AQUIFERWATCH: Operational prediction of groundwater heads and storage during river flow recession in the Wairau Aquifer, New Zealand

Abstract

<p>The Upper Wairau Plain Aquifer serves as the major resource for drinking water and irrigation in the region of Blenheim in Marlborough, New Zealand. Natural recession of groundwater levels and storage occurs annually during the summer months in the upper part of the highly conductive gravel aquifer. Due to a number of particularly dry summers, aquifer storage has reached critical levels several times in the past which could lead to future restrictions on groundwater abstraction to be imposed by the Marlborough District Council (MDC) who manages the resource. The MDC requires at any given time an early warning whether or not these critical levels are likely to be reached. Correspondingly, an operational framework was developed to forecast Wairau Plains Aquifer groundwater levels and storage. The tool creates more lead time for the MDC in decision making and adaptive management of the Wairau Plain groundwater resources.</p><p>A numerical groundwater flow model of the Wairau Plain was previously set up to understand the main drivers of aquifer storage (Wöhling et al. 2018). Since that model posed practical restrictions to be of use for operational management purposes, we tested several data-driven surrogate models with easily attainable inputs that could be derived directly by an automated database query of the MDC monitoring network. Here, a tailor-made version of the Eigenmodel approach (Sahuquillo, 1983) is used to predict Wairau Aquifer groundwater heads and coupled with Markov chain Monte Carlo (MCMC) sampling for model calibration and parameter uncertainty analysis. Several Eigenmodels are embedded in a modular prediction framework that allows for a flexible description of critical model inputs depending on different states of knowledge and on different purposes of the analysis. A Wairau River flow master-recession curve has been derived from historic (observed) time series data to provide the boundary condition for the major recharge source of the aquifer.</p><p>The Eigenmodels perform very well in hind-casting the recessions of historic groundwater levels at selected locations of the Wairau Plain Aquifer. Periods with critical groundwater levels were successfully detected and accurately reproduced. The models are efficient and fast, which is a prerequisite for the operational management support tool. The way how aquifer recharge is described as a function of river discharge proved to be very sensitive to the accuracy of the results. Future plans are investigations on an improved knowledge of this relationship, but also to implement the propagation of input uncertainty through the framework, in addition to the treatment of parametric and predictive uncertainty which is implemented already.</p><p> </p><p><strong>References</strong></p><p>Sahuquillo, A. (1983). An eigenvalue numerical technique for solving unsteady linear groundwater models continuously in time. Water Resources Research 19(1): 87-93.<br>Wöhling T., Gosses, M. Wilson, S., Davidson, P. (2018). Quantifying river-groundwater interactions of New Zealand's gravel-bed rivers: The Wairau Plain. Groundwater, 56(4), 647-666.</p>