After-ripening, the loss of dormancy under dry conditions, is associated with a decrease in mean base water potential for germination of Bromus tectorum L. seeds. After-ripening rate is a linear function of temperature above a base temperature, so that dormancy loss can be quantified using a thermal after-ripening time (TAR) model. To incorporate storage water potential into TAR, we created a hydrothermal after-ripening time (HTAR) model. Seeds from two B. tectorum populations were stored under controlled temperatures (20 or 30 °C) and water potentials (−400 to −40 MPa). Subsamples were periodically removed from each storage treatment and incubated at 15 or 25 °C to determine germination time courses. Dormancy status (mean base water potential) was calculated from each time course using hydrothermal time equations developed for each seed collection. Seeds stored at −400 MPa did not after-ripen. At water potentials from −400 to −150 MPa, the rate of after-ripening increased approximately linearly with increasing water potential. Between −150 and −80 MPa, there was no further increase in after-ripening rate, while at −40 MPa seeds did not after-ripen and showed loss of vigour. These results suggest that the concept of critical water potential thresholds, previously shown to be associated with metabolic activity and desiccation damage in partially hydrated seeds, is also relevant to the process of after-ripening. The HTAR model generally improved field predictions of dormancy loss when the soil was very dry. Reduced after-ripening rate under such conditions provides an ecologically relevant explanation of how seeds prolong dormancy at high summer soil temperatures.