Dynamics of flood water infiltration and ground water recharge in hyperarid desert

A study on flood water infiltration and ground water recharge of a shallow alluvial aquifer was conducted in the hyperarid section of the Kuiseb River, Namibia. The study site was selected to represent a typical desert ephemeral river. An instrumental setup allowed, for the first time, continuous monitoring of infiltration during a flood event through the channel bed and the entire vadose zone. The monitoring system included flexible time domain reflectometry probes that were designed to measure the temporal variation in vadose zone water content and instruments to concurrently measure the levels of flood and ground water. A sequence of five individual floods was monitored during the rainy season in early summer 2006. These newly generated data served to elucidate the dynamics of flood water infiltration. Each flood initiated an infiltration event which was expressed in wetting of the vadose zone followed by a measurable rise in the water table. The data enabled a direct calculation of the infiltration fluxes by various independent methods. The floods varied in their stages, peaks, and initial water contents. However, all floods produced very similar flux rates, suggesting that the recharge rates are less affected by the flood stages but rather controlled by flow duration and available aquifer storage under it. Large floods flood the stream channel terraces and promote the larger transmission losses. These, however, make only a negligible contribution to the recharge of the ground water. It is the flood duration within the active streambed, which may increase with flood magnitude that is important to the recharge process.     

Slug test analysis to evaluate permeability of compressible materials

The line-fitting methods such as the Hvorslev method and the Bouwer and Rice method provide a rapid and simple means to analyze slug test data for estimating in situ hydraulic conductivity ( k ) of geologic materials. However, when analyzing a slug test in a relatively compressible geologic formation, these conventional methods may have difficulties fitting a straight line to the semilogarithmic plot of the test data. Data from relatively compressible geologic formations frequently show a concave-upward curvature because of the effect of the compressibility or specific storage ( S _s). To take into account the compressibility of geologic formations, a modified line-fitting method is introduced, which expands on Chirlin's (1989) approach to the case of a partially penetrating well with the basic-time-lag fitting method. A case study for a compressible till is made to verify the proposed method by comparing the results from the proposed methods with those obtained using a type-curve method (Kansas Geological Survey method [ Hyder et al. 1994 ]).     

Ground water sustainability: methodology and application to the North China Plain

This article analyzes part of a ground water flow system in the North China Plain (NCP) subject to severe overexploitation and rapid depletion. A transient ground water flow model was constructed and calibrated to quantify the changes in the flow system since the predevelopment 1950s. The flow model was then used in conjunction with an optimization code to determine optimal pumping schemes that improve ground water management practices. Finally, two management scenarios, namely, urbanization and the South-to-North Water Transfer Project, were evaluated for their potential impacts on the ground water resources in the study area. Although this study focuses on the NCP, it illustrates a general modeling framework for analyzing the sustainability, or the lack thereof, of ground water flow systems driven by similar hydrogeologic and economic conditions. The numerical simulation is capable of quantifying the various components of the overall flow budget and evaluating the impacts of different management scenarios. The optimization modeling allows the determination of the maximum "sustainable pumping" that satisfies a series of prescribed constraints. It can also be used to minimize the economic costs associated with ground water development and management. Furthermore, since the NCP is one of the most water scarce and economically active regions in the world, the conclusions and insights from this study are of general interest and international significance.     

The performance of benthic indicators of ecological change in Adriatic coastal lagoons: throwing the baby with the water?

The ecological quality of 127 stations from six Adriatic coastal lagoons was assessed using a suite of biotic indices: H', d, 1-lambda', Delta(*), Delta(+), W, AMBI, BOPA and FINE. The analysis indicated the difficulties in deriving and using the existing indices from benthic communities in highly variable environmental conditions in coastal lagoons. Different metrics rendered different results: the use of H' resulted in the classification of all stations as "Moderate/Poor/Bad", whereas BOPA classified the majority of the stations as "Good/High". Using Delta(+), most of the stations resulted as "Not anthropogenically impacted", while the W-statistics gave 82 stations as "Undisturbed" and 45 as "Moderately/Grossly disturbed". AMBI classified 55 stations as "High/Good" and 72 as "Moderate/Poor/Bad", whereas those were 32 and 95 with FINE, respectively. The latter, which was developed just for Italian coastal lagoons, was the most conservative among the indices studied. Yet, it showed the highest correlation with the species/abundance matrix. Using indices developed for coastal waters in lagoons can give the distorted indication that the water body is degraded, when it might be just a natural, low diversity, high abundance community, i.e. a lagoonal community. Moreover, the outcome of the use of a certain index has a financial dimension such that lagoons misclassified as being "poor status" will then require expensive remediation measures. For the lagoons studied, there is probably an adequate quality and quantity of benthic data available for making management decisions, but this study highlights the limits of the existing indices for lagoonal ecosystems. The challenge for the next future is to couple long term conservation of the natural environment with the highly productive activities carried out in lagoonal ecosystems.     

俯冲带火山下岩浆的形成、上升和存储的时间尺度

现在有足够的资料可用来尝试建立一个关于俯冲带火山下熔体的产生、迁移和存储的完整的模型。在喷发前的几十万年到少于一千年的时间里,都可以发生流体从俯冲的洋壳流到地幔楔中的现象。这支持了认为流体的加入是与部分熔融紧密相关的模型,但也有一些证据可以证明流体是降压熔融的结果。流体开始加入的时间可能与俯冲的速率(例如,水供应的速率)、俯冲的角度以及由此决定的地幔楔的热结构有关。相反,俯冲沉积物到俯冲带熔岩源区的贡献在喷发前大约35万年到4百万年的时候看来就发生了。沉积组分的部分熔融证据与短的流体迁移时间、斑晶平衡温度及其他观测结果的结合都指出,地幔楔在接近于俯冲板块边界上的地方具有相当高的温度。新的226Ra资料表明,在流体加入和火山喷发之间只有很短的时间。这就要求快速的熔体分凝、沿通道流动的岩浆上升过程以及在岩石层内最短的停滞时间。一般说来,在上升期间或者在岩浆房里,发生从玄武岩到安山岩的演化是快速的,这可以从一些有关地幔的地球物理资料来推断。矿物等时线资料表明,一些安山质岩浆后来停在了更浅的岩浆房中。它们可以在那里以几万年或更短些的时间尺度通过分异作用演化成英安质成分,通常都伴有同化作用。     

核试验说明了USArray台阵数据的质量

作为USArray台阵部分而部署的地震台站,是美国国家科学基金会资助的主要研究仪器设备“地球透镜”计划的一部分。当全部运行时,USArray台阵将包括400个宽带地震台站的可移动台阵(TA),这些台站将以有规则网格的场点排列部署,台站间距约为70km。在今后的10年时间里,可移动台阵TA的布设区域将在美国境内迁移,最终在美国本土及阿拉斯加形成2000个记录场点。尽管USArray台阵的主要目的是记录探寻北美的构造、演化和地震活动性时所需要应用的地震信号,但可移动台阵TA布设后高质量的地震记录能力,就能够获得和分析用于诸如深地幔研究、地震破裂研究以及进行诸如地下核爆炸异常震源分析等其他目的的遥远小事件的资料。目前,可移动台阵TA正在美国西部进行初期的部署,2006年10月9日北朝鲜起爆地下核爆炸时已有约240个台站在运行(图1)。图1(左上图)事件及台站图的位置。(左下图)北朝鲜核试验及日本海深部事件的位置。(右图)得不到可移动台阵(TA)台站信号的位置(细线空心圆)、记录到低信噪比信号的台站(粗线空心圆)和具有北朝鲜核试验清晰P波到时的台站(实心圆)本文通过提取北朝鲜爆炸产生的高保真信号,说明可移动台阵TA对...