Since the Agno River basin is characterized by mountainous topography, the Ambuklao water reservoir was built to keep it from gushing down and causing devastating effects on the lower levels of the river, the flood runoff estimated annually at about 6,654 million cubic meters reaches the plain in several hours in the river mouth in about a day. Average annual rainfall varies from 2,000 millimetres (79 in) in the neighborhood of Tarlac to 4,000 millimetres (160 in) in the upper reaches of the Agno River.