Impact of channel bar form and grain size variability on estimated stranding risk of juvenile brown trout during hydropeaking

Hydropeaking leads to artificial fluctuations in discharge and corresponding water levels with pronounced dewatering areas between base and peak flow along gravel bars and channel banks. In the present study, 16 hydropeaking reaches in Austria were investigated to assess possible differences in the estimated stranding risk for young of the year brown trout according to different gravel bar types and differences in microtopography roughness. Based on hydrodynamic‐numerical modelling, a predictive habitat modelling approach was implemented in the study design. Accompanied by grain size sampling along the various channel bars, a conceptual stranding risk model (SRM) was developed. The results showed that a high variability in estimated stranding risk exists for the tested sites considering discharge ratios of 1:3, 1:5 and 1:10. With respect to the discussion of establishing legal thresholds for ramping ratios in discharge, it was documented that, exemplarily, a discharge ratio base flow/peak flow of 1:5 (winter base flow conditions) could cause minor differences in the spatial extent of dewatering areas and the related estimated stranding risk for juvenile brown trout compared to a ratio of 1:2 for summer base flow conditions. Microtopographic roughness was addressed due to sampling and analysis of grain size distributions. Statistical testing of grain size distributions revealed significant differences between the surface material compositions of the investigated gravel bars. Those differences are evident, particularly for the coarser fraction (d₉₀), which is important as cover for young of the year brown trout. These aspects of grain size in habitat use and hydraulics have been addressed in the conceptual SRM. The results showed that point bar morphology, in particular, was less sensitive to the risk of stranding compared to, for example, alternating gravel bars. Considering the multiple pressures for alpine rivers, the improvement of structural features due to bar formation and related self‐forming processes is discussed as a possible alternative for future mitigation measures to reduce the negative impacts of hydropeaking. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantifying sediment storage in a high alpine valley (Turtmanntal,Switzerland)

The determination of sediment storage is a critical parameter in sediment budget analyses. But, in many sediment budget studies the quantification of magnitude and time‐scale of sediment storage is still the weakest part and often relies on crude estimations only, especially in large drainage basins (>100 km²). We present a new approach to storage quantification in a meso‐scale alpine catchment of the Swiss Alps (Turtmann Valley, 110 km²). The quantification of depositional volumes was performed by combining geophysical surveys and geographic information system (GIS) modelling techniques. Mean thickness values of each landform type calculated from these data was used to estimate the sediment volume in the hanging valleys and the trough slopes. Sediment volume of the remaining subsystems was determined by modelling an assumed parabolic bedrock surface using digital elevation model (DEM) data. A total sediment volume of 781·3×10⁶–1005·7×10⁶ m³ is deposited in the Turtmann Valley. Over 60% of this volume is stored in the 13 hanging valleys. Moraine landforms contain over 60% of the deposits in the hanging valleys followed by sediment stored on slopes (20%) and rock glaciers (15%). For the first time, a detailed quantification of different storage types was achieved in a catchment of this size. Sediment volumes have been used to calculate mean denudation rates for the different processes ranging from 0·1 to 2·6 mm/a based on a time span of 10 ka. As the quantification approach includes a number of assumptions and various sources of error the values given represent the order of magnitude of sediment storage that has to be expected in a catchment of this size. Copyright © 2009 John Wiley & Sons, Ltd.     

Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part I: Crystallization of zircon and REE-phosphates over three million years—a geochemical and U–Pb geochronological study

Zircon, monazite and xenotime crystallized over a temperature interval of several hundred degrees at the magmatic to hydrothermal transition of the Sn and W mineralized Mole Granite. Magmatic zircon and monazite, thought to have crystallized from hydrous silicate melt, were dated by conventional U–Pb techniques at an age of 247.6 ± 0.4 and 247.7 ± 0.5 Ma, respectively. Xenotime occurring in hydrothermal quartz is found to be significantly younger at 246.2 ± 0.5 Ma and is interpreted to represent hydrothermal growth. From associated fluid inclusions it is concluded that it precipitated from a hydrothermal brine ≤ 600 °C, which is below the accepted closure temperature for U–Pb in this mineral. These data are compatible with a two-stage crystallization process: precipitation of zircon and monazite as magmatic liquidus phases in deep crustal magma followed by complete crystallization and intimately associated Sn–W mineralization after intrusion of the shallow, sill-like body of the Mole Granite. Later hydrothermal formation of monazite in a biotite–fluorite–topaz reaction rim around a mineralized vein was dated at 244.4 ± 1.4 Ma, which distinctly postdates the Mole Granite and is possibly related to a younger hidden intrusion and its hydrothermal fluid system.

Obtaining precise age data for magmatic and hydrothermal minerals of the Mole Granite is hampered by uncertainties introduced by different corrections required for multiple highly radiogenic minerals crystallising from evolved hydrous granites, including ²³⁰Th disequilibrium due to Th/U fractionation during monazite and possibly xenotime crystallization, variable Th/U ratios of the fluids from which xenotime was precipitating, elevated contents of common lead, and post-crystallization lead loss in zircon, enhanced by the fluid-saturated environment. The data imply that monazite can also survive as a liquidus phase in protracted magmatic systems over periods of 10⁶ years. The outlined model is in agreement with prominent chemical core-rim variation of the zircon.     

A review on missing hydrological data processing

Like almost all fields of science, hydrology has benefited to a large extent from the tremendous improvements in scientific instruments that are able to collect long-time data series and an increase in available computational power and storage capabilities over the last decades. Many model applications and statistical analyses (e.g., extreme value analysis) are based on these time series. Consequently, the quality and the completeness of these time series are essential. Preprocessing of raw data sets by filling data gaps is thus a necessary procedure. Several interpolation techniques with different complexity are available ranging from rather simple to extremely challenging approaches. In this paper, various imputation methods available to the hydrological researchers are reviewed with regard to their suitability for filling gaps in the context of solving hydrological questions. The methodological approaches include arithmetic mean imputation, principal component analysis, regression-based methods and multiple imputation methods. In particular, autoregressive conditional heteroscedasticity (ARCH) models which originate from finance and econometrics will be discussed regarding their applicability to data series characterized by non-constant volatility and heteroscedasticity in hydrological contexts. The review shows that methodological advances driven by other fields of research bear relevance for a more intensive use of these methods in hydrology. Up to now, the hydrological community has paid little attention to the imputation ability of time series models in general and ARCH models in particular.     

Vertical and Horizontal Transport of Energy and Matter by Coherent Motions in a Tall Spruce Canopy

In the framework of the EGER (ExchanGE processes in mountainous Regions) project, the contribution of coherent structures to vertical and horizontal transports in a tall spruce canopy is investigated. The combination of measurements done in both the vertical and horizontal directions allows us to investigate coherent structures, their temporal scales, their role in flux transport, vertical coupling between the sub-canopy, canopy and air above the canopy, and horizontal coupling in the sub-canopy layer. The temporal scales of coherent structures detected with the horizontally distributed systems in the sub-canopy layer are larger than the temporal scales of coherent structures detected with the vertically distributed systems. The flux contribution of coherent structures to the momentum and sensible heat transport is found to be dominant in the canopy layer. Carbon dioxide and latent heat transport by coherent structures increase with height and reach a maximum at the canopy height. The flux contribution of the ejection decreases with increasing height and becomes dominant above the canopy level. The flux fraction transported during the sweep increases with height and becomes the dominant exchange process at the upper canopy level. The determined exchange regimes indicate consistent decoupling between the sub-canopy, canopy and air above the canopy during evening, nighttime and morning hours, whereas the coupled states and coupled by sweep states between layers are observed mostly during the daytime. Furthermore, the horizontal transport of sensible heat by coherent structures is investigated, and the heterogeneity of the contribution of coherent events to the flux transport is demonstrated. A scheme to determine the horizontal coupling by coherent structures in the sub-canopy layer is proposed, and it is shown that the sub-canopy layer is horizontally coupled mainly in the wind direction. The vertical coupling in most cases is observed together with streamwise horizontal coupling, whereas the cross-stream direction is decoupled.     

Landscape variations in stream water SO4 and δSSO4 in a boreal stream network

Despite reduced anthropogenic deposition during the last decades, deposition sulphate may still play an important role in the biogeochemical cycles of S and many catchments may act as net sources of S that may remain for several decades. The aim of this study is to elucidate the temporal and spatial dynamics of both SO₄²⁻ and δ³⁴S_SO4 in stream water from catchments with varying percentage of wetland and forest coverage and to determine their relative importance for catchment losses of S. Stream water samples were collected from 15 subcatchments ranging in size from 3 to 6780 ha, in a boreal stream network, northern Sweden. In forested catchments (<2% wetland cover) S-SO₄²⁻ concentrations in stream water averaged 1.7 mg L⁻¹ whereas in wetland dominated catchments (>30% wetland cover) the concentrations averaged 0.3 mg L⁻¹. A significant negative relationship was observed between S-SO₄²⁻ and percentage wetland coverage (r² = 0.77, p < 0.001) and the annual export of stream water SO₄²⁻ and wetland coverage (r² = 0.76, p < 0.001). The percentage forest coverage was on the other hand positively related to stream water SO₄²⁻ concentrations and the annual export of stream water SO₄²⁻ (r² = 0.77 and r² = 0.79, respectively). The annual average δ³⁴S_SO4 value in wetland dominated streams was +7.6‰ and in streams of forested catchments +6.7‰. At spring flood the δ³⁴S_SO4 values decreased in all streams by 1‰ to 5‰. The δ³⁴S_SO4 values in all streams were higher than the δ³⁴S_SO4 value of +4.7‰ in precipitation (snow). The export of S ranged from 0.5 kg S ha⁻¹ yr⁻¹ (wetland headwater stream) to 3.8 kg S ha⁻¹ yr⁻¹ (forested headwater stream). With an average S deposition in open field of 1.3 kg S ha⁻¹ yr⁻¹ (2002-2006) the mass balance results in a net export of S from all catchments, except in catchments with >30% wetland. The high temporal and spatial resolution of this study demonstrates that the reducing environments of wetlands play a key role for the biogeochemistry of S in boreal landscapes and are net sinks of S. Forested areas, on the other hand were net sources of S.     

Estimating the sediment trap efficiency of intermittently dry reservoirs: lessons from the Kruger National Park,South Africa

The assessment of sediment yield from reservoir siltation requires knowledge of the reservoir's sediment trap efficiency (TE). Widely used approaches for the estimation of the long‐term mean TE rely on the ratio of the reservoir's storage capacity (C) to its catchment size (A) or mean annual inflow (I). These approaches have been developed from a limited number of reservoirs (N ≤ 40), most of them located in temperate climate regions. Their general applicability to reservoirs receiving highly variable runoff such as in semi‐arid areas has been questioned. Here, we examine the effect of ephemeral inflow on the TE of 10 small (≤ 280 × 10³ m³), intermittently dry reservoirs located in the Kruger National Park. Fieldwork was conducted to determine the storage capacity of the reservoir basins. The frequency and magnitude of spillage events was simulated with the daily time step Pitman rainfall–runoff model. Different runoff scenarios were established to cope with uncertainties arising from the lack of runoff records and imperfect input data. Scenarios for the relationship between water and sediment discharge were created based on sediment rating curves. Taking into account uncertainties in hydrological modelling, uncertainties of mean TE estimates, calculated from all scenarios (N = 9), are moderate, ranging from ±6 to ±11% at the 95% confidence level. By comparison, estimating TE from the storage capacity to catchment area (C/A) ratio induces high uncertainty (ranges of 35 to 65%), but this uncertainty can be confined (15 to 33%) when the latter approach is combined with hydrological modelling. Established methods relying on the storage capacity to mean annual inflow (C/I) ratio most probably lead to an overestimation of the TE for the investigated reservoirs. The approach presented here may be used instead to estimate the TE of small, intermittently dry reservoirs in semi‐arid climate regions. Copyright © 2017 John Wiley & Sons, Ltd.     

What Can be Expected from the GRACE-FO Laser Ranging Interferometer for Earth Science Applications?

The primary objective of the gravity recovery and climate experiment follow-on (GRACE-FO) satellite mission, due for launch in August 2017, is to continue the GRACE time series of global monthly gravity field models. For this, evolved versions of the GRACE microwave instrument, GPS receiver, and accelerometer will be used. A secondary objective is to demonstrate the effectiveness of a laser ranging interferometer (LRI) in improving the satellite-to-satellite tracking measurement performance. In order to investigate the expected enhancement for Earth science applications, we have performed a full-scale simulation over the nominal mission lifetime of 5 years using a realistic orbit scenario and error assumptions both for instrument and background model errors. Unfiltered differences between the synthetic input and the finally recovered time-variable monthly gravity models show notable improvements with the LRI, on a global scale, of the order of 23 %. The gain is realized for wavelengths smaller than 240 km in case of Gaussian filtering but decreases to just a few percent when anisotropic filtering is applied. This is also confirmed for some typical regional Earth science applications which show randomly distributed patterns of small improvements but also degradations when using DDK4-filtered LRI-based models. Analysis of applied error models indicates that accelerometer noise followed by ocean tide and non-tidal mass variation errors are the main contributors to the overall GRACE-FO gravity model error. Improvements in these fields are therefore necessary, besides optimized constellations, to make use of the increased LRI accuracy and to significantly improve gravity field models from next-generation gravity missions.