CLUES-GW: a simple coupled steady state surface-groundwater model for contaminant transport

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Elliott S, Rajanayaka C, Yang J, White J. 2019. CLUES-GW: a simple coupled steady state surface-groundwater model for contaminant transport. Lower Hutt (NZ): GNS Science. 51 p. (GNS Science report; 2018/44). doi:10.21420/34XR-AG12.

Abstract:
This report presents CLUES-GW, a simple catchment model that provides mean-annual contaminant load estimates in surface water and groundwater. The model was developed by NIWA as part of the SAM (Smart Models for Aquifer Management) research programme, led by GNS Science, which aims to investigate the implications of model simplification for uncertainty and implications for decision-making. CLUES-GW represents an extension of the existing CLUES model —it includes shallow and deep groundwater reservoirs under each surface sub-catchment, each reservoir exchanging water with the stream and adjacent reservoirs (except only the shallow reservoir is recharged from the stream). The model was applied to a case study of nitrogen transport for the Piako-Waihou system in the Hauraki area within the Waikato region. The model relies on defining flow exchange coefficients between each element (stream reach or reservoir) and adjacent elements, as a proportion of the total outflow from the element. We found it impracticable to calibrate the coefficients, even using Monte Carlo parameter conditioning methods. Therefore, we either assumed no lateral exchange between reservoirs (Model A) or extracted suitable coefficient values by interrogating the output of a complex MODFLOW model (Model B). Surprisingly, both models predicted similar nitrogen stream loads. Other approaches could be used as well, for example, interpreting lateral exchange coefficients from the phreatic surface. (auth)