Autocalibration of a one-dimensional hydrodynamic-ecological model (DYRESM 4.0-CAEDYM 3.1) using a Monte Carlo approach: Simulations of hypoxic events in a polymictic lake

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Abstract

Automated calibration of complex deterministic water quality models with a large number of biogeochemical parameters can reduce time-consuming iterative simulations involving empirical judgements of model fit. We undertook auto-calibration of the one-dimensional hydrodynamic-ecological lake model DYRESM-CAEDYM, using a Monte Carlo Sampling (MCS) method, in order to test the applicability of this procedure for shallow, polymictic Lake Rotorua (New Zealand). The calibration procedure involved independently minimising the root-mean-square-error (RMSE), maximizing the Pearson correlation coefficient (<i>r</i>) and Nash-Sutcliffe efficient coefficient (Nr) for comparisons of model state variables against measured data. An assigned number of parameter permutations was used for 10,000 simulation iterations. The <q>optimal</q> temperature calibration produced a RMSE of 0.54&amp;thinsp;°C, Nr-value of 0.99 and <i>r</i>-value of 0.98 through the whole water column based on comparisons with 540 observed water temperatures collected between 13 July 2007&amp;ndash;13 January 2009. The modeled bottom dissolved oxygen concentration (20.5&amp;thinsp;m below surface) was compared with 467 available observations. The calculated RMSE of the simulations compared with the measurements was 1.78&amp;thinsp;mg&amp;thinsp;L<sup>&amp;minus;1</sup>, the Nr-value was 0.75 and the <i>r</i>-value was 0.87. The autocalibrated model was further tested for an independent data set by simulating bottom-water hypoxia events for the period 15 January 2009 to 8 June 2011 (875 days). This verification produced an accurate simulation of five hypoxic events corresponding to DO < 2&amp;thinsp;mg&amp;thinsp;L<sup>&amp;minus;1</sup> during summer of 2009&amp;ndash;2011. The RMSE was 2.07&amp;thinsp;mg&amp;thinsp;L<sup>&amp;minus;1</sup>, Nr-value 0.62 and <i>r</i>-value of 0.81, based on the available data set of 738 days. The auto-calibration software of DYRESM-CAEDYM developed here is substantially less time-consuming and more efficient in parameter optimisation than traditional manual calibration which has been the standard tool practiced for similar complex water quality models.

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Luo, L., Hamilton, D., Lan, J., McBride, C., & Trolle, D. (2018). Autocalibration of a one-dimensional hydrodynamic-ecological model (DYRESM 4.0-CAEDYM 3.1) using a Monte Carlo approach: Simulations of hypoxic events in a polymictic lake. Geoscientific Model Development, 11(3), 903–913. https://doi.org/10.5194/gmd-11-903-2018

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