Ephemeral Gully Research Articles

Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion

Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion

Generation, transport and preservation of armoured mudballs in an ephemeral gully system

Armoured mudballs have been reported from modern fluvial catchments and preserved in ancient (Quaternary and older) sedimentary sequences. Despite many descriptions of mudballs in the literature there is little systematic analysis of their physical properties, genesis, transport and deposition within alluvial sequences. This study uses two adjacent catchments developed on identical geology in southeast Spain that offer a unique opportunity to examine the key controls on armoured mudball development as one catchment produces armoured mudball whilst the second does not. Field survey using a Total Station together with laboratory tests for material composition and aggregate stability were used to examine the catchment and the armoured mudballs. The results show that the generation of the armoured mudballs is heavily dependent on the correct sediment supply (suitable aggregates to act as mudball ‘seeds’, and suitable fragments to source the armour) and good slope-channel coupling within the catchment. Coupling is facilitated by steep gully sides which deliver the weathered blocks of marl (mudball ‘seeds’) to the gully floor. The ‘seeds’ form the core of the mudballs in the ’mudball factory’ zone. If the ‘seeds’ are subjected to sealing by moisture from rainfall the mudball will hold together through negative pore pressures and can be transported by the ensuing gully flow. As the mudball rolls it picks up a surface armour which becomes embedded into the mudball. With longer transport paths the turbulent nature of the flow creates spherical mudballs which are size sorted downstream by the hydraulics of the flow rather than attrition. Armoured mudball deposition occurs within unit braid bars in areas of waning flow (due to transmission losses), within vegetated, point or mid-channel bars. The armoured mudballs are preferentially deposited within the coarser bar head of these depositional units, with smaller marl fragments forming the finer bar tail. Trenching of the bars reveals that the armoured mudballs are preserved in the subsurface although they may become increasingly flattened upon burial at depth, and may be difficult to recognise due to disaggregation by roots. This study suggests that armoured mudballs recognised in ancient alluvial sequences imply seasonality of climate and good slope-channel coupling within the catchment area, and that they are capable of transporting delicate flora undamaged, making them good targets for pollen recovery in ancient alluvial sequences. Where armoured mudballs were present but go unrecognised in the ancient record the mode and energy of transportation and deposition of the fine sediment may be misinterpreted as the fines are in fact being transported as bedload rather than suspended load. The armoured mudballs have also been noted to transport pristine microfauna such as ostracod shells from the source area material. This may provide challenges to palaeoenvironmental interpretations of the mud-dominated unit as environmentally significant fauna may not be contemporaneous with the unit.

Development of gullies and sediment production in the black soil region of northeastern China

Gully erosion is an important environmental hazard in the black soil region of northeastern China. It is a primary sediment source in the region which needs appropriate soil conservation practices. Gully incision in rolling hills typical of this region was monitored using real-time kinematic GPS to assess the rates of gully development and the resultant sediment production. From 2002 to 2005, gully heads in the study area retreated between 15.4 and 33.5 m, giving an average retreat rate of 8.4 m yr − 1 . Field measurements showed that total sediment production due to gully erosion during the three years ranged between 257 and 1854 m 3 yr − 1 , which is equivalent to 326 to 2355 t yr − 1 , with gully-head retreat accounting for 0 to 21.7% (4.4% in average). The sediment delivery ratio was especially high during the summer rainy season (56% in average). Sediment production by ephemeral gullies and permanent gullies was 1.5 times greater than that from surface erosion. Gully heads retreated faster in the spring freeze–thaw period than in the summer. The stage of gully development could be identified based on short-term changes in the gully erosion rate.

A grassed waterway and earthen dams to control muddy floods from a cultivated catchment of the Belgian loess belt

Muddy floods, i.e. runoff from cultivated areas carrying large quantities of soil, are frequent and widespread in the European loess belt. They are mainly generated in dry zero-order valleys and are nowadays considered as the most likely process transferring material eroded from Cultivated hillslopes during the Holocene to the flood plain. The huge Costs Of Muddy flood damages justify the urgent installation of control measures. In the framework of the 'Soil Erosion Decree' of the Belgian Flemish region, a 12 ha-grassed waterway and three earthen dams have been installed between 2002-2004 in the thalweg of a 300-ha cultivated dry valley in the Belgian loess belt. The measures served their purpose by preventing any Muddy flood in the downstream village, despite the Occurrence of several extreme rainfall events (with a maximum return period of 150 years). The catchment has been intensively monitored from 2005-2007 and 39 runoff events were recorded in that period. Peak discharge (per ha) was reduced by 69% between the upstream and the downstream extremities of the grassed waterway (GWW). Furthermore, runoff was buffered for 5-12 h behind the dams, and the lag time at the outlet of the catchment was thereby increased by 75%. Reinfiltration was also observed within the waterway, runoff coefficients decreasing by a mean of 50% between both extremities of the GWW. Sediment discharge was also reduced by 93% between the GWW's inflow and the Outlet. Before the installation of the control measures, specific sediment yield (SSY) of the catchment reached 3.5 t ha(-1) yr(-1) and an ephemeral gully was observed nearly each year in the catchment. Since the control measures have been installed, no (ephemeral) gully has developed and the SSY of the catchment dropped to a mean of 0.5 t ha(-1) yr(-1). Hence, sediment transfer from the cultivated dry valley to the alluvial plain should dramatically decrease. Total cost of the control measures that are built fora 20 year-period is very low (126 (sic) ha(-1)) compared to the mean damage cost associated with muddy floods in the study area (54 (sic) ha(-1) yr(-1)). Similar measures should therefore be installed to protect other flooded villages of the Belgian loess belt and comparable environments. (C) 2008 Elsevier B.V. All rights reserved.

Seasonal discrepancy of ephemeral gully erosion in Northeast China with black soils

Ephemeral gully erosion can be a major sediment source serving as main conduits to convey runoff and sediment from most farmland.Ephemeral gully erosion is similar to but differ from both rill and classical gully erosion.But ephemeral gully erosion is often overlooked,and it is also not estimated with rill-interrill erosion prediction models such as the Revised Universal Soil Loss Equation.Soil erosion is increasingly severe in Northeast China with black soils,in which ephemeral gully erosion is an important type of water erosion.However,the existing literature indicated that little research on ephemeral gully erosion has been done in this area.In this paper,ephemeral gully erosion was investigated in spring and summer of 2005 in black soil regions of two small catchments on Heshan Farm of Heilongjiang Province.The aim of the study was to present the characteristics of ephemeral gullies and their seasonal discrepancies and controlling factors.The results show that soil losses due to ephemeral gully erosion fluctuate along the slope,and the high erosivity belts alternate with the low belt.There are also great differences in the morphology,the erosion intensity and controlling factors of ephemeral gully erosion between spring and summer.Ephemeral gully erosion in spring is mainly resulted from snow storm and is significantly influenced by freeze-thaw and snowmelt runoff.Ephemeral gully erosion in spring is slightly severer than that in summer.Ephemeral gully erosion was caused by storm.Compared with that in spring,ephemeral gullies in summer were shorter and shallower,but wider due to the storm characterized by high intensity and low duration.Furthermore,the depth and distribution of ephemeral gully are significantly influenced by tillage practice and crop type,especially in summer.

Ephemeral gully erosion by preferential flow through a discontinuous soil-pipe

Abstract Subsurface flow through soil-pipes can contribute to ephemeral gully erosion but these soil-pipes are cut-off when tillage fills in the gully. The objective was to determine the effect of flow through discontinuous soil-pipes on ephemeral gully erosion. Experiments were conducted on 150 cm long by 100 cm wide soil beds with an artificial soil-pipe at the upper end that extended 50 cm into the soil immediately above a water-restricting horizon. The combination of rainfall with pipe flow produced extensive (63 kg) soil losses by mass wasting. The mode of mass wasting appeared to be sudden, cataclysmic pop-out failures. Total soil loss by sheet erosion with rainfall and pipe flow combined (13.6 kg) was four times higher than by rainfall alone (3.4 kg). Under rainfall with pipe flow, soil loss by mass wasting was nearly 20 times higher than sheet erosion from rainfall alone. Soil conservation practices that treat surface runoff process alone may be ineffective if subsurface flow is contributing.

Development of ephemeral gully during rainy season in the slope land in rolling-hill black-soil region of Northeast China

Ephemeral gully erosion during rainy season in typical black-soil region is very serious. To establish the effective control measures, it is necessary for analyzing the initiation mechanism of ephemeral gully in the slope land and calculating soil and water losses caused by ephemeral gully. Mechanism of initiation of ephemeral gully during rainy season in the slope land in rolling-hill black soil region of Northeast was analyzed on the basis of three-year field surveys and plots observations. Results show that influx of surface runoff from a certain caused by microtopography relief and unreasonable tillage methods is considered as the main reasons that generate ephemeral gully erosion in rolling-hill black-soil region of Northeast. And connectivity of watercourse caused by silted up furrows in the thaweg can lead to formation of runoff and then give rise to incision directly. And soil loss from their channels is less than that from the sideslopes, so the destructive potential of ephemeral gully is underestimated by measuring its volume of channel only. Therefore, researches on ephemeral gully erosion should be strengthened in future.

Characteristics and factors controlling the development of ephemeral gullies in cultivated catchments of black soil region, Northeast China

Ephemeral gully erosion is an important process in the black soil region, Northeast China and can be responsible for severe damage to agricultural lands. However, little research on gully formation in this area has been published. The study described in this paper attempted to quantify soil losses, the spatial distribution and morphology of the gullies, and the factors that control their development. Ephemeral gullies were measured in spring and summer of 2005 in two small catchments. The critical periods for ephemeral gully formation were late spring and summer in the study area. Mean soil losses due to ephemeral gully erosion were 0.40 and 0.43 kg m −2 year −1 for only croplands despite low slope gradients, and this loss is above the tolerable erosion rates of 0.20 kg m −2 year −1. The erosion rates were greater in spring because the topsoil thawed before deeper layers, reducing infiltration into the soil, and the bare vegetation cover provided no barriers to surface flow. In contrast, summer erosion occurred primarily in response to intense rain events. Development of the gullies was promoted by freeze-thaw cycles in spring and was affected by the type of agricultural operations and crops in summer. A linear regression model for the prediction of ephemeral gully length at the catchment level was established using field data, and although it did not successfully predict the length of individual gullies, it explained 55% of the variation in ephemeral gully length.

Simulating Ephemeral Gully Erosion in AnnAGNPS

Ephemeral gully erosion can cause severe soil degradation and contribute significantly to total soil losses in agricultural areas. Physically based prediction technology is necessary to assess the magnitude of these phenomena so that appropriate conservation measures can be implemented, but such technology currently does not exist. To address this issue, a conceptual and numerical framework is presented in which ephemeral gully development, growth, and associated soil losses are simulated within the Annualized Agricultural Non-Point Source (AnnAGNPS) model. This approach incorporates analytic formulations for plunge pool erosion and headcut retreat within single or multiple storm events in unsteady, spatially varied flow at the sub-cell scale, and addresses five soil particle-size classes to predict gully evolution, transport-capacity and transport-limited flows, gully widening, and gully reactivation. Single-event and continuous simulations demonstrate the model's utility for predicting both the initial development of an ephemeral gully and its evolution over multiple runoff events. The model is shown to recreate reasonably well the dimensions of observed ephemeral gullies in Mississippi. The inclusion of ephemeral gully erosion within AnnAGNPS will greatly enhance the model's predictive capabilities and further assist practitioners in the management of agricultural watersheds.

Morphology parameters of ephemeral gully in characteristics hillslopes on the Loess Plateau of China

The Loess Plateau of North China has one of the highest erosion rates in the world. Ephemeral gullies are an important erosion type found on hill slopes of upland areas, and soil loss due to these gullies equals 30–70% of total soil loss. In this study, we used a high-accuracy global positioning system (GPS) to measure the morphology of characteristic ephemeral gullies near Suide, Shaanxi Province. We discuss the geomorphological characteristics of the gullies (their length, the distance between the head of ephemeral gully and the watershed divide, the distance between two neighboring hill slope ephemeral gullies, the critical slope gradient, the upslope drainage area, and the flow erosivity). The length and average distance between gullies both followed a Pearson IV distribution function, but the function parameters differed. The product of upslope drainage area and the square of the critical slope gradient ranged from 4.74 to 892.66 m 2 and can be used to determine locations where gullies are likely to begin in a digital elevation model with a 2 m × 2 m grid. The relationship between the upslope drainage area ( A) and the critical slope gradient ( S) could be expressed as S = 0.058 A −0.3. For the first time, this paper determined the relationship between gully length ( L) and flow erosivity ( E): L = 0.0305 E + 18.185 ( R 2 = 0.71). These results improve our understanding of the erosion process in ephemeral gullies.

Gully position, characteristics and geomorphic thresholds in an undisturbed catchment in Northern Australia

AbstractGullying is a significant process in the long‐term dynamics and evolution of both natural and rehabilitated (i.e. post‐mining) landscapes. From a landscape management perspective it is important that we understand gully initiation and development, as it is well recognized that catchment disturbance can result in the development of gullies that can be very difficult to rehabilitate. This study examines gully position using geomorphic statistics relating to features such as depth, width and length in a catchment undisturbed by European activity in the Northern Territory, Australia. The results demonstrate that gullying occurs throughout the catchment and that a slope–area threshold does not exist and that gully position broadly follows the catchment area–slope relationship. Simple relationships relating catchment area and slope to gully depth, width and length provide poor results, despite these relationships having been found to apply for ephemeral gullies in cropland. The results suggest that gully initiation thresholds are low as a result of an enhanced fire regime. A threshold model for gully position that uses catchment area and slope to switch between gully and hillslope was evaluated and found broadly to capture gully position. Copyright © 2006 John Wiley & Sons, Ltd.

Accuracy of methods for field assessment of rill and ephemeral gully erosion

To properly assess soil erosion in agricultural areas, it is necessary to determine precisely the volume of ephemeral gullies and rills in the field by using direct measurement procedures. However, little information is available on the accuracy of the different methods used. The main purpose of this paper is to provide information for a suitable assessment of rill and ephemeral gully erosion with such direct measurement methods. To achieve this objective: a) the measurement errors associated to three methods used for field assessment of channel cross sectional areas are explored; b) the influence of the number of cross sections used per unit channel length on the assessment accuracy, is analysed and; c) the effect of the channel size and shape on measurement errors is examined. The three methods considered to determine the cross sectional areas were: the micro-topographic profile meter (1); the detailed measurement of section characteristic lengths with a tape (2); and the measurement of cross section width and depth with a tape (3). Five reaches of different ephemeral gully types 14.0 or 30.0 m long and a set of six 20.4 to 29.4 m long rill reaches were selected. On each gully reach, the cross sectional areas were measured using the three above mentioned methods, with a separation ( s) between cross sections of 1 m. For rills, the cross sectional areas were measured with methods 1 and 3, with s = 2 m. Then, the corresponding total erosion volumes were computed. The volume calculated with method 1 with s = 1 m for gullies and s = 2 m for rills was taken as the reference method. For each channel, and for each one of the possible combinations of s and measurement method ( m), the relative measurement error and the absolute value of the relative measurement error ( E sm r and | E sm r |), defined with respect to the reference one, was calculated. | E sm r | much higher than 10% were obtained very easily, even for small s values and for apparently quasi prismatic channels. Channel size and shape had a great influence on measurement errors. In fact, the selection of the more suitable method for a certain gully shape and size seemed to be much more important than s, at least when s < 10 m. Method 1 always provided the most precise measurements, and its results were the less dependent on s. However, s must be < 5 m to guarantee an error smaller than 10%. Method 2 is not recommended, because it is difficult, time consuming and can lead to large errors. Method 3 seems to be enough for small, wide and shallow gullies, and for small rills, but only if s is shorter than 5 m. Results obtained after the analysis of rill measurement errors were similar to those of gullies. The analysis of E sm r and | E sm r | when calculating channel volumes using a unique representative cross section highlighted the importance of correctly selecting the adequate cross section. Due to the high error values that this method can entail, it is not considered as advisable whenever accurate erosion measurements are pursued.

The cost of soil erosion in vineyard fields in the Penedès–Anoia Region (NE Spain)

On-site and off-site environmental impacts of runoff and erosion are usually stressed in order to bring to the public's attention the importance and implications of soil erosion. However, few studies are aimed at calculating the economic implications of erosion, this being the message that farmers and/or policy makers understand best. In this current work we estimated the cost of erosion in vineyards in the Penedès–Anoia region (NE Spain), in which high intensity rain storms (> 80–100 mm h − 1 ) are frequent. Modern plantations in the region consist of trained vines, usually planted perpendicular to the maximum slope direction. Broadbase terraces are interspersed between vine rows to intercept surface runoff and convey it out of the field. Part of the sediment generated above these terraces is deposited in them and other parts are either deposited beyond the boundaries of the fields or are exported to the main drainage network. High intensity rainfall produces heavy soil losses (up to 207 Mg ha − 1 computed in an extreme event in June 2000, which had a maximum intensity in 30-min periods of up to 170 mm h − 1 ). To estimate the cost of erosion in vineyard fields of this region, two important aspects were considered. These were a) the cost incurred by the maintenance of the broadbase terraces, drainage channels and filling of ephemeral gullies and b) the cost incurred by the loss of fertilisers (mainly N and P) caused by erosion. According to farmers' records, the former was estimated at 7.5 tractor-hour ha − 1 year − 1 (as average), which comprises 5.4% of the income from grape sales. Regarding N and P losses, nutrients exported by runoff were 14.9 kg ha − 1 N and 11.5 kg ha − 1 of P, which, if compared to the annual intakes, represent 6% and 26.1% of the N and P respectively. In economic terms, the replacement value of the N and P lost represents 2.4% for N or 1.2% for P of the annual income from the sale of the grapes.

Gully erosion in the Siwalik Hills, Nepal: estimation of sediment production from active ephemeral gullies

AbstractA simple field‐based monitoring programme was established in a small catchment (area 4·6 km2) to find the rates of gully erosion in the Siwalik Hills, Nepal. The rates are used to estimate the amount of sediment produced by gully erosion in the catchment. Three large and active gullies were selected with areas ranging from 0·44 to 0·78 ha. Aerial photographs taken in 1964, 1978 and 1992 were ortho‐rectified and used to study the dynamics of gully heads. The same gullies were also monitored manually using an orthogonal reference system fixed by erosion pins around the gully heads. Results from the aerial photos indicated that the gullies expanded remarkably over the period from 1964 to 1992, by 34 to 58 per cent. Head‐retreat rates during that period were 0·48, 0·55 and 0·73 m a−1 and average annual sediment evacuation was estimated as 2534 ± 171, 959 ± 60 and 2783 ± 118 m3 a−1 for the three gullies respectively. From the field measurement, estimated volumes were found to vary from 731 ± 57 to 2793 ± 201 m3 a−1 over the monitoring period of two years. It was also found that the gullies produce sediment which accounts for up to 59 per cent of the sediment produced from surface erosion in the headwater catchment. The findings are useful for planning and executing appropriate control measures and constructing a sediment hazard map at the catchment scale. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Study of ephemeral gully erosion in a small upland catchment on the Inner-Mongolian Plateau

Ephemeral gully erosion is an important soil erosion process on the Inner-Mongolia Plateau in North China, and although its damage is very intense, little research on the area has been published. In this paper, a global positioning system (GPS) is used to measure the morphology of ephemeral gullies in a small catchment, the Inner-Mongolia Autonomous Region. First, this paper presents the characteristics of ephemeral gullies and soil loss due to ephemeral gully erosion. The network of ephemeral gullies takes on the shapes of tree branches, and there are 16 hole-ephemeral gullies in the middle of the ephemeral gullies. An average gully length of about 19.6 m ha −1 and an average soil loss of 8.8 m 3 ha −1 due to ephemeral gully erosion were measured. Second, soil erosion influences crop production in cropland and combinations of vegetation in fallow. The difference between vegetation in the middle of ephemeral gullies and in other places is very obvious. Third, this paper discusses hole-ephemeral gullies that are holes locating in the middle of ephemeral gullies whose widths and depths are more than 0.5 m ( Fig. 6) for the first time. The relationship between local hill slope gradient S (m m −1) and upslope contributing area A (ha) can be expressed as S = 0.064 A −0.375 and may be a key indicator for determining the position of existing hole-ephemeral gully heads and for predicting where hole-ephemeral gullies could form in the small watershed on the Inner-Mongolian Plateau.

Field Evaluation of a Multi-Class Sediment Net Deposition Model in Contour Channels

Accumulation of sediment in contour channels or terraces often occurs when overland flow passes from a hillslope to a lesser channel slope and sediment load is reduced. This article describes a field evaluation of an approximate analytic solution to a multi-class net deposition equation in a channel. The approximate analytic solution requires input settling classes to be of equal mass, which can be interpolated from a measured settling velocity distribution. Lateral input of runoff and suspended sediment into the channel was approximated by representing the channel as a series of 20 m segments that received hillslope inputs at their uppermost end via ephemeral gullies. At the beginning of each channel segment, new settling velocity classes (of equal mass) were interpolated, following mixing of inputs from the contributing hillslope segment and the previous channel segment. This procedure was found to be stable over the whole length of the channel only if the number of interpolated settling velocity classes (I) was >60. Precise measurement of such a large I value is infeasible, and users of the interpolation procedure should consider any potential errors that might occur if interpolating from a small number of measured data points. Using the model, the input sediment load was partitioned into deposition in the channel and output suspended sediment, which compared well with field measurements. However, enrichment of slower settling velocity fractions, relative to the input sediment, was overpredicted when compared to the only available measurement of the output settling velocity distribution. This finding is consistent with other applications of the model and with some applications of simple settling theory. Outputs were shown to be relatively insensitive to: surface roughness, threshold stream power beyond which re-entrainment occurs, and the proportion of stream power available for re-entrainment. It is likely that the approximate analytic solution is not applicable where channel segments are 10 m or less, in which case the full solution should be used.

Assessing soil erosion rates in cultivated areas of Navarre (Spain)

AbstractAlthough there is much evidence of intense soil erosion in cultivated areas of Navarre (Spain), information on it is currently scarce. Rill and ephemeral gully volumes can be used as a guide to minimum erosion rates. With the main purpose of determining the annual soil loss rates in cultivated areas of central Navarre, a detailed assessment of rainfall and of rill and gully erosion was made in 19 small catchments from October 1999 to September 2001. Seventeen of them were randomly selected, and were cultivated with winter cereals, vineyards or sunflowers. The other two catchments were selected to represent partially uncultivated lands abandoned for ten years. Channel cross‐sections were measured by using a 1‐m‐wide micro‐topographic profile meter, describing 632 cross‐sections and processing information from 31 600 pins. Erosive events happened every year in the three study areas. For cereal catchments, soil losses occurred in only one or two rainfall events each year, usually at the end of autumn and in some summers, with high erosion rates (0·20–11·50 kg m−2 a−1). In vineyards, soil losses occurred several times per year, and in any season. This is attributed to the small percentage of surface covered by the crop throughout the year. Again, high erosion rates were found (0·33–16·19 kg m−2 a−1), with ephemeral gully erosion causing more loss than rill erosion. No‐till is proposed as an effective conservation measure. From this large data set, it can be stated that rill erosion and ephemeral gully erosion are widespread in Mediterranean regions, and that much more attention should be paid to the problem. Abandoned fields showed very high erosion rates (16·19 kg m−2 a−1 on average), suggesting that the abandonment of marginal lands without implementing any erosion control can lead to severe erosion rates. Copyright © 2005 John Wiley &amp; Sons, Ltd.

RUSLE applied in a GIS framework: Calculating the LS factor and deriving homogeneous patches for estimating soil loss

The RUSLE (Revised Universal Soil Loss Equation) is integrated within a GIS framework to calculate soil loss spatially. For this module, algorithms and procedures were developed to derive the slope length factor (L) and steepness factor (S) from a DEM, then integrated with the R, K, C, and P factors to develop homogeneous patches (sub‐units) within each field or river basin. Soil loss is determined for each patch within a study unit, and then combined to determine the fields' or river basin's average annual and total soil loss. Two case studies are presented. The first case study, in central Massachusetts, compares estimated soil loss values obtained for individual fields using the Idrisi RUSLE module to USDA‐NRCS RUSLE field data. While soil loss results were similar, the RUSLE module allows fields to be partitioned into more similar units than practical in the field. This permits detailed spatial analysis of soil‐loss patterns. The second case study compares soil‐loss estimates for a catchment in southwestern Flanders, Belgium. This model–model comparison contrasts the results from the RUSLE module to the WATEM model—a grid cell based model based on the USLE/RUSLE but conceptualized in a multi‐flow context. Results between the predicted soil losses utilizing the two different approaches are significantly correlated. However, estimated soil losses are consistently higher for the WATEM model. This likely reflects the differences between how the two models compute L as well as the contribution of ephemeral gullies and flow convergence which are incorporated in WATEM but not in RUSLE.

Exploring the role of topography in small channel erosion

AbstractErosion caused by concentrated flows in agricultural areas is responsible for important soil losses, and rapid sediment transfer through the channel network. The main factors controlling concentrated flow erosion rates include the erodibility of soil materials, soil use and management, climate and watershed topography. In this paper, two topographic indices, closely related to mathematical expressions suggested by different authors, are used to characterize the influence of watershed topography on gully erosion. The AS1 index is defined as the product of the watershed area and the partial area‐weighted average slope. The AS2 index is similar to the AS1 but uses the swale slope as the weighting factor. Formally, AS2 is the product of the watershed area and the length‐weighted average swale slope. From studies made using different ephemeral gully erosion databases, it is shown that a high correlation consistently exists between the topographic indices and the volume of eroded soil. The resulting relationships are therefore useful to assess soil losses from gully erosion, to identify the most susceptible watersheds within large areas, and to compare the susceptibility to gully erosion among different catchments. This information is also important in studying the response of natural drainage network systems to different rainfall inputs. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Reconstructing rainfall and land-use conditions leading to the development of old gullies

Knowledge of past erosion events and their controlling factors is an important key to understanding the impacts of environmental change (climate-land use) on the landscape. In this study, knowledge about erosion processes on the development of present-day ephemeral gullies is used for reconstructing conditions leading to the formation of old, permanent gullies. Empirical relations between flow hydraulics and channel geometry have been recently established for gullies. Hence, using measured bottom width Wbttom of old gullies as input, peak flow discharges (Qp) of these gullies can be estimated. In two forested areas in central Belgium, 52 old gullies were mapped. The old gullies had an average Wbottom ranging between 1.1 and 1.5 m. Corresponding calculated Qp values ranged between 0.04 and 0.07 m3/s. Rainfall intensities (I) were also deduced from Qp using the rational formula. By simulating various land use scenarios and thus various runoff coefficient (C) values, I and concentration time (Tc) could be calculated for each land-use class. Using I, Tc and intensity-duration-frequency tables for the study area, the recurrence interval (RI) of the rain events, needed to erode the observed gully channels was assessed. Although analysis of historical documents indicates that both areas have probably been under forest since the Middle Ages, it is unlikely that the old gullies originated under forest vegetation or even degraded forest vegetation, since R&gt;I 200 years were obtained for these land-use scenarios. Cropland is the only land use that provides acceptable values of RI(11-128 years).