava Hill Land is part of the area with the highest density of permanent gullies in Slovakia (up to 11 km km⁻²). Gullies are often 10–15 m deep, and occasionally extend >20 m. The pattern and density of gullies have been controlled primarily by the original land use from the pre-collectivization era. Old maps and historical sources indicate two to three periods of gully formation. The central part of the area, settled between the second half of the 16th and the beginning of the 19th centuries, was affected by gully formation in two periods, the first between the end of the 16th century and the 1730s and the second roughly between the 1780s and 1840s. Marginal parts of the study area to the west and south–west, which were settled earlier, may have been affected by disastrous gullying as early as the 14th century. Gullies were formed during periods of extensive forest clearance and expansion of farmland, but the triggering mechanism of gullying was extreme rainfalls during the Little Ice Age. However, periods of gully formation in the Myjava Hill Land were not fully synchronous with the well-known phases of gullying in other central European countries.     

Development of gullies and sediment fans in last-glacial areas on the example of the Suwałki Lakeland (NE Poland)

The density of gully network in the Suwa?ki Lakeland (northeastern Poland) with typical last-glaciation relief is 0.2 km/km² on average and locally reaches 1.2 km/km². Most gullies are isolated but sporadically they create dendritic patterns. The larger gullies are developed along dellies (bowl-shaped, dry valleys) or melt-out valleys. The smaller and shorter gullies occur on the slopes of melt-out depressions and tunnel valleys. Ages of peat covered by fans at the mouths of larger gullies indicate that gully erosion started between 3520 ± 70 to 2240 ± 100 BP. Two different units build the fans and infilled the gullies. The older unit in the lower part of fans is up to 5 m thick, contains sand and gravel that generally originated from the bottom and bank erosion of the gullies and resembles alluvium. The younger unit, about 2–3 m thick, consists of colluvium. The fans at the mouths of smaller and shorter gullies are mainly built of colluvium. The maximal grain diameter in both units is similar, which testifies to a similar intensity of extreme rainfalls. The analysed sediments have different characteristics, which indicate that the source material and depositional changes are linked to forest clearance and farmland expansion starting in the 7th century AD and continuing in the Middle Ages.     

Using high-resolution aerial images to study gully development at the regional scale in southern China

There are 239,100 gullies in southern China, which cause the degradation of ecological system. However, studies on gully development at the regional scale is relatively lack. The purpose of this study is to analyze the regional gully dynamics and explore how land use and topographic factors affect gully development. Input data consists of land use maps derived from high-resolution images from 2004 to 2014, and topography maps from a digital elevation model with a pixel size of 8 m × 8 m in two 25-km² study sites of Anxi (AX) and Xingning county (XN). The following results were obtained: (1) AX gullies decreased from 2006 to 2014 while XN first increased from 2004 to 2009, and then decreased from 2009 to 2014. Both AX and XN gully area in percentage of the total area ranged from 1% to 3%, which was higher than the average 0.25% of the whole southern China by the artificial survey in 2005. (2) Most of AX and XN gullies occurred along the ridgeline and had the close relationships with the upstream contributing area and slope gradient. (3) New gullies developed on the exposed land and forestland. The lost gullies were converted into forestland and grassland. In fact, most of these gullies were only covered by vegetation, not real disappearance. These results proved that gully decreased, but construction of roads and buildings intensified gully development. Consequently, these findings reveal that vegetation protection and ecological restoration should be adopted in southern China.     

黄土丘陵区典型沟道侵蚀诱发的CO2通量估算

以黄土丘陵区典型侵蚀沟道为对象,基于沟道剖面有机碳和¹³⁷Cs数据,采用碳库重分布模型估算了典型沟道侵蚀诱发的CO₂通量,并通过检验模型预测效率、解析影响因子,提出了模型校正的思路。结果表明:(1)在长期侵蚀作用下,沟道侵蚀区和沉积区均表现为剧烈的侵蚀效应,侵蚀区侵蚀速率介于30.99~46.44 mm/a,沉积区侵蚀速率介于34.20~37.88 mm/a,沉积区土壤流失速率略小于侵蚀区;(2)碳库重分布模型估算显示,侵蚀区与沉积区均表现为较强烈的碳源效应,侵蚀区CO₂通量介于18.41~28.44 g/(m²·a),沉积区CO₂通量介于22.19~29.25 g/(m²·a);(3)侵蚀部位、土壤容重、有机碳含量、侵蚀量、沟道平均坡度、植被地上部与地下部生物量共同解释了碳库重分布模型预测效率的变异特征(R²=0.68),其中侵蚀部位、侵蚀量、有机碳含量、土壤容重、植被地下部对预测效率有强驱动效应;(4)引入被忽略的植被新输入有机碳库参数,有望校正碳库重分布模型,提升模型预测效率。该研究结果明确了碳库重分布模型在沟道侵蚀区相比沉积区有更高的CO₂通量预测效率,为进一步提高模型的预测精度,可以考虑引入植被输入有机碳库作为校正参数。