Silenced Rivers: Modelling and assessing the impacts of large-scale hydropower projects on the ecohydrology of the Myitnge and Myittha rivers in Myanmar

Hydrological modifications to the natural flow regime through the regulation of a river threaten the integrity of river ecosystems. In Myanmar, the exponentially growing hydropower sector poses a threat to the ecohydrology of some of the last large free-flowing rivers in the world. This study investigates to what extent the natural flow regime of the Myitnge in Myanmar has been hydrologically altered by the Yeywa dam, which is currently the largest hydropower dam in Myanmar. The study furthermore examines which ecological processes have been or could potentially be most affected by these hydrological modifications. This is done by modelling the natural flow regime of the dammed Myitnge river in Myanmar using the distributed hydrological rainfall-runoff Wflow\_sbm model, which consists of a set of python programs to perform hydrological simulations. The suitability of this model as a tool for environmental flow management in Myanmar is simultaneously investigated. The model uses PCRaster, which in turn makes use of a dynamic modelling language within a GIS framework. It was forced using static and dynamic data which is mostly globally available from different satellites, and derivatives of this, and calibrated and validated on data collected during field visits in 2018 as well as some secondary data sources. The field data collection focused on river bathymetry and soil properties such as infiltration capacity. Furthermore, scenarios were developed and simulated that varied in dam operational capacity (29\% and 80\%), reservoir management (for flood mitigation), and irrigation demand. Using modelled discharge results, multiple environmental flow assessments were carried out, comparing pre- and post-dam scenarios. The results demonstrated that different elements of the flow regime can be altered slightly depending on the Yeywa operational scenario. For the irrigation scenario, the high water uptake to meet the crop demand alters the magnitudes and duration to the largest extent of all the scenarios. According to the results, the dam alone does not alter most of the components of the natural flow regime (and hence presumably the associated ecological factors) to a very large extent, regardless of the capacity it is operated at. This is because the attainable operational capacity is limited by inflow: raising the capacity at which the dam is operated can only be done for a limited amount of time due to the extreme seasonality. For the current operation and in the simulated scenarios, habitat availability for different species of plants and animals, as well as the river's ability to structure the channel morphology are the elements most at risk due to the Yeywa dam. This is mostly because the occurrence of large floods has significantly reduced or completely disappeared from the flow regime, as also demonstrated by a habitat inundation analysis for the Myitnge. There is potential for the optimisation of the operation of the Yeywa reservoir, but it remains limited to the availability of inflows, which in turn is dependent on the natural seasonality and the size of the reservoir. Therefore, one of the main recommendations is to avoid keeping the outflow below the natural inflow in the dry season, and to run the turbines at maximum capacity during the monsoon period. This is advised in order to protect the downstream channel area from dewatering in the dry season, and to maximize the electricity generation during the wet season, while simultaneously keeping the released discharge very close to the natural flow regime.Water Managemen