Implementing and evaluating models of steady-state groundwater flow under baseflow conditions, Lake Rotorua catchment, New Zealand

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Daughney, C.J.; Toews, M.W.; Ancelet, T.; White, P.A.; Cornaton, F.J.; Stokes, K.; Maxwell, D.; Jackson, B.M.; Tschritter, C. 2015 Implementing and evaluating models of steady-state groundwater flow under baseflow conditions, Lake Rotorua catchment, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/52 121 p. + 1 DVD

Abstract: Lake Rotorua is located in a roughly circular caldera basin in the central North Island, New Zealand. The lake water quality has declined over the past 50 years due to nutrient inputs from the catchment. Increasing nitrate concentrations have been observed in virtually all of the major streams flowing in to the lake during the period 1968–2003, strongly suggesting that nutrient inputs are derived from diffuse sources all over the catchment instead of from a small number of point sources. Groundwater inflows to these streams are significant, indicating that the catchment aquifer system provides a vector for transport of nutrients from the land surface into the streams and ultimately to Lake Rotorua. Management of the lake water quality would therefore be assisted by improved understanding of the flow of groundwater through the catchment’s aquifer system. Several previous investigations have focussed on the groundwater system in the Lake Rotorua catchment. Of particular relevance to the present study, previous investigations by White et al. (2004, 2007), White and Rutherford (2009) and Rutherford et al. (2008, 2009) have developed conceptual and numerical groundwater flow and/or nutrient transport models for the Lake Rotorua catchment. This study builds on the numerical groundwater modelling work previously undertaken for the Lake Rotorua catchment. Specifically, this study develops a set of steady-state finite element groundwater flow models for baseflow conditions in the Lake Rotorua catchment. Twelve different models were compared following automated and manual calibration of different types and combinations of parameters including hydraulic conductivity of the major geological units in the catchment, the percentage of rainfall that contributes to groundwater recharge, and the transfer rates and reference elevations applied to Cauchy boundaries at stream, spring and lake nodes. Automated calibration was undertaken using PEST with and without pilot points. Relative to previously developed groundwater flow models for the Lake Rotorua catchment, the models developed in this study provide increased spatial resolution, more realistic representations of groundwater-stream and groundwater-lake interaction, and use hydraulic heads as well as stream flows for calibration targets. One of the model structures is considered to provide reasonable catchment-scale simulations of groundwater flow. This model invokes a zone of enhanced hydraulic conductivity aligned with the topographic margin of the north-western portion of the caldera surrounding Lake Rotorua. This model provided an acceptable match to observed heads and stream flows and also offered a satisfactory catchment water balance. The other eleven models developed in this study were not considered sufficiently accurate or realistic for catchment-scale simulations of groundwater flow. However, the approach of developing and comparing twelve different models did provide useful insight into the cat chment’s groundwater system by identifying model structural features and parameter sets that, once optimised during calibration, offered significant improvements in goodness of fit. (auth)