Quantifying and modelling the dynamics of groundwater balance in unconfined aquifers under a changing climate

Abstract

The strategic importance of groundwater for water security is increasing under a changing climate, where the projected more frequent and intensive climate extremes could result in a higher vulnerability of surface water compared to groundwater. Nevertheless, groundwater systems are inevitably impacted by climate change directly through changes of replenishment or indirectly through the climate inducing changes in groundwater extraction. It has been reported that a large proportion of aquifers across the world have been experiencing significant groundwater depletion, which in turn poses greater threats not only to human society but also to natural ecosystem. For sustainable groundwater management, one of the essential but challenging hydrogeological missions is to investigate the groundwater dynamics under a changing climate. This research progressively has developed an integrative framework to investigate dynamics of groundwater balance components (including groundwater recharge, discharge and storage) in unconfined aquifers and their connections with climate determinants. The framework combined use of process-based modelling, statistical modelling, hydrograph-based methods and a tracer aided approach to facilitate investigating the dynamics of groundwater balance with consideration of data availability. The methods developed in this research expand the horizon of groundwater research and have been applied to catchments in the Murray-Darling Basin (MDB), the most important agricultural region in Australia that has been suffering from water overallocation for decades. The new findings of this research are expected to contribute to the development of adaption strategies in water resources management and are summarized in the following. (1) This research demonstrates that the combined use of hydrological modelling, hydrograph-based approaches and a tracer-aided approach is applicable to investigate the dynamics of groundwater balance elements in unconfined aquifers at a catchment scale. For the studied catchments in the MDB, catchments with higher precipitation have more recharge and discharge together with higher intra-annual variability. Groundwater storages collectively reach their highest level in winter or early spring, rapidly recede during summer and then continue to be low-level in autumn. (2) This research demonstrates that for the studied catchments, groundwater discharge has decreased significantly and consistently over the past decades and particularly so since the 1990s. The changes in groundwater are strongly associated with climate change and climate variation. Groundwater discharge and storage are found more sensitive to changes in winter precipitation. The decrease in autumn precipitation accounts for 45%-85% of the decrease in groundwater discharge. (3) This research demonstrates that groundwater drought is less resilient but more strongly resistant than precipitation drought. The difference in performance between groundwater drought and precipitation drought narrows down over longer timescales due to a closer relationship between groundwater and precipitation at a longer timescale. Relationships between groundwater drought performance and climate determinants are highly nonlinear and could be distinctly different at different timescales as found by the random forest modelling. (4) This research demonstrates that uncertainties exist in investigating groundwater dynamics owing to the limitation of observations, assumptions in the approaches and the complexity of the groundwater system. The recharge estimates inherit uncertainties from model conceptualization, parameterization and hydrological non-stationarity. The uncertainties from parameterization are comparable with that from model structure. Uncertainties are also discerned in quantifying climate contributions to groundwater reductions based on statistical models without considering hydrological non-stationarity. Show more