Evaluation of MODFLOW-LGR in connection with a synthetic regional-scale model

This work studies costs and benefits of utilizing local-grid refinement (LGR) as implemented in MODFLOW-LGR to simulate groundwater flow in a buried tunnel valley interacting with a regional aquifer. Two alternative LGR methods were used: the shared-node (SN) method and the ghost-node (GN) method. To conserve flows the SN method requires correction of sources and sinks in cells at the refined/coarse-grid interface. We found that the optimal correction method is case dependent and difficult to identify in practice. However, the results showed little difference and suggest that identifying the optimal method was of minor importance in our case. The GN method does not require corrections at the models' interface, and it uses a simpler head interpolation scheme than the SN method. The simpler scheme is faster but less accurate so that more iterations may be necessary. However, the GN method solved our flow problem more efficiently than the SN method. The MODFLOW-LGR results were compared with the results obtained using a globally coarse (GC) grid. The LGR simulations required one to two orders of magnitude longer run times than the GC model. However, the improvements of the numerical resolution around the buried valley substantially increased the accuracy of simulated heads and flows compared with the GC simulation. Accuracy further increased locally around the valley flanks when improving the geological resolution using the refined grid. Finally, comparing MODFLOW-LGR simulation with a globally refined (GR) grid showed that the refinement proportion of the model should not exceed 10% to 15% in order to secure method efficiency.     

Phytoplankton distribution and productivity in a highly turbid, tropical coastal system (Bach Dang Estuary, Vietnam)

Phytoplankton diversity, primary and bacterial production, nutrients and metallic contaminants were measured during the wet season (July) and dry season (March) in the Bach Dang Estuary, a sub-estuary of the Red River system, Northern Vietnam. Using canonical correspondence analysis we show that phytoplankton community structure is potentially influenced by both organometallic species (Hg and Sn) and inorganic metal (Hg) concentrations. During March, dissolved methylmercury and inorganic mercury were important factors for determining phytoplankton community composition at most of the stations. In contrast, during July, low salinity phytoplankton community composition was associated with particulate methylmercury concentrations, whereas phytoplankton community composition in the higher salinity stations was more related to dissolved inorganic mercury and dissolved mono and tributyltin concentrations. These results highlight the importance of taking into account factors other than light and nutrients, such as eco-toxic heavy metals, in understanding phytoplankton diversity and activity in estuarine ecosystems.     

第三次国土调查的图斑自动综合方法

从第三次国土调查数据库快速缩编派生出多尺度数据库,是地图制图的重要任务。本文针对大量图斑,首先通过研究图斑栅格综合,减少碎图斑数量;然后构建图斑语义邻近度模型、空间几何拓扑模型,分析了顾及拓扑和地区地理特征约束下的地类图斑制图综合方法;最后形成了自动化图斑缩编生产工艺流程。基于上述方法,采用计算机编程技术研发国土三调缩编系统,并应用于四川省某县23万图斑量的三调数据并试验验证。试验结果表明,通过本文方法的成果符合专题制图要求,极大地提高了三调图斑的自动综合效率。     

A finite world,earth sciences,and public trust

The beginning of the 21st century has coincided with our recognition that life-sustaining earth cycles are remarkably fine-tuned, and that humans have developed technological abilities to perturb these cycles. Also, inspired bythe gifts of freedom and democracy, humans have given themselves laws to exploit nature for profit. The upshot is that nature's balance, governed by immutable physical laws, is being confronted by social laws driven by human aspirations. This conflict and its implications to the human relevance of the earth sciences are explored in the context of an extraordinary tradition of European culture known as public trust.     

激光测距对静态重力场恢复精度的影响分析

鉴于下一代重力卫星设计中利用更高精度的激光测距技术代替微波测距,对激光测距提高地球重力场探测精度的问题进行了讨论。通过高精度的动力学重力场模型反演方法,推导了线性化的星间变率公式,并以一定的权融合卫星精密轨道与星间变率数据。通过模拟计算结果可知,当卫星轨道、定轨精度、加速度计精度与Gravity Recovery and Climate Experiment(GRACE)相同,星间变率精度依次从1.0×10~(–6) m/s提高到5.0×10~(–7) m/s、1.0×10~(–7) m/s、5.0×10~(–8) m/s、1.0×10~(–8) m/s时,累积大地水准面误差则在120阶时依次从85.14 cm降低为33.09 cm、7.33 cm、3.70 cm、3.59 cm。结果表明当采用高精度的激光测距后,采用低低卫卫跟踪模式,地球静态重力场模型的探测精度有望比GRACE提高1个量级。当激光测距精度提高至10 nm/s时,计算结果与精度为50 nm/s的计算结果无明显差别,这表明过高的星间变率测量精度相对于其他指标而言有冗余。本文建议在其他测量技术精度未有提高的前提下,只需将激光测距的精度提高至50 nm/s即可。