Visualization of aquifer response to periodic forcing

Many studies of periodic forcing and response in aquifers have focused on describing the induced fluctuations in hydraulic head, without much consideration of the time-dependent flows. Visualization techniques presented in this paper can be applied to obtain a more physically intuitive impression of groundwater motion in aquifers that undergo periodic fluctuations of hydraulic head. The concepts of velocity ellipse and displacement ellipse are introduced as methods for visualizing oscillatory flows associated with individual forcing modes. Cyclical trajectories illustrate the potential complexity of flow paths that can arise due to superposition of modal responses. The full periodic motion that results due to superposition of the mean flow and modal flows is visualized using streaklines. An animated time series of streaklines provides an intuitive impression of the flow and affords insight into apparent dispersion phenomenon that can arise due to periodic fluctuations in both the strength and direction of groundwater flow. Electronic animations are available from the authors.