Bali, a small and active volcanic island in the humid tropics, is close to the east of Java, Republic of Indonesia, with an area of about 5, 600km² and a total population of about 2.8 millions. Subak system, a traditional irrigation system of Bali island, is considered to be an example of skillful and developed irrigation system at the foot of volcanic mountains. A large number of studies about the subak system have already been made, but there is no attempt to make clear the relationship between actual condition of subak system and its physical conditions.
In the present paper, a comparative description of the subak system in several rivers is conducted, based on field observation with special reference to physical condition.
The subak system taking river water via long channel including tunnel has been developed under the following hydrological condition : 1) high precipitation without severe drought, 2) geology composed of tephra deposits with high permeability, 3) stable base flow from groundwater, and 4) soft rocks composed of pyroclastic flow deposits for excavating tunnel.
There exists a wide regional difference in water intake and distribution system of the subak, and it would be strongly affected by the different physical conditions. Major findings of the present investigation of subak irrigation systems are summarized as follows :
1) Improvement works of subak system have been actively done under government assistance. A large number of small-sized traditional dam (empelan) made of local materials such as stones and coconut trees, have been changed to large-sized permanent dam (bendung) made of concrete.
2) The permanent dam improved by Public Works of Bali Province, has an average area of about 400ha and primary channel length of 3 km including water tunnel (aungan) of 1 km. The length of tunnel accounts for approximately 30% of primary channel in average.
3) In the south-central region of Bali, there exists the long tunnel with average of 2 km in Gianyar, constituting about 60% of primary channel. In Tabanan it constitutes only 10% of primary channel.
4) In the Pati river basin at the foot of Mt. Agung, which consists of lava and debris flow slope with shallow valley, there exist many small-sized subak systems having no long tunnel.
5) In the Batur pyroclastic flow region, many parallel rivers run in the deepest and narrow valleys and the large sized sawahs are widely distributed on the slope area. Long water tunnels have been constructed as a part of primary channel, with the maximum length of about 5 km in the Bendung Bedulu area.
6) In the Buyan-Bratan pyroclastic flow region, parallel river system is formed in valley. Most of dams have very long primary channels including short tunnel.
7) In the Mt. Batukau volcanic region consisted of lava flow hills and shallow valleys, rice terraces are widely distributed, and subak system has usually long primary channel with out water tunnel.

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