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.

Inferred inflow forecast horizons guiding reservoir release decisions across the United States

“…Forecast reliability, in turn, depends on the available predictive information. An operator might rely on upstream water storage (e.g., soil moisture, snowpack, lake levels) (Shukla and Lettenmaier, 2011), hydrological regime state (Turner and Galelli, 2016), climate indices and teleconnections (Yang et al, 2017;Libisch-Lehner et al, 2019), weather forecasts (Georgakakos et al, 2005;Shukla et al, 2012;Nayak et al, 2018), current river flow rates (Hejazi et al, 2008), knowledge of planned water releases from upstream dams, and perhaps some or all of these in combination (Denaro et al, 2017). This enormous scope for variability in forecast quality and application across dams means there is no obvious way to identify the actual operationalized forecast, or indeed the model used to assimilate it into decision making, for a given system without insight into individual agencies' models and data preferences.…”

Coherent streamflow variability in Monsoon Asia over the past eight centuries---links to oceanic drivers

“…Therefore, water management in Monsoon Asia should take into account the ocean states. A case study of the Angat River, the Philippines, showed that reservoir operating policies informed by ENSO states are more robust than conventional policies (Libisch-Lehner et al, 2019). Operating policies may be improved further if, say, the PDV is also considered.…”

“…These river basins are home to 1.7 billion people (Best, 2019). With high population densities, even smaller basins support the livelihood of millions-e.g., Chao Phraya (Thailand): 25 million, Angat (the Philippines): 13 million, and Citarum (Indonesia): 10 million (Nguyen and Galelli, 2018;Libisch-Lehner et al, 2019;D'Arrigo et al, 2011). River discharge, or streamflow, provides water for domestic and industrial uses, irrigation, and hydropower.…”

Coherent streamflow variability in Monsoon Asia over the past eight centuries---links to oceanic drivers

“…Therefore, water management in Monsoon Asia should take into account the ocean states. A case study of the Angat River, the Philippines, showed that reservoir operating policies informed by ENSO states are more robust than conventional policies (Libisch- Lehner et al, 2019). Operating policies may be improved further if, say, the PDV is also considered.…”