Assessing hydrological regime alterations caused by climate change in reservoir-regulated river basins: a case study in the Mekong Basin

Understanding how hydrological regimes will evolve in response to climate change and anthropogenic activities is important to ensure ecological integrity, agricultural productivity, and energy security in large river basins. While significant research efforts have been spent on understanding the effect of climate change on hydrological processes, only recently researchers have started to account explicitly for the potential effect of hydraulic infrastructures. Here, we contribute to this growing body of literature by studying how the combined effect of climate change and cascade hydropower operations could alter the hydrological processes in the Mekong river basin. First, we calibrated and validated the large-scale distributed hydrological model VIC against multi-site gauged data. When developing the model, we considered two scenarios, namely with and without water reservoirs. Interestingly, we achieved similar performance for both modelling scenarios. Then, we run the VIC models using future rainfall and temperature data retrieved from five Global Circulation Models (GCMs) and two emission scenarios (RCPs 4.5 and 8.5). Results showed that the evaluation of the flow alterations due to climate change varied substantially with the modelling scenarios considered during the calibration/validation phase. In particular, we found that the VIC model calibrated without reservoirs tended to provide a more conservative estimate of the flood volume variability. This is because the fraction of soil moisture, hydraulic conductivity, and infiltration capacity parameters were modified to delay the runoff generation, so as to indirectly account for the presence of the water reservoirs. These modeling results suggest the necessity of including hydraulic infrastructures in modelling studies to provide more robust assessments; they also raise the question on how to employ safety margins in reservoir operations to confront the impact of future climate events.