Soil Erosion Plots Research Articles

Sustainability Analysis of Soil Erosion Control in Rwanda: Case Study of the Sebeya Watershed

Soil erosion is a complex process that results in soil and fertility losses from agricultural land and, ultimately, leads to river sedimentation. This study aimed to assess various influential factors and processes affecting soil erosion and to recommend suitable site-based Soil Erosion Control Measures (SECM) for sustainable agriculture while minimizing the downstream rivers and reservoir sedimentation in the Sebeya watershed of Rwanda. The present research used a literature review, site visits, and focus groups to assess various SECM within the Sebeya watershed. As a result, various site-based SECM were evaluated, recommended, and simulated to alleviate high soil loss rates in the Sebeya watershed using the Universal Soil Erosion Equation (USLE) model. Simulating existing and proposed SECM, soil loss was reduced significantly from 73 t/ha/y to 29 t/ha/y. To highlight the implication of the site-based recommended SECM in improving agricultural productivity, this study suggests field investigations in soil erosion plots and prediction of crop yields from an established linear correlation model between soil loss and crop yields in the Sebeya watershed. For effective action in reducing high soil erosion rates to tolerable rates in the Sebeya watershed, the present research recommends implementing the site-based recommended SECM with mulching and drainage channels on the same farmland. However, lack of money and knowledge are the main limitations in implementing SECM in the Sebeya watershed. Therefore, governmental and non-governmental organizations should technically and financially help farmers in the Sebeya watershed.

Aggregate and Particle Size Distribution of the Soil Sediment Eroded on Steep Artificial Slopes

In this study, the particle size distribution (PSD) of the soil sediment from topsoil obtained from soil erosion experiments under different conditions was measured. Rainfall simulators were used for rain generation on the soil erosion plots with slopes 22°, 30°, 34° and length 4.25 m. The influence of the external factors (slope and initial state) on the particle and aggregate size distribution were evaluated by laser diffractometer (LD). The aggregate representation percentage in the eroded sediment was also investigated. It has been found that when the erosion processes are intensive (steep slope or long duration of the raining), the eroded sediment contains coarser particles and lower amounts of aggregates. Three methods for the soil particle analyses were tested: (i) conventional–sieving and hydrometer method; (ii) PARIO Soil Particle Analyzer combined with sieving; and (iii) laser diffraction (LD) using Mastersizer 3000. These methods were evaluated in terms of reproducibility of the results, time demands, and usability. It was verified that the LD has significant advantages compared to other two methods, especially the short measurement time for one sample (only 15 min per sample for LD) and the possibility to destroy soil aggregates using ultrasound which is much easier than using hexametaphosphate.

Replacing the (inefficient) manual runoff sampling method used in Brazil – a prototype sample splitter

The method used to sample the runoff collected from soil erosion plots can be a significant source of error. In this study, we performed a laboratory experiment to evaluate the efficiency of the runoff sampling method most commonly used in Brazil. It is based on the manual homogenization and sampling of the collected runoff. Using soil material with 583 g kg–1 of sand and 89 g kg–1 of clay, the manual sampling method was tested for its ability to produce representative samples of artificial suspensions with a concentration of 2, 10 and 50 g L–1 of total solids. An underestimation of 30 % or more of the concentration of total solids was observed, with a variation of the same magnitude (CV between 20 and 45 %). We then developed a prototype sample splitter to replace the manual sampling method and tested it using the same artificial suspensions. The splitter was efficient in producing samples representative of the artificial suspensions, even without altering the particle size distribution of the total solids. Both absolute percentage errors (|< 5 %|) and the variation between five replicates (CV < 3 %) were small. The problems with the manual method are due to the inefficient homogenization that facilitates the differential sedimentation of particles of different sizes. If these problems are also found in other areas, then the prototype that we developed is a reasonable alternative.

Soil erodibility indices under different land uses in Ri-Bhoi district of Meghalaya (India)

In absence of soil erosion plots for determination of erodibility index (K) for erosion models like Universal Soil Loss Equation (USLE) or Revised Universal Soil Loss Equation (RUSLE) to estimate soil erosion, empirical relations are used. In the present study, soil erodibility index was determined for entire Ri-bhoi district of Meghalaya based on soil physical and chemical properties through empirical relationship and presented in a map form. Dominant land uses of the district were identified through geo-spatial tools which were viz. agriculture, forest, jhum land and wasteland. Soil samples from surface depth (01–15 cm) were collected from areas of different dominant land uses. Twenty five sampling points were selected under each land use type and geo-coded them on the base map of Ri-bhoi district. Apart from K-index, Clay Ratio, Modified Clay Ratio and Critical Soil Organic Matter were also determined for understanding the effect of primary soil particles on erodibility. In agriculture land use system K-index values were found in the range of 0.08–0.41 with an average of 0.25 ± 0.02. In case of jhum, forest and wasteland these were in the range of 0.08–0.42 with an average of 0.20 ± 0.01; 0.09–0.40 with an average of 0.22 ± 0.02, and 0.10–0.34 with an average value of 0.23 ± 0.02, respectively. Clay ratio (2.74) and Modified clay ratio (2.41) were observed to be higher in forest LUS, lower clay ratio (1.97) and modified clay ratio (1.81) were found in the wasteland indicating erosion susceptibility in forested area. The values of Critical Level of Organic Matter (CLOM) for the district ranged from 4.72 to 16.56. Out of 100 samples, only one sample had CLOM value less than 5 and rest 99 samples had values more than 5 indicating that the soils of the district had moderate to stable soil structure and offer resistance to erosion. All the indices values of geo-coded points were then interpolated in the Arc-GIS environment to produce land use based maps for Ri-bhoi district of Meghalaya. As K-index is a quantitative parameter which is used in models, the index can be then interpolated for estimation of soil erosion through USLE or RUSLE for any given situation.

A comprehensive study of erosivity and soil erosion over a small tropical islet: Round Island, Mauritius

AbstractRound Island, a small (219‐ha) volcanic island 22.5 km north of Mauritius, has steep slopes and thin soils. This small tropical island was previously subjected to overgrazing by goats and rabbits, which detrimentally affected ecosystem functioning. Classified as a nature reserve since 1957, Round Island houses the last remnant of palm savannah that has been lost on mainland Mauritius and is a refuge to several endemic and endangered species. Landscape restoration and ecosystem functioning on Round Island are key conservation objectives, but soil erosion remains a significant hindrance to habitat restoration and ecosystem functioning. Here, we investigate soil erosion on Round Island and assess the implications for conservation and management. Sediment movement was monitored monthly (2015–2018) at seven soil erosion plots coupled with rainfall and erosivity measurements. A mean annual sediment movement rate of 0.2136 t ha−1 yr −1 is calculated. Although this rate of sediment movement is low for humid tropical regions, the barren landscape and low contemporary rates of soil erosion suggest that most of the erodible soil has already been eroded. Interrill erosion of the shallow soils and friable parent material is closely correlated with rainfall erosivity and exacerbated by limited vegetation cover. Continued revegetation is recommended to retard soil erosion and restore ecosystem functioning. The paper also emphasises that rainstorms are the greatest single phenomena affecting earth surface processes on and around the island of Mauritius. As such, long‐term monitoring of the interaction between the surface and climate is required to provide insights for conservation management.

Improving cover and management factor (C-factor) estimation using remote sensing approaches for tropical regions

The Revised Universal Soil Loss Equation (RUSLE)'s cover and management factor (C-factor) is one of the most difficult factors to obtain, mainly because long-term monitoring soil erosion plots under natural rainfall are needed. Therefore, remote sensing approaches have been used as an alternative for obtaining this factor. However, there is a lack of studies comparing values of this factor computed from remote sensing approaches with measured data. In this study, we compare two widely used remote sensing approaches (CrA and CVK) to estimate the C-factor based on the Normalized Difference Vegetation Index (NDVI) with the literature (CLIT) and field experimental data. We also investigated the influence of C-factor methods on the prediction of soil loss and sediment yield (SY) using measured data in the Guariroba basin, Central-West Brazil. We obtained mean C-factor values of 0.032, 0.023 and 0.137 for CLIT, CrA and CVK, respectively. We found an average annual soil loss of 2.20 t ha−1 yr−1, 2.02 t ha−1 yr−1 and 10.07 t ha−1 yr−1 and SY values of 6875 t yr−1, 6468 t yr−1 and 33,435 t yr1, for CLIT, CrA and CVK, respectively. Our results indicated a significant improvement in soil loss and SY estimations by using the CrA approach developed for tropical regions, with a bias of 13% to the measured SY (5709 t yr−1). We conclude that the CrA method present the most suitable alternative to compute soil loss and SY in tropical regions. Furthermore, this approach allows large-scale evaluation and temporal monitoring, therefore enhancing multi spatial and temporal assessment of soil erosion processes.

Land use types control solid wash-off rate and entrainment coefficient of Fukushima-derived 137Cs, and their time dependence

Understanding solid 137Cs wash-off in sediment sources is important for predicting radiological risks in zones contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Yoshimura et al. (2015) studied solid 137Cs wash-off using soil erosion plots representing different land uses in Fukushima. However, temporal trends of 137Cs activity concentration in sediments remained unclear owing to the short duration of their observations. This study is a follow-up to that of Yoshimura et al. (2015) and provides additional observations that test the dependency of temporal variations in 137Cs activity concentration in sediment and solid wash-off processes of 137Cs on land use types. Eight soil erosion plots were observed, and data from 2011 to 2014 were analyzed. A normalized 137Cs solid entrainment coefficient, defined as Sc (m2 kg−1), was calculated by dividing 137Cs activity concentration in sediment by initial 137Cs deposition. A particle size-corrected Sc, defined as Sccorrect (m2 kg−1), was also calculated based on granulometry. Sediment quantity-weighted mean values of Sc and Sccorrect, ranged from 0.0072 to 0.084 m2 kg−1 and 0.0052–0.078 m2 kg−1, respectively. Annual wash-off rates of solid 137Cs were 0.0029–12% year−1. There was no significant decreasing trend in Sc or Sccorrect on most of the plots due to its huge variability. However, on an uncultivated farmland after the removal of surface vegetation, marked surface erosion including formation of rill network was found, and Sccorrect significantly decreased as the cumulative sediment discharge increased. Our follow-up observations suggest that temporal changes in 137Cs activity concentration in sediment at the sediment source should be controlled by soil erosion processes and their intensity.

The age of vines as a controlling factor of soil erosion processes in Mediterranean vineyards

Vineyards incur the highest soil and water losses among all Mediterranean agricultural fields. The state-of-the-art shows that soil erosion in vineyards has been primarily surveyed with topographical methods, soil erosion plots and rainfall simulations, but these techniques do not typically assess temporal changes in soil erosion. When vines are planted they are about 30cm high×1cm diameter without leaves, the root system varies from 2 to over 40cm depth, and sometimes the lack of care used during transplanting can result in a field with highly erodible bare soils. This means that the time since vine plantation plays a key role in soil erosion rates, but very little attention has been paid to this by the scientific community. Thus, the main goal of this research was to estimate soil losses and assess soil erosion processes in two paired vineyard plantations of different ages. To achieve this goal, the improved stock unearthing method (ISUM) was applied to vineyards on colluvial parent materials with similar soil properties, topographical characteristics and land managements in the Les Alcusses Valley, southwestern Valencia province, Spain. Our findings suggested that the old vineyards showed lower erosion rates (−1.61Mgha−1yr−1) than those that were recently planted (−8.16Mgha−1yr−1). This is because of the damage that the plantation of the vines causes to soil. Tillage after planting (4 times per year) resulted in changes in the inter-row and row morphology, promoting the development of a ridge underneath the vines that disconnected the inter-rows and reduced soil losses with time. After the second year and until the 25th year after plantation, soil erosion was approximately 1Mgha−1y−1, which means that most of the erosion took place during the first two years after the plantation. Soil conservation strategies should be applied immediately after the plantation works to allow sustainable grape production. That is when soil erosion most needs to be controlled.

Reduction of soil erosion and mercury losses in agroforestry systems compared to forests and cultivated fields in the Brazilian Amazon

In addition to causing physical degradation and nutrient depletion, erosion of cultivated soils in the Amazon affects aquatic ecosystems through the release of natural soil mercury (Hg) towards lakes and rivers. While traditional agriculture is generally cited as being among the main causes of soil erosion, agroforestry practices are increasingly appreciated for soil conservation. This study was carried out in family farms of the rural Tapajós region (Brazil) and aimed at evaluating soil erosion and associated Hg release for three land uses. Soils, runoff water and eroded sediments were collected at three sites representing a land cover gradient: a recently burnt short-cycle cropping system (SCC), a 2-year-old agroforestry system (AFS) and a mature forest (F). At each site, two PVC soil erosion plots (each composed of three 2 × 5 m isolated subplots) were implemented on steep and moderate slopes respectively. Sampling was done after each of the 20 rain events that occurred during a 1-month study period, in the peak of the 2011 rain season. Runoff volume and rate, as well as eroded soil particles with their Hg and cation concentrations were determined. Total Hg and cation losses were then calculated for each subplot. Erosion processes were dominated by land use type over rainfall or soil slope. Eroded soil particles, as well as the amount of Hg and cations (CaMgK) mobilized at the AFS site were similar to those at the F site, but significantly lower than those at the SCC site (p < 0.0001). Erosion reduction at the AFS site was mainly attributed to the ground cover plants characterizing the recently established system. Moreover, edaphic change throughout AFS and F soil profiles differed from the SCC site. At the latter site, losses of fine particles and Hg were enhanced towards soil surface, while they were less pronounced at the other sites. This study shows that agroforestry systems, even in their early stages of implementation, are characterized by low erosion levels resembling those of local forest environments, thus contributing to the maintenance of soil integrity and to the reduction of Hg and nutrient mobility.

Pinus halepensis M. versus Quercus ilex subsp. Rotundifolia L. runoff and soil erosion at pedon scale under natural rainfall in Eastern Spain three decades after a forest fire

Afforestation aims to recover the vegetation cover, and restore natural ecosystems. The plant species selected for restoration will determine species richness and the fate of the ecosystem. Research focussing on the impact of vegetation recovery on soil quality are abundant, especially on fire affected land and where rehabilitation, afforestation and restoration projects were carried out. However, little is known about how different plants species affect soil erosion and water losses, which are key factors that will impact the fate of the afforested land. Aleppo pine (Pinus halepensis M.) is the species commonly used for afforestation in the Mediterranean and is very successful when natural recovery takes place, however, the original forests were composed of Holm oaks (Quercus ilex subsp. rotundifolia L.). There is little information about the hydrological and erosional impact of this change of vegetation cover stimulated by a millennia old forest use in the Mediterranean, and a century old afforestation policies and natural recovery as a consequence of land abandonment. To get insights in the effect of plant species on runoff generation and soil erosion, individual trees should be selected. Plots of 1 m2 are necessary to identify homogeneous patches, and were installed under Aleppo pine (4 plots) and Holm oaks (4 plots) in a 30(34)-years old plant cover recovered after a forest fire that took place in 1979. A raingauge was installed in the study site to characterize the rainfall. The soil erosion plots were built with metal borders and each plot drained to a collector (gutter) and a 60L container to store the surface runoff. Runoff was measured after each rainfall event and sediment concentration was determined by desiccation. Results show that Aleppo pine covered soils yield six times more runoff (232mm, 8.31%) than Holm oaks (40mm, 1.4%) during the experimental period of 2010–2014, when rainfall amount 2,721.1mm. Runoff sediment concentration was higher in the Aleppo pine plots (4.9gl−1) than in the Holm oaks plots (2.6gl−1). Soil erosion rate was ten times higher in Aleppo pine (2.6Mgha−1 y−1) than in Holm oaks (0.26Mgha−1 y−1).

The impact of standard preparation practice on the runoff and soil erosion rates under laboratory conditions

Abstract. The use of laboratory methods in soil erosion studies, rainfall simulation experiments, Gerlach troughs, and other measurements such as ring infiltrometer has been recently considered more and more because of many advantages in controlling rainfall properties and high accuracy of sampling and measurements. However, different stages of soil removal, transfer, preparation and placement in laboratory plots cause significant changes in soil structure and, subsequently, the results of runoff, sediment concentration and soil loss. Knowing the rate of changes in sediment concentration and soil loss variables with respect to the soil preparation for laboratory studies is therefore inevitable to generalize the laboratory results to field conditions. However, there has been little attention given to evaluate the effects of soil preparation on sediment variables. The present study was therefore conducted to compare sediment concentration and soil loss in natural and prepared soil. To achieve the study purposes, 18 field 1 × 1 m plots were adopted in an 18 % gradient slope with sandy–clay–loam soil in the Kojour watershed, northern Iran. A portable rainfall simulator was then used to simulate rainfall events using one or two nozzles of BEX: 3/8 S24W for various rainfall intensities with a constant height of 3 m above the soil surface. Three rainfall intensities of 40, 60 and 80 mm h−1 were simulated on both prepared and natural soil treatments with three replications. The sediment concentration and soil loss at five 3 min intervals after time to runoff were then measured. The results showed the significant increasing effects of soil preparation (p ≤ 0.01) on the average sediment concentration and soil loss. The increasing rates of runoff coefficient, sediment concentration and soil loss due to the study soil preparation method for laboratory soil erosion plots were 179, 183 and 1050 % (2.79, 2.83 and 11.50 times), respectively.

Comparative Study on Community Characteristics of Desert Non-commercial Forests in Northwest China of Xinjiang after Being Closed 6 Years

Conservation of Desert Non-commercial Forests in Northwest China of Xinjiang is an important and effective way of restoring vegetation and rapid formation of stable community. Being closed the forests is desired for these activities, but tending them can be hampered by lack of community characteristics. We initiated situ experiments in eight serious desertification and soil erosion plots to better unravel community characteristics. We found that variations in characteristics of forest communities studied. The plots are sorted into: Group one: Poplus euphratica and Poplus pruinosa dominated species (Plot I and Plot IV), Group two: dominated species is Tamarix elongata (Plot III and Plot V, Haloxylon ammodendron dominated in Plot VI, Plot VII and Plot VIII as Group three and when Poplus euphratica and Salix psammophila are as dominated ones of Plot II served to Group four. Polynomial model can be used to predict the relationships between Shannon-Wiener Indices and Simpson Diversity Indices, Shannon-Wiener Indices and Monk Richness Indices and Shannon-Wiener Indices and Simpson Dominance Indices. It is power model that can be used to describe the relationship between Shannon -Wiener Indices and MaIntosh Evenness Indices. All the findings are useful for restoring vegetation in Northwest China.

Investigating sampling accuracy to estimate sediment concentrations in erosion plot tanks

Significant errors when sampling from collection tanks installed at the lower end of soil erosion plots may lead researchers toward wrong conclusions. Limited research has been found on sampling accuracy. In this study, a cylindrical sampler is introduced as a new sampling device and its sampling accuracy is investigated and compared with 2 other manual methods, namely bottle and pipette. Three target sediment concentrations were prepared in separate 213.5-L collection tanks as 3 replications with 3 mixing periods. The results using the bottle and pipette in all mixing periods and all target sediment concentrations showed that the concentration of sediment increases with the depth of the tank, which implies that making a uniform sediment concentration in the collection tank for sampling is impossible. The results indicated that the cylindrical sampler had the highest sampling efficiency of 88.68%. Computed sampling efficiencies in tanks of 20, 40, 60, and 80 cm in depth were 34.76%, 43.94%, 54.45%, and 67.21% for the bottle and 34.58%, 43.05%, 49.20%, and 56.42% for the pipette, respectively. There was no significant difference between sampling from the center and from the side of the collection tank and among mixing periods of 1, 2, and 5 min. In order to maximize the accuracy of the sampling, a 10-L bucket was placed in the collection tank to trap the coarse material. Weighing collected coarse material and adding the sampled part improved the accuracy by 9.45%. In fact, using a cylindrical sampler and a bucket together resulted in 98.13% sampling accuracy.

Micro-scale post-fire surface cover changes monitored using high spatial resolution photography in a semiarid environment: A useful tool in the study of post-fire soil erosion processes

Although post-fire soil erosion has been studied, little attention has been paid to changes in soil surface cover following fires, despite this being a key factor in understanding the water and sediment yield. This study, at Peñaflor (Spain), investigated the effect of fire on soil erosion using data from soil erosion plots and high spatial resolution photography (HSRP). Measurements were made from October 2003 to October 2005 in a control plot and a plot experimentally exposed to a fire in October 2004. Ground cover components were identified, including vegetation, bare soil, stones, charcoal and ash. Runoff and sediment concentrations were low because of the absence of intense rainfall events, but the fire contributed to an increase of soil erosion by one order of magnitude. HSRP observations demonstrated that the sediments become mixed with ash and charcoal. Disappearance of the ash two months after the fire and a reduction in the amount of surface charcoal showed that these factors control erosion during the first few rainfall events following a fire. An increase in rock fragment cover was a consequence of the removal of vegetation and litter by the fire, and removal of the ash and charcoal by overland flow.

Plot sizes dependency of runoff and sediment yield estimates from a small watershed

The spatial scale dependency of runoff generation and sediment yield and accuracy to extending the results of experimental plots to surrounding watersheds is rarely taken into account. The present paper aimed to evaluate the accuracy of soil erosion plots with different lengths in estimation of runoff and soil loss from a small watershed in Iran. Towards this attempt, two similar sets of experimental plots with lengths of 2, 5, 10, 15, 20 and 25 and fixed width of 2 m were installed on two north and south facing slopes (i.e. 6 plots on each slope and totally 12 plots) in Sanganeh watershed, Razavi Khorasan Province with an area of ca. 1 ha. Runoff and sediment was collected at the outlet of each plot as well as main outlet of the watershed associated with 12 storm events occurred during November 2006 to June 2007. The results verified that the accuracy of plot estimates on sediment and runoff improved while the plot length increased. The optimal lengths for estimation of sediment and runoff parameters were found 15 and preferably 20 m for plots located on southern slope. No significant difference was also proved among estimates obtained from plots established on north facing slope and those measured at the main outlet of the watershed.

Assessing dominant factors affecting soil erosion using a portable rainfall simulator

Investigating the causes of soil erosion is difficult in natural conditions owing to the presence of other factors. Without simplifying the experimental conditions, studying soil behavior with its numerous parameters while considering factors such as vegetation cover, topography, and rainfall is difficult and in most conditions impossible. The application of simulation approaches is therefore necessary to simplify the prototype. In this research, the effects of physical soil factors such as texture and antecedent soil moisture, along with land slope and vegetation cover were evaluated in the Taleghan watershed, Iran, using a rainfall simulator and soil erosion plots. For this purpose, a 89 u 120 cm rainfall simulator producing 24.5 and 32 mm/h rainfall intensities of 30 min duration, as a common condition of the study area, was used at 144 locations over soil erosion plots with dimensions of 95 u 125 cm. Plots had slope classes of 12–20 and 20–30 %, different soil textures, different antecedent soil moistures, and medium to poor vegetation cover conditions. It was found that for 24.5 and 32 mm/h rainfall intensities, the sediment yield had high correlations of −0.771 and −0.796 with vegetation cover and slight correlations of 0.045 and 0.029 with land slope respectively. Regression equations for predicting the sediment yield were also developed for different conditions.

PRODUTIVIDADE DO TRIGO E DA SOJA EM LATOSSOLO VERMELHO DISTRÓFICO COM DIFERENTES NÍVEIS DE EROSÃO HÍDRICA

Muitos dados estão disponíveis sobre as perdas de solo por erosão hídrica, sobre a influência destas perdas na produtividadedos cultivos há alguns dados obtidos por simulação, muito poucos tem sido obtidos em condições de chuva natural. Este trabalho foiconduzido em 1995, nas parcelas que há 17 anos vinham tendo monitoradas as perdas de solo no Polo Regional do IAPAR/PontaGrossa, PR. Avaliou-se a produtividade das culturas de trigo (Triticum aestivum) e soja (Glycine max) de acordo com a espessura da camada de solo perdida por erosão, em função dos diferentes sistemas de manejo do solo adotados: plantio direto, alternado (direto e cultivo mínimo), cultivo mínimo, convencional e permanentemente descoberto em dois comprimentos de rampa (22 e 11 m). Foram avaliados os atributos químicos e físicos do solo até a profundidade de 30 cm. A produtividade do trigo foi reduzida pela perda de solo por erosão, em média 116 kg.ha-1.cm-1, e a da soja, em 139 kg.ha-1.cm-1 de solo perdido. O trigo, principalmente em plantio direto, foi bastante prejudicado pelo mal-do-pé (Geaumannomyces graminis), dificultando a obtenção de correlações entre os atributos químicos e físicos do solo e a sua produtividade. O teor de carbono orgânico na camada de 10 a 20 cm apresentou correlações significativas comprodutividade de trigo e soja, com coeficiente de determinação menor que 50%.

Factors controlling the erosion rates in a semi-arid zone (Bardenas Reales, NE Spain)

Bardenas Reales is an erosive depression 415 km 2 in area located in the central-western part of the Ebro Depression. Tertiary sediments crop out in the margin of the basin whereas Quaternary sediments, Holocene in age, occur only in its centre. These Holocene sediments (clays and silts) are the result of the erosion and weathering of the Tertiary clays and are made up of 3 different stratigraphic units. Erosion is clearly the dominant process in the centre of the depression, generating gullies up to 10 km in long and 8 m deep. In order to analyse and quantify the erosion rates of the studied area, erosion was measured on two Soil Erosion Plots with volumetric and instrumental techniques that have been supplied for the last 12 years. The results show that the estimated erosion rates depend extrinsically on the used method and intrinsically on the time of the year in which they are measured. The most accurate values are those obtained by means of collector devices. The values obtained by the microtopographic profile gauge are overestimated, while the results of the measurement with erosion pins are clearly lower. The study area is located in a semi-arid environment, with two pluviometric maximums, in spring and autumn, associated with convective storms. In summer and winter, rainfall is related to Atlantic cyclonal fronts. Soil loss is significant during the pluviometric maximums when rainfall quantity and intensity are higher. During cyclonal periods, in contrast, water erosion is null or scarce and soil loss is related to mudflows. In addition, the erosion rates of each Holocene unit are different. The C2 Holocene unit has undergone greater erosion because of its physico-chemical properties. All the stratigraphic units have similar characteristics regarding Tertiary materials. Since 1993 the average erosion rate measured by the collector device on the Tertiary materials is 32 Mg ha − 1 year − 1 and 77 Mg ha − 1 year − 1 in the Holocene.

Measuring soil erosion by field plots: Understanding the sources of variation

Soil erosion plots of different types and sizes are widely used to investigate the geomorphological processes related to soil erosion. This field method has provided a variety of results, depending on the characteristics of the plots, on their suitability to reflect the ecosystem's characteristics and on the objectives of each particular research. The coupling of real soil loss at patch and slope scale within a landscape and the values obtained by field plots depend, among other things, on how good the methodology performs over a set of ecosystem properties, such as those related with temporal and spatial scale issues, disturbance and representation of natural conditions, and the ability to account for the complexity of ecosystem interactions. Here, we present a review of (i) the advantages and limitations of the use of field plots to measure soil erosion; and (ii) the potential sources of variation in the results obtained due to a lack of harmony between methodological conditions and the processes operating in the environment at different scales. As regards the spatial and temporal scale of measurements, topics such as the exhaustion of available material within closed plots in long term measurements, the different erosion processes operating (and measured) at different spatial scales and the problems and alternatives of extrapolation of the results from larger to smaller scales, are the main causes of variation between measurements. The disturbance and inadequate representation of natural conditions, such as the heterogeneity, continuity and connectivity of factors and processes, are also sources of variation in the results of specific measurements. In short, the key factor is the difficulty to encapsulate the complexity of system interactions and to represent these interactions by means of field plots. The complexity concept is translated in the connectivity of water and sediment fluxes in the landscape and the interaction between processes and patterns of vegetation and surface components operating across scales.

Causes and underlying processes of measurement variability in field erosion plots in Mediterranean conditions

AbstractAn understanding of the sources of variation in the use of erosion plots and of their feasibility to meet the objectives of each specific research project is key to improving future field designs, selecting data for modelling purposes and furthering knowledge of soil erosion processes. Our own field experiences from ongoing research on soil erosion processes since 1989, have allowed us to detect several methodological problems that cause measurement variability. Here several examples are presented concerning: (i) differences in long‐term soil erosion data between open and closed plots; (ii) differences in soil loss derived from replica soil erosion plots; and (iii) differences in soil loss data derived from plots at a range of spatial scales. Closed plots are not the most suitable method for long‐term monitoring of soil erosion rates due to the risk of exhaustion of available material within the plot. The difference in time after which exhaustion occurs depends on the surface soil characteristics, the climatological conditions and the size of the plots. We detected four and seven years as ‘time to exhaustion’. Different results are frequently obtained between pairs of replica plots. Differences up to a factor of nine have been detected in total soil loss between replica plots due to different spatial patterns of surface components. Different constraints appear depending on the spatial scale of measurement of soil loss. We obtained lower runoff percentages at coarser scales; however, larger sediment concentrations are observed at coarser scales (1·32 g l−1, catchment; 0·30 g l−1, 30 m2; 0·17 g l−1, 1 m2 scales). The smaller the plot, the larger the hydrological disconnection within the system, the lower the energy flows due to short distances and the quicker the response to runoff due to an artificial decrease of concentration times for continuous flow. Copyright © 2006 John Wiley &amp; Sons, Ltd.