Response of Hydrodynamics and Water-quality Conditions to Climate Change in a Shallow Lake

Lake water resources operation and water quality management come up with higher challenges due to climate change. The frequency and intensity of extreme hydrological events are increasing under global warming, which may directly lead to more uncertainty and complexity for hydrodynamic and water-quality conditions in large shallow lake. However, studies about effects of climate change on lake hydrodynamic and water-quality conditions are not enough. Thus, a coupled model is es-tablished to investigate the potential responses of lake water level, flow field and pollutant migra-tion to the changing climatic factors. The results imply that water flow capacity and self-purification in the Hongze Lake can be improved by west, northwest, north, south and southeast winds indi-cating wind filed change has a great effect on the hydrodynamic and water-quality conditions in large shallow lake. It is further observed that both hydrodynamics and water quality are more sensitive to rainfall change than to temperature change; compared to the effect from temperature and rainfall, the effect from wind field appear to be more pronounced. Moreover, the results verify the feasibility of coupling basin hydrological model with lake hydrodynamic and water quality model. To the best of knowledge, the coupled model should not be used until independent calibra-tions and verifications for hydrodynamics and water quality modeling, the hydrological model and the coupled model.

     

Effects of Anthropogenic Influences on the Trophic State,Land Uses and Aquatic Vegetation in a Shallow Mediterranean Lake: Implications for Restoration

Lake Pamvotis, NW Greece is a shallow Mediterranean eutrophic lake that has changed drastically over the past 50 years. Strong effects, resulted mainly from anthropogenic causes, in the hydrological regime are shown for this area using long term hydrological data and a GIS system for extracting land cover/use changes. A set of aerial imagery acquired in 1945 through 2002 were used to monitor and assess the spatial and temporal changes in land cover/use, focused mainly on the lake’s surface area and its surrounding ecosystem (Natura 2000 area). The significance of the changes in land cover/use distribution within Pamvotis wetland is further discussed depicting the role of the anthropogenic influence on the fragile ecosystem that resulted in the shrinkage of lake’s habitats extent. The purpose of this analysis was to examine the long-term changes on macrophyte community composition, species occurrence and relative abundance with water quality and water level changes over the past century, using historical data, aerial photos and GIS techniques. The results showed that for the last 25 years annual water level fluctuation ranged from 70 to 159 cm. Water level starts decreasing in mid June and increasing again gradually from November until March–April. Intra annual water level fluctuation seems to be affected by land use for agricultural purpose through intensive irrigation and the summer drought as well. A dramatic decline of the submerged vegetation is apparent mainly attributed to anthropogenic pressures. Regarding the land cover/use changes, the most notable and significant alterations are concerning the urban development around the lake, the disappearance of wet meadows and the extension of reed beds. Finally it seems that water budget data as well as the response of the key eutrophication parameters are affected from both hydrological alterations and point/non-point pollution sources.     

Gyres and Seiches in a Large and Shallow Lake

Gyres and seiches are two prominent features of lakes. Gyres largely transport sediments, nutrients, and algae in the horizontal direction. Seiches, on the other hand, can contribute to the vertical mixing in lakes. Theoretical analysis, statistical methods, and numerical models are used to investigate gyres and seiches in Lake Okeechobee, the largest subtropical/tropical lake in North America. The lake has a 1,730-km2 surface water area, a typical length of more than 50 km, and a mean depth of 3.2 m. Both the Empirical Orthogonal Function (EOF) method and the numerical model results indicated that lake circulation is typically dominated by a two-gyre pattern, especially in the winter. The northwest wind or southeast wind leads to a cyclone (a counterclockwise rotation gyre) in the southwest and an anticyclone (a clockwise rotation gyre) in the northeast. Because the mean velocity field in the lake is very weak, the first two EOF modes play an important role in lake transport. The mechanism of gyre formation in the lake is clearly explained in a theoretical analysis. Power spectra analysis on measured and modeled water elevations at four stations revealed that Lake Okeechobee has a seiche signal of 5 hours or so. The seiche range is typically around 10 cm. Results from the theoretical analysis, power spectral analysis, and numerical modeling all agree with each other very well. The findings in this study should be useful to understand the lake processes, to guide field data collection programs, and to assist decision making on lake management.     

Variations in Spectral Slope in Lake Taihu,a Large Subtropical Shallow Lake in China

Spectral slope (S), describing the exponential decrease of the absorption spectrum over a given wavelength range, is an important parameter in the study of of chromophoric dissolved organic matter (CDOM) dynamics, and also an essential input parameter in remote sensing models. Furthermore, S is often used as a proxy for CDOM composition, including the ratio of fulvic to humic acids and molecular weight. The relative broad range in S values reported in the literature can be explained by the different spectral ranges and fitting methods used. A single exponential model is used to fit the S values for 17 investigations involving 458 samples in Lake Taihu from January to October in 2004. The average S value was 15.18 ± 1.39 μm⁻¹ for the range of 280–500 nm, which fell within the range reported in the literature. The frequency distribution of S value basically obeyed a normal distribution. Significant differences in S values between summer and other seasons showed that phytoplankton degradation was one of the important sources of CDOM in summer, whereas CDOM mainly came from the river input in other seasons. Furthermore, the estimated S value decreased with increasing wavelength range used in regression. The maximum and minimum values derived from the regression were 17.89 ± 1.25 μm⁻¹ and 13.62 ± 2.11 μm⁻¹ for the wavelength ranges of 280–380 nm and 400–500 nm, respectively, a decrease of 23.9%. S values significantly decreased with the increase of CDOM absorption coefficients. CDOM absorption coefficients could be more appropriately estimated from exponential model introducing the variation of S with absorption coefficients, making them useful for a remote sensing bio-optical model of Lake Taihu. DOC-specific absorption coefficient a*(λ) and the parameter M describing molecular size of the humic molecules could also be used as a proxy for the sources and types of CDOM. A general relationship was found between S and a*(λ), and M values. S increased with the decrease of DOC-specific absorption coefficient and the increase of M corresponding to the decrease of molecular weight.     

典型洪水和干旱过程对湖泊湿地的生态作用

在分析水文过程、栖息地变化和湿地生态之间关系的基础上,指出洪水和干旱过程是驱动湿地过渡带生态过程的重要因子,对于湖泊湿地生态产生重要作用。选取Okeechobee湖湿地为试验区,研究典型洪水和干旱过程下,不同频率水位的湖泊湿地淹没变化,分别从定量和定性角度,利用栖息地适宜指数模型评价下垫面变化的生态作用,分析产生生态作用的原因,并以实测候鸟资料验证结果。指出一定程度的洪水和干旱过程有利于过渡带生物物种的多样性。     

Temporal Variations in Dissolved Oxygen Concentrations Observed in a Shallow Floodplain Aquifer

Dissolved oxygen (DO) concentrations play an important role in many groundwater biogeochemial processes, yet assessments of temporal variations are lacking. In this study, we examined daily DO concentrations using a continuously‐reading optical DO probe in a shallow floodplain setting in Iowa to (i) quantify fluctuations across two growing seasons; (ii) examine hydrologic controls on DO values; and (iii) model daily DO concentrations using easily measured variables. DO concentrations exhibited both rapid and long‐term changes in concentrations over time, rapidly increasing and decreasing more than 1 mg/l in response to precipitation recharge and stream stage increase over the span of several hours and days. On 40% of the monitoring days in this study, DO concentrations increased, on average, 0.2–0.4 mg/l from one day to the next. DO concentrations decreased approximately 5–6 mg/l from spring through late summer and fall, likely owing to microbial and root respiration. Daily DO concentrations were successfully modelled using a combination of hydrologic (groundwater level and river stage) and temperature variables (r² > 0.7). Improved understanding of temporal controls on groundwater DO patterns is needed to help clarify the dynamics of many biogeochemical processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Three-dimensional Water Quality and SAV Modeling of a Large Shallow Lake

This study presents the Lake Okeechobee Environment Model (LOEM), a three-dimensional (3D) hydrodynamic, sediment, water quality, and submerged aquatic vegetation (SAV) model of Lake Okeechobee, Florida. The LOEM is developed under the framework of the EFDC model (Hamrick 1992). Lake Okeechobee is the largest subtropical lake in North America (1,730 km²), it is very shallow (mean depth 2.7 m), and it includes a littoral habitat that makes up 20% of its area. The LOEM is calibrated, verified, and validated to 3 years of water quality and SAV data. The water quality results are consistent with observed data from 25 locations in the lake. The model is capable of reproducing key water quality characteristics of the lake without having to resort to extensive, site-specific parameter manipulations. The SAV model is calibrated using measured SAV data in the lake. The SAV model is capable of representing the spatial and temporal variations of SAV variations in the lake well. The LOEM is applied to study water quality and SAV processes in the lake. The model results are consistent with observed data indicating that algal growth in the lake is primarily nitrogen limited in the summer and nitrogen and light co-limited in the winter. Lower water elevation generally leads to larger SAV area. SAV can have positive impact on the lake water quality by reducing algae concentration. The calibrated, verified, and validated LOEM model serves as a useful tool to support lake management.     

Temporal variations in the pelagic fish community of Lake Winnipeg from 2002 to 2019

Lake Winnipeg has undergone extensive changes in environmental conditions and fish community structure during recent decades. The presence of cyanobacteria and invasive species as well as eutrophication, flow regulation, fishing pressure, and climate change all have potential impacts on the native fish community. Since 2002, pelagic forage fish species and early life stages of large-bodied fish species in the pelagic zone in Lake Winnipeg (Manitoba, Canada) have been monitored. Catch data were used to study the temporal variation in species-specific occurrence and catch-per-unit-effort (CPUE) and to analyse the effects of lake condition on species occurrence and CPUE estimates. To account for the temporal variation observed for commonly caught prey species in Lake Winnipeg, we used a Bayesian approach, Integrated Nested Laplace Approximation (INLA), to compare the effects of environmental variables on occurrence and CPUE. The pelagic fish community composition varied among years, and CPUE declined by 2.2-fold between 2005–2008 and 2015–2018. Emerald shiner was the most dominate prey fish species but decreased in the catches since the early 2000’s, while rainbow smelt have almost entirely disappeared from the catches in recent years. Overall, we observed a decrease in CPUE in four of six most caught, pelagic species. Important environmental variables explaining occurrence and CPUE of the six most caught species in the survey were temperature and Secchi, trawl, and water depths. The trawl survey serves as a valuable long-term monitoring tool to study trends in responses of both native and non-native species to ecosystem changes in Lake Winnipeg.     

黄河中游干旱指数计算方法探讨

利用近50年水文、气象资料，通过统计分析气温、降水变化对径流影响及气温、降水相互关系，发现近50年黄河中游呈“干暖型”；据此定义流域干旱及其指数计算思路，建立了干旱指数的计算公式，计算给出了1955～2002年黄河中游夏半年及初夏、盛夏、秋季的干旱指数系列。     

Temporal Distribution of Geoaquatic Fungi at a Nearshore Station in Lake Ontario

In order to better understand the role of fungi in large lakes, the temporal distribution of fungi at a nearshore Lake Ontario station was examined. Water samples were taken at three depths during May to December, 1977. Mean numbers of psychrotrophic fungi in the water samples were significantly less than the numbers of mesophilic fungi to which they were correlated. The mesophilic fungal distribution pattern was correlated (P<0.05) with rainfall in the neighbouring shore area. Apart from coincident autumn peaks, little relationship was observed between the fungi and water temperature. No significant correlation was detected between geoaquatic fungi and several other parameters: heterotrophic bacteria, chlorophyll a, total bacteria, paniculate organic carbon, and total paniculate nitrogen.     

Spatial Patterns and Temporal Variability of Drought in Western Iran

An analysis of drought in western Iran from 1966 to 2000 is presented using monthly precipitation data observed at 140 gauges uniformly distributed over the area. Drought conditions have been assessed by means of the Standardized Precipitation Index (SPI). To study the long-term drought variability the principal component analysis was applied to the SPI field computed on 12-month time scale. The analysis shows that applying an orthogonal rotation to the first two principal component patterns, two distinct sub-regions having different climatic variability may be identified. Results have been compared to those obtained for the large-scale using re-analysis data suggesting a satisfactory agreement. Furthermore, the extension of the large-scale analysis to a longer period (1948–2007) shows that the spatial patterns and the associated time variability of drought are subjected to noticeable changes. Finally, the relationship between hydrological droughts in the two sub-regions and El Niño Southern Oscillation events has been investigated finding that there is not clear evidence for a link between the two phenomena.     

常用旱情分析方法在松原地区的应用

’98洪水过后,降水偏少,进入枯水年份,防汛抗旱齐头并重。在现有墒情观测基础上,以区域降水、蒸发为参数,探讨切实可行的旱情分析方法。     

洞庭湖区2000年～2007年农业干旱灾害特点及成因分析

进入21世纪以来,洞庭湖区连年发生不同程度的农业旱灾.在明确农业旱灾定义的基础上,依据2000～2007年湖区雨情、水情、灾情等资料,综合分析了农业旱灾特点及其成因.分析表明:湖区农业旱灾具有发生频率高,灾情重,连季性和连年性以及成灾率高(50%以上),受灾体类型增多等显著特点.导致农业旱灾的主要原因是:农业用水高峰期连年降雨量偏少,蒸发量大;汛期(4～9月)入湖水量减少,水位偏低;受旱体类型,密度、价值以及抗旱资金投入差异较大.在这些影响因素中,三口断流天数增加,引起入湖水量大幅度减少是直接原因之一,这意味着三峡工程蓄水,对洞庭湖区产生了新的环境效应.     

Spatial and Temporal Variation of Ammonium in Lake Superior

Many aspects of Lake Superior's nitrogen cycle are poorly described in spite of the fact that the lake's nitrate concentration has risen dramatically this past century. One important, yet underdescribed parameter is the concentration of ammonium. Here, we present data to resolve spatial and temporal variation along with vertical profiles of ammonium concentration in Lake Superior. Lake-wide average concentrations were low (0.21 μM, n =166) with considerable spatial and temporal variation. During the onset of summer, the western margin of the lake had higher average concentrations than open and eastern parts. Surface layer (<10 m) ammonium concentrations showed an increase from January to October. Relatively higher ammonium concentration in the near bottom waters at a number of sites during August indicated efflux from sediment to be an important process. Subsurface maxima near the thermocline were observed in late August and persisted until September-October suggesting that ammonium might be controlled by food web processes during warm, stratified conditions. The higher potential for ammonium uptake compared to external inputs suggested rapid turnover of ammonium in the lake.     

Temporal shifts in the biodiversity of nearshore small fishes in Lake Simcoe

Nearshore small fish species represent a large proportion of fish biodiversity in Lake Simcoe, a large inland lake in southern Ontario, Canada. Over the past 30 years, Lake Simcoe has experienced several changes to its aquatic habitat, benthic invertebrate communities and predatory fish populations. This study compared samples of the nearshore small fish community in three geographic areas of Lake Simcoe. Fish community data were grouped into two time periods: a contemporary period (2007–2009) and a historical period (1982–1995). The fish community was compared across time periods for each area to assess if observed ecological changes had an impact on the small fish community. Species richness significantly declined between time periods in two areas (Cook's Bay and the southeast shoreline), the number of individuals captured declined between time periods in one area of the lake (Kempenfelt Bay) and Simpson's diversity index declined between time periods in one area of the lake (southeast shoreline). There were no significant differences in the Shannon–Weiner evenness index between time periods in any of the study areas. Additional analyses of intra- and inter-annual variation in fish sampling results generally supported the findings that shifts in the fish community occurred between time periods. Overall, this study suggests that the nearshore small fish biodiversity of Lake Simcoe has shifted over time but these shifts are not clearly related to recent increases in water clarity, macrophyte growth and nearshore benthic invertebrate densities.     

博斯腾湖湖岸线时空变化特征

利用2006-2016年内的5期landsat遥感数据影像来初步研究博斯腾湖湖岸线在时间和空间两个维度的变化特征以及博斯腾湖湖岸线的变化趋势。通过运用水体指数找出博斯腾湖的水体,提取这5期遥感数据中博斯腾湖的面积和周长,进而计算岸线发育系数、形状复杂程度和圆形度来表征岸线的变化特征。研究结果表明:博斯腾湖湖岸线呈阶段性的变化趋势,2006-2013年间博斯腾湖湖岸线的面积从990. 78 km~2减少到902. 25 km~2,2013-2016年期间博斯腾湖湖岸线面积从902. 25 km~2增加到963. 48 km~2。博斯腾湖湖岸线的面积呈现先减少后增加的过程;博斯腾湖湖岸线的周长也呈现阶段性变化,从2006年的325. 32 km减小到2013年269. 33 km,而2013-2016年为增加趋势,从2013年269. 33 km增加到2016年的331. 93 km。在空间上博斯腾湖的西北角的湿地在2006-2011年向东南变化剧烈,2011-2016年表现稳定,在博斯腾湖的南岸和东南角也出现了周期性的变化,岸线2006-2013年呈现向北部缩小的特征,2013-2016年则向南变化,且变化幅度较大。     

Methods to Assess Costs of Drought Damages and Policies for Drought Mitigation and Adaptation: Review and Recommendations

This article examines available methods for assessing all types of drought costs, including both damage costs and costs arising from adopting policy measures to encourage mitigation of, and adaptation to, droughts. It first discusses damage costs, distinguishing between direct, indirect and non-market costs. Then it examines the suitability of existing methods for estimating drought costs in different economic sectors, their underlying theoretical assumptions, complementarity between different methods, and conditions relevant for their application. The latter include precision, ability to deal with future climate change risks, data needs and availability, and required financial and human resources. The article further considers potential policies for drought mitigation and adaptation and different cost types associated with them. It ends with providing recommendations for good practices regarding the use of methods as well as drought mitigation and adaptation policies.     

洞庭湖区浅层地下水环境质量模糊综合评价

本文以2009年完成的 "洞庭湖区新农村建设浅层地下水质量及新农村安全供水研究报告"地下水资料为依据,通过合理选择地下水污染因子(24项),按浓度和毒理性双权重方法,应用模糊综合评价方法对洞庭湖区浅层地下水进行了环境质量评价,并按三分法划分了地下水质量等级,为该区内农村居民科学取用地下水指明了方向,为当地政府开展新农村建设提供了重要依据.