The Global Gradient-Based Groundwater Model (G³M) simulates three-dimensional groundwater flow and groundwater-surface water interactions on a global scale. It operates at a spatial resolution of 5 arc-min (approximately 9×9 km at the equator) and covers all land areas except Greenland and Antarctica. The model is conceptually based on MODFLOW and implemented with the modeling framework G³M-f to enable coupling to hydrological or land surface models. G³M solves the groundwater flow equation using finite differences, accounting for both lateral and vertical flow between cells, as well as exchanges with streams, lakes, wetlands, reservoirs, and oceans. Exchange fluxes between groundwater and surface water bodies are calculated from the hydraulic head gradient between the groundwater cell and the respective surface water body and the vertical and lateral hydraulic conductivity of the groundwater cell. The subsurface is represented by two groundwater layers, each 100 meters thick.
