Kevin R. Green


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Performance improvements to modern hydrological models via lookup table optimizations
Christopher B. Marsh, Kevin R. Green, B. Wang, Raymond J. Spiteri
Environmental Modelling & Software, Volume 139

Distributed hydrological models predict the spatial variability in processes that govern observed mass and energy fluxes. A challenge associated with the use of these models is the computational burden associated with representing the Earth's (sub)surface via millions of computational elements. This burden is exacerbated as more complex process representations are included because their parameterizations involve computationally intensive mathematical functions. Lookup tables (LUTs) approximate a mathematical function by interpolating precomputed values of the function. Highly accurate approximations are possible for substantially reduced computational costs. In this work, a general methodology using the C++ LUT library FunC is applied to identify and replace computationally intensive mathematical function evaluations in the Canadian Hydrological Model (CHM). The use of LUTs introduces a pointwise relative error below 10 − 8 and provides a reduction in run time of almost 20%. This work shows how LUTs can be implemented with relatively little pain and yield significant computational savings for distributed hydrological models. • The Canadian Hydrological Model (CHM) is profiled and expensive mathematical functions identified. • FunC was used to replace the expensive mathematical functions in CHM with lookup tables. • The run-time performance of CHM was improved by approximately 20% on two realistic simulations. • A general methodology for using FunC to replace expensive mathematical functions with lookup tables is given.

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The numerical implementation of land models: Problem formulation and laugh tests
Martyn P. Clark, Reza Zolfaghari, Kevin R. Green, S. J. Trim, Wouter Knoben, Andrew Bennett, Bart Nijssen, A. M. Ireson, Raymond J. Spiteri
Journal of Hydrometeorology

Abstract The intent of this paper is to encourage improved numerical implementation of land models. Our contributions in this paper are two-fold. First, we present a unified framework to formulate and implement land model equations. We separate the representation of physical processes from their numerical solution, enabling the use of established robust numerical methods to solve the model equations. Second, we introduce a set of synthetic test cases (the laugh tests) to evaluate the numerical implementation of land models. The test cases include storage and transmission of water in soils, lateral sub-surface flow, coupled hydrological and thermodynamic processes in snow, and cryosuction processes in soil. We consider synthetic test cases as “laugh tests” for land models because they provide the most rudimentary test of model capabilities. The laugh tests presented in this paper are all solved with the Structure for Unifying Multiple Modeling Alternatives model (SUMMA) implemented using the SUite of Nonlinear and DIfferential/Algebraic equation Solvers (SUNDIALS). The numerical simulations from SUMMA/SUNDIALS are compared against (1) solutions to the synthetic test cases from other models documented in the peer-reviewed literature; (2) analytical solutions; and (3) observations made in laboratory experiments. In all cases, the numerical simulations are similar to the benchmarks, building confidence in the numerical model implementation. We posit that some land models may have difficulty in solving these benchmark problems. Dedicating more effort to solving synthetic test cases is critical in order to build confidence in the numerical implementation of land models.


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Extended BACOLI
Kevin R. Green, Raymond J. Spiteri
ACM Transactions on Mathematical Software, Volume 45, Issue 1

BACOLI is a Fortran software package for solving one-dimensional parabolic partial differential equations (PDEs) with separated boundary conditions by B-spline adaptive collocation methods. A distinguishing feature of BACOLI is its ability to estimate and control error and correspondingly adapt meshes in both space and time. Many models of scientific interest, however, can be formulated as multiscale parabolic PDE systems, that is, models that couple a system of parabolic PDEs describing dynamics on a global scale with a system of ordinary differential equations describing dynamics on a local scale. This article describes the Fortran software eBACOLI, the extension of BACOLI to solve such multiscale models. The performance of the extended software is demonstrated to be statistically equivalent to the original for purely parabolic PDE systems. Results from eBACOLI are given for various multiscale models from the extended problem class considered.

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Direct Function Evaluation versus Lookup Tables: When to Use Which?
Kevin R. Green, Tanner A. Bohn, Raymond J. Spiteri
SIAM Journal on Scientific Computing, Volume 41, Issue 3

The speed of mathematical function evaluations can significantly contribute to the overall performance of numerical simulations. Two common approaches to evaluate a mathematical function are by dir...