Northern Luzon PRSD
The Agno River has a drainage area of 5,809.3 sq km. at the Bañaga station and is the third largest river in Luzon Island next to the Cagayan and Pampanga rivers. A half of its drainage area particularly the watershed in Benguet Province is mountainous having an average elevation of about 2,000 m above MSL. The main branch is the Tarlac River with drainage areas of about 1,900 sq km including Poponto swamp and which joins the Agno River in midstream from the left at Bayambang Pangasinan. The Tarlac River has its source in Mt. Pinatubo in Tarlac province. The Agno river has a course of about 200 km, 90 km of which runs in mountainous areas. The Agno River, after passing through the mountainous areas, forms a vast alluvial fan and a delta, and then flows into Lingayen Gulf. This fan and delta comprises the Pangasinan Plain. Poponto swamp is located in the vicinity of Bayambang, Pangasinan where Tarlac River joins the Agno River, the swamp temporarily retains flood water from the Tarlac River. Dagupan and Sta. Barbara form the center of the Pangasinan Plain, which was formed as a result of floods from the Agno River. Changes on river course as well as river improvements led to the separation of the Agno River from the plain.
The climate in Agno River Basin is characterized by two pronounced seasons, dry from November to April and wet for the rest of the year. In drainage area the dry season is from November to April and the wet season from May to October Annual precipitation varies from 2,000mm in the neighborhood of Tarlac to 4,000mm in th upper stream of the Agno river, such great variation is caused by the influence of mountains on precipitation produced by monsoons.