Hydrological controls on river network connectivity

This study proposes a probabilistic approach for the quantitative assessment of reach- and network-scale hydrological connectivity as dictated by river flow space–time variability. Spatial dynamics of daily streamflows are estimated based on climatic and morphological features of the contributing catchment, integrating a physically based approach that accounts for the stochasticity of rainfall with a water balance framework and a geomorphic recession flow analysis. Ecologically meaningful minimum stage thresholds are used to evaluate the connectivity of individual stream reaches, and other relevant network-scale connectivity metrics. The framework allows a quantitative description of the main hydrological causes and the ecological consequences of water depth dynamics experienced by river networks. The analysis shows that the spatial variability of local-scale hydrological connectivity is strongly affected by the spatial and temporal distribution of climatic variables. Depending on the underlying climatic settings and the critical stage threshold, loss of connectivity can be observed in the headwaters or along the main channel, thereby originating a fragmented river network. The proposed approach provides important clues for understanding the effect of climate on the ecological function of river corridors

Groundwater flooding within an urbanised flood plain

In Europe in recent years, there has been recognition of the need to better understand the risk from groundwater flooding. This recognition has been due both to the occurrence of major flooding events clearly attributable to groundwater and the inclusion of groundwater flooding in European and national legislation. The case study of the city of Oxford on the River Thames flood plain in UK is used to examine the mechanisms for groundwater flooding in urbanised flood plain settings. Reference is made to an extensive data set gathered during a major flood event in 2007. Groundwater flooding of a significant number of properties is shown to occur in areas isolated from fluvial flooding because of high ground created historically to protect property and the transport network from flood inundation. The options for mitigating this form of flooding are discussed; measures to increase the rate of conveyance of flood waters through Oxford, designed to reduce fluvial flood risk, have also been recognised as a means for reducing groundwater flood risk within the city

Hydrological behaviour of the granitic Strengbach catchment (Vosges massif, Eastern France) during a flood event

A field campaign combining monitoring devices and determination of isotopes and chemical elements has been performed during a summer thunderstorm in the small granitic Strengbach catchment (Vosges, France). The collected ground data were used in a hydrological modelling exercise including two conceptual rainfallrunoff models (GR4, TOPMODEL). The predominant role in flood generation of pre-event water coming from the superficial layers of the water saturated area has been shown and a conceptual scheme has been proposed derived from the field observations. The two tested modelling structures and assumptions are not able to take into account fully the complexity of the physical processes involved in flood generation

Using isotopes to constrain water flux and age estimates in snow-influenced catchments using the STARR (Spatially distributed Tracer-Aided Rainfall-Runoff) model

Acknowledgements. This work was funded by the NERC/JPI SIWA project (NE/M019896/1) and the European Research Council ERC (project GA 335910 VeWa). Numerical simulations were performed using the Maxwell High Performance Computing Cluster of the University of Aberdeen IT Service, provided by Dell Inc. and supported by Alces Software. The isotope work in Krycklan is funded by the KAW Branch-Point project together with SKB and SITES. We would like to thank Marjolein van Hui- jgevoort for her help with the STARR code, and Masaki Hayashi and two anonymous reviewers for their insightful suggestions that significantly improved the paper. The Supplement related to this article is available online at https://doi.org/10.5194/hess-21-5089-2017-supplement.Peer reviewedPublisher PD

Geographic Information Systems (GIS)-based spatially distributed model for runoff routing

A method is proposed for routing spatially distributed excess precipitation over a watershed to produce runoff at its outlet. The land surface is represented by a (raster) digital elevation model from which the stream network is derived. A routing response function is defined for each digital elevation model cell so that water movement from cell to cell can be convolved to give a response function along a flow path and responses from all cells can be summed to give the outlet hydrograph. An example application of analysis of runoff on Waller Creek in Austin, Texas, is presented.Waller Creek Working Grou

Simulating the influences of groundwater on regional geomorphology using a distributed, dynamic, landscape evolution modelling platform

A dynamic landscape evolution modelling platform (CLiDE) is presented that allows a variety of Earth system interactions to be explored under differing environmental forcing factors. Representation of distributed surface and subsurface hydrology within CLiDE is suited to simulation at sub-annual to centennial time-scales. In this study the hydrological components of CLiDE are evaluated against analytical solutions and recorded datasets. The impact of differing groundwater regimes on sediment discharge is examined for a simple, idealised catchment, Sediment discharge is found to be a function of the evolving catchment morphology. Application of CLiDE to the upper Eden Valley catchment, UK, suggests the addition of baseflow-return from groundwater into the fluvial system modifies the total catchment sediment discharge and the spatio-temporal distribution of sediment fluxes during storm events. The occurrence of a storm following a period of appreciable antecedent rainfall is found to increase simulated sediment fluxes

Spatial organisation of groundwater dynamics and streamflow response from different hydropedological units in a montane catchment

Funding was provided by the Leibniz Association (SAW-2012-IGB 4167) within the International Leibniz Graduate School: Aquatic boundaries and linkages- Aqualink. We would like to thank the NRI staff for their help during field work.Peer reviewedPostprin

Lithology and the evolution of bedrock rivers in post-orogenic settings: Constraints from the high elevation passive continental margin of SE Australia

Understanding the role of lithological variation in the evolution of topography remains a fundamental issue, especially in the neglected post-orogenic terrains. Such settings represent the major part of the Earth's surface and recent modelling suggests that a range of interactions can account for the presence of residual topography for hundreds of millions of years, thereby explaining the great antiquity of landscapes in such settings. Field data from the inland flank of the SE Australian high-elevation continental margin suggest that resistant lithologies act to retard or even preclude the headward transmission of base-level fall driven by the isostatic response to regional denudation. Rejuvenation, be it episodic or continuous, is ‘caught up’ on these resistant lithologies, meaning in effect that the bedrock channels and hillslopes upstream of these ‘stalled’ knickpoints have become detached from the base-level changes downstream of the knickpoints. Until these knickpoints are breached, therefore, catchment relief must increase over time, a landscape evolution scenario that has been most notably suggested by Crickmay and Twidale. The role of resistant lithologies indicates that detachment-limited conditions are a key to the longevity of some post-orogenic landscapes, whereas the general importance of transport-limited conditions in the evolution of post-orogenic landscapes remains to be evaluated in field settings. Non-steady-state landscapes may lie at the heart of widespread, slowly evolving post-orogenic settings, such as high-elevation passive continental margins, meaning that non-steady-state landscapes, with increasing relief through time, are the ‘rule’ rather than the exception

A high resolution coupled hydrologic–hydraulic model (HiResFlood-UCI) for flash flood modeling

HiResFlood-UCI was developed by coupling the NWS's hydrologic model (HL-RDHM) with the hydraulic model (BreZo) for flash flood modeling at decameter resolutions. The coupled model uses HL-RDHM as a rainfall-runoff generator and replaces the routing scheme of HL-RDHM with the 2D hydraulic model (BreZo) in order to predict localized flood depths and velocities. A semi-automated technique of unstructured mesh generation was developed to cluster an adequate density of computational cells along river channels such that numerical errors are negligible compared with other sources of error, while ensuring that computational costs of the hydraulic model are kept to a bare minimum. HiResFlood-UCI was implemented for a watershed (ELDO2) in the DMIP2 experiment domain in Oklahoma. Using synthetic precipitation input, the model was tested for various components including HL-RDHM parameters (a priori versus calibrated), channel and floodplain Manning n values, DEM resolution (10 m versus 30 m) and computation mesh resolution (10 m+ versus 30 m+). Simulations with calibrated versus a priori parameters of HL-RDHM show that HiResFlood-UCI produces reasonable results with the a priori parameters from NWS. Sensitivities to hydraulic model resistance parameters, mesh resolution and DEM resolution are also identified, pointing to the importance of model calibration and validation for accurate prediction of localized flood intensities. HiResFlood-UCI performance was examined using 6 measured precipitation events as model input for model calibration and validation of the streamflow at the outlet. The Nash–Sutcliffe Efficiency (NSE) obtained ranges from 0.588 to 0.905. The model was also validated for the flooded map using USGS observed water level at an interior point. The predicted flood stage error is 0.82 m or less, based on a comparison to measured stage. Validation of stage and discharge predictions builds confidence in model predictions of flood extent and localized velocities, which are fundamental to reliable flash flood warning

Spatial aggregation of time-variant stream water ages in urbanizing catchments

Date of Acceptance: 25/03/2015 Acknowledgements The help of Clara F. Soulsby with the sampling is gratefully acknowledged.Peer reviewedPostprin