6. Hydrogeochemical Assessment of the Contribution of Caldera Lakes and Paddy Irrigation to River Water Stability

On-site water sampling surveys were carried out from 2012 to 2014 to sample rainwater, river water, and paddy water in the Saba River Basin in Bali Island and three caldera lakes adjacent to this basin, in order to characterize the different isotopic ratios in this range of water samples. The results showed that the water isotopic ratios of river water could be expressed by the linear combination of the isotopic ratios of the constituent waters such as rainwater and paddy water. Furthermore, the results suggested that there was likely a stable inflow from the caldera lakes, which were located outside of the basin area, to the Saba River via its tributaries (the Panas River and the Ling River). Based on the water isotopic relationship, the return flow from the paddy fields was inferred to contribute considerably to the river flow to the extent beyond its area ratio to the total basin area, especially in dry seasons. We concluded that the caldera lakes and the paddy irrigation practices in the Saba River Basin contributed to the stabilization of the river water flow and that this contribution was especially strong in the dry seasons. The water isotopic properties should be regarded as “scientific information for society,” which can provide strong support for the diagnosis of river basins and decision-making for water resources management in the future.