Assessment of soil redistribution in a typical karst catchment using 137Cs

An effective assessment of soil erosion and redistribution is a prerequisite for soil erosion control and is critical to achieving sustainable development goals. The most typical landscape in the karst region of Southwest China is found in the peak-cluster depression area, but little attention has been given to the soil redistribution here. A typical karst peak-cluster depression catchment water area in Southwest China was selected, and ¹³⁷Cs technology was used to evaluate the soil redistribution rate and soil erosion process along a total transect (hillslope, depression and sinkhole) in the catchment. The results showed that the distribution of ¹³⁷Cs had a high spatial variability on the total transect of the catchment (CV = 60.04%), the middle slope was the most severely eroded (highest erosion rate of 13.49 t ha^-1 yr^-1), and the area between the bottom slope and the depression was the primary sedimentary area on the surface in the catchment. The distribution of soil properties on the hillslope was affected by the process of soil redistribution. According to the distribution of the ¹³⁷Cs soil profile, the soil profile of the hillslope was uniform, and signs of historical tillage activities were evident; the historical tillage activities of depressions were in the range of 0-20 cm depth, while the ¹³⁷Cs in the sinkhole was mostly distributed in the shallow layers and decreased exponentially with depth, reflecting the depositional characteristics of noncultivated soil. In addition, this study found evidence of underground soil loss in sinkholes since the 1960s; the shallow sediment of these sinkholes mainly came from depressions, with an average deposition rate of 11.77 t ha^-1 yr^-1. Human disturbance and land-use change controlled recent changes in soil redistribution. The soil erosion rate of the hillslopes in catchments was extremely low (average erosion rate of 1.92 t ha^-1 yr^-1). The rocky desertification of hillslopes occurred before 1960; it was not a short-term contemporary process that occurred only during recent decades. This study showed that underground soil loss mainly occurred through sinkholes for a short period of time (100 years). These research results are of great significance for understanding the evolution of rocky desertification and the process of soil erosion.

Long-term, process-based, continuous simulations for a small, nested rangeland watershed near Tombstone, AZ (USA): Extending model validity to include soil redistribution

Soil or sediment redistribution prediction along hillslopes and within small watersheds is considered to be a great challenge for the application of watershed erosion models in predicting the impact of soil and water conservation measures as well as for the redistribution of pollution such as radioactive fallout. In this study, long-term soil loss and deposition were estimated for two nested semi-arid watersheds within the Walnut Gulch Experimental Watershed in Southeastern Arizona using the process-based Geo-spatial interface of WEPP (GeoWEPP). While soil parameters were previously parametrized and validated through watershed outlet runoff and sediment yields, the channel parameters were adjusted and validated based on reference values of soil redistribution generated from fallout radionuclide ¹³⁷Cs samples within the watersheds. Two methods were applied for the soil redistribution analysis by comparing observed and simulated soil loss/deposition rates (a) at single pixels and reference values at the specific location of each ¹³⁷Cs sample site; and (b) for average values of a 5 m radius around each ¹³⁷Cs sample site to compensate for measurement and model uncertainties. Surprisingly, soil redistribution predictions improved as topographic data resolution increased from 5 m to 3 m and were best at 1 m without changing key model parameters that were originally derived at the watershed scale.

Modifications in the gamma dose rate in air due to downward and lateral mobility of 137Cs in the soil

¹³⁷Cs can be an important environmental contaminant due to fallout from nuclear reactor accidents and atomic weapons testing. Its contribution to the air gamma dose rate at 1 m height above contaminated ground depends on the soil inventory, the migration processes along the soil profile and possible modifications in the spatial distribution. In this paper the diffusion-convection equation is used to approach the ¹³⁷Cs soil vertical migration transport and standard dose rate factors are jointly applied for estimating the air dose rate. In order to calculate the temporal reduction of the external radiation, an empirical attenuation factor is used and its coefficients are analyzed in terms of the effective diffusion coefficient and downward migration rates. Additionally, it is analyze the corrections that should be introduced in dose rate values attributable to soil redistribution processes. If these processes, natural or as a consequence of human activities, are not taken into account, both the air dose rate values and the attenuation period can be noticeably under or overestimated.

Establishing a tracer-based sediment budget to preserve wetlands in Mediterranean mountain agroecosystems (NE Spain)

Mountain wetlands in Mediterranean regions are particularly threatened in agricultural environments due to anthropogenic activity. An integrated study of source-to-sink sediment fluxes was carried out in an agricultural catchment that holds a small permanent lake included in the European NATURA 2000 Network. More than 1000 yrs of human intervention and the variety of land uses pose a substantial challenge when attempting to estimate sediment fluxes which is the first requirement to protect fragile wetlands. To date, there have been few similar studies and those that have been carried out have not addressed such complex terrain. Geostatistical interpolation and GIS tools were used to derive the soil spatial redistribution from point (137)Cs inventories, and to establish the sediment budget in a catchment located in the Southern Pyrenees. The soil redistribution was intense and soil erosion predominated over soil deposition. On the areas that maintained natural vegetation the median soil erosion and deposition rates were moderate, ranging from 2.6 to 6 Mg ha yr(-1) and 1.5 to 2.1 Mg ha yr(-1), respectively. However, in cultivated fields both erosion and deposition were significantly higher (ca. 20 Mg ha yr(-1)), and the maximum rates were always associated with tillage practices. Farming activities in the last part of the 20th century intensified soil erosion, as evidenced by the 1963 (137)Cs peaks in the lake cores and estimates from the sediment budget indicated a net deposition of 671 Mg yr(-1). Results confirm a siltation risk for the lake and provide a foundation for designing management plans to preserve this threatened wetland. This comprehensive approach provides information useful for understanding processes that influence the patterns and rates of soil transfer and deposition within fragile Mediterranean mountain wetlands subjected to climate and anthropogenic stresses.