Drought assessment in the Pampanga River basin, the Philippines – Part 1: Characterizing a role of dams in historical droughts with standardized indices

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Droughts are frequent disasters in the Philippines with the most severe 1998 drought, which caused food shortages nationwide and major losses of rice production in the Pampanga River basin. For the Pampanga River basin, irrigated rice agriculture contributes about 30% of the rice production in the Philippines and relies on the complex water infrastructure system consisting of barrages, irrigation canals, Pantabangan and Angat multi-purpose dams, and trans-basin tunnels to increase inflows into these two dams. Both Pantabangan and Angat dams supply irrigation water for both wet and dry season rice production, which did not exist prior to the construction of these two dams, while the Angat dam supplies water to the Metro Manila City with population of about 12 million people. We conducted drought assessment with standardized indices in the Pampanga River basin by characterizing historical droughts (Part 1), introducing a comparative approach of standardized indices for climate change quantification (Part 2), and evaluating climate change impacts (Part 3). In Part 1, we computed standardized indices with collected field data (i.e., precipitation, reservoir inflows and water volumes, and dam discharges) to identify natural and socio-economic droughts at the Pantabangan and Angat dams. We utilized the standardized precipitation index (SPI) to characterize meteorological droughts and developed a standardized inflow index (SII) from reservoir inflows for hydrological drought assessment. To characterize socio- economic droughts, we developed a standardized reservoir storage index (SRSI), which was computed with reservoir inflow and water volume data, and compared SRSI values with standardized discharge index (SDI) values, which were estimated from dam discharges that were released to meet irrigation and municipal water demands. From the results of our drought assessment, we identified several meteorological, hydrological and socio- economic droughts between 1980 and 2012 with standardized indices. The use of several standardized indices allows us to identify the most extreme conditions based on the combined meteorological, hydrological and socio-economic droughts. The newly developed SII and SRSI indices match historical natural and socio-economic droughts in the Pampanga river basin and reflect increased inflows in the Pantabangan dam after the construction of trans-basin tunnel. As a result, the full-set of standardized indices represents the existing dam infrastructure and operation and could also be utilized for drought forecasting in the Pampanga river basin.