







Monthly Climate Assessment and Outlook (July-August 2018)
Issued: 06 August 2018
Monthly Rainfall Forecast
RAINFALL FORECAST (September 2018 - February 2019)
UPDATED: 29 August 2018 (next update September 26, 2018)
The 4th largest basin in the Philippines and covers an approximate aggregate area of 10,540 sq. km. (includes the allied basin of Guagua River). The basin extends over the southern slopes of the Caraballo Mountains, the western slopes of the Sierra Madre range and the major portions of the Central Plain of Luzon. It encompasses the provinces of Nueva Ecija; part of Bulacan, Tarlac and Quezon; and almost whole of Pampanga. The total length of the main river, the Pampanga River, is about 260 kilometers.The basin is drained through the Pampanga River and via the Labangan Channel into the Manila Bay. The main river is supported by several tributaries, the principal ones of which are the Penaranda and the Coronel-Santor Rivers on the eastern side of the basin and the Rio Chico River from the northwest side. The Angat River joins the Pampanga River at Calumpit in Bulacan via the Bagbag River. The Labangan channel, on the other hand, acts as a cut-off channel for the Angat River into Manila Bay. Somewhere between the middle and lower portion of the basin stands Mount Arayat, about 1,026 meters in elevation. Adjacent to Mount Arayat, across Pampanga River, just on the eastern side is the Candaba swamp, covering an area of some 250 sq. km. absorbing most of the flood flows coming from the eastern sections of the basin (western slopes of a portion of the Sierra Madre mountain range) and the overflowing of the Pampanga River via the Cabiao Floodway. This area is submerged during the rainy season but is relatively dry during summer. At the lower sections of the basin, where the Pampanga delta lies, the Pampanga River system divide into relatively small branches, crisscrossed with fishponds to form a network of sluggish, tidal flats and canals, which eventually find their way to Manila Bay. The main river has a relatively low-gradient channel particularly at the middle and lower sections. With the anticipated completion of the Pampanga Delta Project (DPWH), it is expected that floodflows at the lower section of the Pampanga River will recede at a much faster rate than before.
The basin experiences, on an average, at least one flooding in a year. The dry season generally occurs from December to May, and wet the rest of the year. The wettest months are from July to September. The frequency of tropical cyclone passage over the basin is about 5 in 3 years. (Click for PRB website)
The Cagayan River is the largest river in the Philippines with a drainage area of 27,280 square kilometers at its mouth in Aparri.The basin encompasses parts of Cagayan, Isabela, Mountain Province, Nueva Vizcaya, Quirino and Quezon Provinces. The basin is roughly elliptical in shape with its major axis oriented in a north-south direction. The Cagayan Valley is relatively flat but mountains with elevations up to 2,000m surround the east, south and west of the drainage basin.
The Cagayan River, which is the main drainage channel of the basin, flows in a northerly direction from its head waters in Nueva Vizcaya to its mouth in the Babuyan Channel near Aparri. Its principal tributaries include the Siffu-Malling, Chico, Ilagan and Magat Rivers. The estimated annual discharge is 53,943 million cubic meters.The Magat River is the largest tributary with an estimated annual discharge of 9,808 million cubic meters. It lies in the southwestern portion of the basin, stretching approximately 150 kilometers from Nueva Vizcaya down to its confluence with Cagayan River about 55 kilometers from the river mouth. Both the Magat and the Chico Rivers have extensive drainage areas which comprise about 1/3 of the whole basin. The Ilagan River originates from the western slopes of the Sierra Madre and drains the eastern central portion of the Cagayan River basin with an estimated annual discharge of 9,455 million cubic meters. It flows westward and joins the Cagayan River at Ilagan, Isabela, 200 kilometers from the mouth. The Siffu-Malling River lies on the slope of the Central Cordillera ranges flowing almost parallel to the Magat River. Marshes and swamps are found in some parts of its lower reaches.The average annual rainfall in 1,000mm in the northern part and 3,000mm in the southern mountains. Floods caused by this river flow down very slowly because of surface retention over the extensive flood plain, extremely gentle slope, retardation of flood by several gorges and river meander.The target areas for the flood forecasting and warning system of the Cagayan River Basin are:a)The areas along the lower reaches, from Tuguegarao to Aparrib) The alluvial plain along the river course from Ilagan to Tumauini, Isabela.
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