Using Cs measurements to validate the application of the AGNPS and ANSWERS erosion and sediment yield models in two small Devon catchments

Distributed erosion and sediment yield models are being increasingly used for predicting soil erosion and sediment yields in agricultural catchments. In most applications, validation of such models has commonly been restricted to comparison of the predicted and measured sediment output from a catchment, because spatially distributed information on rates and patterns of soil redistribution within the catchment has been lacking. However, such spatially distributed data are needed for rigorous model testing, in order to validate the internal functioning of a model and its applicability at different spatial scales. The study reported in this paper uses two approaches to test the performance of the agricultural non-point source pollution (AGNPS) and areal non-point source watershed environmental response simulation (ANSWERS) erosion and sediment yield models in two small catchments in Devon, UK. These involve, firstly, comparison of observed and predicted runoff and sediment output data for individual storm events monitored at the basin outlets and, secondly, information on the spatial pattern of soil redistribution within the catchments derived from ¹³⁷Cs measurements. The results obtained indicate that catchment outputs simulated by both models are reasonably consistent with the recorded values, although the AGNPS model appears to provide closer agreement between observed and predicted values. However, the spatial patterns of soil redistribution and the sediment delivery ratios predicted for the two catchments by the AGNPS and ANSWERS models differ significantly. Comparison of the catchment sediment delivery ratios and the pattern of soil redistribution in individual fields predicted by the models with equivalent information derived from ¹³⁷Cs measurements indicates that the AGNPS model provides more meaningful predictions of erosion and sediment yield under UK conditions than the ANSWERS model and emphasises the importance of using information on both catchment output and sediment redistribution within the catchment for model validation.     

Holocene sediment budgets for upland catchments: The problem of soilscape model and data availability

This study presents a Holocene sediment budget for the upland catchment of the Speyerbach in the Palatinate Forest, southwestern Germany. The influences of both, data availability and the choice of an appropriate soilscape model on the sediment budget calculations are investigated. For budget calculations the spatial distribution of soils was derived from the soil map 1:50,000. Thickness values of soil truncation and colluvial burial were extracted from two soil data sets with varying information content. Data processing contained a disaggregation of the soil map with the help of land use data. In order to model the sediment budget a reference soil thickness (a so-called soilscape model), which represents the initial conditions is necessary. As upland soils are developed in periglacial solifluction sheets, the main solifluction sheet (MSS, “Hauptlage”) showing a constant thickness and being affected by soil forming processes was chosen.     

Establishing a Holocene sediment budget for the river Dijle

A Holocene sediment budget was constructed for the 758 km² Dijle catchment in the Belgian loess belt, in order to understand long-term sediment dynamics. Hillslope sediment redistribution was calculated using soil profile information from 809 soil augerings, which was extrapolated to the entire catchment using morphometric classes. As large parts of the forests within the catchment prove to have undergone little or no erosion since medieval times, a correction was applied for the presence of forests. Total Holocene erosion amounts 817 ± 66 Mt for the catchment, of which 327 ± 34 Mt was deposited as colluvium. This corresponds with a net Holocene soil erosion rate of 10.8 ± 0.8 × 10³ Mg ha^− 1 for the entire Dijle catchment. Alluvial deposits were studied through 187 augerings spread over 17 cross-valley transects. The total alluvial sediment deposition equals 352 ± 11 Mt or 42% of total eroded sediment mass. Results indicate that at the scale of a medium-sized catchment the colluvial sediment sink is as important as the alluvial sediment sink and should not be neglected. As a result the estimation of erosion through alluvial storage and sediment export would yield large errors. Dating of sediment units show an important increase in alluvial deposition from medieval times onwards, indicating the important influence of agricultural activities that developed from that period. Mean sediment export rates from the catchment for the last 1000–1200 years range between 0.8 and 1.3 Mg ha^− 1 a^− 1 and are consistent with present suspended sediment measurements in the Dijle. Erosion for agricultural land for this period is 9.2 ± 2.2 Mg ha^− 1 a^− 1. Sediment budgets for the various tributary catchments provide an insight in the sources and sinks of sediment at different scales within the catchment.     

Cesium and soil carbon in a small agricultural watershed

Scientific, political, and social interests have developed recently in the concept of using agricultural soils to sequester carbon. Studies supporting this concept indicate that soil erosion and subsequent redeposition of eroded soils in the same field may establish an ecosystem disequilibrium that promotes the buildup of carbon on agricultural landscapes. The problem is to determine the patterns of soil erosion and redeposition on the landscape and to relate these to soil carbon patterns. Radioactive ¹³⁷cesium (¹³⁷Cs) can be used to estimate soil erosion patterns and, more importantly, redeposition patterns at the field level. The purpose of this study was to determine the relationship between ¹³⁷Cs, soil erosion, and soil carbon patterns on a small agricultural watershed. Profiles of soils from an upland area and soils in an adjacent riparian system were collected in 5 cm increments and the concentrations of ¹³⁷Cs and carbon were determined. ¹³⁷Cs and carbon were uniformly mixed in the upper 15–20 cm of upland soils. ¹³⁷Cs (Bq g⁻¹) and carbon (%) in the upland soils were significantly correlated (r²=0.66). Carbon content of the 0–20 cm layer was higher (1.4±0.3%) in areas of soil deposition than carbon content (1.1±0.3%) in areas of soil erosion as determined by the ¹³⁷Cs technique. These data suggest that measurements of ¹³⁷Cs in the soils can be useful for understanding carbon distribution patterns in surface soil. Carbon content of the upland soils ranged from 0.5 to 1.9% with an average of 1.2±0.4% in the 0–20 cm layer while carbon below this upper tilled layer (20–30 cm) ranged from 0.2 to 1.5% with an average of 0.5±0.3%. Total carbon was 2.66 and 3.20 kg m⁻² in the upper 20 cm and upper 30 cm of the upland soils, respectively. Carbon content of the 0–20 cm layer in the riparian system ranged from 1.1 to 67.0% with an average 11.7±17.1%. Carbon content below 20 cm ranged from 1.8 to 79.3% with an average of 18.3±17.5%. Soil carbon in the upper 20 cm of the riparian profile was 10.1 and 15.0 kg m⁻² in the upper 30 cm of the riparian profiles. This is an increase of organic carbon by a factor of 3.8 and 4.7 for the upper 20 cm and upper 30 cm of the riparian profiles, respectively, when compared to the upland soil profiles.     

Integrated assessment of catchment suspended sediment budgets: a Zambian example

In many developing countries, the management of sediment‐related environmental problems is severely hampered by a lack of information on sediment mobilization and delivery in river basins. The sediment budget concept represents a valuable framework for assembling such information, which can, in turn, be used to assist with the design and implementation of soil erosion and sediment control policies. However, the information necessary to construct a catchment sediment budget is difficult to assemble. Against this background, an integrated approach to establishing a catchment suspended sediment budget, involving a river monitoring station, the use of ¹³⁷Cs measurements to estimate soil erosion and deposition and floodplain accumulation rates within the catchment, and sediment source fingerprinting, has been developed and tested in the 63 km² catchment of the upper Kaleya River in southern Zambia. The approach developed not only provides detailed information on individual components of the suspended sediment delivery system, but also permits the establishment of the overall catchment sediment budget. A sediment budget for the upper Kaleya catchment is presented and both its key features and its wider implications for catchment management are discussed. Copyright © 2001 John Wiley & Sons, Ltd.     

Investigating the effects of afforestation on soil erosion and sediment mobilisation in two small catchments in Southern Italy

Annual soil losses in southern Italy can exceed 100–150 t ha^− 1 year^− 1. Where erosion on agricultural land is particularly severe, land use change and afforestation are frequently seen as the most appropriate means of reducing erosion risk. However, the overall effectiveness of afforestation in reducing soil erosion remains uncertain, due to the poor development of the forest cover in some areas, leading to significant areas with sparse tree cover, and the erosional impact of forest harvesting, which commonly involves clearcutting. The study reported here addresses this uncertainty and focuses on two small catchments (W2 and W3) located in Calabria, southern Italy, for which measurements of suspended sediment yield are available. Both the catchments originally supported a rangeland vegetation cover and they were planted with eucalyptus trees in 1968. Currently, only catchment W3 supports a continuous forest cover. In catchment W2 the forest cover is discontinuous and there is a significant area of the catchment (ca. 20%) where the tree cover is sparse and the vegetation cover is dominated by natural grasses. Two additional erosion plots were established within catchment W2 in 1991, in order to explore the effect of the density of the tree cover on soil erosion. Information on the sediment yields from the two catchments and the plots for 10 storm events that occurred during the period December 2005–December 2006 and associated information on the ¹³⁷Cs and excess ²¹⁰Pb of the sediment, have been used to investigate the effectiveness of afforestation in reducing sediment mobilisation and net soil loss from the catchments involved. The results demonstrate that the areas of greatest soil loss are associated with the slopes where the tree cover is discontinuous, and that forest harvesting by clearcutting causes significant short-term increases in sediment mobilisation and sediment yield. These findings, which are consistent with previous work undertaken within the same area, emphasize the importance of vegetation cover density in influencing rates of soil loss in the study catchments. The study also provided a useful demonstration of the potential for using measurements of the ¹³⁷Cs and ²¹⁰Pb_ex content of sediment, in combination with more traditional sediment monitoring, to investigate sediment sources and to compare the sediment dynamics of catchments subjected to different land management practices.     

Application of catchment scale sediment delivery model INCA-Sed to four small study catchments in Finland

The novel catchment scale erosion and sediment delivery model INCA-Sed was applied to four small study catchments in Finland. Three of these, the Mustajoki, Haarajoki and Luhdanjoki, are headwater catchments located in central Finland. The associated rivers have differing morphological characteristics varying from a ditch to a small river. Soil textures in the area are derived from moraine deposits and are largely sand and gravel. The Mustajoki and Haarajoki catchments are forested and only 10% of the area is under cultivation. In the Luhdanjoki catchment agricultural fields cover 40% of the area. The fourth study site, the Savijoki catchment, represents an intensively cultivated area in south-western Finland. Cultivated fields cover 40% of the catchment area, and they are located on clay soils along the river. The INCA-Sed model was able to capture both the correct magnitude and seasonal behaviour of suspended sediment concentrations in the rivers, as well as the correct magnitude of the sediment load derived from different land use classes. Small differences in river morphology and soil textures between the catchments have a significant influence on suspended sediment concentration in the rivers. Correct timing of suspended sediment concentration peaks is not, however, captured by the INCA-Sed model, which may be due to the stochastic nature of erosion and delivery processes at the catchment scale which are not taken into account in the parameter values used in the modelling. Parameter values were estimated from previous researches based on average process loads. The INCA-Sed model was, however, generally found to be a suitable tool for evaluating effects of land use change on erosion and sediment delivery in Finland as it correctly reproduces spatial and seasonal variations in sediment delivery, in addition to annual averages with spatial and temporal variations.     

Evaluating gully erosion using Cs and Pb/Cs ratio in a reservoir catchment

Water erosion in the hilly areas of west China is the main process contributing to the overall sediment of the Yellow River and the Yangtze River. The impact of gully erosion in total sediment output has been mostly neglected. Our objective was to assess the sediment production and sediment sources at both the hillslope and catchment scales in the Yangjuangou reservoir catchment of the Chinese Loess Plateau, northwest China. Distribution patterns in sediment production caused by water erosion on hills and gully slopes under different land use types were assessed using the fallout ¹³⁷Cs technique. The total sediment production from the catchment was estimated by using the sediment record in a reservoir. Sediment sources and dominant water erosion processes were determined by comparing ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in surface soils and sub-surface soils with those of sediment deposits from the reservoir at the outlet of the catchment. Results indicated that landscape location had the most significant impact on sediment production for cultivated hillslopes, followed by the terraced hillslope, and the least for the vegetated hillslope. Sediment production increased in the following order: top>upper>lower>middle for the cultivated hillslope, and top>lower>upper>middle for the terraced hillslope. The mean value of sediment production declined by 49% for the terraced hillslope and by 80% for the vegetated hillslope compared with the cultivated hillslope. Vegetated gully slope reduced the sediment production by 38% compared with the cultivated gully slope. These data demonstrate the effectiveness of terracing and perennial vegetation cover in controlling sediment delivery at a hillslope scale. Averaged ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in the 0–5 cm surface soil (2.22–4.70 Bq kg⁻¹ and 20.70–22.07, respectively) and in the 5–30 cm subsoil (2.60 Bq kg⁻¹ and 28.57, respectively) on the cultivated hills and gully slopes were close to those of the deposited sediment in the reservoir (3.37 Bq kg⁻¹ and 29.08, respectively). These results suggest that the main sediment sources in the catchment were from the surface soil and subsoil on the cultivated slopes, and that gully erosion is the dominant water erosion process contributing sediment in the study area. Changes in land use types can greatly affect sediment production from gully erosion. An increase in grassland and forestland by 42%, and a corresponding decrease in farmland by 46%, reduced sediment production by 31% in the catchment.     

Soil erosion evaluation in a small basin through the use of Cs technique

Soil erosion significantly affects the most productive lands in Argentina, particularly the region called “Pampa Ondulada”. Quantification of the actual rates and patterns of soil loss is necessary for designing efficient degradation control strategies. The aim of this investigation was to gather using the ¹³⁷Cs technique a reliable set of data of erosion and sedimentation rates, in order to describe the long-term erosive landscape dynamic in a 300 ha basin representative for the “Pampa Ondulada” region of Argentina. The general topography of the basin is undulated with slopes gradients between 0 and 2.5% and slope lengths up to 800 m long. The main land use consisted in annual cropping under conventional tillage.For the soil erosion study in the basin the ¹³⁷Cs technique was used, which is based on the comparison between the ¹³⁷Cs inventories surveyed with a local reference ¹³⁷Cs profile. The sampling strategy was based on a multiple transect approach.The estimated mean soil erosion rates obtained applying Mass Balance Model 2 for the studied hillslopes ranged between −11.5 and −36 t ha⁻¹ per year and fitted the low and moderate erosion classes according to FAO. These values ranged beyond the admitted tolerance. Sedimentation was observed at the lower landscape positions probably related to changes from convex to concave slopes. The application of the ¹³⁷Cs technique in the studied basin proved to be a useful and sensible tool for assessing erosion/deposition rates. In areas with low topographic gradients like the Pampa Ondulada region, the slope length appears to be an important property for predicting spatial patterns of erosion rates.     

Radioactive cesium distribution in bamboo [Phyllostachys reticulata (Rupr) K. Koch] shoots after the TEPCO Fukushima Daiichi Nuclear Power Plant disaster

Abstract

Radioactivity levels of cesium (Cs)-134 and ¹³⁷Cs in bamboo [Phyllostachys reticulata (Rupr) K. Koch] sprouts grown from April to June 2011 over a wide area (including Fukushima Prefecture) were elevated (max. 3100 Bq kg^?1 fresh weight) after the Tokyo Electric Power Company, Inc. (TEPCO) Fukushima Daiichi Nuclear Power Plant disaster in March 2011. Bamboo sprouts in 2012 also contained high radioactivity levels. Radioactivity imaging analysis of bamboo sprouts harvested in 2012 showed increasing concentration gradients of radioactivity from the lower parts to the top of the sprouts. The peels were individually separated from the sprouts, and the inner edible part (trunk) was cross-sectioned at the internodal sections from the top to the lower parts. Each segmented trunk and its corresponding peel were analyzed for radioactive cesium (¹³⁴Cs and ¹³⁷Cs) and stable cesium (¹³³Cs). The concentrations of ¹³⁴Cs and ¹³⁷Cs showed significant increases from the lower part to the top, whereas ¹³³Cs showed an almost constant value in the trunk and peel except in the peel of the top node. We speculated that ¹³⁴Cs and ¹³⁷Cs in newly emerging bamboo sprouts in 2012 were translocated mainly from various plant tissues (where the fallout was layered on the bamboo tissues) in older bamboo, while ¹³³Cs was translocated from the soil through the roots of the new bamboo sprouts and was present in the roots and stems.