Hydrologic and Isotopic Lake Modelling for Palaeoclimate Research

Abstract

Numerical lake hydrological and geochemical models are valuable tools for interpreting and quantifying palaeoclimate reconstructions derived from lake sediment archives, and the sensitivity of lakes under future climate scenarios. This study details the development and calibration of a holistic lake model, alongside related projects focussed on the development of tools and datasets associated with lake modelling. Lake monitoring was conducted over three years at twelve sites in the Newer Volcanic Province, south-eastern Australia on a bi-monthly basis, collecting water levels, and water samples for analysis of oxygen and hydrogen isotopes and major/minor ions. A correlation was identified between the lake morphology and the 18O and 2H isotopic enrichment of lake water, visible as variations in the regression slopes of !18O and !2H (local evaporation lines). Cl-/Br- and HCO3 - /Cl- ratios were used to partition the source of water for each lake between groundwater and direct precipitation. Lakes high in the landscape, above the regional aquifers, were found to have lower Cl-/Br- ratios, suggesting they are predominantly rainfall fed, whereas the other lakes have Cl-/Br- ratios similar to groundwater. HCO3 -/Cl- ratios were ambiguous, likely due to the variability of HCO3 -/Cl- in groundwater. Deuterium excess (d-excess) data were used to assess the degree to which lakes exhibited through-flow or terminal hydrology. The d-excess results showed strong seasonal variability as a function of lake depth, suggesting that a more conservative solute may be better suited to determine the groundwater regime for these lakes. One challenge encountered throughout this research was the need to monitor the isotopic composition of daily precipitation feeding into lake systems. To address this challenge in future research, an autonomous rainfall sampler was developed. A computational method using hydrologic-isotopic modelling to estimate the original isotopic composition and volume of evaporated samples was also described. An important yet poorly constrained component of lake hydrological models is the interaction between lakes and their surrounding groundwater. A single layer, finite difference groundwater model was developed to simulate and investigate this interaction. The model was designed to operate using spreadsheet software, and is able to model transient groundwater flows, confined and unconfined aquifers incorporating recharge, abstraction and injection, no-flow, fixed-head and head-dependent boundary conditions. The model was validated using nine groundwater scenarios and applied to demonstrate that the use of a specified saturated thickness for unconfined aquifers beneath lakes may give more realistic results. A holistic lake hydrological and geochemical model was developed, coupling mass and energy balances, hydrology, groundwater, catchment processes, geochemistry and water isotopes. The model was applied to Lake Bullen Merri and Lake Gnotuk, neighbouring maar crater lakes in Victoria, Australia. The model was able to simulate lake hydrology from 1889–2018, water temperatures and chemistry from ~1965–2018, and water isotopes from 2015–2018. The simulations suggest that both lakes experienced through-flow hydrology at high water levels, transitioning to terminal lakes at lower water levels. The potential for the isotopic composition of the lake water to become disconnected from the hydrological balance of the lake was also identified. The newly developed model offers significant potential to constrain past climates and to forecast the trajectory of lake hydrological and geochemical change under future climate scenarios.