Groundwater modules are critically important to the simulation of low flows in land surface models (LSMs) and rainfall-runoff models. Here, we develop a Groundwater for Ungauged Basins (GrUB) module that uses only physically-based properties for which data are widely available, thus allowing its application without the need for calibration. GrUB is designed to be computationally simple and readily adaptable to a wide variety of LSMs and rainfall-runoff models. We assess the performance of GrUB in 84 US watersheds by incorporating it into HBV, a popular rainfall-runoff model. We compare predictions of low flows by the native (calibrated) HBV groundwater module with those by the (uncalibrated) GrUB module and find that GrUB generates error metrics that are equivalent to (or superior to) those generated by the native HBV groundwater module. To assess whether predictions by GrUB are robust to changes in the structure and parameterization of the overlying hydrologic model, we run tests for two artificial scenarios: Slow Recharge with rates of percolation below 0.1 mm/day, and Fast Recharge with rates of percolation of up to 1000 mm/day. GrUB proves to be robust to these extreme changes, with mean absolute error (MAE) of predictions of low flows only increasing by an average of up to 19%, while average MAE increases by up to 157% when the same tests are performed on HBV without the GrUB module. We suggest GrUB as a potential tool for improving predictions of low flows in LSMs as well as rainfall-runoff models where calibration data are unavailable.