农村宅基地退出研究综述

宅基地是农民生活最基本的保障条件。目前,在我国宅基地存在很多问题,包括面积超标、分布零散、违法占用、长期空闲等现象。根据宅基地存在的各种问题,2016年国家第一次提出宅基地退出,并在全国推行试点工作。本文通过分析影响农村宅基地退出的因素,从中找出农村宅基地退出的困难和阻碍。     

紫金矿区"资源三号"卫星立体影像提取DEM精度评价

作为我国首颗高分辨率立体测绘卫星,"资源三号卫星"(ZY-3)为大区域立体测绘提供了可能.以紫金矿区为研究区,利用ZY-3立体影像提取数字高程模型(Digital Elevation Model,DEM),评价了山区地形条件下该卫星的立体测绘能力.结果显示,无地面控制点情况下DEM精度在20 m以内,1个控制点便可以将DEM的精度提高至5.2 m,3～5个控制点可使DEM的精度达到4.8 m.分析3～8个控制点生成的DEM的精度,结果显示,单个检查点的系统误差对总误差影响较大,随机误差对总误差贡献较小.在有控制点的情况下,ZY-3立体影像能够满足山区地形1:50000 DEM测绘的精度要求.     

Change of collision efficiency with distance in bacterial transport experiments

Understanding the mechanisms of bacterial transport in aquifers is important in developing bioremediation strategies. Collision efficiency (alpha) is one important parameter used in modeling bacterial transport. This study was undertaken to measure change in alpha with distance by performing a bacterial transport experiment in Oyster, Virginia. Following injection of a bacterium, Comamonas sp., into a well, water samples were collected at various distances along the flowpath and injected into columns packed with homogenized South Oyster focus area sediment. Zeta potentials of the bacteria in the samples were measured. Values of alpha were determined at various locations in the field in two ways: based on field breakthrough concentrations at the sampling points and based on column breakthrough concentrations. The alpha values estimated from field breakthrough decreased with distance, whereas those estimated from column breakthrough increased with distance. Bacterial cell surface charge became progressively more negative with distance in the field. We hypothesize that the apparent contradiction between field and column alpha values was caused by differences in the flow of the two systems. Flow in the columns was forced to occur through fine-grained zones of iron and aluminum hydroxide coatings that selectively removed the most negatively charged bacteria. In contrast, in the field, the injected cells did not come into contact with the positively charged coatings because the bulk solution bypassed them due to heterogeneous hydraulic properties. These results suggest that laboratory-based models may underestimate bacterial transport distance in the field. A more realistic approach may be necessary to capture the degree of heterogeneity.