The stability of groundwater flow systems in unconfined sandy aquifers in the Netherlands

Groundwater flow is temporally variable and uncertain, due to climatologically or anthropogenically induced variation in boundary conditions that result in changes in the drainage network, and uncertainties in hydraulic model parameters used in the quantification of groundwater flow. The quantification and mapping of the variation and uncertainty in groundwater flow is especially essential in relatively flat areas where flow direction is sensitive to decimetre-scale head variations. In these areas, the variability and uncertainty of groundwater flow directions may therefore have important implications for the uncertainties in the spatial configuration of groundwater flow systems. In this study we aim to quantify and map the sensitivity of shallow groundwater flow systems to uncertainties in aquifer anisotropy and drainage resistance, and variations in drainage level and groundwater recharge for a sandy unconfined aquifer in the Salland region, the Netherlands. For this purpose, the most probable configuration of current groundwater flow systems was mapped using particle tracking and Monte Carlo analysis. Sensitivity was represented by the membership of each model cell to the defined groundwater flow systems given the uncertainties and variations in the hydraulic parameters and boundary conditions. In addition, the current configuration of groundwater flow systems was compared to the historical situation without artificial drainage. The average groundwater flow system size was found to be in the order of a few square kilometres, with a relatively stable configuration. In contrast to the intrinsic and temporally invariant hydraulic parameters, which were shown to have a minor influence on the spatial configuration of groundwater flow systems, natural variation in recharge and variations in drainage level management exert a large influence.