Surface-subsurface Interactions Research Articles

Multi-biological risk in groundwater-surface water system under landfill stress: Driven by bacterial size and biological toxicity

The variation in bacterial size and biotoxicity leads to different ecological risks. However, the diversity of bacterial size and virulence in bacterial communities has been neglected, leading to limitations and inaccuracies in risk assessment. Surface-subsurface interactions lead to the complexity, variability, and insidiousness of pathogen transmission in the environment. Mountainous landfills are considered as a major source of pathogens in subsurface systems. Although numerous studies have reported the geographic distribution of microbes in landfills, there is a lack of knowledge regarding the impacts on the microbial spatial distribution of bacterial size, which is a key factor affecting microbial migration and risks. In this study, the groundwater, river water, soil and leachate samples were collected inside and around a typical mountainous urban landfill, which is close to a river with a high hydraulic gradient and a strong surface–subsurface interaction. Small-sized (<0.2 μm) and large-sized (>0.2 μm) bacteria were separated to verify the variability of migration potential and spatial distribution of different sizes of bacteria in groundwater-surface water and soil, investigating the driving factors and accessing the biological risk of different sizes of bacteria. The results revealed notable differences in spatial distribution and driving factors among various bacterial sizes. Small-sized bacterial communities in groundwater and soils exhibited clear trends towards leachate along the water flow direction. In contrast, large-sized bacterial communities remained stable and resembled native bacterial communities. Groundwater hydrodynamics was the main driving force affecting the spatial distribution of small-sized bacteria. The small-sized bacteria showed a higher content of virulence genes which were predominantly expressed for invasion. Thus, small-sized bacteria exhibited a stronger migration potential and a higher biological risk. Due to the surface–subsurface interactions, the bacterial communities in river water were more similar to that in groundwater after rain. Our results provide strong support to guide the management of subsurface system safety in mountainous landfills.

The Potential of Isotopic Tracers for Precise and Environmentally Clean Stream Discharge Measurements

Accurate discharge measurement is mandatory for any hydrological study. While the “velocity” measurement method is adapted to laminar flows, the “dilution” method is more appropriate for turbulent streams. As most low-gradient streams worldwide are neither laminar nor turbulent, a methodological gap appears. In this study, we demonstrate that the application of the “dilution” method to a low-gradient small stream gives very satisfactory results in addition to revealing surface/subsurface processes. A variety of chemical and isotopic tracers were injected into the stream (anions, fluorescent dyes, and chloride and hydrogen isotopes). We report the first use of 37Cl for stream discharge measurement and show that 37Cl and 2H can be reliably used as quantitative tracers. Discharge uncertainty calculations show that deuterium is the most accurate tracer method used. We also compare the differences in the tailing part of the restitution curves of tracers and investigate the role of transient surface and hyporheic zones in solute transport in light of a simple transport modelling approach. We conclude that isotopic tracers can be used as “environmentally friendly” tracers for discrete stream discharge measurements and that the application of multi-tracers tests in rivers opens the path to a better understanding of surface–subsurface interaction processes.

Coupling water, solute, and sediment transport into a new computationally efficient hydrologic model

While surface–subsurface interactions strongly impact water quality and quantity in ecosystems, only relatively little attention has been given to solute mass transfers at this boundary. The KINEROS2 (K2) model solves the 1D kinematic wave equation for water flow and the advection–dispersion equation for non-absorbing solute (or sediment) transport at the surface, whereas the HYDRUS-1D (H1D) model solves the Richards equation for flow and the advection–dispersion equation for solute transport in the subsurface. We previously coupled water flow in the surface and subsurface environments using the K2 and H1D models (H1D-K2). Herein, an innovative framework was developed to additionally couple solute transport at the surface–subsurface boundary in the simple, accurate, and computationally efficient H1D-K2 model. To test the H1D-K2 model’s accuracy and robustness, simulation results are compared to analytical solutions, experimental data, and results from the two-dimensional HYDRUS-2D (H2D) code. The relative differences in water and solute balance components between the H2D and H1D-K2 models are less than 1% for homogeneous cases and 4% for heterogeneous cases. The simulated hydrograph and chemograph are almost identical for the two models. Furthermore, simulated Bromide (Br) concentration in runoff is in good agreement with observations. The proposed model also accurately reproduces soil erosion under different rainfall patterns and intensities.

Modeling Aquifer Responses from Flash Flood Events through Ephemeral Stream Beds: Case Studies from Saudi Arabia

The determination of groundwater recharge (GWR) with an appropriate method is among the main subjects of groundwater resource management. This research is an event-scale quantitative assessment and a development of a water balance model of an aquifer response resulting from flash flooding and infiltration from the ephemeral streambed of some representative basins in Saudi Arabia. The methodology employed in the water balance analysis of this study starts with surface runoff rather than rainfall as in the literature, since the recharge process mainly occurs from runoff on the ground surface. A conceptual model of the local surface–subsurface interaction is developed to estimate the effective aquifer response (AR). The annual cumulative groundwater recharge (GWR) depths vary from 1.22 m to 6.97 m, with an average rise of 4.24 m at the studied wells; most of this happened following the May and April flash flood events. The results show also similar profile patterns and timings for most of the water level rises and falls. An average root mean square error (RMSE) of 0.46 m is obtained from the simulations and the coefficient of determination (R2) values range from 0.57 to 0.99. The results verified that the daily simulated groundwater elevations have shown very good agreement with the observed daily groundwater elevations over a time span of one year. In this work, the key issue to interpret the GWR depth from individual well samples is whether the model on the samples fits the measured groundwater level within the experimental runoff reaches (ERRs). The time steps (Δt) of the GWR simulation process are significantly affected by the high variability in the aquifer characteristics, as shown for the hydraulic conductivity values ranging from 5.3 m/day to 438 m/day within ERRs. This study highly recommends that groundwater recharge studies be site-specific. Otherwise, the extrapolation of the results even to adjacent basins is questionable.

Understanding the hydrological response of a headwater-dominated catchment by analysis of distributed surface–subsurface interactions

We computationally explore the relationship between surface–subsurface exchange and hydrological response in a headwater-dominated high elevation, mountainous catchment in East River Watershed, Colorado, USA. In order to isolate the effect of surface–subsurface exchange on the hydrological response, we compare three model variations that differ only in soil permeability. Traditional methods of hydrograph analysis that have been developed for headwater catchments may fail to properly characterize catchments, where catchment response is tightly coupled to headwater inflow. Analyzing the spatially distributed hydrological response of such catchments gives additional information on the catchment functioning. Thus, we compute hydrographs, hydrological indices, and spatio-temporal distributions of hydrological variables. The indices and distributions are then linked to the hydrograph at the outlet of the catchment. Our results show that changes in the surface–subsurface exchange fluxes trigger different flow regimes, connectivity dynamics, and runoff generation mechanisms inside the catchment, and hence, affect the distributed hydrological response. Further, changes in surface–subsurface exchange rates lead to a nonlinear change in the degree of connectivity—quantified through the number of disconnected clusters of ponding water—in the catchment. Although the runoff formation in the catchment changes significantly, these changes do not significantly alter the aggregated streamflow hydrograph. This hints at a crucial gap in our ability to infer catchment function from aggregated signatures. We show that while these changes in distributed hydrological response may not always be observable through aggregated hydrological signatures, they can be quantified through the use of indices of connectivity.

Assessing the effect of flood restoration on surface–subsurface interactions in Rohrschollen Island (Upper Rhine river – France) using integrated hydrological modeling and thermal infrared imaging

Abstract. Rohrschollen Island is an artificial island of the large Upper Rhine river whose geometry and hydrological dynamics are the result of engineering works during the 19th and 20th centuries. Before its channelization, the Rhine river was characterized by an intense hydromorphological activity which maintained a high level of biodiversity along the fluvial corridor. This functionality considerably decreased during the two last centuries. In 2012, a restoration project was launched to reactivate typical alluvial processes, including bedload transport, lateral channel dynamics, and surface–subsurface water exchanges. An integrated hydrological model has been applied to the area of Rohrschollen Island to assess the efficiency of the restoration regarding surface and subsurface flows. This model is calibrated using measured piezometric heads. Simulated patterns of water exchanges between the surface and subsurface compartments of the island are checked against the information derived from thermal infrared (TIR) imaging. The simulated results are then used to better understand the evolutions of the infiltration–exfiltration zones over time and space and to determine the physical controls of surface–subsurface interactions on the hydrographic network of Rohrschollen Island. The use of integrated hydrological modeling has proven to be an efficient approach to assess the efficiency of restoration actions regarding surface and subsurface flows.

Analyzing climate change and surface-subsurface interactions using the Postojna Planina Cave System (Slovenia) as a model system

Worldwide, there are at least 12 ILTER sites with an emphasis on karst, landforms arising from the combination of high rock solubility and well-developed solutional channel porosity underground, but the study of cave ecosystems has been largely neglected. Only two ILTER sites, both in Slovenia, are primarily caves. Caves are under-represented for several reasons, but especially because of the overall difficulty of access and the lack of a clear research agenda for cave ecosystem studies. We review several aspects of long-term studies in Postojna Planina Cave System (PPCS), proposing our approach as a model for ILTER research in caves. In PPCS, analysis of short-term temperature data shows a muted daily cycle and seasonality, and analysis of long-term temperature data shows an increase, largely the result of climate change. Changes in drip rate of epikarst aquifers above the cave are correlated with rainfall but with lags and complications resulting from differences in longer term rainfall patterns. Analysis of discharge rates indicates a rapid response to precipitation not only in the Pivka River at its sinking, but also at Unica Spring, where discharge is augmented from other parts of the aquifer, including epikarst. Quantitative analysis of the obligate epikarst-dwelling copepod community shows that, unlike most cave communities, complete sampling of the fauna is possible. Finally, organic carbon levels in PPCS indicate likely carbon limitation in the system. These five factors (temperature, drip rate, river discharge, epikarst copepod fauna, and organic carbon) are the appropriate variables for capturing the essential long-term trends in cave ecosystems and their causes.

Aquifer configuration and geostructural links control the groundwater quality in thin-bedded carbonate–siliciclastic alternations of the Hainich CZE, central Germany

Abstract. The quality of near-surface groundwater reservoirs is controlled, but also threatened, by manifold surface–subsurface interactions. Vulnerability studies typically evaluate the variable interplay of surface factors (land management, infiltration patterns) and subsurface factors (hydrostratigraphy, flow properties) in a thorough way, but disregard the resulting groundwater quality. Conversely, hydrogeochemical case studies that address the chemical evolution of groundwater often lack a comprehensive analysis of the structural buildup. In this study, we aim to reconstruct the actual spatial groundwater quality pattern from a synoptic analysis of the hydrostratigraphy, lithostratigraphy, pedology and land use in the Hainich Critical Zone Exploratory (Hainich CZE). This CZE represents a widely distributed yet scarcely described setting of thin-bedded mixed carbonate–siliciclastic strata in hillslope terrains. At the eastern Hainich low-mountain hillslope, bedrock is mainly formed by alternated marine sedimentary rocks of the Upper Muschelkalk (Middle Triassic) that partly host productive groundwater resources. Spatial patterns of the groundwater quality of a 5.4 km long well transect are derived by principal component analysis and hierarchical cluster analysis. Aquifer stratigraphy and geostructural links were deduced from lithological drill core analysis, mineralogical analysis, geophysical borehole logs and mapping data. Maps of preferential recharge zones and recharge potential were deduced from digital (soil) mapping, soil survey data and field measurements of soil hydraulic conductivities (Ks). By attributing spatially variable surface and subsurface conditions, we were able to reconstruct groundwater quality clusters that reflect the type of land management in their preferential recharge areas, aquifer hydraulic conditions and cross-formational exchange via caprock sinkholes or ascending flow. Generally, the aquifer configuration (spatial arrangement of strata, valley incision/outcrops) and related geostructural links (enhanced recharge areas, karst phenomena) control the role of surface factors (input quality and locations) vs. subsurface factors (water–rock interaction, cross-formational flow) for groundwater quality in the multi-layered aquifer system. Our investigation reveals general properties of alternating sequences in hillslope terrains that are prone to forming multi-layered aquifer systems. This synoptic analysis is fundamental and indispensable for a mechanistic understanding of ecological functioning, sustainable resource management and protection.

Evolution of Indian Ocean dipole and its forcing mechanisms in the absence of ENSO

The evolution of Indian Ocean dipole (IOD) and its forcing mechanisms are examined based on the analysis of coupled model simulations that allow or suppress the El Nino-Southern Oscillation (ENSO) mode of variability. The model can reproduce the most salient observed features of IOD even without ENSO, including the relationships between the eastern and western poles at both the surface and subsurface, as well as their seasonality. This suggests that ENSO is not fundamental for the existence of IOD. It is demonstrated that cold (warm) sea surface temperature (SST) anomalies in the eastern Indian Ocean associated with IOD can be initiated by springtime Indonesian rainfall deficit (surplus) through local surface wind response. The growth of the SST anomalies depends on the initial local subsurface condition. Both the air–sea interaction and surface–subsurface interaction contribute to the development of IOD. The evolution of IOD can be represented by two leading extended empirical orthogonal function (EEOF) modes of tropical surface–subsurface Indian Ocean temperatures; one stationary and the other non-stationary. The onset, growth, and termination of IOD, as well as the transition to an opposite phase, can be interpreted as alternations between the non-propagating mode (EEOF1) and the eastward-propagating Kelvin wave (EEOF2). The evolution of IOD is also accompanied by a westward-propagating Rossby wave which is captured in the EEOF1 of the subtropical surface–subsurface Indian Ocean temperatures. Therefore, both Bjerknes feedback and a delayed oscillator operate during the evolution of IOD in the absence of ENSO also.

Investigating the respective impacts of groundwater exploitation and climate change on wetland extension over 150 years

Summary Peatlands are complex ecosystems driven by many physical, chemical, and biological processes. Peat soils have a significant impact on water quality, ecosystem productivity and greenhouse gas emissions. However, the extent of peatlands is decreasing across the world, mainly because of anthropogenic activities such as drainage for agriculture or groundwater abstractions in underlying aquifers. Potential changes in precipitation and temperature in the future are likely to apply additional pressure to wetland. In this context, a methodology for assessing and comparing the respective impacts of groundwater abstraction and climate change on a groundwater-fed wetland (135 km 2 ) located in Northwest France, is presented. A groundwater model was developed, using flexible boundary conditions to represent surface–subsurface interactions which allowed examination of the extent of the wetland areas. This variable parameter is highly important for land management and is usually not considered in impact studies. The model was coupled with recharge estimation, groundwater abstraction scenarios, and climate change scenarios downscaled from 14 GCMs corresponding to the A1B greenhouse gas (GHG) scenario over the periods 1961–2000 and 2081–2100. Results show that climate change is expected to have an important impact and reduce the surface of wetlands by 5.3–13.6%. In comparison, the impact of groundwater abstraction (100% increase in the expected scenarios) would lead to a maximum decrease of 3.7%. Results also show that the impacts of climate change and groundwater abstraction could be partially mitigated by decreasing or stopping land drainage in specific parts of the area. Water management will require an appropriate compromise which encompasses ecosystem preservation, economic and public domain activities.

On the implementation of the surface conductance approach using a block-centred surface–subsurface hydrology model

• The surface conductance (SC) approach is explored for rainfall partitioning in a block-centred code. • A discrepancy arises between saturation of the uppermost node and the head reaching the land surface. • Previous research suggests the exchange interface be represented by the uppermost half-cell in MODHMS. • Our work suggests that the previous recommendation prevents accurate simulation of rainfall partitioning. • Adding a thin surface layer improves model accuracy with minimal increase in run time. In physically based catchment hydrology models, dynamic surface–subsurface interactions are often represented using the surface conductance (SC) coupling approach. Guidance on SC parameterisation within block-centred codes is limited, and common practice is to express the SC coefficient as the quotient of the vertical saturated hydraulic conductivity and the half-cell thickness of the uppermost layer. This study evaluates the implementation of the SC approach utilising a popular block-centred, surface–subsurface hydrology model (MODHMS) to simulate one-dimensional infiltration experiments under Hortonian conditions. Results show that defining the SC coefficient based on a half-cell thickness of the uppermost subsurface cell inhibits accurate prediction of infiltration rates ( q e ) and the time to initiate surface runoff ( t ro ) for the adopted rainfall–runoff scenario. Increasing the SC coefficient independently of the grid allows for accurate simulation of q e , but not t ro . The addition of a thin layer at the surface is shown to improve model accuracy substantially, such that q e and t ro approach those obtained using an equivalent mesh-centred model (i.e. where the surface and upper subsurface nodes are coincident). Whilst the addition of a single thin layer in block-centred codes allows improved prediction of surface–subsurface interaction, it does not provide a surrogate for fine discretisation throughout the subsurface that is necessary for accurate simulation of unsaturated zone flow. This study offers guidance on the implementation of the SC approach in a block-centred code and demonstrates the importance of systematic testing of parameters (that are otherwise calibrated) in physically based surface–subsurface hydrology models.

Stream and bed temperature variability in a coastal headwater catchment: influences of surface-subsurface interactions and partial-retention forest harvesting

Stream temperature was recorded between 2002 and 2005 at four sites in a coastal headwater catchment in British Columbia, Canada. Shallow groundwater temperatures, along with bed temperature profiles at depths of 1 to 30 cm, were recorded at 10-min intervals in two hydrologically distinct reaches beginning in 2003 or 2004, depending on the site. The lower reach had smaller discharge contributions via lateral inflow from the hillslopes and fewer areas with upwelling (UW) and/or neutral flow across the stream bed compared to the middle reach. Bed temperatures were greater than those of shallow groundwater during summer, with higher temperatures in areas of downwelling (DW) flow compared to areas of neutral and UW flow. A paired-catchment analysis revealed that partial-retention forest harvesting in autumn 2004 resulted in higher daily maximum stream and bed temperatures but smaller changes in daily minima. Changes in daily maximum stream temperature, averaged over July and August of the post-harvest year, ranged from 1.6 to 3 °C at different locations within the cut block. Post-harvest changes in bed temperature in the lower reach were smaller than the changes in stream temperature, greater at sites with DW flow, and decreased with depth at both UW and DW sites, dropping to about 1 °C at a depth of 30 cm. In the middle reach, changes in daily maximum bed temperature, averaged over July and August, were generally about 1 °C and did not vary significantly with depth. The pre-harvest regression models for shallow groundwater were not suitable for applying the paired-catchment analysis to estimate the effects of harvesting. However, shallow groundwater was warmer at the lower reach following harvesting, despite generally cooler weather compared to the pre-harvest year. Copyright © 2012 John Wiley & Sons, Ltd.

대도시 복합유역의 지표 및 지표하 유출해석기법 개발 (I)- 이론 및 모듈의 개발 -

Surface-subsurface interactions are an intrinsic component of the hydrologic response within a watershed. In general, these interactions are considered to be one of the most difficult areas of the discipline, particularly for the modeler who intends simulate the dynamic relations between these two major domains of the hydrological cycle. In essence, one major complexity is the spatial and temporal variations in the dynamically interacting system behavior. The proper simulation of these variations requires the need for providing an appropriate coupling mechanism between the surface and subsurface components of the system. In this study, an approach for modelling surface-subsurface flow and transport in a fully intergrated way is presented. The model uses the 2-dimensional diffusion wave equation for sheet surface water flow, and the Boussinesq equation with the Darcy`s law and Dupuit-Forchheimer`s assumption for variably saturated subsurface water flow. The coupled system of equations governing surface and subsurface flows is discretized using the finite volume method with central differencing in space and the Crank-Nicolson method in time. The interactions between surface and subsurface flows are considered mass balance based on the continuity conditions of pressure head and exchange flux. The major module consists of four sub-module (SUBFA, SFA, IA and NS module) is developed.

Reply to comment by Keith J. Beven and Hannah L. Cloke on “Hyperresolution global land surface modeling: Meeting a grand challenge for monitoring Earth's terrestrial water”

WATER RESOURCES RESEARCH, VOL. 48, W01802, doi:10.1029/2011WR011202, 2012 Reply to comment by Keith J. Beven and Hannah L. Cloke on ‘‘Hyperresolution global land surface modeling: Meeting a grand challenge for monitoring Earth’s terrestrial water’’ Eric F. Wood, 1 Joshua K. Roundy, 1 Tara J. Troy, 1 Rens van Beek, 2 Marc Bierkens, 2 Eleanor Blyth, 3 Ad de Roo, 4 Petra Doll, 5 Mike Ek, 6 James Famiglietti, 7 David Gochis, 8 Nick van de Giesen, 9 Paul Houser, 10 Peter Jaffe, 1 Stefan Kollet, 11 Bernhard Lehner, 12 Dennis P. Lettenmaier, 13 Christa D. Peters-Lidard, 14 Murugesu Sivapalan, 15 Justin Sheffield, 1 Andrew J. Wade, 16 and Paul Whitehead 17 Received 25 July 2011; revised 9 November 2011; accepted 3 December 2011; published 21 January 2012. Citation: Wood, E. F., et al. (2012), Reply to comment by Keith J. Beven and Hannah L. Cloke on ‘‘Hyperresolution global land surface modeling: Meeting a grand challenge for monitoring Earth’s terrestrial water’’ Water Resour. Res., 48, W01802, doi:10.1029/ 2011WR011202. Introduction [ 1 ] The authors of Wood et al. [2011, hereafter W2011] would like to thank Beven and Cloke [2012, hereafter BC2012] for furthering the discussion about the pathway to- ward a global-scale hyper-resolution water-energy-biogeo- chemistry land surface modeling capability: its need, feasibility and development. Their comment brings focus to the discussion and shows that the proposed challenge to our community is one element in a long history of hydrology Department of Civil and Environmental Engineering, Princeton Uni- versity, Princeton, New Jersey, USA. Department of Physical Geography, University of Utrecht, Utrecht, Netherlands. Centre for Ecology and Hydrology, Wallingford, UK. Institute for Environment and Sustainability, European Commission Joint Research Center, Ispra, Italy. Institute of Physical Geography, J. W. Goethe University, Frankfurt am Main, Germany. Environmental Modeling Center, National Centers for Environmental Protection, Suitland, Maryland, USA. UC Center for Hydrologic Modeling, University of California-Irvine, Irvine, California, USA. Research Applications Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA. Department of Water Management, Delft University of Technology, Delft, Netherlands. Department of Geography and GeoInformation Science, George Mason University, Fairfax, Virginia, USA. Meteorological Institute, University of Bonn, Bonn, Germany. Department of Geography, McGill University, Montreal, Quebec, Canada. Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington, USA. Hydrological Sciences Laboratory, NASA Goddard Space Flight Cen- ter, Greenbelt, Maryland, USA. Department of Civil and Environmental Engineering and Department of Geography, University of Illinois Urbana-Champaign, Urbana, Illinois, USA. School of Human and Environmental Science, University of Reading, Reading, UK. School of Geography and the Environment, Oxford University, Oxford, UK. Copyright 2012 by the American Geophysical Union 0043-1397/12/2011WR011202 model developments with the goal to improving hydrologic predictions and understanding. [ 2 ] What is laid out in W2011 is, first and foremost, a grand challenge because (1) there is a grand need, (2) there are great new opportunities, and (3) if the hydrologic com- munity does not do it someone else will do it, albeit poorly. The reader is directed to W2011 for a discussion of the growing need for continental-scale land surface models that consider improved, scale-appropriate parameteriza- tions of the water, energy and biogeochemical cycles at resolutions on the order of 10 2 to 10 3 m grid resolutions. Some examples are presented, which were not meant be to comprehensive in their scope of detail, that include surface-subsurface interactions, land-atmospheric interac- tions and coupling, water quality that includes nonpoint pollution, and human impacts that include water manage- ment, land cover change and the effects of climate change. [ 3 ] The commentary by BC2012 focuses on just one chal- lenge or building block described in W2011: the issue of parameterization of subgrid heterogeneity and the resulting uncertainty—what they refer to as ‘‘epistemic uncertainty.’’ BC2012 interprets the Grand Challenge in W2011 as ‘‘simply moving to finer resolutions.’’ This is not what W2011 says or proposes. There are many new building blocks available for the research into hyper-resolution modeling: (1) new data sources and measurement techniques for precipitation, topog- raphy, vegetation cover, soils, but also soil moisture, evapo- transpiration, water storages (rivers, lakes, groundwater storages, soil moisture); (2) new physics—new sets of gov- erning equations, including new approaches to developing closure relations; (3) new approaches to handling known and unknown uncertainties in model structure, variables and numerics, including characterizing subgrid heterogeneity (including new ways to capture their effects) based on new insights into ecohydrology and hydropedology and approaches that utilize the coevolution of climate, soils, vege- tation and topography; (4) new approaches that can better include nonlinear feedbacks between various subsystems, and local, regional and global cycles and teleconnections; (5) new regionalization efforts aimed at learning from compara- tive analysis across climatic, geologic and human-impact W01802 1 of 3

Influence of the first-order exchange coefficient on simulation of coupled surface–subsurface flow

Summary Presently, there is little guidance for model users on the selection of the first-order exchange coefficient (FOEC; or “conductance”) commonly used in simulating surface–subsurface interactions (e.g. infiltration). In this study, relationships between the FOEC and surface–subsurface exchange flux, surface-subsurface head difference and time to initiate overland flow are systematically explored using 1D soil column simulations with the fully integrated code HydroGeoSphere. Numerical experiments adopt five different hydrological scenarios and nine different soil profiles. Results converge on the more accurate, but sometimes more computationally intensive, continuity of pressure (COP) coupling approach as the coupling length (le) parameter within the FOEC is decreased (i.e. FOEC increased). Threshold le values that produce results converged on the COP approach vary considerably with hydrological scenario, soil type and total obstruction height (Hs; accounting for sub-grid depression storage), with most threshold le values ⩽10−2 m. Lower le values are required for infiltration under Hortonian conditions, under non-Hortonian conditions in lower permeability soils, and to capture timing of initiation of overland flow. The condition le > Hs precludes top-down saturation under Hortonian conditions. Steady-state exchange flux and time to initiate overland flow are within 0.05% and 24%, respectively, of COP results when le = Hs = 1 mm. 3D simulation of a hypothetical catchment demonstrates that the general FOEC sensitivities obtained through 1D simulation are transferrable to the 3D case. This study shows that a value of le = Hs provides an appropriate initial value for modelling applications. We suggest a FOEC parameter sensitivity assessment on a case-by-case basis to ensure adequately converged results and to avoid unrealistic model behaviour.

Coupling water flow and solute transport into a physically-based surface–subsurface hydrological model

A distributed-parameter physically-based solute transport model using a novel approach to describe surface–subsurface interactions is coupled to an existing flow model. In the integrated model the same surface routing and mass transport equations are used for both hillslope and channel processes, but with different parametrizations for these two cases. For the subsurface an advanced time-splitting procedure is used to solve the advection–dispersion equation for transport and a standard finite element scheme is used to solve Richards equation for flow. The surface–subsurface interactions are resolved using a mass balance-based surface boundary condition switching algorithm that partitions water and solute into actual fluxes across the land surface and changes in water and mass storage. The time stepping strategy allows the different time scales that characterize surface and subsurface water and solute dynamics to be efficiently and accurately captured. The model features and performance are demonstrated in a series of numerical experiments of hillslope drainage and runoff generation.

Furrow Irrigation Advance Simulation Using a Surface–Subsurface Interaction Model

A model for the simulation of furrow irrigation advance was developed based on the Saint-Venant equations for the one-dimensional surface flow and the two-dimensional Richards equation for porous media flow. Solutions are computed numerically using finite differences for the surface flow and finite elements for the subsurface flow. Computations are internally coupled through an iterative procedure. Infiltration is computed with the Richards equation every five nodes used in the surface flow computations and by linear interpolation at the remaining nodes. In addition, the Richards equation is solved at the boundaries of the surface flow domain and in the vicinity of the wave front. The time step is calculated using the Courant–Friedrich–Lewy condition and a stability criterion that accounts for friction effects. This combined criterion prevents numerical instabilities and convergence problems, especially in cases of high friction coefficient, low discharge rates, and/or high infiltration rates resulting ge...

A comparison of two physics-based numerical models for simulating surface water–groundwater interactions

Problems in hydrology and water management that involve both surface water and groundwater are best addressed with simulation models that can represent the interactions between these two flow regimes. In the current generation of coupled models, a variety of approaches is used to resolve surface–subsurface interactions and other key processes such as surface flow propagation. In this study we compare two physics-based numerical models that use a 3D Richards equation representation of subsurface flow. In one model, surface flow is represented by a fully 2D kinematic approximation to the Saint–Venant equations with a sheet flow conceptualization. In the second model, surface routing is performed via a quasi-2D diffusive formulation and surface runoff follows a rill flow conceptualization. The coupling between the land surface and the subsurface is handled via an explicit exchange term resolved by continuity principles in the first model (a fully-coupled approach) and by special treatment of atmospheric boundary conditions in the second (a sequential approach). Despite the significant differences in formulation between the two models, we found them to be in good agreement for the simulation experiments conducted. In these numerical tests, on a sloping plane and a tilted V-catchment, we examined saturation excess and infiltration excess runoff production under homogeneous and heterogeneous conditions, the dynamics of the return flow process, the differences in hydrologic response under rill flow and sheet flow parameterizations, and the effects of factors such as grid discretization, time step size, and slope angle. Low sensitivity to vertical discretization and time step size was found for the two models under saturation excess and homogeneous conditions. Larger sensitivity and differences in response were observed under infiltration excess and heterogeneous conditions, due to the different coupling approaches and spatial discretization schemes used in the two models. For these cases, the sensitivity to vertical and temporal resolution was greatest for processes such as reinfiltration and ponding, although the differences between the hydrographs of the two models decreased as mesh and step size were progressively refined. In return flow behavior, the models are in general agreement, with the largest discrepancies, during the recession phase, attributable to the different parameterizations of diffusion in the surface water propagation schemes. Our results also show that under equivalent parameterizations, the rill and sheet flow conceptualizations used in the two models produce very similar responses in terms of hydrograph shape and flow depth distribution.

Using a fluctuating tracer to estimate hyporheic exchange in restored and unrestored reaches of the Truckee River, Nevada, USA

AbstractThe goal of this research was to compare hyporheic activity in recently restored and adjacent un‐restored reaches of the Truckee River downstream from the Reno/Sparks metropolitan area. The installation of rocky riffles and raised channel bed elevations in the restored reaches may have increased the degree of surface–subsurface interaction. A fluctuating chloride concentration signal served as the tracer, induced by the variable influx of higher salinity water several miles upstream from the study reach. The solute transport model, OTIS, was used in conjunction with the hydrodynamic model, DYNHYD5, to estimate transient storage parameters under unsteady flow conditions. The model was calibrated to chloride concentrations measured over a period of three days at six in‐stream locations representing restored and un‐restored reaches. An automated parameter estimation algorithm (SCE‐UA) was used to optimize parameters for multiple reaches simultaneously and generate a distribution of parameter estimates. Results suggest that the transient storage zone cross‐sectional area (As) is larger in the restored reaches than in the unrestored reaches, but the exchange coefficient (α) is smaller, leading to increased hyporheic residence time and hydrologic retention in the vicinity of channel reconstructions. Scenarios were used to simulate the potential effects of increased subsurface residence time on denitrification and in‐stream NO3‐N concentrations. Monte Carlo analysis was performed to assess uncertainty in the simulation results and show the potential for greater nutrient retention in the lower Truckee River as a result of channel restoration. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Coupled Finite-Volume Model for 2D Surface and 3D Subsurface Flows

Surface-subsurface interactions are an intrinsic component of the hydrologic response within a watershed; therefore, hydrologic modeling tools should consider these interactions to provide reliable predictions, especially during rainfall-runoff processes. This paper presents a fully implicit coupled model designed for hydrologic evaluation in wetlands, agricultural fields, etc. The model uses the depth-averaged two-dimensional (2D) diffusion wave equation for shallow surface water flow, and the three-dimensional (3D) mixed-form Richards equation for variably saturated subsurface flow. The interactions between surface and subsurface flows are considered via infiltration in dynamic equilibrium. A general framework for coupling the surface and subsurface flow equations is adopted, based on the continuity conditions of pressure head and exchange flux rather than the traditional conductance concept. The diffusion wave surface water equation is used as an upper boundary condition for the initial-boundary value ...