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.     

Long‐term root uptake of radiocesium by several crops

Abstract

A greenhouse study was conducted to evaluate the long‐term availability of radiocesium (Cs‐137) to various crops grown on contaminated soil that were limed to pH 6.8 (LIME) or were limed and Zn‐ and Mn‐EDTA chelates added (LIME + CHELATE). Crops were grown either continuously or followed a cropping sequence. Continuously grown clover and bahiagrass accumulated the most Cs‐137 with levels exceeding 2,000 pCi/g dry weight in bahiagrass. Uptake of Cs‐137 was depressed by both the LIME and LIME + CHELATE treatments. Uptake was usually greatest during the first three years but only in unlimed soil. Cesium‐137 is about 3 to 8 times higher in soybean beans than in wheat grain. Uptake of Cs‐137 by plants from contaminated soil can be expected to be high in acidic soils, especially in the absence of lime treatment.     

Slope Basis Deposits Linked to Long-Term Land Use in NE-Germany

In the absence of long-term records of soil erosion by land use change in the moraine landscape, NE-Germany, slope basis deposits offer considerable potential for reconstructing the changing soil erosion rates at a catena scale in response to the environmental change. A pit with a length of 61m and an average depth of 1.5 to 3m was opened along the catena, located 60km east of Berlin, NE Germany. The 137 Cs and 210 Pb ex dating in conjunction with soil profile analysis methods were used to define a chronology for the accumulated soil materials at slope basis of the catena. The temporal patterns of sediment accumulation rates at study catena were subdivided into five time periods. The deposited depth of sediments in the slope basis has increased eight to ten - fold during the periods between 1954 and 1986 compared to the deposit depth over the past 600 years. Over the last 100 years, deposited depth at the slope basis of the catena increased from 0.49 cm/yr to 1.04 cm/yr and then decreased to 0.45 cm/yr. Increase in sedimentation rate in the time period from 1897 to 1986 corresponded to intensive agricultural tillage operations whereas a decrease in sedimentation after 1986 corresponded to a land use change from arable land to grassland of the study catena.     

北京山区果园水平阶整地措施的水土保持效益

坡地果树产业已成为北京山区市县农业的重要组成部分.坡地果园坡度大,地表覆盖低,土壤侵蚀严重.水平阶整地是北京山区坡地果园的重要水土保持措施之一,定量测算坡地果园土壤侵蚀速率和水平阶整地措施的水土保持效益对于实现资源环境和经济的可持续协调发展具有重要的实际意义.分别采集北京山区无措施（昌平）和水平阶整地措施（密云、怀柔）后的土壤样品,通过测定137 Cs面积活度换算土壤侵蚀速率,估算北京山区坡地果园水平阶整地措施的水土保持效益.结果表明：昌平、密云和怀柔坡地果园的土壤侵蚀速率分别为5 331、2 494和1 970 t/（km2·a）,水平阶整地措施减少50％的土壤侵蚀量,可起到降低土壤侵蚀的作用;但是由于年久失修,没能完全发挥其应有的水土保持功效.     

人工林植被细根防蚀拦沙作用的有效性

选择我国西南山地典型人工林为对象,调查全山坡人工林的植被盖度及根系密度,利用137Cs示踪技术确定人工林坡地土壤侵蚀速率,探讨人工林坡地植被盖度及其细根与土壤侵蚀的关系.结果表明:整个人工林坡地表现为严重土壤侵蚀,侵蚀速率达25.25 t/(hm2·a).不同类型植被覆盖下土壤侵蚀再分布速率为裸地(-36.32 t/(hm2·a))＜乔木(-0.57 t/(hm2·a))＜灌木(0.53 t/(hm2·a))＜草(0.60t/(hm2·a)).回归分析表明,人工林坡地土壤侵蚀速率与植被盖度无显著相关性,土壤侵蚀速率随＜1mm的植被细根密度的增加而降低,二者之间呈显著负相关关系(P＜0.01),但与＞10 mm径级的植被粗根密度没有显著相关性.人工林地植被细根控制土壤侵蚀的有效性可以用植被细根密度与土壤再分布速率的关系方程来定量评价.当植被细根密度为60～100 g/m2时,可以看作人工林植被防蚀拦沙的有效根密度.结果表明,植被细根在控制坡地土壤侵蚀比植被盖度更为重要.因此,禁牧保护植被细根是控制人工林地土壤侵蚀与土地退化的有效措施.     

COMPARISON OF THE DEPTH DISTRIBUTION PROCESSES FOR 137Cs AND lePbu IN CULTIVATED SOILS

探讨了^137Cs与^210Pbex在农耕地土壤深度分布过程的差异。基于^137Cs与^210Pbex的不同沉降过程,考虑到核素由犁耕层向犁底层的扩散,对农耕地土壤^137Cs与^210Pbex的深度分布过程进行了理论推导,并以杨凌符家庄麦田剖面的实测数据予以验证,同时讨论了实测符家庄麦田剖面^137Cs与^210Pbex深度分布的规律特征及其原因,以此阐明了^137Cs与^210Pbex在农耕地土壤深度分布过程的差异。^137Cs源于大气核试爆,没有持续沉降补充,犁耕层和犁底层土壤^137Cs深度分布一直处于随时间变化的非稳定态;而210Pbex是天然核素,存在大气沉降的持续补充,犁耕层和犁底层土壤^210Pbex深度分布最终呈稳定态。农耕地土壤^137Cs与^210Pbex深度分布的实测值曲线与理论值曲线的差异,尤其^210Pbex可能与耕作深度的变化历史或土地利用（覆被）变化有关。     

黄土丘陵区水平梯田保水保土效益分析

水平梯田是防止水土流失的一种有效方式。在韭园沟流域的支沟——王茂沟流域分别选取梯田和坡耕地进行土壤采样,测定土壤含水量,结合¹³⁷Cs示踪,对比分析了梯田长期的保水保土效益。结果表明:梯田土壤水分变化分为土壤水分剧变层（0—20 cm）、土壤水分活跃层（20—30 cm）和土壤水分调节层（30 cm以下）;梯田不同深度土壤含水量均表现为内侧（第一级） > 中部 > 外侧（第五级）;坡向对梯田土壤水分的影响并不是十分显著;梯田0—20 cm土壤蓄水量高于坡耕地7.2%;0—40 cm平均土壤蓄水量高出坡耕地1.8 mm;研究流域坡耕地与梯田侵蚀程度分别属于极强度侵蚀和强度侵蚀,梯田平均水土保持效益为53%。建议在该地区修建梯田时,提高梯田的设计标准,对梯田田面、田埂认真维护,实现梯田长期的水土保持效益。     

用Cs-137示踪法研究浙江天台县典型坡面的土壤侵蚀规律

Cs-137示踪法具有简单、快速、准确的优点,已广泛应用于土壤侵蚀研究.本文在非耕地土壤侵蚀模型的基础上,在浙江省天台县典型区域内,得出Cs-137基准值为2382.02 Bq/m2,土壤Cs-137剖面分布函数为 Cs = 243.73e-0.113z,同时分析了非耕地典型坡面上不同部位的侵蚀与沉积规律.结果表明:土壤侵蚀主要发生在土壤表层20 cm以内的范围,坡地土壤净流失上部大于中下部,在距坡脚附近有土壤沉积现象.坡面不同部位的土壤侵蚀速率并不随着坡长的增加而简单地增大,而是随地表植被结构和覆盖度的不同而变化.     

坡耕地土壤侵蚀对土壤化学性质的影响

运用 137 Cs示踪了陕北安塞试区坡耕地土壤侵蚀量,并以室内分析的方法研究了黄土高原 北部坡耕地土壤侵蚀与土壤性状的关系。研究结果表明,黄土高原坡耕地土壤侵蚀量在 山坡中、上 部是侵蚀最严为强烈的地带,而在坡顶侵蚀较弱,在坡下有土壤堆积;土壤全N、碱解N和 速效K 与水蚀及耕作侵蚀成线性相关;有机质、速效P、阳离子代换量及土壤质地则与水蚀及耕 作侵蚀无 显著相关。     

青海湖布哈河口沉积物粒度特征及其环境意义

以青海湖西北部布哈河口湖泊沉积物为研究对象,用激光粒度仪对其进行粒度分析,并用137Cs定年法测定了该区沉积速率。结果表明:沉积物的粒度特征随着水动力条件的波动呈有规律的变化。QH06和QH08的深度堆积速率分别是0.137 cm·a~(-1)、0.078 cm·a~(-1),质量堆积速率分别是0.105 g·cm~(-2)·a~(-1)、0.030 g·cm~(-2)·a~(-1)。靠近湖岸,137Cs通量较大,沉积速率较大,分选性也较好。结合粒度特征分析得出,青海湖西北部布哈河口气候变化分为3个阶段:1853—1886年,该区气候向干旱方向发展;1886—1940年向湿润方向发展;1940—2013年向干旱方向发展。其中,1978年和1989年粒度特征的明显变化和布哈河水文资料相对应,说明了粒度特征指示该区气候干湿变化的可靠性。布哈河口环境受人类活动的影响较大。