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.