Water Resources Management in the Philippines during El Niño Episodes

Leonardo Q. Liongson

National Hydraulic Research CenterUniversity of the Philippines

Diliman, Quezon City, Philippines

FIJI-2003 International Conference on

Managing Water Resources under Climatic Extremesand Natural Disasters

The Outrigger Hotel, Sigatoka, Fiji27 October – 31 October 2003

Aims of the paper :

• To present the main impacts of recent El Niño episodes to the water resources situation in the Metro Manila and Laguna de Bay Basins in Luzon island, in terms of the rainfall magnitudes and distributions and the consequent streamflow values and lake and reservoir water levels attained in affected areas.

• To describe the effects on water quality indices such as lake salinity, transparency and turbidity.

• To present hydrological and water-quality data time-series and their statistical correlations.

• To analyze the management options which were exercised by the state under the El Niño conditions of water scarcity for allocation to various multipurpose uses.

Outline: 1. Introduction

2. Rainfall pattern during El Niño episodes3. Impact of El Niño on aquaculture and fisheries4. Impact of El Niño on water supply and irrigation5. Conclusion and recommendations

Map of the Philippines (upper left), Metro Manila (upper right),

& the relative locations (lower portion) of Pasig and Marikina Rivers and Laguna de Bay.

Basin maps: top - Angat River Basin (A=568 km2) ; bottom - Metro Manila and Laguna de Bay Basins, A = 4500 km2 = 900 km2 (lake) + 3600 km2 (land) with annual isohyetal lines, rainfall station locations and drainage network.

Monthly and annual time-series of rainfall data at Science Garden station, Quezon City, Metro Manila and at UPLB, Los Baños station.

Annual Rainfall

Science Garden Mean = 2418 mm (1961-2000) El Niño episodes: 1982-1983: 1970 mm, 1656 mm1987-1988: 1628 mm (1987)1997-1998: 2141 mm (1997)

UPLB Mean = 2010 mm (1977-2000)El Niño episodes: 1982-1983: 1373 mm, 1676 mm 1987-1988: 1280 mm (1987)1997-1998: 1355 mm (1997)

Rainfall pattern during El Niño episodes

Fish pens (top) &Fish cages (bottom)used for aquaculture in Laguna de Bay.

Small fisherman engagedin open lake fishing.

Impact of El Niño on aquaculture and fisheries

Rainfall (in drought conditions),

lake stage (severe drawdown),

&

salinity (maximized conditions)

during the El Niño months of 1997-1998.

Impact of El Niño on aquaculture and fisheries

This situation was most advantageousfor the traditional brackish-water aquaculture and fisheries, but very disadvantageous forpotential water-supply and irrigation uses.

Monthly measurements of salinity, transparency and turbidity at Laguna de Bay West-Bay-Istation during the years 1997-1999. (a). Time series plots and (b). Scatter plots and fitted regression lines of salinity versus transparency and turbidity.

Impact of El Niño on aquaculture and fisheries

Water-supply sources of Metro Manila from Angat Reservoir in the north

to Novaliches Reservoir and water treatment plants to the south.

AngatReservoir

NovalichesReservoir

Metro Manila

Impact of El Niño on water supply and irrigation

aa

Mean annual basin rainfall = 3155 mm. (for years 1981- 2000) reservoir inflow = 1724 MCM (for years 1981 -1999).

El Niño episodes:

1982-1983basin rainfall = 1430 mm, 1594 mm. reservoir inflow = 1587 MCM, 1279 MCM

1987-1988basin rainfall = 3311 mm, 2831 mm. reservoir inflow =1311 MCM,2025 MCM

1997-1998basin rainfall = 2384 mm,2509 mm. reservoir inflow =906 MCM,1800 MCM,

Impact of El Niño on water supply and irrigation

Scatter plots and regression lines for:

(a). Current reservoir inflow, Q(t), versus previous inflow, Q(t-1),

& (b). Residual error, e(t), versus basin rainfall, P(t).

Monthly plots of

Angat Reservoir monthly inflows,

releases for irrigation & water supply,

and ending water surface elevation, together with the lower rule curve,

for the years 1997-2003.

Summary of the history and pattern of decisions for monthly releases of Angat Reservoir for irrigation and water supply for the years 1997-2003, in terms of scatter plots and regression curves.

Conclusion:

(a). Inspite of the observed unpredictability and low correlation of rainfall in many parts of the Philippines with respect to sea surface temperatures (SST Index), whenever El Niño episodes did indeed happen, remarkable localized cases of prolonged drought conditions had registered in terms of the secondary effects of increased salinity and transparency (and decreased turbidity) in Laguna de Bay, and the reduced inflows and excessive drawdown of Angat Reservoir water level below the lower rule curve.

(b). The occurrence of El Niño episodes such as in 1997-1998 had tested the resolve and will of the state to enforce the provision of the Water Code, which places higher priority to releases for water supply, to the extent of reducing the release for irrigation down to zero for extended period of time. With this conscious policy, reductions in irrigation releases have been made for briefer periods during mild El Niño events in the years 2001-2003.

(c). The prolonged drought conditions and the highly localized rainfall variations during El Niño episodes pose serious challenges to the capability of local experts to model and forecast reliable rainfall and streamflow amounts, given already the spatial inadequacy in hydrometric networks, shortness of most hydrologic records, and the lack of understanding of the meteorological phenomena .

Recommendations:

(a). More and closer cooperation between meteorologists, hydrologists, and other natural scientists and engineers in order to exchange ideas, data and research results to better understand the El Niño and La Niña phenomena and be able to translate the acquired knowledge to better-informed decisions, strategies and programs of actions in water resources management.

(b). Encourage and support cooperative hydrological research and development, in workable local, regional or international groupings, to bridge the gaps in the theory and practice related to predicting rainfall during El Niño episodes and how this effort relate to modeling (use of global circulation models, rainfall forecasting, rainfall-runoff modeling, streamflow forecasting), as well as tackle the related practical and technical issues of hydrometry, data archiving, software tools, data sharing and research dissemination with publications, conferences and the Internet.

Thank you.