Comparison of 15 evaporation methods applied to a small mountain lake in the northeastern USA

Few detailed evaporation studies exist for small lakes or reservoirs in mountainous settings. A detailed evaporation study was conducted at Mirror Lake, a 0.15 km² lake in New Hampshire, northeastern USA, as part of a long-term investigation of lake hydrology. Evaporation was determined using 14 alternate evaporation methods during six open-water seasons and compared with values from the Bowen-ratio energy-budget (BREB) method, considered the standard. Values from the Priestley–Taylor, deBruin–Keijman, and Penman methods compared most favorably with BREB-determined values. Differences from BREB values averaged 0.19, 0.27, and 0.20 mm d⁻¹, respectively, and results were within 20% of BREB values during more than 90% of the 37 monthly comparison periods. All three methods require measurement of net radiation, air temperature, change in heat stored in the lake, and vapor pressure, making them relatively data intensive. Several of the methods had substantial bias when compared with BREB values and were subsequently modified to eliminate bias. Methods that rely only on measurement of air temperature, or air temperature and solar radiation, were relatively cost-effective options for measuring evaporation at this small New England lake, outperforming some methods that require measurement of a greater number of variables. It is likely that the atmosphere above Mirror Lake was affected by occasional formation of separation eddies on the lee side of nearby high terrain, although those influences do not appear to be significant to measured evaporation from the lake when averaged over monthly periods.     

Morphology and paleoclimatic significance of Pleistocene Lake Bonneville spits

Pleistocene Lake Bonneville of western Utah contains a variety of spits associated with shorelines and other features that formed between 21,000 and 12,000 ¹⁴C yr BP. Field studies in the low-lying mountain ranges of the central portion of Lake Bonneville identified 17 spits of various types. The spits are connected to small mountain ranges and islands, vary in size from 0.02 to 0.5 km², and are composed of coarse-grained, well-rounded, poorly-sorted sedimentary material. Sixteen of the 17 spits have a northeasterly to southwesterly orientation implying that winds were from the northwest to northeast, approximately 180° out of phase with modern winds in the eastern Great Basin. Lake Bonneville spit orientation is best explained as the result of persistent northerly winds caused by the high atmospheric pressure cell of the continental ice sheet and passage of low pressure extratropical storms south of the lake. Similar, strong persistent winds are a common feature of modern continental ice sheets and passing low pressure systems. If so, the North American jet stream tracked south of Lake Bonneville as recently as 12,000 ¹⁴C yr BP, well past the height of the last glacial maximum.     

基于手持GPS洞庭湖区植被景观格局外业调绘研究

本文以1：50000的地形图为工作底图，采用手持GPS技术、利用实地调查与分解森林资源分布图相结合的方法，对洞庭湖区植被分布状况进行了外业调绘的研究。建立了外业调绘过程中植被群落斑块最小面积和边界的确定原则及其斑块类型与编码。探讨了手持GPS实现WGS-84坐标系向1954北京坐标系的转化方法。并进行了精度估算，其点位误差≤5．8m，完全可以满足洞庭湖区植被分布外业调绘的精度要求。运用该方法进行植被分布的外业调绘，可以解决运用传统的定位测量方法在湖区定位难的问题，提高了外业作业效率。     

风浪作用下水生植物对水流结构的影响——以太湖中两种典型沉水植物为例

为研究风浪作用下水生植物对水流结构的影响,选取太湖中两种典型沉水植物(苦草与马来眼子菜)为研究对象,分别对苦草植物斑块与马来眼子菜植物斑块内外水体的瞬时流速进行野外现场测量,利用瞬时流速的能量谱分布将波浪流速与紊动流速分离,分别分析水生植物对时均流速、波浪流速以及紊动能的影响。风浪影响下,水体中存在流向与测量时近水面处盛行风向一致的水流;波浪流速以垂向流速为主,且波浪流速自水面向床底逐渐减小;紊动能在水面处达到最大值,并向床底方向逐渐减小。与无植被条件相比,苦草与马来眼子菜的存在减小了时均流速、波浪流速以及紊动能。两种植物形态上的差异,导致其对水流结构的影响不同:苦草叶片阻流面积在冠层中部达到最大,使得时均流速与波浪流速在苦草中部位置的减小程度最大;马来眼子菜叶片主要集中于冠层顶部(水面附近),其对时均流速以及波浪流速的减小作用在水面处达到最大。     

Survey on PCDDs and PCDFs in sediments and soils in Ya-Er Lake area, China

INTRODUCTIONOverthepastseveralycars,considerableinteffethasbocenteIdontheenvir0nmentalbehaviouroftoxicandpersistritcomP0undssuchashalogenatalaromat-ies,inparticularthepolychiorinataldibopdioxins(PCDDe)andpolydil0rinateddi~fUtansrpCDFs).ManyofthesecomPounds,espedllythosewithahighdegere0fchiorinesubstitution,arehighlylipophiliccontawhnantSwithlowWhtersolubility,highre-sistancet0chdricaltransformationsandlowwhcrobiologhaldegradatfonpeatnnann,l988,Bromanetal.,l989,Hites,l990)whichexplain…     

察尔汗盐湖卤水中钾的放射性测定误差原因初步分

盐湖卤水中钾的放射性测定是一个复杂的工作过程,有许多因素影响着测量精度和稳定性,必须对各个方面综合考虑,最大限度地降低可能产生的干扰影响。本文分析了引起测定误差的主要因素,并提出了提高测量精度采取的措施。     

Potential salinity limitations on nitrogen fixation in sediments from Mono Lake, California

Mono Lake is a hypersaline alkaline lake in the high altitude Great Basin desert of eastern California. Algal productivity of the lake is nitrogen-limited, and a contributing source is derived from benthic nitrogen fixation. Lake level and salinity have fluctuated with natural climatic variations but have also been affected by the diversion of tributary streams. This research examines the influence of varied salinity and lake level on the potential for benthic nitrogen fixation in Mono Lake. A sediment-surface microbial mat community was exposed directly, and in acclimated cultures, to a range of Mono Lake salinities under anaerobic incubations and the activity of nitrogenase assayed by acetylene reduction. Activity was stimulated in light, but also occurred in darkness. Over an experimental salinity range from 50 to 150 g L⁻¹ TDS, nitrogenase activity was reduced by 90 per cent, with the activity persisting at the highest salinity being attributable to dark fixation alone. Between a salinity of 50 g L⁻¹, occurring in Mono Lake over 50 years ago, and 100 g L⁻¹, nitrogenase activity was reduced by nearly half. Changes in the area of the littoral zone at varied lake levels also affect the total amount of potential benthic nitrogen fixation in the lake. An accounting of yearly inputs of nitrogen to Mono Lake suggests N₂-fixation could contribute as much as 76–81 percent of the total. Inhibition of nitrogen fixation rates by increased salinity could limit the long-term nutrient supply and benthic primary productivity of this ecosystem.     

Lake Winnipeg: geological setting and sediment seismostratigraphy

Lake Winnipeg, the seventh largest lake in North America, is located at the boundary between the Interior Plains and the Canadian Shield in Manitoba, Canada. Seismic profiles were obtained in Lake Winnipeg on two geoscientific cruises in 1994 and 1996. These data indicate the morphology of the bedrock surface. In most cases, a clear distinction between low relief Paleozoic carbonate rock and high relief Precambrian rock can be made. In northern Lake Winnipeg, the eastern limit of Paleozoic rock is clearly demarcated 30 km west of the previous estimate of its position. In southern Lake Winnipeg, all or most of the Paleozoic sequence terminates at a prominent buried escarpment in the centre of the lake. This indicates that Paleozoic rock on the eastern shore, known from drilling and outcrops, is an outlier. Major moraines are apparent as abrupt, large ridges having a chaotic internal reflection pattern. These include the Pearson Reef Moraine, the George Island Moraine and the offshore extension of The Pas Moraine. Little evidence for extensive or thick till was observed. Instead, fine-grained sediments deposited in glacial Lake Agassiz rest directly on bedrock over most of the lake basin. Hence an episode of erosion to bedrock was associated with glaciation and/or deglaciation. The Agassiz Sequence sediments are well-stratified, drape underlying relief and in some areas are over 100 m thick. In places, stratification in these sediments is disrupted, perhaps by dewatering. Evidence of erosion of Agassiz Sequence sediments by recent currents was observed. The contact between the Agassiz Sequence and the overlying Winnipeg Sequence sediments is a marked angular unconformity. The Agassiz Unconformity indicates up to 10 m of erosion in places. The low-relief character of this unconformity precludes subaerial erosion and the lack of till, moraines, or extensive deformation precludes glacial erosion. Waves appear to be the most likely erosional agent, either in waning Lake Agassiz or early Lake Winnipeg time. Winnipeg Sequence sediments, in places very thin, mantle most of the lakefloor. These sediments were deposited in the present Lake Winnipeg and are faintly stratified to massive and reach about 10 m in thickness in deep water. On the surface of the Winnipeg Sequence, vigorous, episodic currents are thought to contribute to the construction of flow-transverse sand waves as much as 6 m high in a deep, narrow constriction in the lake.     

Magnetic properties of recent sediments in Lake Baikal, Siberia

Mineral magnetic measurements of six ²¹⁰Pb-dated surface cores from different basins of Lake Baikal, Siberia, show temporal records controlled by a range of internal and external processes. With the exception of sediments on the Academician Ridge, there is clear evidence for widespread reductive diagenesis effects on the ferrimagnetic component coupled with neo-formation of paramagnetic iron minerals. Greigite formation, bacterial magnetosome accumulation and turbidite layers may affect the properties of some sediment levels. Concentrations of canted antiferromagnetic minerals (eg. haematite) appear to increase from the 19th century onwards. These minerals are less affected by dissolution processes and probably represent detrital minerals delivered by catchment fluvial processes. The magnetic evidence for recent atmospheric pollution by fossil-fuel combustion processes is weak in all the cores, and supports the findings from studies of spherical carbonaceous particles (SCPs) and heavy metals that pollution is largely restricted to the southern basin. Correlations between recent sediments based on magnetic data may be insecure over long distances or between basins.     

Mineral magnetic and physical properties of surficial sediments and onshore samples from the southern basin of Lake Baikal, Siberia

Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using ²¹⁰Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM₂₁₀ suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core.