MODFLOW-NWT groundwater model demonstrating groundwater model calibration with repeat microgravity measurements

A numerical model was developed using MODFLOW-NWT and FloPy to simulate groundwater flow and demonstrate a practical method for incorporating repeat microgravity observations (i.e., small changes in the acceleration due to Earth's gravity) as a new type of calibration target for groundwater-flow models. The single-layer, 250-m cell size model was kept relatively simple to focus on the value of repeat microgravity data in the East Mesa area of the Imperial Valley, in the vicinity of the All American Canal, southeast California, USA. The method is demonstrated with repeat microgravity data collected over a 10-year period following the lining of the All American Canal with concrete. This lining of the canal resulted in the removal of a large source of seepage (recharge) causing groundwater levels, groundwater storage, and gravity all to declined considerably. The study area and field data are used to demonstrate a novel forward gravity modeling application linked to MODFLOW input and output files. The analysis demonstrates that including gravity data, compared to drawdown data alone, improves model parameter identifiability and reduces uncertainty in parameter estimates. This USGS data release contains all of the input and output files for the simulations described in the associated journal article (https://doi.org/10.1111/gwat.13167).