Hydrologic factors are examined concerning their role in the determination of watershed yields. A primary research effort was directed to the measurement of evaporation at Quabbin Reservoir in central Massachusetts. Conventional energy budget computations were used to evaluate the evaporative mass-transfer coefficient. Studies were also conducted on methods for determining the reservoir storage necessary to sustain safe yields under specified variable demands. The relatively new concept of the hydrologic response function was examined for its applicability to Massachusetts watersheds. This hydrologic response function should be capable of providing a ready source of information on generalized precipitation-runoff relationships to be of assistance in delineating areas of high and low flooding potential.