The efficient operation of water resources is one of the major challenges in modern society. One of the main characteristics of the hydrological cycle corresponds to its temporal variability, expressed as fluctuations in the precipitation regime as well as modifications in the rates of evapotranspiration as a result of changes in atmospheric conditions. A better knowledge of the relationships between the main components of the hydrological cycle and atmospheric and/or oceanic indices that can be monitored and forecasted, represents useful information for management decisions. In Chile, rainfall variability has been studied and associated to the El Niño-Southern Oscillation concluding that abundant precipitation is observed while el Niño phase is present, and relatively low precipitation is expected in La Niña events. Evapotranspiration, on the other hand, has received little attention by researchers even though there is evidence supporting sea surface temperature anomalies (SSTA) are influencing meteorological variables that play an important role in determining rates of evapotranspiration. The purpose of this work is to describe the main relationships between reference evapotranspiration (ETo), estimated by means of the Penman-Monteith equation, and the phases of el Niño phenomenon. A weather generator conditioned on El Niño phases is used to generate synthetic series of daily meteorological variables and to get estimates of the main statistical characteristics of the variability of ETo and its association to SSTA. Such knowledge can be used in hydrological forecast and operation of water resources since ETo can be translated into agricultural water demands, and there are skillful models that forecast sea surface temperature conditions in the eastern Equatorial Pacific.