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A Subak irrigation scheme, primarily in Bali, Indonesia concerns an irrigation system of which the construction, operation and maintenance are based on agreed principles of technology, management of agriculture and a religious community. Subak irrigation schemes are an example of water resources management, distribution and supply of irrigation water in a perfect vision on social welfare in the river basin. The decisionmaking process in a Subak irrigation scheme takes into account political, economic, social and cultural (religious) elements. Multifunctional ecosystems in a sustainable way of agriculture are implemented in the Subak irrigation schemes, particularly in the technology of such systems. On June 29, 2012 the UNESCO World Heritage Committee formally added the Subak systems as a manifestation of the Tri Hita Karana philosophy to the World Heritage List. Subak systems have been well known since the 9th Century. These systems are managed by a Subak Association based on the Tri Hita Karana philosophy - harmony between human beings and God, harmony between people and nature, and harmony between people and people. This philosophy underlies every activity of the Subak farmers. For managing the Subak irrigation schemes, Subak Associations and farmers pursue the Subak regulation called Awig-awig Subak as the togetherness consensus that in the past was announced by the King, nowadays by the Head of the Regency. Paddy terraces in Bali create important cultural landscapes. Traditionally, the flow within a river basin has been managed using the Subak irrigation system technology. Unfortunately, this traditional technology is facing challenges: problems of water shortage and competition with other water users, that make it complicated to sustain the agriculture production of the Subak irrigation schemes. In the middle of the twentieth century, as consequence of ongoing population growth and land conversion, Balinese farmers were having difficulties in meeting the ever-growing demand for rice. In addition, the problem of insufficient water in the dry season developed. In order to increase the irrigation water supply in Yeh Ho River Basin the Government constructed the Telaga Tunjung Dam, based on the cultivated hectares of paddy fields (sawah). However, since the dam was build the downstream river flow was reduced quite significantly and the process of water sharing among the Subak irrigation systems was changed. Due to this, there are serious conflicts on the water sharing among the farmers within Yeh Ho River Basin. Therefore, the main objective of this study was to develop an optimal reservoir operation strategy in relation to the operation of the Subak irrigation systems, capable to support agricultural productivity at upstream, midstream and downstream level. Detailed objectives of study were to: • identify the type of river basin and to evaluate the land use; • identify the contribution of the existing hydraulic structures in the main river system in supplying water to the Subak irrigation schemes; • identify and analyze the reliability of discharge in the main river system and the inflow to the reservoir; • analyze and determine the optimal outflow from the reservoir based on the needs of the Subak irrigation schemes within the river basin; • simulation of the reservoir operation and operation of the related Subak irrigation systems based on the needs of the Subak irrigation schemes at upstream, midstream and downstream level to achieve optimal productivity of agriculture in a sequence time of operation related to the cropping patterns of the Subak irrigation schemes; • to formulate recommendations on the future operation of the reservoir and related Subak irrigation systems. Initially the study presents a literature review to portray centuries of experience with ancient Subak irrigation system management, in which Participatory Irrigation Management (PIM) is represented by three linked elements: PIM in irrigation system operation and maintenance; PIM with respect to socio-culture and economics of agriculture; PIM in light of a religious community. While there have been dynamic changes in human civilization, there are concerns how the values of the Tri Hita Karana philosophy can be applied consistently. The lessons learned are based on natural resources of topography, especially in the paddy terraces landscape, water resources and soils based on the principles using the Tri Hita Karana philosophy that influences all activities of participatory irrigation system management related to those three linked elements in Bali. As associations for irrigation system management, Subak Associations have been already naturally adapted to Participatory Irrigation Management (PIM). The technological and socio-agricultural elements, the application of local cropping patterns and indigenous water management are regarded as the building blocks, which have been further developed in this study as scenarios of Telaga Tunjung Reservoir operation and supply to the related Subak irrigation systems. Using the Weibull formula, the historic supply data of several diversion weirs were analyzed. This analysis was able to determine the water balance of the Subak irrigation schemes behind each diversion weir. In line with this, a system approach has been applied based on the managed flows within the river basin and the characteristics of the Subak irrigation schemes. Irrigation and drainage (irrigation-drainage) of Subak irrigation schemes concern an operational approach, which has been adapted naturally to the topography, soil and water resources of Bali. Irrigation and drainage of terrace systems became an essential element in water distribution. Subak farmers have been able to operate and manage irrigation and drainage from one scheme to another for more than a thousand years. They did also know when irrigation water reached from the highest level to the lowest level of the paddy terraces within the schemes in a river basin. In order to sustain the agriculture production of Subak irrigation schemes there was a need for study based on farmer's perspective related to irrigation and drainage from one scheme to another. This research took place during the cultivation periods by observing the water levels at the inlet and outlet of a paddy terraces block, followed by an analysis of the trend and the amount of water being drained. Latosol soil is the soil layer of the study region. It is one of two types of volcanic soil cover, which is the oldest of soils in Bali. The soil characteristics of Latosol can be categorized by silty clay with medium until high plasticity and brown to yellowish colours. The average value of bulk density is 0.91gr/m3 , the particle density 2.58 gr/m3 , the porosity 0.65, the average hydraulic conductivity 9.7 x 10-4 cm/hour or 2.7 x 10-9 m/s. The average root zone permeability of the paddy terraces is 75 cm/hour. The results prove that the Subak farmers irrigate for saturation of the root zone before land preparation, which requires a large amount of water. In order to achieve an optimal scenario a technical system analysis was needed. Two common techniques have been applied, namely simulation and optimisation. The RIBASIM (RIver BAsin SIMulation) model was applied to identify the best distribution of water resources in the Yeh Ho River Basin. The focus of this study was to obtain the highest productivity related to the operation of the Telaga Tunjung Reservoir and the Subak irrigation systems with several weirs. The approach by representing the Yeh Ho River Basin with the RIBASIM model offered the possibility to find optimum results for the agriculture production. However, there was reduced availability of water for irrigation from upstream sources and from the river. The river basin simulation uses the 80% dependable discharge, and the simulation and optimisation of the Subak schemes in Yeh Ho River Basin. While the period of land preparation is critical with respect to the water needs the scenario analysis and optimisation have been based on shifting of the starting time of land preparation (nyorog). The results show that especially in the upstream and midstream of Yeh Ho River the allocation of irrigation water to the Subak irrigation schemes experienced a deficit since the water of Gembrong Spring was primarily withdrawn for domestic use, and the distribution of water from the spring was not based on applying the agreement on water sharing in practice. Therefore the first step in improved water supply would have to be to really apply the agreement about it in practice. When this will be done effective operation of the Telaga Tunjung Reservoir does not necessarily completely solve the problem of water shortage in the river basin. However, when a cropping pattern based on the fifth scenario will be applied the water deficit and yield reduction will be quite limited. The results further show that the cropping pattern of the fifth scenario would result in an optimal overall agriculture production at 100% and feasibility of farming of 2.3 of the potential field level production for the Subak schemes. The recoverable flow considered in the RIBASIM model plays an important role in the simulation and optimisation of Yeh Ho River Basin. Two values have been applied, being 62% for the average and 21% for the minimum recoverable flow. This is supported by the Tri Hita Karana philosophy on harmony among people that overall could be applied in the schemes in accordance with the cropping pattern of the fifth scenario, so that agricultural productivity will be optimal. Nevertheless, under normal hydro-climatological conditions it is also possible to apply the other scenarios, especially the first scenario. This reflects the applicability of the Tri Hita Karana philosophy of harmony among people and harmony of people and nature. At last, it is important how Subak farmers are able to maintain harmony within the irrigation systems. This research started quantitatively at the river basin scale and at paddy terraces block scale by composing the Yeh Ho River Basin with the Subak irrigation schemes in it. The fieldwork was done by orientation on the water sources and hydraulic structures of and within Yeh Ho River Basin. Then a closed paddy terraces block with inlets and outlets that relatively easy could be studied was analysed. These studies were based on the third element of the Tri Hita Karana philosophy that is the material subsystem in relation to the natural elements. This aspect implies that every paddy block of one farmer has one inlet and one outlet and that the boundaries of the Subak schemes are naturally clear. These observations were conducted during two wet and dry seasons (April 2013 - April 2015). In addition data on agriculture production within Subak Caguh upstream of Telaga Tunjung Reservoir and Subak Meliling downstream of the reservoir were collected, and soil samples were taken to determine the type and the characteristics of the soils. Based on the river basin and Subak irrigation schemes model, a scenario analysis based on the Subak irrigation systems and the Tri Hita Karana philosophy and Subak regulation was done. The results can be applied as a recommendation for the farmers. Although the Subak farmers are reluctant to change to new irrigation practices, such practices can in principle be useful for them as shown in two recent studies. One of these studies concerned the application of the System of Rice Intensification (SRI) combined with intermittent irrigation (ngenyatin). In the farmer's perspective the SRI method is not easy applicable in practice. Nevertheless, the Subak farmers agreed that 15 days before harvesting they will dry their fields, which is one element of the SRI method. The reservoir operation rule as applied in the fifth scenario gives the best result. The reservoir has especially an important role to increase water supply for the downstream schemes: Meliling, Gadungan and Sungsang. It also has a direct impact to sustain water conservation at the river basin scale. |
