ASSESSING THE PERFORMANCE OF A SPATIALLY DISTRIBUTED SOIL EROSION AND SEDIMENT DELIVERY MODEL (WATEM/SEDEM) IN NORTHERN ETHIOPIA

Most regional‐scale soil erosion models are spatially lumped and hence have limited application to practical problems such as the evaluation of the spatial variability of soil erosion and sediment delivery within a catchment. Therefore, the objectives of this study were as follows: (i) to calibrate and assess the performance of a spatially distributed WATEM/SEDEM model in predicting absolute sediment yield and specific sediment yield from 12 catchments in Tigray (Ethiopia) by using two different sediment transport capacity equations (original and modified) and (ii) to assess the performance of WATEM/SEDEM for the identification of critical sediment source areas needed for targeting catchment management. The performance of the two model versions for sediment yield was found promising for the 12 catchments. For both versions, model performance for the nine catchments with limited gully erosion was clearly better than the performance obtained when including the three catchments with significant gully erosion. Moreover, there is no significant difference (alpha 5 per cent) between the performances of the two model versions. Cultivated lands were found to be on average five times more prone to erosion than bush–shrub lands. The predicted soil loss values in most parts of Gindae catchment are generally high as compared with the soil formation rates. This emphasises the importance of implementing appropriate soil and water conservation measures in critical sediment source areas prioritising the steepest part of the catchment (i.e. areas with slope >50 per cent). The applicability of the WATEM/SEDEM model to environments where gully erosion is important requires the incorporation of permanent gully and bank gully erosion in the model structure. Copyright © 2011 John Wiley & Sons, Ltd.     

Thresholds for channel initiation at road drain outlets

Slope–Area relationships have been widely used to identify channel initiation points in the landscape. More recently, the importance of road-related rill and gully erosion has been highlighted. Previous studies indicate that channel initiation at road drain outlets can be predicted from the contributing road area and hillslope gradient at the drain outlet. In this study the validity and usefulness of channel initiation thresholds for road runoff management has been evaluated using field data from three catchments in south-east Australia. None of these catchments show a clear separation between channelised and non-channelised sites based on contributing road area and hillslope gradient. Many drains without channel have similar contributing road area and slope gradients as drain with a channel at the outlet. This demonstrates a relatively large variability within our dataset and limits the identification of a slope–area threshold for these catchments. Logistic regression using road area and slope gradient shows that both variables contribute significantly to channel initiation in the Albert and Tyers River catchments, while for the Sandy Creek catchment only slope gradient is significant at the 0.05 level. Stepwise logistic regression on a wider range of variables shows the significance of cut-batter height in two of the catchments, indicating that flow interception at cut-and-fill roads may be important. Comparison of road-related channel initiation thresholds to previously published topographic thresholds, indicates that channels at road outlets tend to form at relatively small contributing areas and steep slopes. Current forest management guidelines do not consider the risk of channel initiation at road drain outlets in road design and drain spacing. The slope–area thresholds are still considered useful in minimising the risk of gully erosion, especially in forestry areas where it appears most likely that channel initiation occurs during a significant rainfall event immediately following hillslope disturbance or road construction.     

7Be示踪坡耕地次降雨细沟与细沟间侵蚀

对于坡面细沟与细沟间侵蚀过程的了解是建立侵蚀预报模型的基础,但传统方法难以对其进行深入研究。利用7Be示踪技术并结合人工模拟降雨,考虑坡脚沉积作用,研究了25°坡耕地径流小区次降雨过程中细沟与细沟间侵蚀动态。结果表明：根据流出径流小区泥沙7Be含量变化计算坡面明显细沟出现时间,由于坡脚沉积作用使得A、B两试验小区这一时间比实际细沟出现分别延迟了45 min和11 min;根据坡面-侵蚀泥沙中7Be总量守恒和泥沙质量平衡原理,坡面细沟间侵蚀及细沟侵蚀在坡面总侵蚀、坡脚沉积区泥沙及流出径流小区泥沙中的比例被定量区分开;总体上,细沟间侵蚀量在径流泥沙中的比例逐渐减少,而细沟侵蚀量逐渐增加。两试验小区中7Be示踪计算坡面细沟侵蚀量和坡脚沉积量与实测值相比相对误差均较小,因此7Be示踪技术可以对土壤侵蚀进行较为准确地定量研究。     

Improving precision in sediment source and erosion process distinction in an upland catchment,south-eastern Australia

Analysis of sediment sources is an important component in the development of catchment sediment budgets and in determining links between erosion from sources and sediment delivery to catchment outlets. In this study ¹³⁷Cs and ²¹⁰Pb_ex were used to determine surface and sub-surface source contributions of fine sediment in a small upland headwater catchment (1.6 km²) in south-eastern Australia. The findings from this analysis are employed in an adjustment procedure to better differentiate sediment source erosion processes by utilising channel survey and erosion pin data. This improved the precision of estimates of sediment-source erosion-process contributions from hillslopes and channel/gully walls. A mean of 74% of in-channel deposits and suspended sediment exiting the study catchment was derived from sub-surface sources and when adjusted for erosion process this increased to 81%, which may be attributed to channel and gully wall erosion alone. Net erosion of the channel floor was low and constitutes only a small part of the total channel source input to sediment flux. Variability in sediment source contributions within the catchment was high, with rapid transition from hillslope to channel source dominance of sediment flux with distance downstream in the study catchment.     

Fingerprinting sediment sources in the outlet reservoir of a hilly cultivated catchment in Tunisia

Purpose

Approximately 74 % of agricultural soils in Tunisia are affected by water erosion, leading to the siltation of numerous human-made reservoirs and therefore a loss of water storage capacity. The objective of this study was to propose a methodology for estimating the relative contributions of gully/channel bank erosion and surface topsoil erosion to the sediment accumulated in small reservoirs.

Materials and methods

We tested an approach based on the sediment fingerprinting technique for sediments collected from a reservoir (which has been in operation since 1994) at the outlet of a catchment (Kamech, 2.63 km²). Sampling concentrated on the soil surface (in both cropland and grassland), gullies and channel banks. A total of 17 sediment cores were collected along a longitudinal transect of the Kamech reservoir to investigate the origin of the sediment throughout the reservoir. Radionuclides (particularly caesium-137, ¹³⁷Cs) and nutrients (total phosphorus, total nitrogen and total organic carbon (TOC)) were analysed as potential tracers.

Results and discussion

The applications of a mixing model with ¹³⁷Cs alone or ¹³⁷Cs and TOC provided very similar results: The dominant source of sediment was surface erosion, which was responsible for 80 % of the total erosion within the Kamech catchment. Additionally, we showed that the analysis of a single composite core provided information on the sediment origin that was consistent with the analysis of all sediment layers in the core. We demonstrated the importance of the core sampling location within the reservoir for obtaining reliable information regarding sediment sources and the dominant erosion processes.

Conclusions

The dominance of surface erosion processes indicates that conservation farming practices are required to mitigate erosion in the agricultural Kamech catchment. Based on the results from 17 sediment cores, guidelines regarding the number and location of sampling cores to be collected for sediment fingerprinting are proposed. We showed that the collection of two cores limited the sediment source apportionment uncertainty due to the core sampling scheme to <10 %.     

Evaluating gully erosion using Cs and Pb/Cs ratio in a reservoir catchment

Water erosion in the hilly areas of west China is the main process contributing to the overall sediment of the Yellow River and the Yangtze River. The impact of gully erosion in total sediment output has been mostly neglected. Our objective was to assess the sediment production and sediment sources at both the hillslope and catchment scales in the Yangjuangou reservoir catchment of the Chinese Loess Plateau, northwest China. Distribution patterns in sediment production caused by water erosion on hills and gully slopes under different land use types were assessed using the fallout ¹³⁷Cs technique. The total sediment production from the catchment was estimated by using the sediment record in a reservoir. Sediment sources and dominant water erosion processes were determined by comparing ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in surface soils and sub-surface soils with those of sediment deposits from the reservoir at the outlet of the catchment. Results indicated that landscape location had the most significant impact on sediment production for cultivated hillslopes, followed by the terraced hillslope, and the least for the vegetated hillslope. Sediment production increased in the following order: top>upper>lower>middle for the cultivated hillslope, and top>lower>upper>middle for the terraced hillslope. The mean value of sediment production declined by 49% for the terraced hillslope and by 80% for the vegetated hillslope compared with the cultivated hillslope. Vegetated gully slope reduced the sediment production by 38% compared with the cultivated gully slope. These data demonstrate the effectiveness of terracing and perennial vegetation cover in controlling sediment delivery at a hillslope scale. Averaged ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in the 0–5 cm surface soil (2.22–4.70 Bq kg⁻¹ and 20.70–22.07, respectively) and in the 5–30 cm subsoil (2.60 Bq kg⁻¹ and 28.57, respectively) on the cultivated hills and gully slopes were close to those of the deposited sediment in the reservoir (3.37 Bq kg⁻¹ and 29.08, respectively). These results suggest that the main sediment sources in the catchment were from the surface soil and subsoil on the cultivated slopes, and that gully erosion is the dominant water erosion process contributing sediment in the study area. Changes in land use types can greatly affect sediment production from gully erosion. An increase in grassland and forestland by 42%, and a corresponding decrease in farmland by 46%, reduced sediment production by 31% in the catchment.     

东北黑土区土壤侵蚀研究进展与展望

东北黑土区是我国重要的商品粮生产基地,强烈水土流失导致的黑土厚度下降及土壤质量退化,直接威胁国家粮食安全。系统分析黑土区土壤侵蚀研究成果,对阻控黑土退化、维持土地生产力、保障国家粮食安全具有重要意义,论述了黑土区土壤侵蚀环境与侵蚀环境效应、土壤侵蚀过程与机理、土壤侵蚀时空变化和未来研究展望。多营力复合、缓坡长坡是黑土区独特的侵蚀环境,黑土厚度下降与坡耕地土壤质量退化是该区最典型的侵蚀环境效应。细沟间、细沟、浅沟、切沟及冻融侵蚀发育的动力机制与主控因素差异明显,输沙耗能驱动的泥沙输移对土壤分离的反馈效应是侵蚀强度沿坡长呈强弱交替的根本原因,浅沟和切沟主要发育在坡耕地,侵蚀强度受耕作方式等人类活动的显著影响。冻融侵蚀强度显著小于水力侵蚀,但可通过降低土壤抗蚀性能和促进坡面径流及壤中流发育影响水力侵蚀。土壤侵蚀类型与强度在小流域尺度上沿坡面呈明显的垂直分带特征,在区域尺度上呈南北与东西递变的纬度和经度地带性。全球气候变化可能导致黑土区水力侵蚀加强、冻融侵蚀减弱。未来亟需加强黑土厚度及土地生产力对侵蚀响应机理、复合侵蚀动力过程与耦合机制、典型侵蚀类型时空变化与主控因素等方面的研究。     

Soil resistance to interrill erosion: Model parameterization and sensitivity

Interrill erosion, which is less visible in the landscape than rill and gully erosion, may cause major sediment deposits in the lower part of cultivated fields. It is often associated with runoff resulting from sealing and crusting, and soil properties such as soil detachability or soil aggregate stability have been used to express soil resistance to interrill erosion processes, i.e., interrill erodibility. From a literature review including more than fifteen erosion models, we have identified three main methods used to measure these properties: aggregate stability and splash cup detachability, methods performed in the laboratory using only a few grams of soil, and standard plot methods that are based on field plot measurements. This difference makes the parameters involved in assessing interrill erodibility dependent upon the scale and the hydrological processes involved and difficult to compare. According to the literature, the sensitivity of actual erosion models to interrill erodibility is lower than the sensitivity to hydrological properties and rill erodibility parameters. This numerical study shows that erodibility measurements from the three major assessment methods give different results regarding the contribution of interrill erosion and show that the sensitivity of erosion modeling to interrill erodibility may in fact be greater than shown in the literature on global sensitivity analysis.     

A strategy for fine sediment retention with bioengineering works in eroded marly catchments in a mountainous mediterranean climate (Southern ALPS,France)

In eroded catchments in mountainous and Mediterranean‐type ecosystems, erosion can be responsible for high yields of fine sediment at the exit of catchments, which can cause different types of damage such as reservoir silting or ecological pollution. To avoid this sediment transport, low‐cost rehabilitation actions such as involving bioengineering techniques within small gullies can be used to trap and retain eroded materials before they reach the catchment exit. Based on current results of research carried out in the French Southern Alps, a specific strategy for such interventions is proposed, with rules for bioengineering structure use and methods for determining priority actions. This strategy for fine sediment retention with bioengineering works is currently being applied on eroded marly catchments in the French Southern Alps, in a mountainous Mediterranean climate. Copyright © 2009 John Wiley & Sons, Ltd.     

黄土坡面侵蚀方式演变与侵蚀产沙过程试验研究

利用自行设计的双土槽径流小区系统和人工模拟降雨试验研究方法,研究了坡面侵蚀方式演变对坡面侵蚀产沙量的影响,分析了沟蚀发育不同阶段对坡面侵蚀产沙的贡献。结果表明,坡面侵蚀发育初期,沟头溯源侵蚀占主导地位,侵蚀产沙量呈增加趋势;坡面侵蚀发育中期,沟蚀的宽度和深度变化已属于切沟形态发育的范围,导致了侵蚀产沙量的急剧增加;坡面侵蚀发育后期,沟蚀发育过程处于后期稳定阶段,侵蚀产沙量变化伏动较小。在坡面侵蚀发育初期、中期和后期,沟蚀分别占坡面总侵蚀产沙量的28．7％,70．4％和37．1％。     

黄土坡面侵蚀方式演变与侵蚀产沙过程试验研究

利用自行设计的双土槽径流小区系统和人工模拟降雨试验研究方法,研究了坡面侵蚀方式演变对坡面侵蚀产沙量的影响,分析了沟蚀发育不同阶段对坡面侵蚀产沙的贡献。结果表明,坡面侵蚀发育初期,沟头溯源侵蚀占主导地位,侵蚀产沙量呈增加趋势;坡面侵蚀发育中期,沟蚀的宽度和深度变化已属于切沟形态发育的范围,导致了侵蚀产沙量的急剧增加;坡面侵蚀发育后期,沟蚀发育过程处于后期稳定阶段,侵蚀产沙量变化伏动较小。在坡面侵蚀发育初期、中期和后期,沟蚀分别占坡面总侵蚀产沙量的28.7%,70.4%和37.1%。     

黄土丘陵沟壑区小流域水蚀预报模型构建

根据黄土丘陵沟壑区侵蚀产沙垂直分带性规律,提出小流域土壤侵蚀方式发生部位界定方法。基于黄土丘陵沟壑区侵蚀环境特点,设计了GIS支持下的小流域分布式水蚀预报模型结构,模型考虑的侵蚀过程包括雨滴击溅侵蚀、片蚀、细沟侵蚀、浅沟侵蚀、切沟侵蚀以及沟道侵蚀。同时。提出模型各模块的计算方法,根据动态物质平衡原理。完成了流域侵蚀产沙过程演算。     

USE OF RESERVOIR DEPOSITS AND CAESIUM-137 MEASUREMENTS TO INVESTIGATE THE EROSIONAL RESPONSE OF A SMALL DRAINAGE BASIN IN THE ROLLING LOESS PLATEAU REGION OF CHINA

This paper reports the results of an investigation of the erosional response of the 3·86 km² Zhaojia Gully catchment in the rolling loess plateau region of Zichan County, Shaanxi Province, China. In the absence of direct measurements, information derived from reservoir deposits and from caesium-137 measurements on both the catchment soils and reservoir deposits was used to obtain a retrospective assessment of the longer-term (ca. 30 year) erosional response of the catchment and of the relative contributions of the rolling plateau surface and the gully areas to the sediment output from the basin. Net erosion rates on cultivated land occupying the gentle crest slopes and steeper lower slopes of the rolling plateau and the steep gully slopes were estimated to be 4500 t km⁻² year⁻¹, 8584 t km⁻² year⁻¹ and 15851 t km⁻² year⁻¹, respectively. Estimates of annual sediment yield from the study catchment based on analysis of sediment deposits in the two sediment-trap reservoirs ranged between 4627 and 32472 t km⁻² year⁻¹. Almost all the sediment transported from the catchment was contributed by 2–4 large floods each year. Measurements of the caesium-137 content of recent sediment deposits in a sediment-trap reservoir allowed the relative contributions of the total sediment yield derived from the rolling plateau and gully areas of the catchment to be estimated at 23 and 77 per cent, respectively. Analysis of the sediment deposits dating from 1973–1977 in another sediment trap reservoir allowed individual flood event couplets to be identified and indicated that the sediment associated with the first one or two floods in a season, when the soils of the plateau area were relatively dry, was derived primarily from the gully areas. The cultivated soils of the rolling plateau contributed an increased proportion of the total sediment yield during the latter stages of the flood season when the soils were wetter, and surface runoff and erosion were more widespread. Based on analysis of the caesium-137 content of the sediment deposited in this sediment-trap reservoir, the relative contributions of sediment from the rolling plateau and gully areas over the period 1973–1977 were estimated to be 21 and 79 per cent, respectively. The results obtained demonstrate the potential for using caesium-137 measurements and analysis of reservoir deposits to document the erosional response of a drainage basin. © 1997 John Wiley & Sons, Ltd.     

Hydrological structure and erosion damage caused by concentrated flow in cultivated catchments

This study analyses the between-catchment variability of rill volumes produced by concentrated flow erosion during winter in the northern part of the Paris Basin. The working hypotheses were that (i) runoff concentrates along channels determined by topography or by agricultural practices; (ii) rill length is a major component of rill volume variability on a catchment scale; (iii) rill cross-sectional areas are controlled by the size of upslope runoff-contributing areas connected to the corresponding channels. Two samples, one of 20 zero order catchments and the other of 15 catchments, were surveyed. For each catchment, the runoff collector network was modelled from topographical and agricultural information, and split into homogeneous segments. Each segment was characterized by its slope gradient (SL), the soil susceptibility to rill erosion (SSE) and the size of the upslope runoff-contributing areas (RCA) connected to it. These areas were identified by the structural state of their soil surface. The frequency of rill occurrence was highly correlated with RCA and SL. The rill cross-sectional areas of eroded channels were correlated with RCA, SL and SSE. Catchment erosion rates were estimated by adding together the predicted rill volumes for each segment within the catchment. These estimations were closely correlated with observed rill erosion rates. The relative spatial position of runoff collectors must be taken into account when examining the damage caused by concentrated flow erosion.     

Comparison between rainfall simulator erosion and observed reservoir sedimentation in an erosion-sensitive semiarid catchment

Estimating catchment scale soil loss based on rainfall simulators is often hampered by the difficulty to scale up simulator results. Our objective was to develop and test a method for estimating catchment scale soil loss based on observed rainfall using a variable intensity rainfall simulator in an erosion-sensitive catchment in semiarid Tunisia. A 7-year period, 1992–1999, with observed sedimentation amounts in a downstream reservoir was chosen to test a methodology. The methodology was based on (1) energy adjustment for the used simulator due to the difference in kinetic energy of simulated and natural rainfall at equal intensities and (2) upscaling of simulated erosion in which rill erosion was estimated by adjusting the difference between slope lengths for the plots versus the catchment after onset of runoff. The comparison between calculated soil loss from rainfall simulator experiments and observed sedimentation in the downstream reservoir displayed good overall results. Calculated soil loss was found to be about 96%, 36%, and 80% for different observed subperiods, respectively. The observed low value for the second period was probably due an exceptionally intense rainfall event during this period, which appears to have led to gully erosion, soil slide, and riverbank collapse. Therefore, during this event, siltation in the reservoir may essentially be due to unaccounted erosion processes such as gully erosion. Overall, however, it appears that plot-scale variable intensity rainfall simulators can rather successfully estimate catchment scale soil losses.     

黄土区坡耕地细沟间侵蚀和细沟侵蚀的研究

利用人工模拟降雨试验,通过在径流小区上覆盖纱网消除雨滴动能和增加雨滴除落高度来增加雨滴功能,并采用翻耕裸露作对照的试验处理,对黄土区坡耕地细沟间侵蚀和细沟侵蚀过程及其机理进行了研究。结果表明,坡面侵蚀产沙过程可明显的分为四个阶段,即溅蚀,细沟间侵蚀、细沟侵蚀和雨后径流侵蚀阶段。     

Validating the Use of 137Cs Measurements to Derive the Slope Component of the Sediment Budget of a Small Rangeland Catchment in Southern Italy

The sediment budget is a key concept and tool for characterizing the mobilization, transfer and storage of fine sediment within a catchment. Caesium‐137 measurements can provide valuable information on gross and net erosion rates associated with sheet and rill erosion that can be used to establish the slope component of a catchment sediment budget. However, there is a need to validate the use of ¹³⁷Cs measurements for this purpose, because their reliability has sometimes been questioned. The study reported focuses on a small (3·04 ha) steepland (mean slope 37%) catchment in Southern Italy. It exploits the availability of information on the medium‐term sediment output from the catchment provided by the construction of a reservoir at its outlet in 1978 and the existence of estimates of soil redistribution rates derived from ¹³⁷Cs measurements made on 68 replicate soil cores collected from the slopes of a substantial proportion of the catchment in 2001, to validate the use of ¹³⁷Cs measurements to construct the slope component of the catchment sediment budget. An additional 50 replicate soil cores were collected from the catchment slopes for ¹³⁷Cs analysis, to complement the data already available. Nine cores collected from the area occupied by the reservoir were used to estimate the mean annual sediment input to the reservoir. In the absence of evidence that the poorly developed channel system in the catchment was either a significant sediment source or sink, it was possible to directly compare the estimate of net soil loss from the catchment slopes (7·33 Mg ha⁻¹ y⁻¹) with the estimate of sediment output from the catchment provided by the reservoir deposits (7·52 Mg ha⁻¹ y⁻¹). Taking account of the uncertainties involved, the close agreement of the two values is seen as providing a convincing validation of the use of ¹³⁷Cs measurements to both estimate soil redistribution rates and as a basis for constructing the slope component of the sediment budget of a small catchment. Copyright © 2015 John Wiley & Sons, Ltd.     

Run-off and sediment yield in relation to differential gully-plugging schemes in micro-catchments of Shiwaliks in the lower Himalayas

Gully-management strategies adopted in the Shiwalik range of hills in the lower Himalayas of India, which involve treatment of only higher order gullies, have not been successful. A study was conducted in the four selected micro-catchments in the region to monitor run-off and sediment yield variations in relation to differential gully-plugging schemes. One of the study catchments (catchment III) with highest run-off and sediment yield was selected for an alternative scheme of gully management. In this, all the first-order and 25–30% of the second-order gullies were plugged with locally available stones, and native grass vegetation wasplanted on the upstream side. Other catchments (catchments I, II and IV) were treated as per the the conventional method of gully plugging involving the installation of loose stone check dams in the highest order gullies. The highest run-off coefficient (15.5%) for the period 2000–2006 was registered in catchment III. However, this decreased to 12.8% during 2007, when the catchment was treated using an alternative gully-management scheme. The sediment yield from catchment III decreased by 20% in 2007 from 15.2 t ha^?1 during 2000–2006. The results indicate that to control gully erosion, lower order gullies should be treated as a priority.     

Establishing sediment budgets for two small lowland agricultural catchments in the UK

Sediment budgets have been established for two small (<4 km²), lowland, agricultural catchments, by using ¹³⁷Cs measurements, sediment source fingerprinting and more traditional monitoring techniques to quantify the individual components of the budget. The gross and net erosion rates for the fields on the catchment slopes were estimated using ¹³⁷Cs measurements within selected fields, which encompassed a representative range of slope angles, slope lengths and land use. These estimates were extrapolated over the entire catchment, using a simple topographically driven soil erosion model (Terrain-Based GIS, TBGIS) superimposed on a DEM, to derive catchment average gross and net erosion rates. Suspended sediment yields were measured at the catchment outlets and sediment source fingerprinting techniques were used to establish the relative contributions from the catchment surface, subsurface tile drains and eroding channel banks to the sediment yields. In-channel and wetland storage were quantified using both direct measurements and ¹³⁷Cs measurements. The sediment budgets established for the catchments highlighted the importance of subsurface tile drains as a pathway for sediment transfer, accounting for ca. 60% and 30% of the sediment output from the two catchments. Erosion from channel banks contributed ca. 10% and 6% of the sediment output from the two catchments. Although the suspended sediment yields from these catchments were considered high by UK standards (ca. 90 t km⁻² year⁻¹), the sediment delivery ratios ranged between 14% and 27%, indicating that a major proportion of the mobilised sediment was stored within the catchments. In-field and field-to-channel storage were shown to be of similar magnitude, but storage of sediment in the channel system and associated wetlands was relatively small, representing <5% of the annual suspended sediment yield.     

Gully erosion and environmental change: importance and research needs

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales.Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates.In this respect, various research questions can be identified. The most important ones are:

What is the contribution of gully erosion to overall soil loss and sediment production at various temporal and spatial scales and under different climatic and land use conditions?

What are appropriate measuring techniques for monitoring and experimental studies of the initiation and development of various gully types at various temporal and spatial scales?

Can we identify critical thresholds for the initiation, development and infilling of gullies in different environments in terms of flow hydraulics, rain, topography, soils and land use?

How does gully erosion interact with hydrological processes as well as with other soil degradation processes?

What are appropriate models of gully erosion, capable of predicting (a) erosion rates at various temporal and spatial scales and (b) the impact of gully development on hydrology, sediment yield and landscape evolution?

What are efficient gully prevention and gully control measures? What can be learned from failures and successes of gully erosion control programmes?

These questions need to be answered first if we want to improve our insights into the impacts of environmental change on gully erosion. This paper highlights some of these issues by reviewing recent examples taken from various environments.