Numerical simulations of the bi-level and branched structure of intracloud lightning flashes

Intracloud (IC) lightning is used to mean those lightning flashes which channels do not strike the ground. It is an important scientific problem to inves-tigate the IC flash features and the discharge physics.Measurements from the electric field change arrange-ment[1,2] and VHF radiation events[3,4] have provided ample evidence that IC flashes have branches with substantial horizontal extents. The VHF interferomet-662 Science in China: Series D Earth Sciences ric observations[5] also s…     

Numerical simulations of land surface physical processes and land-atmosphere interactions over oasis-desert/Gobi region

A land-surface physical process model was coupled with a mesoscale atmospheric model. This cou- pled model was then used to simulate the interactions between land and the atmosphere, including surface temperature, net radiation, sensible heat flux and latent heat flux over a desert/Gobi with an oasis in northwestern semiarid regions in China. Comparisons between observations and simulations were made over the oasis and the desert/Gobi, respectively. Both cold island effect and wet island ef- fect, the so-called oasis effect, were observed and simulated. Lower temperature, higher specific hu- midity and weaker turbulent transfer were present over the oasis than the desert/Gobi. A subsidence occurred over the oasis, leading to a thermally-generated mesoscale circulation.     

How does DEM resolution affect microtopographic characteristics,hydrologic connectivity,and modelling of hydrologic processes?

The resolution of a digital elevation model (DEM) is a crucial factor in watershed hydrologic and environmental modelling. DEM resolution can cause significant variability in the representation of surface topography, which further affects quantification of hydrologic connectivity and simulation of hydrologic processes. The objective of this study is to examine the effects of DEM resolution on (1) surface microtopographic characteristics, (2) hydrologic connectivity, and (3) the spatial and temporal variations of hydrologic processes. A puddle‐to‐puddle modelling system was utilized for surface delineation and modelling of the puddle‐to‐puddle overland flow dynamics, surface runoff, infiltration, and unsaturated flow for nine DEM resolution scenarios of a field plot surface. Comparisons of the nine modelling scenarios demonstrated that coarser DEM resolutions tended to eliminate topographic features, reduce surface depression storage, and strengthen hydrologic connectivity and surface runoff. We found that reduction in maximum depression storage and maximum ponding area was as high as 97.56% and 76.36%, respectively, as the DEM grid size increased from 2 to 80 cm. The paired t‐test and fractal analysis demonstrated the existence of a threshold DEM resolution (10 cm for the field plot), within which the DEM‐based hydrologic modelling was effective and acceptable. The effects of DEM resolution were further evaluated for a larger surface in the Prairie Pothole Region subjected to observed rainfall events. It was found that simulations based on coarser resolution DEMs (>10 m) tended to overestimate ponded areas and underestimate runoff discharge peaks. The simulated peak discharge from the Prairie Pothole Region surface reduced by approximately 50% as the DEM resolution changed from 2 to 90 m. Fractal analysis results elucidated scale dependency of hydrologic and topographic processes. In particular, scale analysis highlighted a unique constant–threshold–power relationship between DEM scale and topographic and hydrologic parameters/variables. Not only does this finding allow one to identify threshold DEM but also further develop functional relationships for scaling to achieve valid topographic characterization as well as effective and efficient hydrologic modelling. Copyright © 2016 John Wiley & Sons, Ltd.     

Definitions of climatological and discharge days: do they matter in hydrological modelling?

The performance of hydrological models is affected by uncertainty related to observed climatological and discharge data. Although the latter has been widely investigated, the effects on hydrological models from different starting times of the day have received little interest. In this study, observational data from one tropical basin were used to investigate the effects on a typical bucket-type hydrological model, the HBV, when the definitions of the climatological and discharge days are changed. An optimization procedure based on a genetic algorithm was used to assess the effects on model performance. Nash-Sutcliffe efficiencies varied considerably between day definitions, with the largest dependence on the climatological-day definition. The variation was likely caused by how storm water was assigned to one or two daily rainfall values depending on the definition of the climatological day. Hydrological models are unlikely to predict high flows accurately if rainfall intensities are reduced because of the day definition.     

The settling dynamics of flocculating mud and sand mixtures: part 2—numerical modelling

Estuarine and coastal sediment transport is characterised by the transport of both sand-sized particles (of diameter greater than 63?μm) and muddy fine-grained sediments (silt, diameter less than 63?μm; clay, diameter less than 2?μm). These fractions are traditionally considered as non-cohesive and cohesive, respectively, because of the negligible physico-chemical attraction that occurs between sand grains. However, the flocculation of sediment particles is not only caused by physico-chemical attraction. Cohesivity of sediment is also caused by biology, in particular the sticky extra-cellular polymeric substances secreted by diatoms, and the effect of biology in binding sediment particles can be much larger than that of physico-chemical attraction. As demonstrated by Manning (2008) and further expanded in part 1 of this paper (Manning et al., submitted), the greater binding effect of biology allows sand particles to flocculate with mud. In many estuaries, both the sand and fine sediment fractions are transported in significant quantities. Many of the more common sediment transport modelling suites now have the capability to combine mud and sand transport. However, in all of these modelling approaches, the modelling of mixed sediment transport has still essentially separated the modelling of sand and mud fractions assuming that these different fractions do not interact except at the bed. However, the use of in situ video techniques has greatly enhanced the accuracy and reliability of settling velocity measurements and has led to a re-appraisal of this widely held assumption. Measurements of settling velocity in mixed sands presented by Manning et al. (2009) have shown strong evidence for the flocculation of mixed sediments, whilst the greater understanding of the role of biology in flocculation has identified mechanisms by which this mud-sand flocculation can occur. In the first part of this paper (Manning et al., submitted), the development of an empirical flocculation model is described which represents the interaction between sand and mud particles in the flocculation process. Measurements of the settling velocity of varying mud-sand mixtures are described, and empirical algorithms governing the variation of settling velocity with turbulence, suspended sediment concentration and mud-sand content are derived. The second part of this paper continues the theme of examination of the effects of mud-sand interaction on flocculation. A 1DV mixed transport model is developed and used to reproduce the vertical transport of mixed sediment fractions. The 1DV model is used to reproduce the measured settling velocities in the laboratory experiments described in the part 1 paper and also to reproduce measurements of concentration of mixed sediments in the Outer Thames. In both modelling exercises, the model is run using the algorithms developed in part 1 and repeated using an assumption of no interaction between mud and sand in the flocculation process. The results of the modelling show a significant improvement in the ability of the 1DV to reproduce the observed sediment behaviour when the empirical equations are used. This represents further strong evidence of the interaction between sand and mud in the flocculation process.     

Acoustic and petrophysical properties of mechanically compacted overconsolidated sands: Part 2 – Rock physics modelling and applications

Part one of this paper reported results from experimental compaction measurements of unconsolidated natural sand samples with different mineralogical compositions and textures. The experimental setup was designed with several cycles of stress loading and unloading applied to the samples. The setup was aimed to simulate a stress condition where sediments underwent episodes of compaction, uplift and erosion. P-wave and S-wave velocities and corresponding petrophysical (porosity and density) properties were reported. In this second part of the paper, rock physics modelling utilizing existing rock physics models to evaluate the model validity for measured data from part one were presented. The results show that a friable sand model, which was established for normally compacted sediments is also capable of describing overconsolidated sediments. The velocity–porosity data plotted along the friable sand lines not only describe sorting deterioration, as has been traditionally explained by other studies, but also variations in pre-consolidation stress or degree of stress release. The deviation of the overconsolidated sands away from the normal compaction trend on the V_P/V_S and acoustic impedance space shows that various stress paths can be predicted on this domain when utilizing rock physics templates. Fluid saturation sensitivity is found to be lower in overconsolidated sands compared to normally consolidated sands. The sensitivity decreases with increasing pre-consolidation stress. This means detectability for four-dimensional fluid saturation changes can be affected if sediments were pre-stressed and unloaded. Well log data from the Barents Sea show similar patterns to the experimental sand data. The findings allow the development of better rock physics diagnostics of unloaded sediments, and the understanding of expected 4D seismic response during time-lapse seismic monitoring of uplifted basins. The studied outcomes also reveal an insight into the friable sand model that its diagnostic value is not only for describing sorting microtextures, but also pre-consolidation stress history. The outcome extends the model application for pre-consolidation stress estimation, for any unconsolidated sands experiencing similar unloading stress conditions to this study.     

Mechanisms of Cenozoic deformation in the Bohai Basin, Northeast China: Physical modelling and discussions

The Bohai Basin is a petroliferous Cenozoic basin in northeast China (Fig. 1(a)) and has apparent geo- metrical and kinematic similarities with the other Meso-Cenozoic extensional basins located along the eastern margin of the Eurasian Plate[1,2]. Its architec- ture and Cenozoic stratigraphy have been well under- stood from several decades of petroleum exploration. Previous studies have suggested that the Bohai Basinis a typical extensional basin and has two tectonic evolution phases, rift…     

Numerical modelling of the impacts of water abstraction for hydraulic fracturing on groundwater–surface water interaction: a case study from northwestern Alberta,Canada

ABSTRACT

The temporal dynamics of groundwater–surface water interaction under the impacts of various water abstraction scenarios are presented for hydraulic fracturing in a shale gas and oil play area (23 984.9 km²), Alberta, Canada, using the MIKE-SHE and MIKE-11 models. Water-use data for hydraulic fracturing were obtained for 433 wells drilled in the study area in 2013 and 2014. Modelling results indicate that water abstraction for hydraulic fracturing has very small (<0.35%) negative impacts on mean monthly and annual river and groundwater levels and stream and groundwater flows in the study area, and small (1–4.17%) negative impacts on environmental flows near the water abstraction location during low-flow periods. The impacts on environmental flow depend on the amount of water abstraction and the daily flow over time at a specific river cross-section. The results also indicate a very small (<0.35%) positive impact on mean monthly and annual groundwater contributions to streamflow because of the large study area. The results provide useful information for planning long-term seasonal and annual water abstractions from the river and groundwater for hydraulic fracturing in a large study area.     

Age-depth modelling and the effect of including – or not – shared errors across sets of OSL samples: The case study of Beg-er-Vil (Brittany,France)

The coastal site of Beg-er-Vil (Brittany, France) has yielded remains of a dwelling site from the Mesolithic period, dating back to ca. 8000 years ago. These archaeological remains were covered by a marine sand dune. Among the research questions raised by recent excavations, the timing of the dune formation with respect to the human occupations is of particular interest: how much time elapsed between these two events? To resolve this question, we employed radiocarbon dating to infer the timing of human occupations and OSL dating to date the dune aggradation. We combined radiocarbon and OSL data in a Bayesian framework, including stratigraphic constraints and measurement errors shared across OSL samples, to build a robust and precise chronological model of the site. We then built an age-depth model for the dune to determine the onset of dune formation. We conclude that we cannot detect a time gap between the latest human occupations and dune aggradation. Finally, we demonstrate how stratigraphic constraints and shared errors affect – and improve – the precision of the chronology inferred from our measurements.     

The construction of a reliable absolute chronology for the last two millennia in an anthropogenically disturbed peat bog: Limitations and advantages of using a radio-isotopic proxy and age–depth modelling

The main aim of this paper is to present the pitfalls connected with the construction of reliable chronologies for anthropogenically disturbed peatlands over the last two millennia based on ²¹⁰Pb and ¹⁴C dating, i.e. the period of the strongest human impact on these ecosystems. The following hypotheses have been formulated: i) parts of peatlands suspected to be affected by peat extraction may possess traces of mechanical disturbances undetectable using different analyses based on biota proxy; ii) failure to consider information included in radionuclide date inversions may contribute to the establishment of misleading chronologies. To test these hypotheses, different scenarios of chronology based on high resolution ²¹⁰Pb and ¹⁴C dating from a peat core retrieved from the Puścizna Krauszowska bog (southern Poland) have been analysed. Nowadays, this mire is intensively exploited by humans; however, it contains remains of dome considered undisturbed, from which the core presented in this paper was collected. The set of dates revealed the presence of marked ¹⁴C date inversions (mechanical disturbances) which, if inappropriately interpreted before the age–depth modelling process, may lead to the establishment of misleading chronologies, and thus an incorrect interpretation of biota proxy records, e.g. pollen. Those sections of peat profiles with prominent age inversions and/or strong discrepancies between the peat accumulation rate and bulk density should be rejected from age–depth modelling, even if interpretable chronologies can be obtained.     

On width function-based unit hydrographs deduced from separately random self-similar river networks and rainfall variability: Discussion of “Coding random self-similar river networks and calculating geometric distances: 1. General methodology” and “2. Application to runoff simulations”

Abstract

Hung & Wang (2005a,b) base their approach on successive steps related to a kind of geomorphometric modelling, the deduction of a rainfall—runoff transfer function, and the application to a Taiwanese basin subject to typhoons. Several conceptual points of each of these steps and their propagation through the whole approach are discussed; referring to the likelihood of the proposed separately random self-similar river networks, the deduction of width function-based unit hydrographs, and the accounting for variability of rainfall and of induced runoff.     

Distributed hydrologic and hydraulic modelling with radar rainfall input: Reconstruction of the 8–9 September 2002 catastrophic flood event in the Gard region,France

On 8–9 September 2002, an extreme rainfall event caused by a stationary mesoscale convective system (MCS) occurred in the Gard region, France. Distributed hydrologic and hydraulic modelling has been carried out to assess and compare the various sources of data collected operationally and during the post-event field surveys. Distributed hydrological modelling was performed with n-TOPMODELs and assessed for ungauged basins with the discharge estimates of the post-event surveys. A careful examination of the occurrence in time and space of the flash floods over the head watersheds indicates that flooding was controlled by the trajectory of the convective part of the MCS. Stationarity of the MCS over the Gardon watershed (1858 km² at Remoulins) for 28 h was responsible for the exceptional magnitude of the flood at this scale. The flood dynamics were characterized by an extensive inundation of the Gardonnenque plain upstream of the Gardon Gorges resulting in a significant peak flow reduction downstream. One-dimensional unsteady-flow hydraulic modelling was found to be required to reproduce these dynamics. Hydraulic modelling also proved to be potentially useful for the critical analysis and extrapolation of operational discharge rating curves.     

Moving socio-hydrologic modelling forward: unpacking hidden assumptions,values and model structure by engaging with stakeholders: reply to “What is the role of the model in socio-hydrology?”*

The arguments presented in Melsen et al. advance ideas in the “Panta Rhei” decade (2013–2022) of the International Association of Hydrological Sciences, which focuses on change in hydrology and society. While we reiterate that, despite acknowledged shortcomings, the enterprise of integrating societal feedbacks into hydrological models is beneficial in prediction and adaptive management, we also agree with the sentiments of the authors. In response, we offer concrete steps the socio-hydrologic community can take to educate modellers to become aware about unconscious biases embedded in model structure and clearly communicate assumptions. We stress the need for “knowledge brokers” that can help modellers work with stakeholders, instead of doing everything themselves. We also caution, however, against the danger of over-reaching. Young scholars already pay a big price by having to master both the natural and social sciences. As coupled human–water problems increase in societal importance, along with calls for more holistic thinking, we also need to promote an academic culture that rewards reaching across the aisle.