Environmental Flow Assessment using multiple criteria: A case study in the Kumbih river, West Sumatra (Indonesia)

Water withdrawal from rivers is a key resource for agriculture, industry, and power generation. However, in order to maintain riverine ecosystem, withdrawal should not exceed certain thresholds, and normally a requirement is made that a certain amount of water be left in the river, to be assessed via Environmental Flow Assessment (EFA) procedure. In spite of the crucial importance of this task, little or no international standard exists to assess minimum values of the EF. Rather, general guidelines are provided, with large subjectivity entailed. Here, focusing on the case study Kumbih river, in Sumatra, Indonesia, we apply a multiple criteria approach to EFA procedure, testing the values of EF assessed with hydrological methods in a previous study, with a focus on a relevant target species, i.e. the fish Tor Soro. Based upon a two-dimensional hydraulic model, we explore 6 indicators, covering some most important criteria to ensure the welfare of the fish fauna. Indicator I_I, weighed usable volume, slightly modifies the well-known method PHABSIM. Indicators I_Ii.IIIII target water depth needed to ensure welfare of target species, whereas criteria I_IV-VI target flow velocity, and sediment removal. Use of multiple indicators as shown here, albeit still subjective, may provide more consistent values of EF, and even an outlook of the disparity in terms of flow requirements given by the several criteria. Here we found that the maximum estimated EF under I_V (flow velocity), is over twice as large as the flow estimate from I_I. Overall however, initial EF estimates from the hydrological criterion seem adequate against the criteria proposed. Our study is among the few covering EF for Indonesia, and the only one regarding Kumbih river, and our results are of interest, also as a benchmark for other EFA studies in the region.