Jeroen Nachtergaele

Gully erosion and environmental change: importance and research needs

Abstract 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: (1) 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? (2) 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? (3) 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? (4) How does gully erosion interact with hydrological processes as well as with other soil degradation processes? (5) 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? (6) 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.

Sediment export by water from an agricultural catchment in the Loam Belt of central Belgium

Abstract An agricultural drainage basin of 250 ha was selected in the Belgian Loam Belt to evaluate sediment export by water. Water discharge was continuously measured at the outlet of the catchment and suspended sediment samples were taken proportional with discharge during rainfall events. From these samples supended sediment concentration and grain-size distribution were determined. Within the catchment, regular surveys were conducted in order to obtain detailed data on vegetation cover, soil surface parameters and erosion and sedimentation features. Important variations in sediment yield were detected at various time-scales, ranging from a year, over a season to one event. These variations could be linked with processes occurring within the catchment. Due to the development of a vegetation cover, the suspended sediment concentrations were lower during the summer period than during the winter period for the same discharge. The evolution in both suspended sediment concentration and grain-size distribution during the winter period is explained by variations in the soil surface state of the fields as well as the development of a rill and gully network. The positive hysteresis observed within a single event is explained by taking into account the distance between the sediment source and the catchment outlet and not by sediment flushing. Our analysis shows that the role of the main factors controlling the geomorphic response of such a catchment can be identified only if measurements at the outlet are complemented by detailed data on the conditions within the catchment. Catchment model calibration and validation are therefore impossible if this information is lacking.

THRESHOLDS FOR GULLY INITIATION AND SEDIMENTATION IN MEDITERRANEAN EUROPE

InMediterraneanareasthedynamicsofgullydevelopmentactasanimportantindicatorofdesertification.However,littleis known about the influence of climate and land-use changes, and almost no field data exist to assess the sensitivity of a landscapetogullyerosion.Twoimportantcomponentsofgullyerosionstudiesarethepredictionofwheregulliesbeginand where they end. To address some of these issues, topographical thresholds for gully initiation and sedimentation in six differentMediterraneanstudyareaswereestablished.Fieldmeasurementsoflocalsoilsurfaceslope(S)anddrainage-basin area(A)atthepointofinitiationofephemeralgulliesinintensivelycultivatedfields(fivedatasets)andpermanentgulliesin rangelands (three datasets) were carried out. A negative power relationship of the form S = aAb was fitted through all datasets, and defined as the mean topographical threshold for gullying in the respective area. Topographically controlled slopesofsedimentationatthegullybottomwerealsomeasured.Comparedtotheoreticalrelationshipsforchannelinitiation by overland flow, relatively low values for b are obtained, suggesting a dominance of overland flow and an influence of subsurfaceflow.Theinfluenceoflandslidingatsteeperslopesappearedfromtheflatteningoftheoverallnegativetrendin thehighersloperange(S> 030)oftheintegrateddataset.Comparingthethresholdlinesofourdatasetstotheaveragetrend lines through data found in literature revealed that vegetation type and cover could better explain differences in topographical thresholds level than climatic conditions. In cultivated fields, soil structure and moisture conditions, as determined by the rainfall distribution, are critical factors influencing topographical thresholds rather than daily rainfall amounts of the gully-initiating events. In rangelands, vegetation cover at the time of incision appears to be the most importantfactordifferentiatingbetweentopographicalthresholds,overrulingtheeffectofaverageannualrainfallamounts. Soil texture and rock fragment cover contributed little to the explanation of the relative threshold levels. Differences in regressionslopes(b)betweentheS-Arelationshipsfoundinthisstudyhavebeenattributedtothesoilcharacteristicsinthe differentstudyareas,determiningthe relativeimportanceofsubsurface flowandHortonianoverlandflow.Sedimentation slopeswherebothephemeralandpermanentgulliesendweregenerallyhighbecauseofthehighrockfragmentcontentofthe transportedsediment.Apositiverelationshipwasfoundbetweentherockfragmentcontentattheapexofthesedimentation fan and the slope of the soil surface at this location. Copyright # 2000 John Wiley & Sons, Ltd.

The value of a physically based model versus an empirical approach in the prediction of ephemeral gully erosion for loess-derived soils

Abstract A data set on soil losses and controlling factors for 58 ephemeral gullies has been collected in the Belgian loess belt from March 1997 to March 1999. Of the observed ephemeral gullies, 32 developed at the end of winter or in early spring (winter gullies) and 26 ephemeral gullies developed during summer (summer gullies). The assessed data have been used to test the physically based Ephemeral Gully Erosion Model (EGEM) and to compare its performance with the value of simple topographical and morphological indices in the prediction of ephemeral gully erosion. Analysis shows that EGEM is not capable of predicting ephemeral gully cross-sections well. Although conditions for input parameter assessment were ideal, some parameters such as channel erodibility, critical flow shear stress and local rainfall depth showed great uncertainty. Rather than revealing EGEMs inability of predicting ephemeral gully erosion, this analysis stresses the problematic nature of physically based models, since they often require input parameters that are not available or can hardly be obtained. With respect to the value of simple topographical and morphological indices in predicting ephemeral gully erosion, this study shows that for winter gullies and summer gullies, respectively, over 80% and about 75% of the variation in ephemeral gully volume can be explained when ephemeral gully length is known. Moreover, when previously collected data for ephemeral gullies in two Mediterranean study areas and the data for summer gullies formed in the Belgian loess belt are pooled, it appears that one single length ( L )–volume ( V ) relation exists ( V =0.048 L 1.29 ; R 2 =0.91). These findings imply that predicting ephemeral gully length is a valuable alternative for the prediction of ephemeral gully volume. A simple procedure to predict ephemeral gully length based on topographical thresholds is presented here. Secondly, the empirical length–volume relation can also be used to convert ephemeral gully length data extracted from aerial photos into ephemeral gully volumes.

Assessment of soil losses by ephemeral gully erosion using high‐altitude (stereo) aerial photographs

The objective of this study is to explore in a critical way the potential of high-altitude (stereo) aerial photographs for the assessment of ephemeral gully erosion rates. On 28 May 1995, an intensive rainfall event (30 mm h−1 during 30 min, return period = 3 years) occurred in central Belgium. Ephemeral gullies formed within an area of 218 ha (study area 1) were mapped and measured both in the field and by high-altitude aerial photos taken at the same time. Comparison of these two methods shows that if only one of the two surveying techniques had been used, only 75 per cent of the total ephemeral gully length would have been detected, so that the combination of aerial and field data leads, in fact, to the best possible determination of total gully length within the selected area. A correction factor (C) is proposed, so that the results of an ephemeral gully erosion survey based on high-altitude (stereo) aerial photos can be adjusted for the undetected gullies. Next, a sequential series of high-altitude stereo aerial photographs, taken in six different years, was analysed in order to determine ephemeral gully erosion rates in three selected study areas (study areas 2, 3 and 4). Selection criteria were chosen so that these three areas were similar to study area 1 and representative for the cultivated areas in central Belgium where intense soil erosion regularly occurs. Ephemeral gullies were mapped and their total length was measured from the aerial photos. Using a mean gully cross-section of 0·2635 m2 (determined in study area 1), the average eroded volume is 1·89 m3 ha−1 in six months for study area 1, 0·86 m3 ha−1 in six months for area 2, 1·44 m3 ha−1 in six months for area 3, and 2·37 m3 ha−1 in six months for area 4. According to the correction factor (C), these mean ephemeral gully erosion volumes have to be increased by 44 per cent. The ephemeral gully erosion rates based on high-altitude stereo aerial photos, correspond well with the results of other surveys carried out in the Belgian loess belt. Copyright

Concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content

Abstract Concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover (Rc) on sediment yield for an initially wet and an initially air-dry loamy topsoil. The experimental results indicate that Rc reduces concentrated flow erosion rates (E) in an exponential way (i.e., E=e−bRc), which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet-rill erosion measured on runoff plots. The decay rate (b) of this exponential relationship increased throughout the experiments because of scour-hole development and bed armouring. The concentrated flow erosion rates and b-values also depend on the initial moisture content of the topsoil. Depending on Rc, mean concentrated flow erosion rates were 20% to 65% less on initially wet compared to initially air-dry topsoils. The mean value for b was 0.032 for the initially wet, but only 0.017 for the initially air-dry topsoil, indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air-dry than when it is initially wet. The results help explain the data scatter in reported relationships between Rc and interrill–rill erosion rates. They also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils, which are very common in Mediterranean environments. Event-based water erosion models should incorporate effects of antecedent soil moisture content as well as those of Rc on concentrated flow erosion rates.

Evaporation from cultivated soils containing rock fragments

Evaporation from bare soil surfaces represents an important water loss for agriculture in semi-arid regions. Numerous efforts have been undertaken to modify the topsoil characteristics (mulching, tillage) in order to create a thin dry topsoil that reduces evaporation. However, little attention was paid to the role of natural rock fragments in topsoils with respect to evaporation. This paper presents the results of laboratory experiments simulating evaporation from initially wet and air-dry soils containing a range of rock fragment contents, and compares them to field conditions. Evaporation was stimulated by blowing fans at one (high) evaporative demand (Eo = 7.7–9.2 mm day−1). Time domain reflectometry (TDR) was used to investigate the relation between the water content of the topsoil and the actual evaporation rate. For soils at field capacity, initial fine earth water content decreases with rock fragment content, and consequently evaporation rates decrease in the same order. For air-dry soils that received a limited amount of rain (10 and 20 mm), an opposite behaviour was observed. Initial fine earth water content and evaporation rates increase with rock fragment content. A strong positive sigmoidal relation between relative evaporation rate (actual over open-pan evaporation rates) and fine earth water content in the centre of the wetted soil section was observed during the laboratory experiments. Except for the columns covered with a mulch, there were no systematic differences in this relation between the treatments. A rock fragment mulch reduces evaporation rate at a given soil water content significantly. These trends explain the often ambivalent effects of rock fragments on evaporation rates in the field.

Characteristics of sediment deposits formed by intense rainfall events in small catchments in the Belgian Loam Belt

Erosion and deposition patterns within two agricultural catchments in the Belgian Loam Belt were mapped and their volumes measured after an intense rainfall event. From these data, the total erosion and deposition budget was calculated. The surveys clearly indicated that deposits could be differentiated according to the type of process that caused deposition. For most deposits topography was the controlling factor. However, important deposits were also found at field borders where a vegetation barrier caused deposition. Vegetation-controlled deposition occurs at significantly higher slope gradients than slope-controlled sediment deposition. This implies that vegetation-controlled deposition has an important effect on the spatial distribution of deposited sediment and on the sediment delivery ratio at the catchment outlet. The undispersed aggregate-size distribution of sediment deposits in front of vegetation barriers is finer than the sediment deposited under topographically-controlled conditions. However, the dispersed particle-size distributions of both types of sediment are very similar and only slightly coarser than the dispersed particle-size distribution of the source material. During these extreme rainfall events, sediment is eroded, transported and deposited in aggregated form. The aggregates themselves have a particle size distribution, closely resembling the source material. Consequently, considerable quantities of fine material and associated pollutants, which are expected to be exported to the river system, are trapped within the catchment.

Testing the Ephemeral Gully Erosion Model (EGEM) for two Mediterranean environments

Few models can predict ephemeral gully erosion rates (e.g. CREAMS, EGEM). The Ephemeral Gully Erosion Model (EGEM) was specifically developed to predict soil loss by ephemeral gully erosion. Although EGEM claims to have a great potential in predicting soil losses by ephemeral gully erosion, it has never been thoroughly tested. The objective of this study was to evaluate the suitability of EGEM for predicting ephemeral gully erosion rates in Mediterranean environments. An EGEM-input data set for 86 ephemeral gullies was collected: detailed measurements of 46 ephemeral gullies were made in intensively cultivated land in southeast Spain (Guadalentin study area) and another 40 ephemeral gullies were measured in both intensively cultivated land and abandoned land in southeast Portugal (Alentejo study area). Together with the assessment of all EGEM-input parameters, the actual eroded volume for each ephemeral gully was also determined in the field. A very good relationship between predicted and measured ephemeral gully volumes was found (R2 = 0·88). But as ephemeral gully length is an EGEM input parameter, both predicted and measured ephemeral gully volumes have to be divided by this ephemeral gully length in order to test the predictive capability of EGEM. The resulting relationship between predicted and measured ephemeral gully cross-sections is rather weak (R2 = 0·27). Therefore it can be concluded that EGEM is not capable of predicting ephemeral gully erosion for the given Mediterranean areas. A second conclusion is that ephemeral gully length is a key parameter in determining the ephemeral gully volume. Regression analysis shows that a very significant relation between ephemeral gully length and ephemeral gully volume exists (R2 = 0·91). Accurate prediction of ephemeral gully length is therefore crucial for assessing ephemeral gully erosion rates. Copyright

Characteristics and controlling factors of old gullies under forest in a temperate humid climate: a case study from the Meerdaal Forest (Central Belgium)

Abstract In many forests of Northwestern Europe old gullies can be found, but few studies have reported their genesis and characteristics. This study investigates these old gullies under forest in the large case-study area of Meerdaal Forest, in the Central Belgian loess belt. The objectives are (1) to determine the spatial distribution of these gullies, (2) to measure their morphological and topographical characteristics and (3) to reconstruct the factors that led to their development. In the 1329-ha study area, 252 channel-like incisions were mapped. Different types of incisions could be distinguished. Besides small and large gullies, many incisions were sunken lanes or road gullies. These road gullies are aligned along north–south oriented lines, whereas the concentration of old gullies is strongly related to the distribution of archaeological sites. Out of the 252 mapped incisions, 43 large gullies and 21 representative road gullies were selected for detailed morphological and topographical measurements. The characteristics of these two types of incisions were compared with ephemeral gullies formed under nearby cropland. Significant differences in morphology between the three types could be demonstrated. Ephemeral gullies under cropland and large gullies under forest differ significantly in all measured parameters, except bottom width. Both the old gullies and road gullies under forest have a significantly larger cross section and total eroded volume compared with the ephemeral gullies observed under cropland. This indicates that once formed, the old gullies were not ploughed in nor were they filled by sediment originating in their drainage areas, because of limited sediment production. Comparing topographical characteristics (i.e. slope at the gully head and runoff contributing area) of forest gullies and ephemeral gullies that formed under cropland yields important indications about their formation. The larger sedimentation slope of forest gullies, compared with ephemeral gullies and road gullies, suggests that the forest gullies incised on vegetated slopes as a consequence of runoff from the adjacent plateau, where the forest cover was disturbed. For the old gullies under forest, no relation between slope at the gully head and runoff contributing area is observed, probably because most gullies occur on very steep slopes. When simulating arable land-use in the study area, zones where ephemeral gullies are expected to develop can be predicted using published topographical threshold relationships. Comparing the zones where ephemeral gullies are predicted with the position of old gullies under forest leads to the conclusion that gully incision was most probably not triggered by extreme rainfall events and that they are not of periglacial origin. The observed gully pattern can best be explained by local, anthropogenically determined land-use changes.