Water stored in lakes and underground is a crucial component of the global hydrological cycle, with impacts on climate and sea level. However, long-term changes in the global distribution of this water are not well understood. Here we present the Water Table Model (WTM), which is capable of computing water-table elevation at large spatial scales and over long temporal scales. The WTM comprises two components: groundwater and dynamic lakes. The inclusion of a dynamic lake component allows us to incorporate surface-water movement and evaporation into water-table elevation estimates. We share sample results from both an artificial topography, and for the North American continent. These results indicate the close interactions between changes to water levels in lakes and the surrounding groundwater tables. The open-source code for the WTM is available on Github and Zenodo.