Liesbeth Vandekerckhove

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.

Spatial distribution of gully head activity and sediment supply along an ephemeral channel in a Mediterranean environment

Abstract In this study, we examined the factors that control the spatial distribution of bank gully heads along a reach of an ephemeral river (Rambla Salada) in an area threatened by desertification in Southeast Spain. The activity of 458 gully heads was assessed in the field by pre-defined criteria such as sharp edges, presence of a plunge pool, tension cracks, recent deposited sediments, flow marks, and vegetation re-growth. The results showed that land use has a significant impact on bank gully head activity. Recent land-use changes involving the extension of almond cultivation appears to intensify bank gully head activity. Also, lithology has a clear impact on the bank gully extension. It was further investigated whether the gully heads were important sediment sources that contributed to reservoir sedimentation. The density of (very) active bank gully heads along the study reach was one per 17 m of channel length. Average annual retreat volumes were derived from measurements at 46 active gully heads (4.0 m3 y−1). By selecting all the channel sections in the catchment of the Puentes Reservoir with a similar pattern of bank gullies using aerial photographs, an estimate of basin-wide sediment production of bank gully heads was established. It was estimated that the retreat of active bank gully heads alone in the 12,760 ha study area (representing 12% of the total catchment area of the Puentes Reservoir) produced 6% of the sediment filling up the reservoir. Considering that the sediment is also derived from other sources such as channel walls, channel beds, and hillslopes, the overall conclusion is that bank gully expansion in Southeast Spain is a major point source of sediment and therefore, a major process of land degradation.

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 . .

Medium-term gully headcut retreat rates in Southeast Spain determined from aerial photographs and ground measurements

This paper deals with gully retreat rates at different time scales, whereby the short-term time scale may span a time interval of 1–5 years, the medium-term time scale a time interval of 5–50 years, and the long-term time scale a time interval of more than 50 years. An analysis of high-altitude aerial photographs in combination with ground measurements allowed us to quantify volumetric gully-head retreat rates for 12 permanent gullies in Southeast Spain (Guadalentin and Guadix study areas) over a 40–43 year time interval (medium-term time scale). This resulted in an average retreat rate (Ve) of 17.4 m3 year−1. A power relationship between drainage-basin area (A) and medium-term volumetric gully-head retreat rate, Ve=0.069A0.380 (R2=0.51, n=21), was found by combining the gully-head retreat rates obtained in this study with those obtained by a dendrochronological method. The exponent (b) and the coefficient of determination (R2) of the power relationships Ve=aAb increases from the short-term to the long-term time scale, expressing the increasing importance of drainage-basin area in gully development with time. Considerable differences between gully-head retreat rates measured at the short and medium-term time scales at individual gullies showed the importance of land-use changes and unsuccessful management practices on gully-head retreat, and the episodic nature of gully-head retreat when piping and tension cracking are involved. Higher gully-head retreat rates are obtained at the medium-term time scale compared to the short-term time scale but the differences are not significantly different at α=10%. The medium-term methods tend to measure proportionally more high gully retreat rates, but less extreme values compared to the short-term method. This is explained by a more equal distribution of extreme rainfall events both in space and time at a longer-term time scale, and hence a higher probability of measuring the average effect of both small and extreme rainfall events at each gully.

Characteristics and controlling factors of bank gullies in two semi-arid mediterranean environments

Abstract Bank gullies are gullies that are formed due to a height drop caused by a terrace or a river bank, which develop by headward retreat in erodible hillslopes. This study aims (i) to investigate the morphology of actively eroding bank gullies, i.e., geometrical characteristics resulting from past erosion and active erosion processes shaping the gully, and, (ii), to find relationships with environmental site characteristics, such as topographical parameters, material properties and climate. The ultimate goal is to identify the most important controlling factors of past and present bank gully erosion. Fifty-five active bank gullies formed in different lithologies by various erosion processes have been selected in the Guadalentin basin and the surroundings of Guadix (Southeast Spain). For each bank gully site, geometrical and topographical parameters of both the channel and the drainage basin were measured. Erosion features indicating activity at the gully head, such as tension cracks, plunge pools, undercutting, fluting, piping and rill or sheet erosion on sloping side walls were mapped, and samples were taken from distinct lithological layers that were considered to influence the type and intensity of erosion processes. A relationship could be shown between the presence of piping and fluting and a number of material characteristics, including particle size distribution, dispersion behaviour and electrical conductivity. On the other hand, lithology appeared not to be a differentiating factor on gully development in the long run, as expressed by the total eroded volume ( V ). This parameter was most strongly related to the drainage basin area in which the entire bank gully had been formed ( A o ), explaining 66% of the variance. The relationship is V =1.75* A o 0.59 . No significant difference was found between regression lines through sub-datasets of different soil textural classes. Finally, multiple regression was used to include both topographical parameters and material characteristics in an explanatory and/or predictive equation for the total eroded bank gully volume. The results of the analyses using the entire dataset, including the sites in the Guadalentin as well as in the Guadix area, have been compared with the results for the separate study areas. Differences are not only related to topographical and lithological characteristics, but may also be the consequence of a different climate in the two areas.

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

Short-term bank gully retreat rates in Mediterranean environments

Abstract 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 m 3 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 =0.04 A p 0.38 ( R 2 =0.39) and R l =0.01 A p 0.23 ( R 2 =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=1.71 A o 0.60 ( R 2 =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 m 3 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

Abstract 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 m 3 year −1 ( n =9). For gully sidewall processes, the average minimum erosion rate per unit sidewall length amounted 0.1 m 3 year −1 m −1 ( n =9). A strong correlation was found between the headcut retreat rate ( v m(ortho) , m 3 year −1 ) and the drainage–basin area ( A , m 2 ) of the gullies, expressed by v m(ortho) =0.02 A 0.57 ( R 2 =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.

Medium-term evolution of a gully developed in a loess-derived soil

Field surveys in the Belgian loess belt revealed the presence in many forested areas of large, permanent gully systems, most of which are currently inactive. In cultivated areas, such gullies can only be observed in cross-sectional soil profiles through hollows, as virtually all such large gullies are currently infilled with colluvium. Little is known about the spatial distribution, initiation and temporal evolution of these large, permanent gully systems on loess-derived soils. Therefore, the medium-term evolution of a gully initiated in a cultivated area on loess-derived soils southwest of Leuven (Belgium) in May–June 1986, was studied over 13 years. Two intense rainfall events created this (ephemeral) gully, which was not erased by subsequent tillage. Between June 1986 and the December 1999, eight field surveys were conducted to measure gully dimensions. During two surveys, topographic indices (e.g., slope and drainage area) were also measured. Daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully. Analysis of rainfall data showed that no extreme rainfall event was required to initiate such large (permanent) gullies, as observed in forested areas and through cross-sectional profiles in cultivated fields in the Belgian loess belt. Return periods of the event that caused the gully varied between <1 year and 25 years, depending on the assumptions used for defining event rain intensity. Once established, length, surface area and volume of the studied gully evolved with time, cumulative rainfall or cumulative runoff, following a negative exponential relation. This accords with observations reported for gullies in Australia and the USA. This study shows that a degressive increase of gully extension, can be largely explained by the evolution of a “slope–drainage area” factor (S×A, which is proportional to stream power) with time. While gully length and gully surface area asymptotically evolve towards a final value, gully volume decreased at a given point in time. From this, it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it. From this moment on, the combined effect of water and tillage erosion in the gully drainage area, will lead towards rapid infilling. This expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern Belgium. The permanent gullies observed under forest are attributed to the fact that after severe gully erosion, this area was reforested or abandoned. Therefore, the sediment source was cut off and the gully was not filled in by sediment deposition.