Rewetting approximation for a simulator of flow in a surficial aquifer overlain by seasonally inundated wetlands

An approximation is presented to rectify situations occurring as part of the three-dimensional simulation of surficial aquifer flows in which surface grid cells become dry (zero saturated thickness) and then cannot readily receive flows that would again partially saturate them. The approximation permits the cells to be rewetted by adjusting dry cell pressures by the same amount as the pressure change computed for the uppermost nondry cells in the vertical column. This is continued in successive timesteps until the dry cells once again become partially or fully saturated.

The rewetting procedure was tested in a recent transient simulation of flows in the surficial Biscayne aquifer of Dade County, Florida, which is partly covered by seasonally inundated wetlands. The uppermost layer of the model (layer 1) was used to represent overland sheetflow and grid cells of high equivalent hydraulic conductivity in this layer became dry and were rewetted seasonally, requiring a relatively robust representation of rewetting. The rewetting procedure made possible a simulation of overland sheetflow stages and aquifer water-table altitudes between 1945 and 1989 that was generally correct to within about 1 foot in inundated and noninundated regions.

In a 6-year time period (1962–67) in which the behavior of the rewetting procedure was studied in detail, there were 4,723 simulated cell rewettings, of which 3,172 were in the overland sheetflow layer and 1,551 were in the uppermost aquifer layer (layer 2). The average head adjustment in a cell rewetting was 0.10 foot in layer 1 and 0.15 foot in layer 2. This study demonstrates an empirical approach to the problem of cell rewetting that might prove useful in other models of flow in surficial aquifers.