Triple coupling random forest approach for bias correction of ensemble precipitation data derived from Earth system models for Divandareh-Bijar Basin (Western Iran)

Climate change is expected to change the frequency, duration, intensity, and pattern of precipitation, underscoring the need for accurate predictive tools. Earth system models (ESMs) serve as invaluable instruments in this endeavour, simulating climate variable variations across temporal and spatial dimensions. This study aims to develop a methodology for generating precise daily precipitation maps by rectifying biases inherent in ESM outputs. The proposed methodology includes downscaling ESM outputs to simulate historical daily grid-based precipitation, thereby enhancing the fidelity of daily precipitation representation. For this purpose, 14 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) were employed. Random forest (RF) machine learning method was used to correct biases in these ESM outputs. This study's novelty lies in integrating results of a grid-based RF classification model, employed to distinguish between rainy and non-rainy days, with those obtained by two RF regression models, to estimate precipitation amounts for grid cells receiving extreme and non-extreme precipitation, to generate an ensemble of ESM outputs. The resulting method, termed the triple coupling method (EN-RF), was validated using precipitation data from the Divandareh-Bijar Basin in western Iran to simulate historical climate conditions. Furthermore, the accuracy of the developed triple coupling approach was compared with that of a commonly used single machine learning-based downscaling model (EN-Single-RF). Comparative analysis against a commonly used single machine learning-based downscaling model (EN-Single-RF) revealed the superior performance of the EN-RF approach in replicating the intensity and distribution of daily precipitation. Furthermore, within the triple coupling framework, support vector machine (SVM) was utilized to simulate daily historical precipitation (EN-SVM), while the quantile mapping (QM) method served as a benchmark. Comparison of the results showed superiority of the EN-RF to other methods (EN-Single-RF, EN-SVM, and QM) in terms of various accuracy metrics (Kling-Gupta Efficiency = 0.95, mean square error = 0.22). The findings indicated the capability of the proposed triple coupling framework using the RF algorithm to simulate the spatio-temporal distribution of precipitation using the ESM precipitation outputs. The developed framework can be used to produce reliable projections to gain deeper insights into the potential impacts of climate change on regional precipitation patterns.