Brunner, Philip

2019-2, Schilling, Oliver, Cook, Peter G., Brunner, Philip

Traditionally, groundwater and surface water flow models have been calibrated against two observation types: hydraulic heads and surface water discharge. It has repeatedly been demonstrated, however, that these classical observations do not contain sufficient information to calibrate flow models. To reduce the predictive uncertainty of flow models, the consideration of other observation types constitutes a promising way forward. Despite the ever‐increasing availability of other observation types, however, they are still unconventional when it comes to flow model calibration. By reviewing studies that included nonclassical observations in flow model calibration, benefits and challenges associated with their integration in flow model calibration were identified, and their information content was analyzed. While explicit simulation of mass transport processes in flow models poses challenges, even simplified approaches to integrate tracer concentrations yield significantly better calibration results than using only classical observations. For a majority of calibrated flow models, observations of tracer concentrations and of exchange fluxes were beneficial. Temperature observations improved the simulation of heat transport but often worsened all other model outcomes. Only when temperature observations were made within 2 m of the surface water‐groundwater interface did they have the potential to also improve flow and mass transport simulations. Surprisingly, many models were calibrated manually rather than with the widely available, mathematically robust and automated tools. There is a clear need for more systematic implementation of unconventional observations and automated flow model calibration as well as for more systematic quantification of the information content of unconventional observations.

2014-5-18, Lamontagne, S., Taylor, A. R., Cook, Peter G., Crosbie, R. S., Brownbill, R., Williams, R. M., Brunner, Philip

In semi-arid and arid river basins, understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources. The type of connection present (gaining, losing-connected, transitional and losing-disconnected) was assessed at 12 sites along six Murray-Darling Basin river reaches. The assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure () in the riverbeds. The rationale for the latter was that will always be greater than or equal to zero under connected conditions (either losing or gaining) and always lesser than or equal to zero under losing-disconnected conditions. A mixture of losing-disconnected, losing-connected and gaining conditions was found among the 12 sites. The losing-disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer, usually in the form of a silty clay or clay unit 0.5-2m in thickness. The riparian water tables were 6 to 25m below riverbed level at the losing-disconnected sites but never lower than 1m below riverbed level at the losing-connected ones. The contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study, when a severe regional drought had generated a widespread decline in regional water tables. This decline was apparently compensated near losing-connected rivers by increased infiltration rates, while the decline could not be compensated at the losing-disconnected rivers because the infiltration rates were already maximal there. Copyright (c) 2012 John Wiley & Sons, Ltd.

2011, Brunner, Philip, Cook, Peter G., Simmons, Craig T.

When describing the hydraulic relationship between rivers and aquifers, the term disconnected is frequently misunderstood or used in an incorrect way. The problem is compounded by the fact that there is no definitive literature on the topic of disconnected surface water and groundwater. We aim at closing this gap and begin the discussion with a short introduction to the historical background of the terminology. Even though a conceptual illustration of a disconnected system was published by Meinzer (1923), it is only within the last few years that the underlying physics of the disconnection process has been described. The importance of disconnected systems, however, is not widely appreciated. Although rarely explicitly stated, many approaches for predicting the impacts of groundwater development on surface water resources assume full connection. Furthermore, management policies often suggest that surface water and groundwater should only be managed jointly if they are connected. However, although lowering the water table beneath a disconnected section of a river will not change the infiltration rate at that point, it can increase the length of stream that is disconnected. Because knowing the state of connection is of fundamental importance for sustainable water management, robust field methods that allow the identification of the state of connection are required. Currently, disconnection is identified by showing that the infiltration rate from a stream to an underlying aquifer is independent of the water table position or by identifying an unsaturated zone under the stream. More field studies are required to develop better methods for the identification of disconnection and to quantify the implications of heterogeneity and clogging processes in the streambed on disconnection.

2009-5-26, Brunner, Philip, Simmons, Craig Trevor, Cook, Peter G., Therrien, René

2014-5-18, Trigg, M. A., Cook, Peter G., Brunner, Philip

The successful management of groundwater dependent shallow seasonal wetlands requires a sound understanding of groundwater fluxes. However, such fluxes are hard to quantify. Water volume and solute mass balance models can be used in order to derive an estimate of groundwater fluxes within such systems. This approach is particularly attractive, as it can be undertaken using measurable environmental variables, such as; rainfall, evaporation, pond level and salinity. Groundwater fluxes estimated from such an approach are subject to uncertainty in the measured variables as well as in the process representation and in parameters within the model. However, the shallow nature of seasonal wetland ponds means water volume and surface area can change rapidly and non-linearly with depth, requiring an accurate representation of the wetland pond bathymetry. Unfortunately, detailed bathymetry is rarely available and simplifying assumptions regarding the bathymetry have to be made. However, the implications of these assumptions are typically not quantified. We systematically quantify the uncertainty implications for eight different representations of wetland bathymetry for a shallow seasonal wetland pond in South Australia. The predictive uncertainty estimation methods provided in the Model-Independent Parameter Estimation and Uncertainty Analysis software (PEST) are used to quantify the effect of bathymetric uncertainty on the modelled fluxes. We demonstrate that bathymetry can be successfully represented within the model in a simple parametric form using a cubic Sexier curve, allowing an assessment of bathymetric uncertainty due to measurement error and survey detail on the derived groundwater fluxes compared with the fixed bathymetry models. Findings show that different bathymetry conceptualisations can result in very different mass balance components and hence process conceptualisations, despite equally good fits to observed data, potentially leading to poor management decisions for the wetlands. Model predictive uncertainty increases with the crudity of the bathymetry representation, however, approximations that capture the general shape of the wetland pond such as a power law or Bezier curve show only a small increase in prediction uncertainty compared to the full dGPS surveyed bathymetry, implying these may be sufficient for most modelling purposes. (C) 2014 Elsevier B.V. All rights reserved.

2012, Doble, Rebecca, Brunner, Philip, McCallum, James, Cook, Peter G.

Recognizing the underlying mechanisms of bank storage and return flow is important for understanding streamflow hydrographs. Analytical models have been widely used to estimate the impacts of bank storage, but are often based on assumptions of conditions that are rarely found in the field, such as vertical river banks and saturated flow. Numerical simulations of bank storage and return flow in river-aquifer cross sections with vertical and sloping banks were undertaken using a fully-coupled, surface-subsurface flow model. Sloping river banks were found to increase the bank infiltration rates by 98% and storage volume by 40% for a bank slope of 3.4° from horizontal, and for a slope of 8.5°, delay bank return flow by more than four times compared with vertical river banks and saturated flow. The results suggested that conventional analytical approximations cannot adequately be used to quantify bank storage when bank slope is less than 60° from horizontal. Additionally, in the unconfined aquifers modeled, the analytical solutions did not accurately model bank storage and return flow even in rivers with vertical banks due to a violation of the dupuit assumption. Bank storage and return flow were also modeled for more realistic cross sections and river hydrograph from the Fitzroy River, Western Australia, to indicate the importance of accurately modeling sloping river banks at a field scale. Following a single wet season flood event of 12 m, results showed that it may take over 3.5 years for 50% of the bank storage volume to return to the river.

2010, Brunner, Philip, Simmons, Craig T., Cook, Peter G., Therrien, René

The accuracy with which MODFLOW simulates surface water-groundwater interaction is examined for connected and disconnected losing streams. We compare the effect of different vertical and horizontal discretization within MODFLOW and also compare MODFLOW simulations with those produced by HydroGeoSphere. HydroGeoSphere is able to simulate both saturated and unsaturated flow, as well as surface water, groundwater and the full coupling between them in a physical way, and so is used as a reference code to quantify the influence of some of the simplifying assumptions of MODFLOW. In particular, we show that (1) the inability to simulate negative pressures beneath disconnected streams in MODFLOW results in an underestimation of the infiltration flux; (2) a river in MODFLOW is either fully connected or fully disconnected, while in reality transitional stages between the two flow regimes exist; (3) limitations in the horizontal discretization of the river can cause a mismatch between river width and cell width, resulting in an error in the water table position under the river; and (4) because coarse vertical discretization of the aquifer is often used to avoid the drying out of cells, this may result in an error in simulating the height of the groundwater mound. Conditions under which these errors are significant are investigated.

2014-5-18, Xie, Y. Q., Cook, Peter G., Brunner, Philip, Irvine, Dylan J., Simmons, Craig Trevor

Decline in regional water tables (RWT) can cause losing streams to disconnect from underlying aquifers. When this occurs, an inverted water table (IWT) will develop beneath the stream, and an unsaturated zone will be present between the IWT and the RWT. The IWT marks the base of the saturated zone beneath the stream. Although a few prior studies have suggested the likelihood of an IWT without a clogging layer, most of them have assumed that a low-permeability streambed is required to reduce infiltration from surface water to groundwater, and that the IWT only occurs at the bottom of the low-permeability layer. In this study, we use numerical simulations to show that the development of an IWT beneath an unclogged stream is theoretically possible under steady-state conditions. For a stream width of 1m above a homogeneous and isotropic sand aquifer with a 47m deep RWT (measured in an observation point 20m away from the center of the stream), an IWT will occur provided that the stream depth is less than a critical value of 4.1m. This critical stream depth is the maximum water depth in the stream to maintain the occurrence of an IWT. The critical stream depth decreases with stream width. For a stream width of 6 m, the critical stream depth is only 1mm. Thus while theoretically possible, an IWT is unlikely to occur at steady state without a clogging layer, unless a stream is very narrow or shallow and the RWT is very deep.

2011-5-26, Brunner, Philip, Cook, Peter G., Simmons, Craig Trevor

When describing the hydraulic relationship between rivers and aquifers, the term disconnected is frequently misunderstood or used in an incorrect way. The problem is compounded by the fact that there is no definitive literature on the topic of disconnected surface water and groundwater. We aim at closing this gap and begin the discussion with a short introduction to the historical background of the terminology. Even though a conceptual illustration of a disconnected system was published by Meinzer (1923), it is only within the last few years that the underlying physics of the disconnection process has been described. The importance of disconnected systems, however, is not widely appreciated. Although rarely explicitly stated, many approaches for predicting the impacts of groundwater development on surface water resources assume full connection. Furthermore, management policies often suggest that surface water and groundwater should only be managed jointly if they are connected. However, although lowering the water table beneath a disconnected section of a river will not change the infiltration rate at that point, it can increase the length of stream that is disconnected. Because knowing the state of connection is of fundamental importance for sustainable water management, robust field methods that allow the identification of the state of connection are required. Currently, disconnection is identified by showing that the infiltration rate from a stream to an underlying aquifer is independent of the water table position or by identifying an unsaturated zone under the stream. More field studies are required to develop better methods for the identification of disconnection and to quantify the implications of heterogeneity and clogging processes in the streambed on disconnection.

2010, McCallum, James L., Cook, Peter G., Brunner, Philip, Berhane, Dawit

Chemical base flow separation is a widely applied technique in which contributions of groundwater and surface runoff to streamflow are estimated based on the chemical composition of stream water and the two end-members. This method relies on the assumption that the groundwater end-member can be accurately defined and remains constant. We simulate solute transport within the aquifer during and after single and multiple river flow events, to show that (1) water adjacent to the river will have a concentration intermediate between that of the river and that of regional groundwater and (2) the concentration of groundwater discharge will approach that of regional groundwater after a flow event but may take many months or years before it reaches it. In applying chemical base flow separation, if the concentration in the river prior to a flow event is used to represent the pre-event or groundwater end-member, then the groundwater contribution to streamflow will be overestimated. Alternatively, if the concentration of regional groundwater a sufficient distance from the river is used, then the pre-event contribution to streamflow will be underestimated. Changes in concentration of groundwater discharge following changes in river stage predicted by a simple model of stream-aquifer flows show remarkable similarity to changes in river chemistry measured over a 9 month period in the Cockburn River, southeast Australia. If the regional groundwater value was used as the groundwater end-member, chemical base flow separation techniques would attribute 8% of streamflow to groundwater, as opposed to 25% if the maximum stream flow value was used.