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Recent developments in speeding Up MODFLOW-USG simulations: the truncated Newton Method and GPU acceleration
Damian Merrick 1 , Marshall Clifton 1
1. HydroAlgorithmics, Hawker, ACT, Australia
Long simulation run-times are the eternal nemesis of the numerical modeller: they impact schedules, prolongate sensitivity analyses, and place practical limits on model scale and the ability to perform comprehensive statistical uncertainty quantification.
We present recent software developments in accelerating numerical simulations to combat this. These developments have so far been applied to MODFLOW-USG but may in future also be implemented in other software that uses Newton-Raphson linearisation. The first development is the truncated Newton method, which computes and applies a dynamic residual-based termination criterion to limit the number of linear solution iterations necessary to solve nonlinear problems. The second development is a new port of the entire MODFLOW outer iteration loop to run on NVIDIA GPU devices, including matrix formulation, nonlinear residual reduction and linear solver logic. The two developments may be used together or separately, and we demonstrate significant speedups in simulation run-time through their use – particularly on modern GPU devices.
Reconstruction of baseline groundwater levels using backwards Cumulative Rainfall Departure and highly-parameterised parameter inversion
Eduardo De Sousa 1
1. DHI, Como, WA, Australia
Groundwater recharge estimates using the Cummulative Rainfal Deparure (CRD) method have been widely used in literature (Bredenkamp, 1995), in areas where groundwater levels and rainfall present a strong correlation. This study further explores the utilisation of this method for hindcasting and estimation of baseline groundwater levels in areas where monitoring data is scarce.
The proposed methodology has been applied in the Lake Muir-Unicup Natural Diversity Recovery Catchment (MUNDRC). The catchment consists of a complex system of lakes, swamps and flood plains, located in southwestern Australia. Long term rainfall time-series in the area have shown a systematic decline over the past decades, with an abrupt change since the 70’s.
Conceptual and numerical groundwater-surface water models have been developed to assess the effects of rainfall decline in the lakes and surrounding environment.
The lack of groundwater and lake level monitoring data prior to the rainfall decline makes the establishment of baseline levels and initial conditions difficult.
The CRD method has been utilised in a backward form to estimate groundwater levels prior to the decline of rainfall rates. The simplified nature this method has inherent non-uniqueness between rainfall ratios and evapotranspiration. To address that, multiple CRD models were carried at different locations within the catchment and calibrated simultaneously using regularized parameter inversion.
39 | P a g e The calibration was undertaken with PEST-HP using 91 models, following by
uncertainty analysis through the generation and sampling of the posterior covariance matrix, with a total of 500 realisations per model.
The results from the optimisation indicated a good agreement between CRD results and respective observations. Furthermore, the distribution of the calibrated
parameters values is reasonable and in agreement with the conceptual model.
1. BREDENKAMP DB, BOTHA LJ, VAN TONDER GJ and VANRENSBURG HJ (1995) Manual on Quantitative Estimation of Groundwater Recharge and Aquifer Storativity. WRC Report No TT73/95
Generating hydrogeological virtual realities for hypothesis testing in groundwater modelling
Jeremy Bennett 1 2 , Samuel Scherrer 3
1. Tonkin & Taylor Ltd, Newmarket, Auckland, New Zealand
2. Centre for Applied Geosciences, University of Tübingen, Tübingen, Baden-Württemberg, Germany
3. Institute for Modelling Hydraulic and Environmental Systems, University of Stuttgart, Stuttgart, Baden-Württemberg, Germany
Objectives: Heterogeneity of subsurface hydraulic properties controls groundwater flow and contaminant transport. However, the depositional processes which account for the creation of clastic aquifers are often neglected in hydrogeological modelling.
Many existing methods for simulating subsurface heterogeneity do not honour depositional concepts or cannot simulate heterogeneous bedding structures present in fluvial deposits.
Design and methodology: We have implemented a hierarchical modelling framework for simulating sedimentary deposits in the Hydrogeological Virtual Realities (HyVR) simulation package. The package uses an object-based modelling approach to model hydraulic parameter fields with multiple scales of heterogeneity.
40 | P a g e Outputs: HyVR outputs are three-dimensional parameter fields that can include hydraulic conductivity, porosity, anisotropic ratios, and bedding parameters (dip and azimuth). The last three parameters can be used to simulate hydraulic anisotropy through rotation of full hydraulic conductivity tensors. HyVR has been designed with the groundwater modeller in mind, and as such, simulation outputs can be used in forward flow-and-transport numerical modelling tools (e.g., MODFLOW), allowing qualitative geological concepts to be tested in quantitative hydrogeological models.
Conclusion: HyVR (https://github.com/driftingtides/hyvr) is an openly available Python module that is comprehensively documented
(https://driftingtides.github.io/hyvr/), for ease of use. It allows hydrogeological researchers to develop object-based models for use in standard groundwater modelling software. It forms an open codebase that can be further extended and developed by the hydrogeological community.
1. Bennett, J. P., Haslauer, C. P., Ross, M. and Cirpka, O. A. (2019), An Open, Object‐Based Framework for Generating Anisotropy in Sedimentary Subsurface Models. Groundwater, 57: 420-429. doi:10.1111/gwat.12803
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Comparing modelling approaches for salinity impact assessment of irrigation in SA Mallee
Juliette Woods 1 , Tariq Laatoe 2 , Dougal R. Currie 3 , Tony J. Smith 4 , Kittiya Bushaway 1 , Virginia Riches 1 , Glen R. Walker 5
1. South Australian Department for Environment and Water, Adelaide, SA, Australia 2. Water Technology, Adelaide, SA, Australia
3. CDM Smith, Adelaide, SA, Australia 4. CDM Smith, Perth, WA, Australia
5. Grounded in Water, Adelaide, SA, Australia
Impacts of past and present irrigation practices on River Murray salinity is a priority for river management in the Mallee region. A 2017 review examined modelling approaches to estimate salinity impacts from irrigation and found they could
introduce biases. Additionally, there is no direct modelling of the unsaturated zone to account for perching on clays which has a significant influence on the magnitude and timing of salt loads to the river. These limitations create potential for bias when linking on-ground actions (such as irrigation efficiency improvements) to salt loads in the river but could be addressed by (1) developing unsaturated zone models to deal with perching; and (2) incorporating and calibrating such models within an
accredited groundwater model. This talk addresses the latter step.
A pilot trial was conducted for the Loxton-Bookpurnong irrigation districts of South Australia, comparing two modelling approaches. The first was that of a 2011 groundwater model, where some assumptions were made about hydraulic conductivity, and then the recharge was inferred, via an inverse method. An agronomic water balance was used as a check but was not used to estimate the recharge.
The second approach was developed and calibrated during the pilot trial. An agronomic water balance was integrated with a semi-analytic unsaturated zone model and a numerical groundwater model. The integrated model used a recharge time series generated by the water balance and unsaturated zone model. This was applied to distinct recharge zones, based on lithology, date of irrigation development and drainage practices. The outputs, especially recharge and salt loads to river, were compared to those with the 2011 accredited model. , in which an integrated
agronomic water balance-unsaturated zone-groundwater model was developed and calibrated; and the modelling outputs, especially salt loads to river, were compared to those with the 2011 accredited model.
The integrated model was fit-for-purpose in that it enabled estimation of salt loads for various scenarios. Irrigation actions were able to be more directly linked to groundwater responses and salt loads than with the inverse method. Drainage information was important to constrain soil properties and vertical fluxes.
The conclusions from the study are
1. Collating historical data on drainage and irrigation development helps constrain the degrees of freedom in calibration;
2. Salt loads varied in magnitude from original model, although timing was similar.
The results of the pilot trial are being used to develop recommendations for modelling salinity impacts for Mallee irrigation districts.
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The early days of groundwater modelling in Australia, 25 years from 1970 to 1994
Noel P. Merrick 1 , Frans R. Kalf 2
1. SLR Consulting, Gerringong, NSW, Australia 2. Kalf & Associates, Sydney, NSW, Australia
This talk is a nostalgic journey through the first 25 years of groundwater modelling in Australia, roughly the years 1970 to 1994 (when PEST was born). The early years were before MODFLOW, before Excel, before SURFER, before Windows, and long before GIS. Back then, every modelling activity was an innovation. Our perspective is coloured by personal experience in New South Wales (Australia) when the authors were working together in the State Government Water Conservation and Irrigation Commission. The game-changer was acquisition of a Digital PDP8/e in 1972 with 8 kb of memory, which supported only Assembler and FOCAL programming languages, followed by BASIC and eventually FORTRAN. I/O was via hardware toggles, a
teletypewriter keyboard (10 cps), and paper tape which doubled as the initial storage device, followed by serial cassette tape, 8-inch floppy discs and eventually large hard discs with 5 Mb capacity. The focus was on analytical models for the first decade until distribution of PLASM (from USA) in the mid-1970s, AQUIFEM in the early 1980s, and MODFLOW in the mid-1980s, supplemented by bespoke code. Our story ends with the creation of PEST, a great Australian invention. In keeping with the theme of this conference, we witnessed groundwater changing from obscurity to prominence over a quarter of a century, and analytical and numerical techniques developed by us and others in those early years were truly “emerging”.
A novel approach to representing longwall induced fracturing in finite difference groundwater models
Neil Manewell 1 , Will Minchin 2 1. AGE, Bowen Hills, QLD, Australia 2. Watershed Hydrogeo, NSW, Australia
Longwall mining induces a significant change to the overlying groundwater system by creating an interconnected fracture network above the collapsed longwall cavity.
This change occurs in response to land subsidence and delamination of sedimentary beds. Representing the effects of mining induced subsidence in MODFLOW models has traditionally been simulated as an increase to permeability, loosely based on several subsidence studies (Tammetta 2017, Guo 2016, & Booth 2002). AGE has developed a method where the newly created fracture network above each longwall panel is implicitly coupled in the MODFLOW model. Using a derived equation to calculate the total sum of fractures above the spent coal seam, a modified version of the ‘Connected Linear Network’ package (CLN) flow equation was used in the Drain package (DRN). A Fortran executable was developed to calculate the rate of
groundwater drawdown using the total aperture of the fracture network (A), the in- situ host vertical permeability (Kz), and the length the fracture network extends into each cell (l). The maximum height of connective fracturing is automatically
calculated using an option of either the Ditton-Merrick (2014) and Tammetta (2016) equations. The DRN package attempts to lower the groundwater level down to the bottom of each fractured cell, i.e. toward zero pressure, at a calculated conductance rate. This method solves the common issue of model convergence when directly representing extremely high fracturing intensity (high permeability), adjacent to in-
43 | P a g e situ cells with low permeability. Case studies presenting historic longwall mine dewatering of several major projects in New South Wales indicate excellent
calibration to pressure and inflow using this technique. Future studies could utilize this method in combination with fracture enhancement predictions from COSFLOW (Adhikary 2007) modelling.