Water temperature has increasingly become a matter of concern for California rice (Oryza sativa L.) growers due to a need for public water agencies to improve habitat for fish. Prudent management of water resources to balance the needs of environmental and agricultural interests requires the quantification of water temperature effects on rice productivity. Our objective was to evaluate two alternative thermal unit models for the effect of low water temperature on yield. One model was based on the total number of hours below a given threshold water temperature Tb (abbreviated TNHB Tb) and the other was based on the concept of inverse degree days (i.e., degree days below a given threshold water temperature) (abbreviated IDD). We tested these models at a range of values of Tb between 10 and 25°C on data from two commercial fields during 2 yr. Results showed that the effect of low water temperature may be much greater than would be apparent from the visual appearance of the rice plants. Values of IDD and TNHB Tb were very highly correlated for 4 of the 4-yr field combinations. A logistic curve model based on TNHB 20°C provided the best fit to the aggregated data.