Daniel Partington

A hydraulic mixing-cell method to quantify the groundwater component of streamflow within spatially distributed fully integrated surface water-groundwater flow models

The complexity of available hydrological models continues to increase, with fully integrated surface water-groundwater flow and transport models now available. Nevertheless, an accurate quantification of streamflow generation mechanisms within these models is not yet possible. For example, such models do not report the groundwater component of streamflow at a particular point along the stream. Instead, the groundwater component of streamflow is approximated either from tracer transport simulations or by the sum of exchange fluxes between the surface and the subsurface along the river. In this study, a hydraulic mixing-cell (HMC) method is developed and tested that allows to accurately determine the groundwater component of streamflow by using only the flow solution from fully integrated surface water-groundwater flow models. By using the HMC method, the groundwater component of streamflow can be extracted accurately at any point along a stream provided the subsurface/surface exchanges along the stream are calculated by the model. A key advantage of the HMC method is that only hydraulic information is used, thus the simulation of tracer transport is not required. Two numerical experiments are presented, the first to test the HMC method and the second to demonstrate that it quantifies the groundwater component of streamflow accurately.

Framework for assessing and improving the performance of recursive digital filters for baseflow estimation with application to the Lyne and Hollick filter

Baseflow is often regarded as the streamflow component derived predominantly from groundwater discharge. The estimation of baseflow is important for water supply, water allocation, investigation of contamination impacts, low flow hydrology and flood hydrology. Baseflow is commonly estimated using graphical methods, recursive digital filters (RDFs), tracer based methods, and conceptual models. Of all of these methods, RDFs are the most commonly used, due to their relatively easy and efficient implementation. This paper presents a generic framework for assessing and improving the performance of RDFs for baseflow estimation for catchments with different characteristics and subject to different hydrological conditions. As part of the framework, a fully integrated surface water/groundwater (SW/GW) model is used to obtain estimates of streamflow and baseflow for catchments with different properties, such as soil types and rainfall patterns. An RDF is then applied to the simulated streamflow to assess how well the baseflow obtained using the filter matches the baseflow obtained using the fully integrated SW/GW model. In order to improve the performance of the filter, the user-defined parameter(s) controlling filter operation can be adjusted in order to obtain the best match between the baseflow obtained using the filter and that obtained using the fully integrated SW/GW model (i.e. through calibration). The proposed framework is tested by applying it to a common SW/GW benchmarking problem, the tilted V-catchment, for a range of soil properties. HydroGeoSphere (HGS) is used to develop the fully integrated SW/GW model and the Lyne and Hollick (LH) filter is used as the RDF. The performance of the LH filter is assessed using the commonly used value of the filter parameter of 0.925, as well as calibrated filter parameter values. The results obtained show that the performance of the LH filter is affected significantly by the saturated hydraulic conductivity (Ks) of the soil and that calibrated LH filter parameter can result in significant improvements in filter performance.

Performance assessment and improvement of recursive digital baseflow filters for catchments with different physical characteristics and hydrological inputs

Recursive digital filters (RDFs) are one of the most commonly used methods of baseflow separation. However, how accurately they estimate baseflow and how to select appropriate values of filter parameters is generally unknown. In this paper, the output of fully integrated surface water/groundwater (SW/GW) models is used to obtain optimal parameters for, and assess the accuracy of, three commonly used RDFs under a range of physical catchment characteristics and hydrological inputs. The results indicate that the Lyne and Hollick (LH) filter performs better than the Boughton and Eckhardt filters, over a larger range of conditions. In addition, the optimal values of the filter parameters vary considerably for all three filters, depending on catchment characteristics and hydrological inputs. The dataset of the 66 catchment characteristics and hydrological inputs, as well as the corresponding simulated total streamflow and baseflow hydrographs obtained using the SW/GW model, can be downloaded as Supplementary material.

Blueprint for a coupled model of sedimentology, hydrology and hydrogeology in streambeds

The streambed constitutes the physical interface between the surface and the subsurface of a stream. Across all spatial scales, surface water-groundwater interactions are controlled by the physical properties of the streambed. Streambed properties such as topography or hydraulic conductivity are continuously altered through erosion and sedimentation processes. Recent studies from the fields of ecology, hydrogeology and sedimentology provide field evidence that sedimentological processes themselves can be heavily influenced by surface water-groundwater interactions, giving rise to complex feedback mechanisms between sedimentology, hydrology and hydrogeology. More explicitly, surface water-groundwater exchanges play a significant role in the deposition of fine sediments, which in turn modify the hydraulic properties of the streambed. We explore these feedback mechanisms and critically review the extent of current interaction between the different disciplines. We identify opportunities to improve current modeling practices. For example, hydrogeological models treat the streambed as a static rather than a dynamic entity, while sedimentological models do not account for critical catchment processes such as surface water-groundwater exchange. A blueprint for a new modeling framework is proposed that bridges the conceptual gaps between sedimentology, hydrogeology and hydrology. Specifically, this blueprint (1) fully integrates surface-subsurface flows with erosion, transport and deposition of sediments, and (2) accounts for the dynamic changes in surface elevation and hydraulic conductivity of the streambed. The opportunities for new research within the coupled framework are discussed.

Fully integrated modeling of surface-subsurface solute transport and the effect of dispersion in tracer hydrograph separation

Tracer hydrograph separation has been widely applied to identify streamflow components, often indicating that pre-event water comprises a large proportion of stream water. Previous work using numerical modeling suggests that hydrodynamic mixing in the subsurface inflates the pre-event water contribution to streamflow when derived from tracer-based hydrograph separation. This study compares the effects of hydrodynamic dispersion, both within the subsurface and at the surface-subsurface boundary, on the tracer-based pre-event water contribution to streamflow. Using a fully integrated surface-subsurface code, we simulate two hypothetical 2-D hillslopes with surface-subsurface solute exchange represented by different solute transport conceptualizations (i.e., advective and dispersive conditions). Results show that when surface-subsurface solute transport occurs via advection only, the pre-event water contribution from the tracer-based separation agrees well with the hydraulically determined value of pre-event water from the numerical model, despite dispersion occurring within the subsurface. In this case, subsurface dispersion parameters have little impact on the tracer-based separation results. However, the pre-event water contribution from the tracer-based separation is larger when dispersion at the surface-subsurface boundary is considered. This work demonstrates that dispersion within the subsurface may not always be a significant factor in apparently large pre-event water fluxes over a single rainfall event. Instead, dispersion at the surface-subsurface boundary may increase estimates of pre-event water contribution. This work also shows that solute transport in numerical models is highly sensitive to the representation of the surface-subsurface interface. Hence, models of catchment-scale solute dynamics require careful treatment and sensitivity testing of the surface-subsurface interface to avoid misinterpretation of real-world physical processes.

Advancing Physically-Based Flow Simulations of Alluvial Systems Through Atmospheric Noble Gases and the Novel 37Ar Tracer Method

To provide a sound understanding of the sources, pathways and residence times of groundwater water in alluvial river-aquifer systems, a combined multi-tracer and modelling experiment was carried out in an important alluvial drinking water wellfield in Switzerland. 222Rn, 3H/3He, atmospheric noble gases and the novel 37Ar-method were used to quantify residence times and mixing ratios of water from different sources. With a half-life of 35.1 days, 37Ar allowed to successfully close a critical observational time gap between 222Rn and 3H/3He for residence times of weeks to months. Covering the entire range of residence times of groundwater in alluvial systems revealed that, to quantify the fractions of water from different sources in such systems, atmospheric noble gases and Helium isotopes are tracers suited for end-member mixing analysis. A comparison between the tracer-based mixing ratios and mixing ratios simulated with a fully-integrated, physically-based flow model showed that models, which are only calibrated against hydraulic heads, cannot reliably reproduce mixing ratios or residence times of alluvial river-aquifer systems. However, the tracer-based mixing ratios allowed the identification of an appropriate flow model parameterization. Consequently, for alluvial systems we recommend the combination of multi-tracer studies that cover all relevant residence times with fully-coupled, physically-based flow modelling to better characterize the complex interactions of river-aquifer systems.