Comparison between statistical and dynamical downscaling of rainfall over the Gwadar-Ormara basin, Pakistan

This paper evaluated and compared the performance of a statistical downscaling method and a dynamical downscaling method to simulate the spatial-temporal rainfall distribution. Outputs from RegCM4 Regional Climate Model (RCM) and the CanESM2 Atmosphere-Ocean General Circulation Model (AOGCM) were selected for the data scarce Gwadar-Ormara basin, Pakistan. The evaluation was based on the climatological average and standard deviation for historic (1971-2000) and future (2041-2070) time periods under Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios. The performance evaluation showed that statistical downscaling is preferred to simulate and project rainfall patterns in the study area. Additionally, the Statistical DownScaling Model (SDSM) showed low R2 values in calibration and validation of the simulations with respect to observed data for the historic period. Overall, SDSM generated satisfactory results in simulating the monthly rainfall cycle of the entire basin. In this study, RegCM4 showed large rainfall errors and missed one rainfall season in the historic period. This study also explored whether the grid-based rainfall time series of the Asian Precipitation-Highly Resolved Observational Daily Integration Towards Evaluation (APHRODITE) dataset could be used to enlarge and complement the sample of in situ observed rainfall time series. A spatial correlogram was used for observed and APHRODITE rainfall data to assess the consistency between the two data sources, which resulted in rejecting APHRODITE data. For the future time period (2041-2070) under RCPs 4.5 and 8.5 scenarios, rainfall projections did not show significant difference for both downscaling approaches. This may relate to the driving model (CanESM2 AOGCM) and not necessarily suggests poor performance of downscaling; either statistical or dynamical. Hence, the study recommends evaluating a multi-model ensemble including other GCMs and RCMs for the same area of study. The comparison between the two downscaling techniques gives an idea about which technique performs better in the data scarce area like Gwadar-Ormara basin, Pakistan. As this may help in future research to simulate and predict the rainfall pattern over the area, knowing the advantages and limitations of each technique. Graphical Abstract Image by Ghulam Murtaza-Own work, CC BY-SA 4.0, .image