1. Challenge of runoff regulation to supply paddy terraces in Subak irrigation schemes

2. Introduction Paddy terraces are typical landscapes of Subak irrigation schemes in Bali Indonesia Bali has 5630 km 2 area, and 57 % have topography slope between 15 % - > 40 % Bali has 2 active volcanoes and 22 no active mountains

3. A Subak irrigation scheme, primarily in Bali Indonesia is an irrigation scheme of which the construction, operation and maintenance is based on agreed principles of technology, management of agriculture and religious community Subak irrigation schemes are based on the continuous flow system (tektek/kecoran).

4. Aya Weir Penebel Weir Caguh Weir Meliling Weir Telaga Tunjung Dam Gadungan Weir Sungsang Weir Rejasa Weir Gembrong Spring Yeh Ho River Basin Cathment area : 123.75 km 2 Main river : 35.5 km Irrigated fields : 5,268 Ha Hydraulic structures: In the upstream (ngulu), Broncaptering of Gembrong Spring is utilized for domestic needs by PDAM since 1987. Other structures are Aya Weir, Empelan Benana, Penebel Weir. In the midstream (mangoin), Telaga Tunjung Reservoir has operated since 2006. Other structures are Caguh Weir, Meliling Weir. In the downstream (ngesep), Gadungan Weir has operated since 1938. Other structure is Sungsang Weir.

5. Subak Agung Yeh Ho Base on in river basin system, Subak Agung Yeh Ho is formed as an organization water user of farmer fields. In the empelan/weir system, the name of Subak is appropriate with the empelan/weir name. Irrigation scheme of Subak Yeh Ho has three genera, which are called ngulu (upstream), maongin (midstream) and ngesep (downstream) These genera represent the cropping patterns of paddy fields in the river basin

6. The cropping patterns In Yeh Ho irrigation schemes, the cropping patterns which fall on specific months have been determined and agreed by Subak For many centuries, this cropping patterns have been used and believed as a fair system of water distribution

7. The main objective of this study is to analyze runoff regulation. This analysis can support the system arrangement of water supply to paddy terraces in Subak irrigation schemes in order to sustain agricultural productivity at upstream, midstream and downstream level. Reasons : Under conditions of water shortage the operation is being challenged. Hence, regulation of the supply needs to be evaluated. The Main Objective of Study

8. Empirical flow frequency analysis A popular method of studying the variability of stream flow is through flow duration curves which can be regarded as standard reporting output from hydrological data processing (DHV Consultants BV & Delft Hydraulic, 1999). Methods and Materials The Weibull formula A compromise with more statistical justification. If the n values are distributed uniformly between 0 and 100 percent probability, then there must be n + 1 intervals, n – 1 between the data points and 2 at the ends (Chow, et.al, 1988):

9. The analysis procedure is as follows: The frequency or number of occurrence m in n year of 365 daily mean data discharge is selected from maximum to minimum. The overflow, intake and daily mean discharge data have the same procedure. The 80 % of failure probability of data is analyzed to determine the availability of minimum discharge in water system. The percentage of failure probability as much as 50 % can be considered as mean discharges availability. The overflow, intake and daily mean discharge are plotted against the 365 days of time. The water balance of each weir is represented by a graph. Methods and Materials

10. The advantages of Weibull analysis are in the ability to provide reasonably accurate failure analysis and failure forecasts with extremely small samples, as well as provide a simple and useful graphical plot, which is important to engineers and managers (Robert B. Abernethy, 2002). Methods and Materials

11. Subak is generally associated with the rice product in Bali, especially in Tabanan Regency, where is Subak Agung Yeh Ho organised. Due to the potential of good soil fertility, Tabanan Regency successfully produced paddy harvest in 22,455 hectares of rice fields in 2010. The paddy field product is approximately 5 ton/ha of unhusked rice or 2.85 ton/ha of rice. Although the changing function of land use from paddy fields to housing, farm land and dry field has increased to 0.3 % within the last two years, the paddy field product is relatively stable every year (Statistical Central Agency, 2010). Results & Discussion

12. Results & Discussion Water Balance

15. In above analysis, there is the seven weirs, however, only have complete data collected from the year 2002 – 2010.

16. The same trends of all weirs in the upstream, midstream and downstream prove that most Q runoff get into the intake, so there are no off-takes controlled during the period of June until October in the upstream, May until November in the midstream, and May to December in the downstream. This is indeed based on the agreement among Subak farmers under the supervision of an irrigation observer who represents the government. In wet season, the same trends show that the return flows increase sharply to the downstream. Results & Discussion

17. The sustainability of Subak schemes in managing the water distribution in whole system depends on the availability of the runoff in the river. However, it should be noticed that the availability should logically be in order from the upstream, midstream to downstream. This system should be ensured to maintain in dry season, although in wet season, the return flows increase sharply to the downstream Conclusion

18. As a result, the source of water in the upstream is extremely important to sustain the river basin system. This has become the main reason why the 35 % of Gembrong Spring in the upstream is claimed by Subak. In contrast, most of Gembrong Spring has been managed by PDAM Tabanan Regency to supply domestic needs. It has become a big challenge for Subak to supply irrigation water for their paddy terraces. Besides, there is no precise measurement in relation to the water intake by each weir, which is associated with the number of paddy fields. This will be an interesting topic for further research. Conclusion