Spatial and temporal patterns of dissolved organic matter quantity and quality in the Mississippi River Basin, 1997–2013

Recent studies have found insignificant or decreasing trends in time‐series dissolved organic carbon (DOC) datasets, questioning the assumption that long‐term DOC concentrations in surface waters are increasing in response to anthropogenic forcing, including climate change, land use, and atmospheric acid deposition. We used the weighted regressions on time, discharge, and season (WRTDS) model to estimate annual flow‐normalized concentrations and fluxes to determine if changes in DOC quantity and quality signal anthropogenic forcing at 10 locations in the Mississippi River Basin. Despite increases in agriculture and urban development throughout the basin, net increases in DOC concentration and flux were significant at only 3 of 10 sites from 1997 to 2013 and ranged between ?3.5% to +18% and ?0.1 to 19%, respectively. Positive shifts in DOC quality, characterized by increasing specific ultraviolet absorbance at 254 nm, ranged between +8% and +45%, but only occurred at one of the sites with significant DOC quantity increases. Basinwide reductions in atmospheric sulfate deposition did not result in large increases in DOC either, likely because of the high buffering capacity of the soil. Hydroclimatic factors including annual discharge, precipitation, and temperature did not significantly change during the 17‐year timespan of this study, which contrasts with results from previous studies showing significant increases in precipitation and discharge over a century time scale. Our study also contrasts with those from smaller catchments, which have shown stronger DOC responses to climate, land use, and acidic deposition. This temporal and spatial analysis indicated that there was a potential change in DOC sources in the Mississippi River Basin between 1997 and 2013. However, the overall magnitude of DOC trends was not large, and the pattern in quantity and quality increases for the 10 study sites was not consistent throughout the basin.     

Quantification of the hydraulic diffusivity of a bentonite-sand mixture using the water head decrease measured upon sudden flow interruption

This paper presents a new modelling approach to quantify the hydraulic diffusivity of low-permeability unconsolidated porous media under confined saturated-flow conditions in the laboratory. The derived analytical solution for the transient variation of the hydraulic head after flow interruption was applied to experimental data obtained from continuous measurements of the water pressure at two locations in the soil column. Three soil samples made of a mixture of natural bentonite (at different mass fractions) and medium sand were studied during a series of stepwise constant flow rates of water. The numerical results well fit the experimentally measured decrease of the dimensionless hydraulic head. The study shows that the increase of the mass fraction of bentonite in the soil sample from 10 to 30% is accompanied by a strong decrease of the hydraulic diffusivity from 2.4 × 10⁻² to 1.1 × 10⁻³ m² s⁻¹, which is clearly due to the decrease of the hydraulic conductivity of the soil sample. The specific storages obtained for each of the three samples are in the same order of magnitude and seem to decrease with the increase of mass fraction of bentonite. However, they clearly reflect the predominant portion of the compressibility of the porous medium compared with that of water. Compared with reported literature values for clayey soils, the specific storage values in this study are slightly higher, varying within the range of 2 × 10⁻³ to 8.1 × 10⁻³ m⁻¹._. The experimental results also give insight into the limitations of the modelling approach. In the case of low-permeability soils (K < 2 × 10⁻⁶ ms⁻¹) and steady-flow conditions with low Reynolds numbers, for example, Re < 0.003, it is recommended to choose a time duration for flow interruption between subsequent flow rate steps of longer than 5 s. For high-permeability porous media, to increase the precision of the quantified hydraulic diffusivity, it might be useful to select a measuring frequency significantly higher than 1 Hz.     

Molecular and stable carbon isotopic characterization of PAH contaminants at McMurdo Station, Antarctica

The molecular and stable carbon isotopic compositions of contaminant polycyclic aromatic hydrocarbons (PAHs) at McMurdo Station, Antarctica were analyzed in samples collected from land and sub-tidal area. PAHs in the study areas were characterized by high amounts of naphthalene and alkylated naphthalenes from petroleum products introduced by human activities in the area. Principal component analysis (PCA) of PAH composition data identified multiple sources of PAH contamination in the study area. Compositional assignments of origins were confirmed using compound specific stable carbon isotopic analysis.     

特殊动力作用在海洋生物中的研究进展

由摄食而引起的特殊动力作用(SDA)使动物在处理和转化食物时需要额外消耗能量。阐述了SDA的含义及其研究参数,讨论了目前国内外对SDA研究的主要途径及不同的研究重点,探讨了影响SDA耗能的主要因素,指出了SDA与动物生长之间的关系。从两方面的研究角度考虑,展望了研究SDA的前景和意义。     

含水层水平二维弹性形变下的位移与水流方程

在垂直方向零应变与垂直方向总荷载不变、忽略骨架颗粒本身的压缩性及骨架颗粒与水分别为均质的假定条件下,推导出均质各向同性多孔介质水平面弹性位移方程及水流方程。它们与传统假定（Jacob假定）下的位移及水流方程在形式上没有区别,只是方程中的系数定义式有所不同。新水流方程中的储水率比旧水流方程中的储水率偏大,但仍可按常数对待。新位移方程的系数也比传统假定下的位移方程的系数大．但也仍为常数。如果采用野外抽水试验来确定储水率,则传统假定下水流方程与新水流方程等价,但利用抽水试验确定的骨架弹性储水率计算地面沉降,则计算值偏大。     

Coupled impact of decadal precipitation and evapotranspiration on peatland degradation in the Zoige basin,China

ABSTRACT

This study quantified the temporal properties of precipitation and actual evapotranspiration (ETa) in the Zoige basin of the Yellow River source region, China during the 1967–2011 period, as well as their influence to the area reduction of peatland. We extracted areas of different land-use and land-cover (LULC) types and obtained daily precipitation data. Then, we calculated annual precipitation (AP) and specific cumulative precipitation (SCP), which is the sum of precipitation to the date when an image was taken, and showed that the peatland areas were strongly affected by SCP. Using a modified Penman–Monteith equation, we calculated ETa for each LULC type and the area-weighted ones to show that the area-weighted total ETa was mainly contributed from grassland and peatland, which was between 450 and 550 mm. Temporal trends of the ratio of SCP to evapotranspiration showed that precipitation was generally greater than evapotranspiration rate not only during the summer but also over the 1967–2011 period. This trend failed to completely explain the continuous decrease of peatland area in the Zoige basin in decades. The draining effect of artificial ditches and natural gullies might play an additional role in causing peatland degradation but requires further process-based studies.     

云雾对电磁传输特性影响的研究

在瑞利散射、液态水介电常数模型、雾滴谱分布基础上,分析了单个云雾粒子和群雾滴的电磁传输特性.数值模拟计算表明,电长度是影响云雾粒子电磁传输特性的主要因素,温度的影响相对较小.平流雾分布的电磁传输特性强于辐射雾的传输特性,影响群雾滴传输特性的因素由大到小依次为人射波频率、含水量和温度.所得结论对提高降水测量的精度,减小卫...     

氯氧镁水泥在特定水环境中的强度变化规律试验研究

为了研究氯氧镁水泥(MOC)试件分别在老卤、原卤、海水和淡水4种水环境中强度变化规律,本文以掺加一定量复合改性剂的MOC为基体,将其养护至28天后,分别置于老卤、原卤、海水和淡水中浸泡一定期龄,分析其在4种特定水环境中的强度变化规律。实验结果表明,MOC试件在4种特定水环境中抗压强度变化规律不同。相较于空气中养护,MOC在老卤、原卤、海水和淡水浸泡下,抗压强度逐渐降低,其抗压强度降低程度为老卤<原卤<海水<淡水。经4种特定水环境浸泡后,MOC抗压强度保留相在老卤中最高,其强度保留相高达95.60%,证明MOC试件在老卤中具有较好的抗盐卤腐蚀性能。     

Recent changes in channel morphology of a highly engineered alluvial river – the Lower Mississippi River

Changes in channel morphology provide relevant insights into sediment transport and deposition in alluvial river systems. This study assessed three to four decades of morphological changes at seven locations along a 327-km reach of the Lower Mississippi River (LMR) to better understand channel adjustment processes of this large alluvial river. The assessment included analysis of three cross-sectional areas at each location during the period 1992–2013, as well as analysis of the changes in river stage and maximum surface slopes under four flow conditions over the last three to four decades . We found that the first 20–25 km LMR reach below its diversion to the Atchafalaya River and the reach from 80 to 140 km experienced significant riverbed aggradation, while the reach in between (i.e. from 20 to 80 km) experienced riverbed degradation. The lower 187-km reach (i.e. from 140 to 327 km) showed negligible sediment trapping. These findings may have relevant implications for management of river sediment diversions along the LMR and other large alluvial rivers in the world.