Variability of reference evapotranspiration and water demands. Association to ENSO in the Maipo river basin, Chile

The growing competition for water across economic sectors, and the natural variability of the hydrological system are two of the major issues that a society has to face when designing efficient water management policies. A better knowledge of the relationships between the main components of the hydrological cycle and atmospheric–oceanic indices that can be monitored and forecasted represents useful information for management decisions. El Niño–Southern Oscillation (ENSO) is a commonly used index to study the main features of climate fluctuations. In Central Chile, rainfall variability has been associated to sea surface temperature anomalies (SSTA), concluding that relatively abundant precipitation is observed when warm SSTA are present (El Niño phase). Evapotranspiration, on the other hand, has received little attention by researchers, although there is evidence supporting SSTA affect meteorological variables that play an important role in determining water demands. The purpose of this work is to describe the variability of reference evapotranspiration (ETo), and irrigation needs at a basin level conditioned on the phases of El Niño. A weather generator conditioned on ENSO phases, and a soil water budget algorithm are used to estimate the main features of reference evapotranspiration, and to calculate regional water demands in the Maipo river basin (Central Chile). This study shows that there is an ENSO footprint on reference evapotranspiration as well as on total water demands. During austral winter and spring seasons, reference evapotranspiration can show differences up to 30% between La Niña and El Niño years. Total water demands in the basin are increased by 20% during La Niña years.