On the Value of ENSO State for Urban Water Supply System Operators: Opportunities, Trade‐Offs, and Challenges

Abstract The El Niño Southern Oscillation (ENSO) is a major driver of global hydro‐climatic variability, with well‐known effects on floods, droughts, and coupled human‐natural systems. Its impact on urban settlements depends on both level of exposure and preparedness; two factors that are responsible for severe cuts on millions of people in developing countries, where urban water supply relies almost entirely on rainfall‐dependent sources. To understand whether information on the ENSO state could help mitigate the effects of droughts, we use Metro Manila's water supply system as exemplifying case study, for which we design “traditional” and adaptive management policies. The former are based on information typically available to operators, such as reservoir storage; the latter complement such information with the Oceanic Niño Index (ONI)—an indicator used for monitoring El Niño and La Niña state. Results obtained by comparing the policy performance on a large set of stochastic streamflow and ONI replicates show that ENSO‐informed policies are more robust, meaning that they attain a minimum performance level across a broader set of replicates. We show that the primary cause of this behavior is the information on the ENSO state. To further quantify the value of the ONI, we then compare the performance of a representative ENSO‐informed policy and the system's current operating rules on the period 1968–2014. The comparison shows that the severe water supply restrictions caused by the existing management system could have been partially avoided through a sequence of smaller restrictions implemented at the onset of the main El Niño events.