Trends and Variability of Future PMP and PMF considering Climate Change: A Study on Dams of Godavari River Basin

The future variation of design flood inputs, i.e., Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF) for fifteen dams located in the Godavari River Basin (GRB) are assessed by considering the effect of climate change. The raw GCM data outputs from the CNRM-CM6-1-HR model from Coupled Model Intercomparison Project phase 6 (CMIP6) over historical rainfall (1971-2014) and future rainfall data (2021-2100) is considered for the analysis. Also, the monthly rainfall data at 0.25° × 0.25° grid size are obtained from the India Meteorological Department (IMD), Pune. The Quantile Mapping (QM) bias correction method is applied to improve the raw GCM data and the bias corrected outputs are used for the analysis. Firstly, the association between the bias corrected raw GCM data versus observed data is checked during the historical period. Upon obtaining the satisfactory performance, the bias corrected GCM data is used for the future analysis. Next, the PMP values at each dam location are computed using the Hershfield approach for historical period. Further, the Gumbel Extreme Value (GEV) method is employed to assess the rainfall extreme value (EV) over different return periods (50, 100, and 1000 years), and its magnitude is compared with the PMP value. It is clearly evident that the PMP values at most of the dam locations are falling below 100-year return period. Further, the PMF is estimated using the Soil Conservation Service Curve Number (SCSCN) method. The future variation of PMP and, subsequently, PMF are projected for the future period (2021-2100) considering Shared Socioeconomic Pathway (SSP) scenarios, viz. SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. The results revealed that the increasing trend in PMP and PMF is noticed for SSP5-8.5 in the future period. The Bhupathipalem dam, which is designed for a spillway capacity of 2314 m³/sec, shows projected PMF values of 3024 m³/sec for epoch-1 (2021-2050), and 3340 m³/sec for epoch-2 (2051-2080) under the SSP5-8.5 scenario. This indicates that the PMF is expected to increase by 31% during epoch-1, and 45% during epoch-2. Therefore, the findings of the study indicate an alarming for significant concern regarding dam safety. From this analysis, crucial insights about the variation in PMP and PMF values can be drawn for the selected dam sites. It is worth noting that this study also offers a valuable information for stakeholders regarding the occurrence of critical situations in the near future. However, the GCM data sets are associated with several uncertainties and have limitations to adopt for regional scale studies. Hence, we recommend to downscale the GCM outputs and use it for local scale impact assessments.