SEDIMENT POLLUTION IN A GRAVITY IRRIGATION SYSTEM AND ITS EFFECTS ON RICE PRODUCTION

The Agno River Irrigation System (ARIS), which was constructed in 1958 to irrigate 18 500 ha of rice, suffers from a continuous process of h igh sediment deposition rates. The sediment is mostly composed of mine tailings from gold and copper mining activities upstream of the ARIS dam that diverts the river flow for irrigating ricelands downstream. I t is estimated that about 269 200 t of sedimentary materials enter ARI S irrigation canal network annually, causing severe canal aggradation. Consequently, the water conveyance network of the system has been red uced by 24% and irrigation to rice farmers was discontinued in 40% of the service area between 1974 and 1985. On average, the area benefited by irrigation declined by about 590 ha year-1, which caused rice prod uction to decline by more than 1650 t year-1 during 1978-1989.