Competition for water resources in agriculturally based economies: A case study of an assessment approach used in Bali

The island of Bali in Indonesia has a long tradition of irrigated agriculture. The irrigation network is complex, and there is a high degree of drainage reuse. Evaluating the water resource of such systems is difficult. Drain flows are seldom measured, and drainage may also appear as a gain in downstream canal flows. This paper describes evaluation of the water resource of the Tukad Ayung, which is the largest river in Bali. The Tukad Ayung supports over 14,500 ha of irrigated agriculture, and was thought to have potential as a source to meet the growing potable water demands of the city of Denpasar and neighbouring tourist resorts. Increased abstraction from the river for potable water supply could, however, have been to the detriment of existing irrigated agriculture. An assessment of the available resource was required as well as an evaluation of current and likely future irrigation demands in the basin. The approach adopted incorporated a combination of hydrological models, an irrigation system simulation model and agricultural and economic production models. This paper demonstrates how relative impact assessments can be made when addressing a practical problem with limited data availability.