Investigation of runoff generation in a pristine,poorly gauged catchment in the Chilean Andes I: A multi‐method experimental study

Catchment scale hydrological process studies in southern Chile are of special interest as little research at this scale has been carried out in this region. In particular, the young volcanic ash soils, which are typical for this area, are not well understood in their hydrological behaviour. In addition, extensive land use changes require detailed knowledge of hydrological processes in disturbed as well as undisturbed catchments in order to estimate resulting risks of erosion, eutrophication, floods and droughts. This study focuses on data collection and experimental determination of relevant processes in an undisturbed forested catchment in the Andes of southern Chile. The here gained understanding of runoff generation can serve as a reference for comparison with sites subject to human intervention, improving estimation of the effects of land use change. Owing to the lack of long‐term data for this catchment it was necessary to replace long time series by a multitude of experimental methods covering as many aspects of the runoff generation process as possible. The methods used in this investigation include: measurements of streamflow, rainfall, throughfall, water chemistry, soil water dynamics, groundwater dynamics, soil physics, soil mineralogy, geo‐electrical sounding, and tracer techniques. Methods and equipment used during field campaigns are described and evaluated for usefulness versus expenditure (labour and financial costs). Selected results and the hypotheses developed from these findings are presented. The results suggest the importance of fast processes for rainfall runoff response on the one hand as well as considerable dampening effects of a large subsurface storage on the other hand. Copyright © 2008 John Wiley & Sons, Ltd.     

Testing and comparison of four ionic tracers to measure stream flow loss by multiple tracer injection

The ionic tracers lithium, sodium, chloride and bromide were used to measure flow loss in a small stream (≈? 10 ls^?1). An injectate containing all four tracers was added continuously at five sites along a 507 m study reach of St Kevin Gulch, Lake County, Colorado to determine which sections of the stream were losing water to the stream bed and to ascertain how well the four tracers performed. The acidity of the stream (pH 3.6) made it possible for lithium and sodium, which are normally adsorbed by ion exchange with stream bed sediment, to be used as conservative tracers. Net flow losses as low as 0.8 ls^?1, or 8% of flow, were calculated between measuring sites. By comparing the results of simultaneous injection it was determined whether subsections of the study reach were influent or effluent. Evaluation of tracer concentrations along 116 m of stream indicated that all four tracers behaved conservatively. Discharges measured by Parshall flumes were 4–18% greater than discharges measured by tracer dilution.     

Assessment of magnetite as a magnetic tracer for sediments in the study of ephemeral gully erosion: Conditioning factors of magnetic susceptibility

In gully erosion, the detached soil can be transported over long distances along the landscape. The eroded material can be redistributed and/or deposited on the soil surface along the landscape and then eventually be buried by newly eroded and deposited sediment. There can be significant variability of the soil conditions (e.g., texture and moisture content) over which the eroded material travels. The eroded material can be detected through the use of magnetic tracers attached to or mixed with the eroded soil. In this study we evaluated the degree to which the magnetic signal of the magnetite is conditioned by (i) burial depth of tracer, (ii) condition of soil covering the tracer and (iii) tracer concentration. In the laboratory containers were filled with a specific soil. In the filling process, a 0.5-cm layer of a soil–magnetite mixture was interspersed in the soil profile at a certain depth. Experiments encompassed three different soil–tracer concentrations (1000:1, 200:1, 100:1), four burial depths of tracer (0 cm, 3 cm, 5 cm and 10 cm from soil surface), and two different soils. In each case, the magnetic susceptibility was measured with a susceptometer. Experiments were repeated with different soil moisture contents. If the tracer is located under the soil surface, a minimum soil–tracer concentration of 200:1 is required for its correct detection. The intensity of the magnetic signal decreases dramatically with the vertical distance of the tracer from the soil surface. The maximum detection depth for the tracer's magnetic signal is strongly dependent on the natural magnetic susceptibility of the soil, which masks the tracer's signal. Variation in soil moisture content does not significantly affect the magnetic signal. For extensive field studies, the soil–tracer volume to be handled would be very high and therefore, it is necessary to explore new tracer application techniques.     

Linkages between bedload displacements and topographic change

Changes in bed topography that build and maintain channel morphology are driven by the displacements of individual particles, either though their entrainment or deposition. However, the linkages between these topographic changes and individual grain displacements have not been comprehensively addressed, as many historical tracer studies have not included coincident topographic data. In this study, we compare the movements of bedload tracers to the differences in repeat topographic surveys across four gravel-bed river reaches. To do this, we apply a 1-D Bayesian survival process model to the starting and ending locations of tracers. This model estimates downstream trapping probabilities, which represent the likelihood that a given segment of channel will “trap” an entrained particle. We then adapt this model to estimate downstream trapping probabilities using digital elevation models of difference and compare the results. The estimates from the tracer and topographic trapping models showed general alignment, meaning that tracers were preferentially trapped in segments that experienced deposition along the channel. Thus, tracers in this study were able to identify downstream differences in bedload transport. The comparison also highlighted that tracer-estimated trapping probabilities were larger than topographically estimated ones. This supports previous observations that sediment travel distances estimated using tracers are shorter than those estimated using morphological methods. We find that the differences between these two estimates vary systematically across study environments. These variations are attributable to either study design (i.e., tracers being larger than the median size of the sediment that deforms the bed) or differences in compensating scour and fill. We explore potential causes for differences in compensating scour and fill, including hydrograph shape, sediment delivery regime, channel deformation style, and channel width, highlighting that morphodynamics needs to be considered in designing bedload tracer studies.     

Contrasting behaviors of 210Po, 210Pb and 234Th in the East China Sea during a severe red tide: Enhanced scavenging and promoted fractionation

210Po, 210Pb, and 234Th were determined in water columns of the East China Sea (ECS) to investigate their biogeochemical behaviors during a severe red tide event. Dissolved 210Po, 210Pb, and 234Th accounted for large fractions of the total phases. The abnormally high concentrations of dissolved 210Pb were observed. Partition behaviors of these radionuclides were influenced by particle content effect and particle composition based on distribution coefficient (K_d) vs. total suspended matter (TSM) content and K_d vs. ratios of particulate organic carbon and total suspended matter contents (POC/TSM content ratios). The peaks of mass specific activities of 210Po, 210Pb, and 234Th indicated that degraded particles could have an intensified enrichment ability for radionuclides compared with the surficial suspended matters. Fractionation factor of 210Po and 210Pb (F_Po/Pb) (>1) and fractionation factor of 210Po and 234Th (F_Po/Th) (>1) were much higher at algal blooming regions than that at non-blooming stations, indicating that algal blooms promoted the fractionation of 210Po against 210Pb and 234Th, and proving that 210Po exhibited a stronger affinity for biogenic particles than 210Pb and 234Th when POC content increased in the sea. POC/210Po, POC/210Pb and POC/234Th ratios (content/activity ratios) sharply decreased with depth in both algal bloom and non-bloom stations. The outbreak of algal bloom promoted the complexity of suspended particles and increased the variability of POC/tracer ratios (content/activity ratios) in the different depth of the shallow seas. More considerations should be taken to the difficulty of the selection of export interface and the suitable tracers when algal blooming occurs.     

Impacts of forest conversion and agriculture practices on water pathways in Southern Brazil

Land‐use/cover change (LUCC), and more specifically deforestation and multidecadal agriculture, is one of the various controlling factors of water fluxes at the hillslope or catchment scale. We investigated the impact of LUCC on water pathways and stream stormflow generation processes in a subtropical region in southern Brazil. We monitored, sampled and analysed stream water, pore water, subsurface water, and rainwater for dissolved silicon concentration (DSi) and ¹⁸O/¹⁶O (δ¹⁸O) signature to identify contributing sources to the streamflow under forest and under agriculture. Both forested and agricultural catchments were highly responsive to rainfall events in terms of discharge and shallow groundwater level. DSi versus δ¹⁸O scatter plots indicated that for both land‐use types, two run‐off components contributed to the stream discharge. The presence of a dense macropore network, combined with the presence of a compact and impeding B‐horizon, led to rapid subsurface flow in the forested catchment. In the agricultural catchment, the rapid response to rainfall was mostly due to surface run‐off. A 2‐component isotopic hydrograph separation indicated a larger contribution of rainfall water to run‐off during rainfall event in the agricultural catchments. We attributed this higher contribution to a decrease in topsoil hydraulic conductivity associated with agricultural practices. The chemical signature of the old water component in the forested catchment was very similar to that of the shallow groundwater and the pore soil water: It is therefore likely that the shallow groundwater was the main source of old water. This is not the case in the agricultural catchments where the old water component had a much higher DSi concentration than the shallow groundwater and the soil pore water. As the agricultural catchments were larger, this may to some extent simply be a scale effect. However, the higher water yields under agriculture and the high DSi concentration observed in the old water under agriculture suggest a significant contribution of deep groundwater to catchment run‐off under agriculture, suggesting that LUCC may have significant effects on weathering rates and patterns.     

南黄海表层沉积物中氮的分布特征及其在生物地球化学循环中的功能

本文报道了南黄海表层沉积物中不同形态氮的区域分布特征及其在生物地球化学循环中的作用。结果显示 ,在粒径 <31 μm的粒度组分含量 >6 5 %( )、35 %～ 6 5 %( )和 <35 %( )的三个区域中 ,沉积物中不同形态氮的含量比值分别为 :1 .6 5 :1 .2 6 :1 (IEF- N) ,1 .2 3:1 .1 0 :1 (WAEF- N) ,1 .4 1 :1 .0 4 :1 (SAEF- N) ,2 .0 8:1 .4 5 :1 (SOEF- N) ,TN(1 .70 :1 .2 6 :1 ) ,即在细粒度组分 (<31 μm)含量较高的区域 ,不同形态氮的含量也相对较高 ;在三个分区内 ,不同形态氮的埋藏通量非常相近 ,而埋藏效率自 至 区逐渐递增 ,在细粒度组分 (<31 μm)含量最低的 区内 ,埋藏效率最高 ,TN的埋藏效率可达 30 .2 1 %,即南黄海表层沉积物中 70 %以上的氮在适当的条件下可释放进入水体参与其生物地球化学循环 ,能提供海洋新生产力所需氮的 6 .5 4 %,对海洋生产力具有一定的补充和调控作用。     

应用环境同位素方法研究黑河源区水文循环特征

维持黑河流域中、下游绿洲生态和社会经济发展的水资源主要形成于祁连山区。该文在野外采样的基础上,通过测试分析不同地段、不同水体中的环境同位素(δD、δ^18O和^3H)和水化学组成特征,利用同位素混合模型,识别黑河源区地表水和地下水的来源、组成和径流过程,揭示源区的水文循环特征,在此基础上探讨源区水文循环模式。结果表明：1)黑河源区存在两类地下径流系统：局部径流系统接受局地降水和冰雪融水入渗补给,就近向河流排泄,水循环交替积极,据同位素估算结果,降水补给占76％,冰雪融水补给占24％;区域径流系统接受中高山带的降水和冰雪融水补给(补给高程大于3600m),在出山口附近的断裂带以泉的形式排泄,水循环交替缓慢。2)山区地表水由大气降水、冰雪融水和地下水组成,出山河水的构成为：降水占69％～72％,冰雪融水占23％～24％,深部地下水占5％～7％。     

Au的生物地球化学异常和对植物细胞的变异效应

对从金矿区和背景区采集和试验种植的植物样品的研究表明,在杉木、马尾松和蕨的叶灰分中,矿区相对于背景区,前者的Au含量是后者的135倍、111倍和147倍;前者的Cu是后者的12倍、8倍和11倍;前者的Pb是后者的11倍、11倍和14倍.试验的高Au含量土壤中生长的青菜和芥菜Au含量分别是对照自然土壤中生长的229倍和205倍.Au等元素高含量土壤中生长的植物的细胞,在电子显微镜下变异效应明显,主要表现为细胞中出现大量电子致高密度体物质或内部组织的微结构遭受损伤.金矿区的植物样品中电子致高密度体物质含有20.160%～37.321%的Au、3.828%～23.324%的Cu及较多的Pb和Ag.种植的样品电子致高密度体物质含Au34.969%.说明Au等元素以电子致高密度体物质的"形式"淀积在细胞的内部组织结构中.     

真菌-玄武岩相互作用过程中真菌属种差异对元素释放行为的影响

真菌在矿物、岩石风化和元素溶解过程中发挥着重要作用。土壤中分布和生长着多种真菌,这些不同真菌在矿物和岩石风化,以及元素溶解过程中发挥的作用可能存在显著差异。为限定此差异,本研究从玄武岩风化土壤中分离获取了4种真菌(Aspergillus pesudofelis、Aspergillus viridinutans、Aspergillus undagawae和Aspergillus clavatus),设计批式溶解实验,探究了实验室条件下(28℃、30天内)4种真菌对玄武岩的风化作用,主要通过分析元素(包括Mg、Al、Si、Ca、Ti、Mn、Fe、Ni和Sr)的溶解速率和机制揭示不同属种真菌造成的玄武岩风化差异,并选取其中两种典型真菌所在的体系进行分析。实验结果显示不同属种真菌对玄武岩的风化和元素溶解的作用确实存在显著差异:1)与无机对照组相比,真菌A.pseudofelis的生长使得溶液pH值发生显著变化,元素的线性释放速率(Ril)显著升高;2)不同种真菌对体系pH值和Ril的影响差别巨大,且并非所有真菌均能加快玄武岩风化,如经过30天反应,含真菌A.pseudofelis体系溶液pH值上升了2.1,元素Mg的线性释放速率是无机对照组的22.5倍;而含真菌A.viridinutans体系溶液pH值仅上升了0.1,Mg的线性释放速率与无机对照组近似相等;3)Mg、Al、Ca、Ti元素的线性释放速率Ril最大值均出现在溶液最低pH值条件下,表明这4种元素的释放行为很大程度上受控于质子交换作用,而Fe和Ni的释放则由真菌代谢的草酸主导。由此可知,不同种真菌对有机酸代谢水平的差异影响了玄武岩中元素的释放行为。这也可能表明,在岩石早期风化过程中,具有较强有机酸(如草酸)代谢能力的真菌在Fe和Mg等生命必需元素的释放过程中起着重要作用。