Estimation Of Soil Erosion Rates Research Articles

DEM spatial resolution sensitivity in the calculation of the RUSLE LS-Factor and its implications in the estimation of soil erosion rates in Ecuadorian basins

DEM spatial resolution sensitivity in the calculation of the RUSLE LS-Factor and its implications in the estimation of soil erosion rates in Ecuadorian basins

Reconstruction of historical soil surfaces and estimation of soil erosion rates with mound measurements and UAV photogrammetry in Mediterranean olive groves

Soil water erosion is one of the most important environmental problems for the sustainability of Mediterranean olive groves on hillsides. Governments and public agencies recognize the need to control this process in order to improve soil conservation, especially in vulnerable areas. In the present study, a simple, inexpensive method using Structure-from-motion (SfM) and Unmanned Aerial Vehicle (UAV) technology was applied to quantify the soil loss rates provoked by water erosion and tillage in mountain olive groves, according to a reconstruction of their historical surface features. Specifically, the main study aims were: i) to quantify the historical soil loss in olive groves, by analysing residual tree mounds; ii) to consider how soil relief and management can influence the erosion process; iii) to determine the degree to which the proposed method achieves the above aims.Analysis revealed a mean erosion rate in the study area of 127.69 t ha−1 years−1, with a linear relationship between soil truncation and slope (R2 = 0.64, p < 0.001). The highest soil loss rates (-1.67 ± 0.48 m) occurred in areas where the slopes were steepest (22.36 ± 4.46 %) and the longitudinal profile rectilinear. Erosion rates are determined not only by the slope, but also by its position and distance from the tree mound. This study highlights the need for differential management based on slope-feature considerations, in order to limit soil losses. Overall, the tool presented to support decision making provides an effective method for calculating erosion rates.

Relating 137Cs and sediment yield from uncultivated catchments: the role of particle size composition of soil and sediment in calculating soil erosion rates at the catchment scale

PurposeSoil loss is considered one of the most important consequences of land degradation as it affects the production of agricultural and forested areas, and the natural equilibrium of aquatic ecosystems downstream. For these reasons, the availability of tools and techniques able to identify areas at risk of land degradation is essential. Over the last 3–4 decades, theoretical models, based on the use of 137Cs, an anthropogenic radiotracer, proved to be very effective for this purpose. However, these models require specific information on soil and sediment particle size to provide estimates of soil erosion or deposition and this information is summarised by a particle size correction factor ‘P’. Empirical methods of calculation of this factor assume the basic hypothesis that a particle size selectivity takes place in erosion processes and this results in a general enrichment of the fine component in sediments and a corresponding higher radionuclide activity. In this contribution, we demonstrate that this hypothesis is not valid everywhere, and consequently, the P factor cannot be estimated using traditional approaches.Materials and methodsA long-term experiment, conducted in Southern Italy and based on two small experimental catchments (approximately 1.5 ha in size), for which measurements of sediment yield are available for the period 1978–2020, is used in this work. More specifically, 137Cs measurements carried out within the catchments and on a reference area provided the basis to obtain long-term estimates of soil erosion rate in these sites. Combined measurements of 137Cs activity and particle size on both soils and sediments, obtained for 46 events, were also carried out to explore possible particle size effects on the final estimates of soil loss.Results and discussionParticle size analyses of soil and sediments showed that there is evidence of a general enrichment of the eroded soil in the finer size fractions. Conversely, radiometric analyses revealed that 137Cs activity in sediments is generally lower than that in surface soil. These results reflect both the decreasing 137Cs activity associated with depth in undisturbed soils and the higher specific surface area of the deeper horizon in these soils. These findings preclude the application of the available empirical models to calculate P, and suggest the opportunity to use, for long-term estimates of soil erosion, a particle size correction factor P = 1. This assumption and an uncertainty analysis associated with the spatial variability of the 137Cs reference value were incorporated into the Diffusion and Migration Model (DMM) to obtain estimates of soil erosion rates for the study catchments.ConclusionsThe final estimates of soil erosion provided by the DMM showed values very close to the measurements of sediment yield obtained for the two catchments during the study period. The overall results demonstrated that the DMM, if properly calibrated using specific information of particle size and of 137Cs reference value, can be considered a useful tool to individuate areas more prone to risks of land degradation and to identify appropriate strategies able to reduce soil loss in forested sites.

Integrating Source and Transport Factors with Best Management Practices to Derive an Index for Assessing Phosphorus Mobilization Risks from Mauritius Sugarcane Fields

Application of phosphorus (P) fertilisers to sugarcane fields in Mauritius increased almost four-fold per unit area over the past 60 years. Some of the applied P accumulated in the soils and can therefore be transported eventually to surface waters resulting in the eutrophication thereof. Precaution measures such as an appropriate P index as a management tool is required. Source factors (dissolved P, particulate P, P application rates, methods and timing), transport factors (soil erosion, runoff potential and precipitation factor) and a best management practices multiplier were integrated to derive an index for assessing risks of P mobilisation from the island’s sugarcane fields. Farmers and their advisors can use the proposed P index during the planning process before sugarcane fields are planted and will be applicable for the whole crop cycle of 6 - 7 years if factors in the index do not change. The index can be also valuable in the selection of alternative management practices that could reduce the risks of P losses from sugarcane fields where the potential of P movement is initially high. Sensitivity analyses and edge-to-plot field tests showed that the P index needs further improvement, especially the estimation of soil erosion rates. The P index can, however, be applied by farmers and their advisors if they are well informed about the index’s capability.

Estimates of soil erosion rates in a principal watershed in Gozo, Malta under current and future climatic conditions

The aim of the study was to estimate the current and future annual soil erosion rate in the Ramla watershed situated in Gozo, Malta. The RUSLE-GIS was applied using innovative approaches, such as the inclusion of the highest possible DEM data as well as reflecting climatic changes to the R Factor based on the latest climate projections for Gozo derived from a total of eight CMIP6 climate change models. A climate change impact analysis was carried to estimate the likely future risk of soil erosion in this local watershed for the first time. The estimated annual soil loss, as a percentage of the total area of study varies from low (85.1%), to moderate (13.3%) and high (1.6%). Within a climate change context, a reduction in the annual soil erosion rate has been estimated for the period 2080–2100, ranging from −5 ton ha−1 yr−1 under SSP126 to −6 ton ha−1 hr-1 under SSP585. The estimated decrease of annual soil loss within the high soil erosion risk category is attributed to an increased aridity of the local climate as projected by eight CMIP6 climate models. Specific hydrological sub-watersheds were identified within the study area that warrant urgent considerations. The outcomes of this study are based on the inclusion of micro-topographical information derived from high resolution 1 m LiDAR DEM. This modelling work is seen as the first study which can be used to provide some objective guidance on the minimization of potential soil losses within the Ramla watershed. Our estimated soil loss and its spatial distribution can assist in a fuller management plan for the sustainability of this watershed. These results should not be interpreted in absolute terms but as an understanding of the underlying controlling processes, the relative spatial and temporal variations, and of possible future scenarios.

137Cs estimates of soil erosion rates in a small catchment on a channelized river floodplain in the lower reaches of Yangtze River, China

Channelization significantly affects soil erosion in river floodplains. The object of this study was to use 137Cs as a tracer to determine the 137Cs inventory and derived soil erosion rates under various land use types in a catchment on channelized river floodplain in the lower reaches of Yangtze River, China. Sampling was carried out to establish a137Cs reference inventory in a 70-year old paddy field located on the shoulder-slope of a local hill. The mean reference inventory of 137Cs was 1275 Bq m−2, whereas the 137Cs inventory within the catchment ranged from 284 to 1150 Bq m−2 and the soil erosion rates from −33.3 to −2.4 t ha−1 yr−1, respectively. The dominated land use of paddy in cultivated soils contributed relative low soil erosion. Bamboo and castanea mollissima were preferential for local land uses in uncultivated soils in comparison with woodland and Pinnus massoniana. The rates of soil erosion rates in old tea garden were higher than that in new tea garden. Overall, severe soil erosion and no deposition in the entire catchment occurred in the entire catchment due to the human-induced channelization in the 1970s. Our results suggest that restricting farmland being returned to tea plantations, thereby maintaining the current land use types would reduce soil erosion in river floodplain in the future.

Use of caesium-137 re-sampling and excess lead-210 techniques to assess changes in soil redistribution rates within an agricultural field in Nakhla watershed

The Nakhla watershed was one of the most affected areas by water erosion in Northwest Morocco. This phenomenon caused the sedimentation of the Nakhla dam, an important water reservoir that contributed to water supply in the region. The motivation behind our investigation is to know the evolution of the situation in this watershed. The objective of this study is to use fallout Caesium-137 and excess Lead-210 to assess changes in soil redistribution rates during the last century and the impact of recent soil conservation practices within an agricultural field in the Nakhla watershed. The Caesium-137 technique, which has proved its efficiency in estimating erosion rates over the last 50–60 years, was associated with the re-sampling approach in the current study to reveal the response of soil redistribution rates to land use change over the last fifteen years within an upland agricultural field in Northwest Morocco under Mediterranean conditions. The excess Lead-210 was used to generate data on soil movement over a longer time window of 100 years. Subsequently, the combined use of Caesium-137 re-sampling with excess Lead-210 techniques allowed for the estimation of soil erosion rates over three periods: 1954–2002, 1954–2017 and 1917–2017. Quantification of water erosion with excess Lead-210 indicated a rate of 27 t ha−1 yr−1. Based on Caesium-137 measurements, the comparative analysis of the soil redistribution estimates over two periods (1954–2002 and 1954–2017) shows that soil loss rates have significantly decreased from 36 t ha−1 yr−1 to 29 t ha−1 yr−1 during the period between the two sampling campaigns (2002–2017) due to the beneficial impact of agricultural practices. This decrease is consistent with the available records of sediment yield in the Nakhla dam during these periods. The main findings show that the sustainable agricultural practices and the implemented soil erosion control strategy, based on more frequent fallow with natural vegetation and olive plantations, were effective in reducing soil erosion in the investigated area.

A Review on Rainfall Erosivity (R factor) in Universal Soil Loss Equation

Land degradation caused by soil erosion by water can endanger the environment around the world. The estimation of soil erosion rates is mainly dependent on modeling rainfall erosivity, land use changes and influences of management on soil erosion rates. The model of soil erosion prediction often used by researchers around the world is Universal Soil Loss Equation (USLE). Rainfall amount and intensity are the most important variable for rainfall erosivity and written as the R factor in the USLE model. Among the factors in the USLE, rainfall erosivity is of high importance as precipitation is the main driver for soil erosion by water and has a direct impact on the detachment of soil particles, the breakdown of aggregates and the transport of eroded particles via runoff. USLE erosivity combines the energy of the rainfall and the maximum continuous 30-min intensity in the rainfall event. Energy of rainfall is estimated as a function of the storm intensity through the rainfall event.

Estimation of soil erosion rates in dehesas using the inflection point of holm oaks

The exposure of tree or shrub roots represents a clear indicator of soil loss in the surrounding area. Previous studies have used this type of botanical evidence to estimate soil erosion rates, preferring roots at some distance from the tree stem because the inflection point (or root collar) undergoes a vertical displacement caused by basal growth. The aim of this paper is to develop a method that combines the inflection point of holm oaks and high-resolution digital images obtained by means of a terrestrial laser scanner to estimate soil erosion rates in Mediterranean dehesas. To correct for vertical displacement, the basal growth of each tree was determined in the field, differentiating the part of the roots that had been exposed by erosion from that generated by wood growth. Two datasets obtained through different approaches were used for validating the basal growth measured in the field survey: (i) a growth study analysing stems and cores of holm oaks and (ii) field measurements of basal growth of holm oaks located in an area without soil erosion. The results obtained by each method offered similar mean values (12.7 and 8.4 cm, respectively) as compared to the values estimated in the field survey (10.2 cm). Afterwards, soil erosion rates were corrected, reducing the values on average by 40%, with a mean loss of 26.7 t ha‐1 y−1. No relationship was found between tree age and basal growth. On the other hand, soil erosion rates showed a slightly negative trend with relation to tree age, indicating an upward tendency of soil losses from the year 1640 to 2014. Finally, it was possible to define three periods with varying erosion rates: 10.4 t ha−1 y−1 (1640–1830), 19.2 t ha−1 y−1 (1831–1888) and 27.2 t ha−1 y−1 (1889–2014).

An assessment of the global impact of 21st century land use change on soil erosion

Human activity and related land use change are the primary cause of accelerated soil erosion, which has substantial implications for nutrient and carbon cycling, land productivity and in turn, worldwide socio-economic conditions. Here we present an unprecedentedly high resolution (250 × 250 m) global potential soil erosion model, using a combination of remote sensing, GIS modelling and census data. We challenge the previous annual soil erosion reference values as our estimate, of 35.9 Pg yr−1 of soil eroded in 2012, is at least two times lower. Moreover, we estimate the spatial and temporal effects of land use change between 2001 and 2012 and the potential offset of the global application of conservation practices. Our findings indicate a potential overall increase in global soil erosion driven by cropland expansion. The greatest increases are predicted to occur in Sub-Saharan Africa, South America and Southeast Asia. The least developed economies have been found to experience the highest estimates of soil erosion rates.

Improving stock unearthing method to measure soil erosion rates in vineyards

Vineyard soils experience high erosion rates compared to soils from other agricultural land uses. The high soil losses in vineyards limits the sustainability of traditional production schemes and warrants comprehensive research aimed at thwarting the main erosion processes affecting vineyard systems. However, long-term measurements, which include spatial variability of soil erosion rates at the plot scale, are uncommon, as most of the measurements have taken place either at the hillslope or watershed scales. Against this background, the stock unearthing method (SUM) can be considered a useful methodology. However, the current method falls short because it assumes that the topography between the vine lines (inter-rows) remains planar. Therefore, we propose a new methodology (ISUM: improved stock unearthing method) that includes three measurements between in the inter-row areas. By taking inter-row measurements, we hypothesized that the spatial patterns of sediment detachment, transport and deposition features in the inter-rows would be detected. The ISUM costs 20% more time to conduct than the SUM, but greatly improved the utility of the field survey. ISUM allowed for: i) the creation of maps that identified linear soil erosion features and accumulation sites; ii) measures the amount of soil accumulated under the vines; iii) estimation of soil erosion rates with higher accuracy at long-term periods in a specific moment. This study compared the ISUM with the SUM in a vineyard in Spain. Soil erosion rates with ISUM were −2.5 Mg ha yr−1 while the SUM calculated rates of +4.9 Mg ha yr−1.Results showed that the traditional method underestimated soil rates a −25.7%. Maps created with the ISUM technique could also be used to provide insights about sediment connectivity.

GIS-based estimation of soil erosion rates and identification of critical areas in Anambra sub-basin, Nigeria

Estimation of soil loss through water erosion is an essential exercise which can help decision makers and planners determine the severity of soil loss through rill and sheet erosion and also curtail the development of further gullies in an area already ravaged by gully erosion. While Universal Soil Loss Equation (USLE) is the most commonly adopted model because it provides a straight forward approach for qualitative estimation of soil loss, however its rainfall erosivity component is found incompetent in most parts of the world. To overcome this deficiency, the Revised Universal Soil Loss Equation (RUSLE) was implemented using rainfall erosivity (R) values peculiar to tropical environment of the Anambra area of Nigeria. Rainfall erosivity (R-factor), soil erodibility (K-factor), slope factor (LS-factor), and cover management (C-factor) were generated in GIS environment and then integrated based on RUSLE equation to estimate the rate of soil erosion. The study indicated that about 1804.39 km2 (39.49 %) of the study area have slight erosion rate of 0–10 t ha−1 year−1, while the rates of erosion in 746.60 km2 (16.34 %), 1025.38 km2 (22.44 %), 659.55 km2 (14.43 %), 287.08 km2 (6.28 %), and 46.59 km2 (1.02 %) of the study area are 10.6–85.3, 85.4–235.2, 235.3–608, 608.1–2200 and >2200.1 t ha−1 year−1 respectively. The study revealed that high rainfall erosivity combined with moderate to high slope factor and decreasing vegetal cover are the major factors driving soil loss in the area.

Estimation of Soil Erosion Rates in Oil Palm Plantation with Different Land Cover

Soil losses from hill slopes in oil palm plantation in Sedenak Estate, Johor were measured using runoff plot and rainfall simulator. The plot was designed to be removable but the size was fixed at 8 x 3.75m. Four types of surface covers were investigated for the plots, i.e. half bare soil and half grass cover (HGC), half bare soil and half dry frond (HDF), fully grass cover (FG), and fully bare soil (BS). The influence of initial soil moisture, saturated hydraulics conductivity, Ks, bulk density and slope on rates of soil loss were also evaluated. The rainfall simulator produced rainfall intensities between 90 and 160 mm/hr with durations from 45 to 60 min per run. BS plot exhibited the highest Ks value among all plots but the percentage of initial soil moisture on this surface was low. BS plot recorded the highest runoff coefficient (C) and soil loss values of 73.6 ± 4 percent and 5.26 ± 3.2 t/ha respectively, while the lowest was from plot FG with 41.7 ± 5.7 percent and soil loss of 2.85 ± 2.1 t/ha. Meanwhile, the results suggested that the ground cover had the ability to reduce soil loss by 67% and 17%, respectively for plots BS-HGC and BS-HDF. Overall, soil erosion control such as surface is effective measures in reducing level of runoff and soil erosion.

Assessing water erosion in Mediterranean tree crops using GIS techniques and field measurements: the effect of climate change

In this work, a dynamic GIS modeling approach is presented that incorporates: a) geoinformatic techniques, b) 55-year historical meteorological data, and c) field measurements, in order to estimate soil erosion risk in intensively cultivated regions. The proposed GIS-based modeling approach includes the estimation of soil erosion rates due to surface water flow under current and future climate change scenarios A2 and B1 for the years 2030 and 2050. The soil erosion was estimated using the Universal Soil Loss Equation (USLE). The proposed soil erosion model was validated using field measurements at different sites of the study area. The results show that an extended part of the study area is under intense erosion with the mean annual loss to be 4.85 t/ha year−1. Moreover, an increase in rainfall intensity, especially for scenario B1, can generate a significant increase (32.44 %) in soil loss for the year 2030 and a much more (50.77 %) for the year 2050 in comparison with the current conditions. Regarding the scenario A2, a slight decrease (1.85 %) in soil loss was observed for the year 2030, while for 2050 the results show an adequate increase (7.31 %) in comparison with the present. All these approaches were implemented at one of the most productive agricultural areas of Crete in Greece dominated by olive and citrus crops.

Erosion rate predictions from PESERA and RUSLE at a Mediterranean site before and after a wildfire: Comparison & implications

Mediterranean ecosystems are commonly vulnerable to wildfires. Accelerated erosion processes due to wildfires in those environments constitute a major restrictive factor in their sustainability. This study aims at evaluating the use of the Revised Universal Soil Loss Equation (RUSLE) and the Pan-European Soil Erosion Risk Assessment (PESERA) models in predicting the changes in spatial variability of soil erosion following a wildfire event. A site in Greece on which a wildfire occurred in the summer of 2007 is used as a case study. Soil erosion rates for the site before and after the fire outbreak were estimated by the two models. Inputs for both models included climatic, land-use, soil type, topography, Earth Observation (EO) as well as management and other ancillary data.Both models showed a substantial increase of soil erosion rates in the affected area as a result of the fire, particularly towards the steeper slopes and on areas of high burning severity. Yet, there were noticeable differences in the predictions between the 2 models in terms of absolute estimates of soil erosion rates before and after the fire event. Mean pre-fire erosion rates from RUSLE were ~2.5 times higher than those from PESERA. RUSLE predicted considerably higher mean erosion in comparison to PESERA for the post-fire conditions, yet of much less spatial variability. Average post-fire soil loss value, compared to pre-fire, was about nine and six times greater when using the RUSLE and the PESERA model respectively. RUSLE predictions were most sensitive to topographic and rainfall erosivity factors. PESERA showed high sensitivity to the vegetation coverage as well as to the soil characteristics inserted as crusting and erodibility variables.To our knowledge, this is the first study performing an intercomparison of soil erosion rate predictions between these two models, particularly so in the context of the influence of wildfire events. This study provides a key contribution towards our ability to better understand the effect of fire on soil erosion in the Mediterranean and elsewhere, as well as the ability of those widely used models as efficient tools to be used for this purpose. The latter is of key significance and practical value for research and policy decision making purposes alike, where information on spatiotemporal estimates of soil erosion rates may be required.

Estimation of Soil Erosion Rates and Eroded Sediment in a Degraded Catchment of the Siwalik Hills, Nepal

The Siwalik Hills is one of the most fragile and vulnerable ecosystems in the Nepalese Himalaya where soil erosion and land degradation issues are fundamental. There is very limited knowledge on soil erosion processes and rates in this region in comparison to other regions of the Himalaya. The aims of the present paper are to document, measure and interpret key soil erosion processes and provide an estimate of erosion rates in the Khajuri Stream catchment located in the eastern Siwalik Hills. We used erosion pins to monitor sheet erosion, gully erosion, landslides and stream bank erosion over the period from 2002 to 2004. Sheet erosion from forest and shrubs generally varied from 0.8–1.2 mm·yr−1 with a mean erosion rate of ~16 t·ha−1·yr−1. Gully erosion rate was estimated to be ~14 t·ha−1·yr−1. Erosion from landslides was significantly higher which was estimated to be ~26 t·ha−1·yr−1. Stream bank erosion varied widely from 0.03 to 0.25 m·yr−1 with a mean erosion rate of ~8 t·ha−1·yr−1. Based on these rates, it was estimated that ~21,000 m3 (64 t·ha−1) of sediment was being eroded within the catchment annually. In comparison to the erosion rates of other regions of the Himalaya these rates are significantly higher.

Sediment tracers in water erosion studies: current approaches and challenges

Introduction Interest in the use of sediment tracers as a tool to complement traditional water erosion or deposition measurements has increased due to the additional information they provide, such as sediment source identification, tracking of sediment movement across the landscape at various temporal and spatial scales, and estimation of soil erosion rates. For these reasons, the utility and robustness of sediment tracing approaches using a wide range of substances and soil properties have been evaluated in numerous studies. Conclusions A review of established tracing approaches identified five distinct groups of tracing approaches: fallout radionuclides, rare earth elements, soil magnetism and magnetic substances, other tracers, and sediment fingerprinting techniques. This paper describes the basic theory of each tracing approach in assessing soil erosion and sediment redistribution, describing their methodology and main applications, and summarizing the commonalities and differences between the approaches. It also identifies research gaps and future trends.

Estimation of soil erosion rates in a subtropical mountain watershed using 137Cs radionuclide

The formulation of watershed management strategies to protect water resources threatened by soil erosion and sedimentation requires a thorough understanding of sediment sources and factors that drive soil movement in the watershed. This paper describes a study of medium-term water-driven soil erosion rates in a mountainous watershed of the Shihmen Reservoir in Taiwan. A total of 60 sampling sites were selected along a hillslope. At each sampling site, the inventory 137Cs activity was determined and then calculated with the diffusion and migration model to derive soil erosion rates. The rates are one to two orders of magnitude lower than estimates using the Universal Soil Loss Equation, a soil erosion model often used in Taiwan. Results of multiple regression analysis indicate that the spatial variability of soil erosion rates is associated with the relative position of a sampling site to the nearest ridge and soil bulk densities (r 2 = 0.33, p < 0.01). Finally, the patterns of soil redistribution rates on the hillslope follow the 137Cs hillslope model as soil erosion increases in the downslope direction. No deposition site is found at footslope because soil deposition is swept away by regular flooding along the stream channel. This study is an important first step in using 137Cs as a tracer of soil redistribution in mountainous watersheds of Taiwan.

Estimation of soil erosion on cultivated fields on the hilly Meghalaya Plateau, North-East India

The estimation of soil erosion rates in complex subtropical agricultural systems of hilly environment is difficult and most of the traditional methods have serious limitations. The 137Cs technique allows to obtain relatively quickly retrospective medium term soil erosion results. The objective of this study was using 137Cs approach to quantify soil loss under agricultural system which develops under growing human pressure on the hilly terrain of the Meghalaya Plateau. The measured values of caesium inventory for all sampling points are between 2% and 63% of the reference value of caesium inventory. The estimated annual soil loss for sampling points located on the slope are between 29 and 79 Mg·ha−1·yr−1 in the case of an improved mass balance model. It means that soil erosion in this manually tilled agricultural area cannot be neglected.

Validating erosion rate estimates provided by caesium‐137 measurements for two small forested catchments in Calabria, southern Italy

AbstractIncreasing concern for problems of soil degradation and the off‐site impacts of accelerated erosion has generated a need for improved methods of estimating rates and patterns of soil erosion by water. The use of environmental radionuclides, particularly 137Cs, to estimate erosion rates has attracted increased attention and the approach has been shown to possess several important advantages. However, the use of 137Cs measurements to estimate erosion rates introduces one important uncertainty, namely, the need to employ a conversion model or relationship to convert the measured reduction in the 137Cs inventory to an estimate of the erosion rate. There have been few attempts to validate these theoretical conversion models and the resulting erosion rate estimates. However, there is an important need for such validation, if the 137Cs approach is to be more widely applied and reliance is to be placed on the results obtained. This paper reports the results of a study aimed at validating the use of two theoretical conversion models, namely the exponential depth distribution model and the diffusion and migration model, that have been used in several recent studies to convert measurements of 137Cs inventories on uncultivated soils to estimates of soil erosion rates. The study is based on data assembled for two small catchments (1.38 and 1.65 ha) in Calabria, southern Italy, for which measurements of sediment output are available for the catchment outlet. The two catchments differ in terms of the steepness of their terrain, and this difference is reflected by their sediment yields. Because there is no evidence of significant deposition within the two catchments, sediment delivery ratios close to 1.0 can be assumed. It is therefore possible to make a direct comparison between the estimates of the mean annual erosion rates within the two catchments derived from 137Cs measurements and the measured sediment outputs. The results of the comparison show that the erosion rate estimates provided by both models are reasonably consistent with the measured sediment yields at the catchment outlets. However, more detailed assessment of the results shows that the validity of the erosion rate estimates is influenced by the magnitude of the erosion rates within the catchment. The exponential depth distribution model appears to perform better for the catchment with higher erosion rates and to overestimate erosion rates in the other catchment. Similarly, the basic migration and diffusion model performs better for the catchment with lower erosion rates and overestimates erosion rates in the other catchment. However, the improved migration and diffusion model appears to perform satisfactorily for both catchments. There is a need for further studies to extend such independent validation of the 137Cs technique to other environments, including cultivated soils, and to other conversion models and procedures. Copyright © 2003 John Wiley &amp; Sons, Ltd.