D Oostwoud Wijdenes

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.

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 . .

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.

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.

Chiselling effects on the vertical distribution of rock fragments in the tilled layer of a Mediterranean soil

Abstract When a mixture of dry particles with different diameters is disturbed, a segregation will occur with the finest particles moving downwards and the largest particles moving upwards. The objective of this paper was to determine whether this principle could be used to explain the high stoniness of topsoils of cultivated fields in the Mediterranean. A dense cover of rock fragments reduces soil erosion and promotes infiltration and hence, maintaining a coarse top layer may be an important management practice in areas threatened by desertification. Field experiments were conducted to determine the change in vertical rock-fragment size distribution after a certain number of tillage passes, until steady state (no more change) was reached. Soil pits were prepared and filled with four layers of rock fragments. Each 4-cm-thick layer contained a known distribution of rock-fragment sizes. One series of pits contained the coarsest layer initially at the bottom and the finest on top, while a second series showed an even distribution of coarse and fine particles throughout the profile. The pits were subjected to different tillage frequencies (max. of eight passes, all to a depth of 16 cm) by a caterpillar tractor pulling a chisel with a duckfoot. After the tillage operations, rock-fragment content (by mass) of each layer was determined by sieving and weighing, and rock-fragment cover (percentage) at the soil surface was assessed by photographical interpretation respectively. The experiments, conducted under dry conditions, demonstrated that there was a strong tendency for the coarsest fraction to accumulate in the top layers and for the finest fraction to accumulate at the bottom of the tilled layers. After two events, the coarsest fraction was already the dominant fraction in the surface layer for the first series of experiments (coarsest rock fragments initially at bottom). For the second series (equal distribution of rock-fragment sizes with depth), this already occurred after one event. The photographical interpretation of the surface rock fragment cover showed initially a rapid increase of intermediate sized fragments (0.6–5.0 cm) during the first series of experiments. After five tillage passes, the coarsest rock fragments were dominant at the surface. The rapid grading due to tillage observed in these experiments may explain the high rock fragment content of the topsoil in many cultivated shallow soils in the Mediterranean area. These results also suggest that in soils in which rock fragments on the surface are known to improve soil and water conservation, these benefits may be obtained after only one or two tined tillage operations in dry soil.