Embedding isochronous cells overland flow module into MODFLOW

This study integrates a simple overland flow module (isochronous cells model) with the river module of MODFLOW such that temporal and spatial interactions between stream flow and groundwater can be simulated using net rainfall data of a watershed. The isochronous cells model is an efficient travel time runoff approach based on geographic information system (GIS) that considers both spatial and temporal variations of net rainfall through hill slope of the watershed. This overland module is easily coupled with MODFLOW river routing module. Specifically, the stream flow from the isochronous cells model is directly assigned to both sides of river cells of the MODFLOW model. Such an integration of MODFLOW and isochronous cells model is especially useful in watersheds where river flow data are limited. The feasibility of this integrated model was demonstrated using a case study in the middle and downstream regions of the Yitong River watershed, China. Copyright © 2012 John Wiley & Sons, Ltd.     

The Impact on the Positioning Accuracy of the Frequency Reference of a GPS Receiver

Despite the pervasive use of the global positioning system (GPS) as a positioning technology for its high efficiency and accuracy, several factors reduce its performance. This study examines to which extent the frequency offset and the frequency stability of the internal quartz oscillator or of an externally supplied rubidium oscillator have an influence. Observations were made at the Taiwan Ching Yun University (TCYU) tracking station, where a quartz oscillator and a rubidium oscillator were applied alternatively on a monthly basis throughout a 16-month period. Moreover, the accuracy of the local oscillator used in this study was calibrated by the National Standard Time and Frequency Laboratory, Taiwan. The frequency offset and frequency stability calculated via the remote method at the TCYU station were compared with values (uncertainty is 3.0E?13) measured directly at the National Standard Time and Frequency Laboratory, Taiwan. Analytical results show that the two methods vary by 1.4E?10 in terms of frequency offset and by 6.5E?12 in terms of frequency stability, demonstrating that the remote method can yield computational results almost as accurate as direct measurement. Positioning precision results also show that rubidium oscillator accuracy improved by 5, 11, and 15 % for short-, medium-, and long-baseline positioning, respectively, indicating that clock quality is more influential for long-baseline GPS relative positioning and that the frequency stability of a receiver clock is far more critical than the frequency offset. On the other hand, the positioning performance noted is essentially independent (max. 15 % change) of the reference frequency stability, which indeed differed by 4 orders of magnitude.     

New analytical solutions for groundwater flow in wedge-shaped aquifers with various topographic boundary conditions

The hydraulic head distribution in a wedge-shaped aquifer depends on the wedge angle and the topographic and hydrogeological boundary conditions. In addition, an equation in terms of the radial distance with trigonometric functions along the boundary may be suitable to describe the water level configuration for a valley flank with a gentle sloping and rolling topography. This paper develops a general mathematical model including the governing equation and a variety of boundary conditions for the groundwater flow within a wedge-shaped aquifer. Based on the model, a new closed-form solution for transient flow in the wedge-shaped aquifer is derived via the finite sine transform and Hankel transform. In addition, a numerical approach, including the roots search scheme, the Gaussian quadrature, and Shanks’ method, is proposed for efficiently evaluating the infinite series and the infinite integral presented in the solution. This solution may be used to describe the head distribution for wedges that image theory is inapplicable, and to explore the effects of the recharge from various topographic boundaries on the groundwater flow system within a wedge-shaped aquifer.     

Parameter Estimation/Sensitivity Analysis for an Aquifer Test with Skin Effect

Aquifer information carried by aquifer test data may be affected by the presence of a finite thickness skin around the wellbore. The mathematical treatment for an aquifer accounting for the skin zone can be characterized by five parameters, that is, the outer radius of the skin zone and the transmissivity and storativity for each of the skin and aquifer zones. Sensitivity analysis was performed to examine the ground water flow behavior in the skin and aquifer zones in terms of the constant-head test (CHT) data. The simulated annealing procedure was applied to simultaneously determine the skin and aquifer parameters from the analysis of CHT data. Toward the previously mentioned goals, four suites of CHT data were analyzed in this article. The analyses of wellbore flow rate at the test well and the specific drawdown at the observation well gave accurate estimates for the skin and aquifer parameters, respectively. Only the skin thickness and both the skin and the aquifer diffusivities could be accurately estimated from the analysis of drawdown data in the observation well. The estimates for all skin and aquifer parameters from the composite analysis of flow rate and drawdown data were the most accurate. The results of sensitivity analyses and parameter estimations provide instructive references in the analysis of the skin-affected CHT data.     

Experiment and analysis of a leverage‐type stiffness‐controllable isolation system for seismic engineering

Owing to the fixed design parameters in traditional isolation systems, the optimal isolation performance may not always be achieved when a structure is subjected to a nondesign earthquake. At the same time, even though an active isolation system (AIS) can offer a better reduction for different seismic waves, in practice the control energy required still constrains its application. To solve this problem, a novel semi‐active isolation system called the Leverage‐type Stiffness Controllable Isolation System (LSCIS) is proposed in this paper. By utilizing a simple leverage mechanism, the isolation stiffness and the isolation period of the LSCIS can be easily controlled by adjusting the position of the pivot point of the leverage arm. The theoretical basis and the control law for the proposed system were first explained in this work, and then a shaking table test was conducted to verify the theory and the feasibility of the LSCIS. As shown in the experiment, the seismic behavior of the LSCIS can be successfully simulated by the theoretical model, and the isolation stiffness can be properly adjusted to reduce the seismic energy input in the LSCIS system. A comparison of the LSCIS with the other systems including passive isolation and AISs has demonstrated that based on the same limitation of base displacement, better acceleration reduction can be achieved by the LSCIS than by any of the other isolation systems. In addition, the control energy required by the LSCIS is lower than that for an AIS using the traditional LQR control algorithm. Copyright © 2010 John Wiley & Sons, Ltd.     

Stochastic modeling of variations in stream flow discharge induced by random spatiotemporal fluctuations in lateral inflow rate

Due to natural heterogeneity in runoff processes, the analysis of response of stream channels to the variation of lateral inflow is therefore viewed in terms of stochastic spatiotemporal processes. Based on the representation theorem, a closed-form expression is derived to describe the spectral response characteristic of stream subject to spatiotemporal fluctuations in lateral inflow. It provides a basis for evaluating the induced discharge variability in stream channels. It is found that the evolutionary power spectrum of the stream flow discharge process and therefore the variance is increased with the distance from the upstream boundary and the characteristic length scale of the lateral inflow process. Flow discharge prediction in the downstream region has a high degree of uncertainty by solving the deterministic partial differential equation.     

Reaction-based modeling of quinone-mediated bacterial iron(III) reduction

This paper presents and validates a new paradigm for modeling complex biogeochemical systems using a diagonalized reaction-based approach. The bioreduction kinetics of hematite (α-Fe₂O₃) by the dissimilatory metal-reducing bacterium (DMRB) Shewanella putrefaciens strain CN32 in the presence of the soluble electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) is used for presentation/validation purposes. Experiments were conducted under nongrowth conditions with H₂ as the electron donor. In the presence of AQDS, both direct biological reduction and indirect chemical reduction of hematite by bioreduced anthrahydroquinone-2,6-disulfonate (AH₂DS) can produce Fe(II). Separate experiments were performed to describe the bioreduction of hematite, bioreduction of AQDS, chemical reduction of hematite by AH₂DS, Fe(II) sorption to hematite, and Fe(II) biosorption to DMRB. The independently determined rate parameters and equilibrium constants were then used to simulate the parallel kinetic reactions of Fe(II) production in the hematite-with-AQDS experiments. Previously determined rate formulations/parameters for the bioreduction of hematite and Fe(II) sorption to hematite were systematically tested by conducting experiments with different initial conditions. As a result, the rate formulation/parameter for hematite bioreduction was not modified, but the rate parameters for Fe(II) sorption to hematite were modified slightly. The hematite bioreduction rate formulation was first-order with respect to hematite ”free“ surface sites and zero-order with respect to DMRB based on experiments conducted with variable concentrations of hematite and DMRB. The AQDS bioreduction rate formulation was first-order with respect to AQDS and first-order with respect to DMRB based on experiments conducted with variable concentrations of AQDS and DMRB. The chemical reduction of hematite by AH₂DS was fast and considered to be an equilibrium reaction. The simulations of hematite-with-AQDS experiments were very sensitive to the equilibrium constant for the hematite-AH₂DS reaction. The model simulated the hematite-with-AQDS experiments well if it was assumed that the ferric oxide “surface” phase was more disordered than pure hematite. This is the first reported study where a diagonalized reaction-based model was used to simulate parallel kinetic reactions based on rate formulations/parameters independently obtained from segregated experiments.     

Contributions of solar and greenhouse gases forcing during the present warm period

Due to the dramatic increase in the global mean surface temperature (GMST) during the twentieth century, the climate science community has endeavored to determine which mechanisms are responsible for global warming. By analyzing a millennium simulation (the period of 1000–1990 ad) of a global climate model and global climate proxy network dataset, we estimate the contribution of solar and greenhouse gas forcings on the increase in GMST during the present warm period (1891–1990 ad). Linear regression analysis reveals that both solar and greenhouse gas forcing considerably explain the increase in global mean temperature during the present warm period, respectively, in the global climate model. Using the global climate proxy network dataset, on the other hand, statistical approach suggests that the contribution of greenhouse gas forcing is slightly larger than that of solar forcing to the increase in global mean temperature during the present warm period. Overall, our result indicates that the solar forcing as well as the anthropogenic greenhouse gas forcing plays an important role to increase the global mean temperature during the present warm period.     

Mantle source heterogeneity of the Early Jurassic basalt of eastern North America

International Journal of Earth Sciences - One of the defining characteristics of the basaltic rocks from the Early Jurassic Eastern North America (ENA) sub-province of the Central Atlantic Magmatic...