Current regional and global climate models generally do not represent groundwater flow between grid cells as a component of the water budget. We estimate the magnitude of between cell groundwater flow as a function of grid cell size by aggregating results from a numerical model of equilibrium groundwater flow run and validated globally. We find that over a broad range of cell sizes spanning that of state of the art regional and global climate models, mean between cell groundwater flow magnitudes scale with the reciprocal of grid cell length. We also derive this scaling a priori from a simple statistical model of a flow network. We offer operational definitions of significant groundwater flow contributions to the grid cell water budget in both relative and absolute terms (between cell flow magnitude exceeding 10% of local recharge or 10 mm y1, respectively). Groundwater flow is a significant part of the water budget, as measured by a combined test requiring both relative and absolute significance, over 42% of the land area at 0.1 grid cell size (typical of regional and mesoscale models), decreasing to 1.5% at 1 (typical of global models). Based on these findings, we suggest that between cell groundwater flow should be represented in regional and mesoscale climate models to ensure realistic water budgets, but will have small effects on water exchanges in current global models. As well, parameterization of subgrid moisture heterogeneity should include the effects of within cell groundwater flow.