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

Groundwater flow is temporally variable and uncertain, due toclimatologically or anthropogenically induced variation in boundaryconditions that result in changes in the drainage network, anduncertainties in hydraulic model parameters used in the quantificationof groundwater flow. The quantification and mapping of the variation anduncertainty in groundwater flow is especially essential in relativelyflat areas where flow direction is sensitive to decimetre-scale headvariations. In these areas, the variability and uncertainty ofgroundwater flow directions may therefore have important implicationsfor the uncertainties in the spatial configuration of groundwater flowsystems. In this study we aim to quantify and map the sensitivity ofshallow groundwater flow systems to uncertainties in aquifer anisotropyand drainage resistance, and variations in drainage level andgroundwater recharge for a sandy unconfined aquifer in the Sallandregion, the Netherlands. For this purpose, the most probableconfiguration of current groundwater flow systems was mapped usingparticle tracking and Monte Carlo analysis. Sensitivity was representedby the membership of each model cell to the defined groundwater flowsystems given the uncertainties and variations in the hydraulicparameters and boundary conditions. In addition, the currentconfiguration of groundwater flow systems was compared to the historicalsituation without artificial drainage. The average groundwater flowsystem size was found to be in the order of a few square kilometres,with a relatively stable configuration. In contrast to the intrinsic andtemporally invariant hydraulic parameters, which were shown to have aminor influence on the spatial configuration of groundwater flowsystems, natural variation in recharge and variations in drainage levelmanagement exert a large influence.