Simple Automatic Time‐Stepping for Improved Simulation of Groundwater Hydrographs

An automatic time-stepping algorithm is presented, based on the intensity of driving groundwater recharge, that improves the simulation of groundwater level fluctuations in regional models while maintaining model run-times. The algorithm is implemented in the ZOOMQ3D finite difference groundwater flow code and controls the discretization of time using two user-defined criteria: a maximum time-step length and a maximum recharge per time-step. Daily recharge is accumulated in time until either of these criteria is violated when the model then calculates a solution. The efficiency and accuracy of the algorithm is tested using an idealized groundwater model and an existing regional groundwater model of a UK aquifer. The approach is illustrated using simulations of high groundwater levels and associated groundwater flood events in a responsive, high-diffusivity aquifer. Simulations using the automatic time-stepping technique are presented that reduce the maximum absolute error in groundwater level by 45% and the run-time by 51% compared to models using conventional, a priori defined stress-periods and time-steps.