Field studies of river flow dynamics in the lower pool of a hydrosystem

Results of field studies of Istra River current dynamics over a 35-km reach in the lower pool of a hydrosystem are analyzed. Data on the distribution of current velocities in cross-sections with different morphometry and the characteristics of bottom deposits at significantly different water discharges, and data on variations in the water surface slope are presented. The coefficients of hydraulic friction are assessed.     

Assessment of environmental flows under limited data availability: case study of the Acheloos River,Greece

Abstract

The lower course of the Acheloos River is an important hydrosystem in Greece, heavily modified by a cascade of four hydropower dams upstream, which is now being extended by two more dams in the upper course. The design of the dams and hydropower facilities that are in operation has not considered any environmental criteria. However, in the last 50 years, numerous methodologies have been proposed to assess the negative impacts of such projects to both the abiotic and biotic environment, and to provide decision support towards establishing appropriate constraints on their operation, typically in terms of minimum flow requirements. In this study, seeking a more environmentally-friendly operation of the hydrosystem, we investigate the outflow policy from the most downstream dam, examining alternative environmental flow approaches. Accounting for data limitations, we recommend the basic flow method, which is parsimonious and suitable for Mediterranean rivers, whose flows exhibit strong variability across seasons. We also show that the wetted perimeter–discharge method, which is an elementary hydraulic approach, provides consistent results, even without using any flow data. Finally, we examine the adaptation of the proposed flow policy (including artificial flooding) to the real-time hydropower generation schedule, and the management of the resulting conflicts.

Editor Z.W. Kundzewicz; Guest editor M. Acreman

Citation Efstratiadis, A., Tegos, A., Varveris, A., and Koutsoyiannis, D., 2014. Assessment of environmental flows under limited data availability: case study of the Acheloos River, Greece. Hydrological Sciences Journal, 59 (3–4), 731–750.     

Effects of permafrost degradation on thermokarst lake hydrochemistry in the Qinghai-Tibet Plateau,China

Thermokarst lakes play a key role in the hydrological and biogeochemical cycles of permafrost regions. Current knowledge regarding the changes caused by permafrost degradation to the hydrochemistry of lakes in the Qinghai-Tibet Plateau (QTP) is limited. To address this gap, a systematic investigation of thermokarst lake water, suprapermafrost water, ground ice, and precipitation was conducted in the hinterland of the QTP. The thermokarst lake water in the QTP was identified to be of the Na-HCO₃-Cl type. The mean concentrations of HCO₃⁻ and Na⁺ were 281.8 mg L⁻¹ (146.0–546.2 mg L⁻¹) and 73.3 mg L⁻¹ (9.2–345.8 mg L⁻¹), respectively. The concentrations of Li⁺, NH₄⁺, K⁺, F⁻, NO₂⁻, and NO₃⁻ were relatively low. Freeze-out fractionation concentrated the dissolved solids within the lake water during winter, which was deeply deepened on lake depth and lake ice thickness. Owing to solute enrichment, the ground ice was characterized by high salinity. Conversely, repeated replenishment via precipitation led to lower solute concentrations in the ground ice near the permafrost table compared to that within the permafrost. Although lower solute concentration existed in precipitation, the soil leaching and saline ground ice melting processes enhanced the solute load in suprapermafrost water, which is considered an important water and solute resource in thermokarst lakes. The influencing mechanism of permafrost degradation on thermokarst lake hydrochemistry is presumably linked to: (1) the liberation of soluble materials sequestered in ground ice; (2) the increase of solutes in suprapermafrost water and soil pore water; and (3) the changes in lake morphometry. These results have major implications on the understanding of the effects of ground ice melting on ecosystem functions, biogeochemical processes, and energy balance in a rapidly changing climate.     

Glacial conditions that contribute to the regeneration of Fountain Glacier proglacial icing,Bylot Island,Canada

Proglacial icings are one of the most common forms of extrusive ice found in the Canadian Arctic. However, the icing adjacent to Fountain Glacier, Bylot Island, is unique due to its annual cycle of growth and decay, and perennial existence without involving freezing point depression of water due to chemical characteristics. Its regeneration depends on the availability of subglacial water and on the balance between ice accretion and hydro‐thermal erosion. The storage and conduction of the glacial meltwater involved in the accretion of the icing were analyzed by conducting topographic and ground penetrating radar surveys in addition to the modelling of the subglacial drainage network and the thermal characteristics of the glacier base. The reflection power analysis of the geophysical data shows that some areas of the lower ablation zone have a high accumulation of liquid water, particularly beneath the centre part of the glacier along the main supraglacial stream. A dielectric permittivity model of the glacier – sediment interface suggests that a considerable portion of the glacier is warm based; allowing water to flow through unfrozen subglacial sediments towards the proglacial outwash plain. All these glacier‐related characteristics contribute to the annual regeneration of the proglacial icing and allow for portions of the icing to be perennial. Copyright © 2013 John Wiley & Sons, Ltd.     

The Seasonal Variation of Large Volume Airgun Signals in Hutubi, Xinjiang

In order to study the seasonal variation of large volume airgun signals in Hutubi, Xinjiang, we analyzed 2,936 signals of airgun source excitations during 2015-2016 received by a seismograph on the bank of the excitation pool. Firstly, the RMS value of the signal amplitude and the daily average temperature were compared after linearly superimposing the signal in days, to analyze the influence of the surface ice cover on the excitation energy release of the airgun source. The result shows that the ice cover will reduce the excitation energy, and the thicker the ice cover is, the more obvious the excitation energy reduces. Secondly, the time-frequency analysis method was used to analyze the influence of the surface ice cover on the signal frequency. It is concluded that the existence of the ice cover has little effect on the signal frequency, but it will affect the intensity of the signal around 4Hz between 1-2s after excitation. The cause of these phenomena is that the ice cover affects the bubble oscillation, which in turn affects the energy conversion. The study shows that when using the cross-correlation delay method to calculate the wave velocity, the signals can be divided into two periods according to the daily average temperature:with or without ice cover on the upper surface of the excitation pool. This can help eliminate the influence of the source variation and improve the accuracy of the monitoring results.     

Deep water circulation, residence time, and chemistry in a karst complex

We investigated the hydrochemistry of a complex karst hydrosystem made of two carbonate units along a coastal lagoon. Ground water emerges on the lagoon floor from a submarine spring. In addition, thermal waters circulate through the limestone and mix with karst water near the lagoon shore. A distinction between the water from the two carbonate units is related to marine influences and human activities. In one of the massifs, the data show an incongruent dissolution of dolomite with time. In the other system, a slight contamination by saline fluids from the thermal reservoir has led to high calcium and magnesium concentrations. 36Cl, 14C, and 3H data constrain the residence time of the water, and allow for the distinguishing of four circulation types: (1) shallow surface circulation (primarily above sea level) in the karstic units with short residence times (<20 years); (2) shallow subsurface circulation (approximately 0 to -50 m) below the karstic units with residence time in the order of 50 years; (3) deep circulation at depth of 700 to 1500 m in the Jurassic limestones below thick sedimentary cover, with residence time of several thousand years for a part of the water; and (4) deep circulation at a depth of approximately 2500 m, which represents the thermal reservoir in the Jurassic units with residence time of approximately 100,000 years. An interpretative hydrogeological framework is based on the constraints of the geochemical analyses of the deep thermal system, and by water flow from the surface to the deep parts of the carbonate formations.     

Ice cover of the Amur River and its impact on channel processes

Special features of the Amur R. ice regime in its middle and lower reaches are discussed along with the role of river ice in bank erosion, transport of coarse fragmental material, bottom washout, and water flow redistribution among branches. Numerous and various consequences of the ice drift impact on the banks and floodplain, as well as the intensity of erosion processes in the Amur River channel during winter have been revealed. The water flow redistribution among the Amur River branches near Khabarovsk noticeably changed the ice regime, which was rehabilitated as a result of implementation of a complex of special measures.     

Pressure‐driven,shoreline currents in a perennially ice‐covered,pro‐glacial lake in Antarctica,identified from a LiCl tracer injected into a pro‐glacial stream

The distribution of streamwater within ice‐covered lakes influences sub‐ice currents, biological activity and shoreline morphology. Perennially ice‐covered lakes in the McMurdo Dry Valleys, Antarctica, provide an excellent natural laboratory to study hydrologic–limnologic interactions under ice cover. For a 2 h period on 17 December 2012, we injected a lithium chloride tracer into Andersen Creek, a pro‐glacial stream flowing into Lake Hoare. Over 4 h, we collected 182 water samples from five stream sites and 15 ice boreholes. Geochemical data showed that interflow travelled West of the stream mouth along the shoreline and did not flow towards the lake interior. The chemistry of water from Andersen Creek was similar to the chemistry of water below shoreline ice. Additional evidence for Westward flow included the morphology of channels on the ice surface, the orientation of ripple marks in lake sediments at the stream mouth and equivalent temperatures between Andersen Creek and water below shoreline ice. Streamwater deflected to the right of the mouth of the stream, in the opposite direction predicted by the Coriolis force. Deflection of interflow was probably caused by the diurnal addition of glacial runoff and stream discharge to the Eastern edge of the lake, which created a strong pressure gradient sloping to the West. This flow directed stream momentum away from the lake interior, minimizing the impact of stream momentum on sub‐ice currents. It also transported dissolved nutrients and suspended sediments to the shoreline region instead of the lake interior, potentially affecting biological productivity and bedform development. Copyright © 2014 John Wiley & Sons, Ltd.     

A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements

Data from three years of MOZAIC measurements made it possible to determine a distribution law for the relative humidity in the upper troposphere and lower stratosphere. Data amounting to 13.5% of the total were obtained in regions with ice supersaturation. Troposphere and stratosphere are distinguished by an ozone concentration of 130 ppbv as threshold. The probability of measuring a certain amount of ice supersaturation in the troposphere decreases exponentially with the degree of ice supersaturation. The probability of measuring a certain relative humidity in the stratosphere (both with respect to water and ice) decreases exponentially with the relative humidity. A stochastic model that naturally leads to the exponential distribution is provided. Mean supersaturation in the troposphere is about 15%, whereas ice nucleation requires 30% supersaturation on the average. This explains the frequency of regions in which aircraft induce persistent contrails but which are otherwise free of clouds. Ice supersaturated regions are 3-4 K colder and contain more than 50% more vapour than other regions in the upper troposphere. The stratospheric air masses sampled are dry, as expected, having mean relative humidity over water of 12% and over ice of 23%, respectively. However, 2% of the stratospheric data indicate ice supersaturation. As the MOZAIC measurements have been obtained on commercial flights mainly between Europe and North America, the data do not provide a complete global picture, but the exponential character of the distribution laws found is probably valid globally. Since water vapour is the most important greenhouse gas and since it might enhance the anthropogenic greenhouse effects via positive feedback mechanisms, it is important to represent its distribution correctly in climate models. The discovery of the distribution law of the relative humidity makes possible simple tests to show whether the hydrological cycle in climate models is represented in an adequate way or not.     

Formation of Frazil Ice and Ice Jams on the Rivers of the Yenisei River Basin

The effect of different factors on the duration of spring ice jams and the flood water level in the Siberian rivers is considered. Observational data on the frequency of ice jams of different probabilities within some reaches of the Yenisei River are presented.     

Hydrogeomorphic processes of thermokarst lakes with grounded‐ice and floating‐ice regimes on the Arctic coastal plain,Alaska

Thermokarst lakes cover > 20% of the landscape throughout much of the Alaskan Arctic Coastal Plain (ACP) with shallow lakes freezing solid (grounded ice) and deeper lakes maintaining perennial liquid water (floating ice). Thus, lake depth relative to maximum ice thickness (1·5–2·0 m) represents an important threshold that impacts permafrost, aquatic habitat, and potentially geomorphic and hydrologic behaviour. We studied coupled hydrogeomorphic processes of 13 lakes representing a depth gradient across this threshold of maximum ice thickness by analysing remotely sensed, water quality, and climatic data over a 35‐year period. Shoreline erosion rates due to permafrost degradation ranged from < 0·2 m/year in very shallow lakes (0·4 m) up to 1·8 m/year in the deepest lakes (2·6 m). This pattern of thermokarst expansion masked detection of lake hydrologic change using remotely sensed imagery except for the shallowest lakes with stable shorelines. Changes in the surface area of these shallow lakes tracked interannual variation in precipitation minus evaporation (P ? E_L) with periods of full and nearly dry basins. Shorter‐term (2004–2008) specific conductance data indicated a drying pattern across lakes of all depths consistent with the long‐term record for only shallow lakes. Our analysis suggests that grounded‐ice lakes are ice‐free on average 37 days longer than floating‐ice lakes resulting in a longer period of evaporative loss and more frequent negative P ? E_L. These results suggest divergent hydrogeomorphic responses to a changing Arctic climate depending on the threshold created by water depth relative to maximum ice thickness in ACP lakes. Copyright © 2011 John Wiley & Sons, Ltd.     

Reassessing Lake Vostok’s behaviour from existing and new ice core data

Interpretation of new ice core data and reappraisal of existing data, both from the basal part of the Vostok ice core, give strong support to a kind of thermohaline circulation in Lake Vostok. Although the salinity of the lake is considered as weak (less than 1‰), the prominent influence of salinity at high pressure and low temperature on water density makes such a circulation possible. As a consequence, subglacial melting along the northern shores of the lake is balanced, further south, by frazil ice production in the upper water column, its accretion and consolidation at the ice–water interface followed by accreted ice export out of the system together with the southeasterly glacier flow. The dynamics of the system is documented by a stable water isotope budget estimate, by inferences concerning accreted ice formation and by an investigation of ice properties at the transition between meteoric ice and accreted ice. This complex behaviour is the controlling factor on water, biota and sediment fluxes in the lake environment.     

Factors influencing the growth of miniature ice lenses

Experiments involving six soil types indicated that the production of miniature ice lenses is dependent on soil moisture content and grain size with lenses being produced in finer-grained soils at lower moisture contents than is possible in coarser soils. Explanation of the results involves consideration of the unsaturated hydraulic conductivity of the unfrozen soil through which water migrated to the freezing plane to allow ice lens growth.     

Data mining methods for hydroclimatic forecasting

Skillful streamflow forecasts at seasonal lead times may be useful to water managers seeking to provide reliable water supplies and maximize hydrosystem benefits. In this study, a class of data mining techniques, known as tree-structured models, is investigated to address the nonlinear dynamics of climate teleconnections and screen promising probabilistic streamflow forecast models for river–reservoir systems. In a case study of the Lower Colorado River system in central Texas, a number of potential predictors are evaluated for forecasting seasonal streamflow, including large-scale climate indices related to the El Niño–Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO), and others. Results show that the tree-structured models can effectively capture the nonlinear features hidden in the data. Skill scores of probabilistic forecasts generated by both classification trees and logistic regression trees indicate that seasonal inflows throughout the system can be predicted with sufficient accuracy to improve water management, especially in the winter and spring seasons in central Texas.     

Experimental investigation of sediment transport in partially ice-covered channels

It is important to understand the effects of ice cover on sediment transport in cold climates, where sub-freezing temperatures affect water bodies for a significant part of the year. The literature contains many studies on sediment transport in open channel flow, and several studies on sediment transport in completely ice-covered flow. There has been little or no research on sediment transport in partially ice-covered channels. In the current study, laboratory experiments were done in a rectangular flume to quantify the impact of border ice presence on the sediment transport rate. The effects of ice cover extent and changing flow strengths on sediment transport distribution also were investigated, and the results were compared to those for fully ice-covered and open channel flow. The ice coverage ratios considered were 0 (representing the open water condition), 0.25, 0.50, 0.67, and 1 (representing fully ice-covered flow). The partial ice cover was found to impact the sediment transport distribution within the channel. The effect of ice coverage extent on sediment transport distribution was more significant at lower flow strengths and became negligible at higher flow strengths. The conventional equations for sediment transport in open channel flow and fully ice-covered flow that relate the dimensionless bedload transport rate to the flow strength were found to be applicable to estimate the total cross-section-averaged bedload transport for partially ice-covered flow when modified appropriately. Empirical coefficients for these equations were determined using the experimental data.     

Modeling of nitrobenzene in the river with ice process in high-latitude regions

To deal with the accidental release of pollutants into frozen rivers, those rivers in high-latitude regions should be treated differently. Pollutants could be stored up in the ice and released when the ice melts in spring, perhaps resulting in secondary pollution in the river. A water quality model of nitrobenzene has been developed in this paper to assess the influence on river water quality due to the freezing process. The model is made up of three modules: thermodynamic module, hydrodynamic module and water quality module. The thermodynamic module considers the complex heat exchange processes between the water body and the atmosphere, the water body and the river bed, the water body and the ice cover, and so on. The growth of the ice cover is simulated in a simplified form whilst taking consideration of heat balance. The hydrodynamic model uses the Saint-Venant equations in non-constant flow and the influence of the ice cover is measured. The degradation, diffusion, absorption and desorption, and the influence of the freezing process are incorporated in the water quality module and the concept of Continuously Stirred Tanked Reactors (CSTRs) model is applied in the model construction. The model has been applied to supporting the management of accidental pollution in the Songhua River. The model was calibrated and validated with monitored data. Regional sensitivity analysis based on a task-based Hornberger-Spear-Young (HSY) algorithm was carried out to examine the model structure and the result showed that the model could describe the system very well. Then the conditioned model was applied to predicting the concentration of nitrobenzene in the water when the ice melted in spring. The predictive result showed that the release of pollutants in the ice could lead to an increase in the concentration of pollutants in the water, but the increase would be very small because there was only a small amount of pollutants stored in the ice. A secondary pollution could therefore be avoided. Also, with necessary modifications, the model established in this study could be applied to the water quality management in rivers that freeze in winter.     

Ice cover,landscape setting,and geological framework of Lake Vostok,East Antarctica

Lake Vostok, located beneath more than 4 km of ice in the middle of East Antarctica, is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment. Melting and freezing at the base of the ice sheet, which slowly flows across the lake, controls the flux of water, biota and sediment particles through the lake. The influx of thermal energy, however, is limited to contributions from below. Thus the geological origin of Lake Vostok is a critical boundary condition for the subglacial ecosystem. We present the first comprehensive maps of ice surface, ice thickness and subglacial topography around Lake Vostok. The ice flow across the lake and the landscape setting are closely linked to the geological origin of Lake Vostok. Our data show that Lake Vostok is located along a major geological boundary. Magnetic and gravity data are distinct east and west of the lake, as is the roughness of the subglacial topography. The physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake’s circulation. Lake Vostok is a tectonically controlled subglacial lake. The tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake.     

The structure of ice and ice cover in lower Amur reaches with branched channel

The study is focused on the specific features of ice regime in Amur lower reaches and the structure of ice surface in branches of different size and dynamic state. The main features of ice stratigraphy in different parts of the river have been identified, and ice cover characteristics have been found to show considerable heterogeneity across the channel. Quantitative data on the concentration of terrigenous material in the ice and its distribution within the ice mass have been collected. The amount of terrigenous material was estimated in the ice in Amur lower reaches, from where it is largely exported into the Sea of Okhotsk during spring ice drift.     

Interrogating the time and processes of development of the Las Liebres rock glacier,central Chilean Andes,using a numerical flow model

The Las Liebres rock glacier is a large (~2.2 km long) Andean rock glacier whose internal composition and kinematics are known from previous studies. We investigate its development by posing and testing the following null hypothesis: the rock glacier has developed from a constant supply of debris and ground ice in periglacial conditions and resulting creep of the ice‐rock mixture. A rheological model was formulated based on recent advances in the study of ice‐rock mixture rheology, and calibrated on the known surface velocities and internal composition of the rock glacier. We show that the rock glacier viscosity is inversely related to both water and debris fractions, in agreement with recent field and theoretical studies of ice‐rock mixture rheology. Taking into account the possible variations in water fraction, the model was used to estimate the time spans of development (0.91–7.11 ka), rates of rock wall retreat (0.44–4.18 mm/a), and rates of ground ice formation (0.004–0.026 m/a) for the rock glacier. These results support the null hypothesis of a periglacial origin of the Las Liebres rock glacier. Copyright © 2016 John Wiley & Sons, Ltd.     

联合GRACE、Swarm、GRACE-FO卫星观测确定格陵兰岛冰盖质量时空变化特征

GRACE(Gravity Recovery and Climate Experiment)重力卫星任务的成功实施,极大推进了极地冰盖质量平衡、全球水循环和海水质量变化等领域的研究,其后续任务GRACE-FO(Gravity Recovery and Climate Experiment Follow-On)于2018...