A simple model was developed to estimate area-averaged evapotranspiration (ET) at the watershed scale, using widely available records of streamflow, precipitation, and basic meteorological data. The pivotal assumption of the model is that area-averaged basin-wide water storage (V) can be used to determine basin-wide ET efficiency. With this assumption, ET was modeled as a function of watershed storage, Priestley-Taylor potential evapotranspiration, and one free parameter (α) controlling the relation between basin-wide ET efficiency and storage. Watershed storage was found by integrating the water balance equation forward in time with observed precipitation and streamflow. By exploiting a hypothesized positive correlation between storage and streamflow, a method was developed to estimate the parameter α without calibration to measured ET, thus allowing the model to be applied in any watershed with measured precipitation, streamflow, and meteorological variables. The model was tested at sites within the AmeriFlux network in a variety of climates and ecosystems, using downstream U.S. Geological Survey streamflow to define the watershed boundary and measured AmeriFlux evapotranspiration to judge model performance. At most sites, the dynamics of modeled ET closely matched those of measured ET: daily root-mean-square errors averaged 0.067 cm d−1. In general, water storage and ET efficiency, especially during dry downs, were captured by the model, and the free parameter determined from the storage-streamflow correlation criteria was close to the optimal fit found through direct calibration. The performance of both the model and the indirect calibration strategy was best in arid to semiarid sites.