Drought and salinity intrusion in the Lower Chao Phraya River: variability analysis and modeling mitigation approaches

Abstract. Saltwater intrusion in the Lower Chao Phraya River (LCPYR) is a significant national concern for Thailand, requiring a thorough understanding and the development of effective prediction systems for current and future management. This study investigates the key drivers influencing saltwater intrusion in the LCPYR. Cross-wavelet analysis was applied to examine the interactions between tidal forces, drought conditions represented by the rolling standardized discharge anomaly (RSDA) and rolling standardized precipitation anomaly (RSPA), and salinity levels. The results reveal that saltwater intrusion in the LCPYR is controlled by two interacting mechanisms: a drought-dependent mechanism and a drought-relaxation mechanism. The drought-dependent process, driven by regional hydro-climatic variability and quantified using the RSDA, dominates sub-annual to annual salinity fluctuations. Extreme salinity peaks are primarily modulated by non-tidal sea-level anomalies, underscoring the crucial influence of sea-level oscillations. In turn, the drought-relaxation mechanism, captured by the RSPA, reflects transient wet periods that can temporarily reduce salinity levels, at times weakening the usual correlation between salinity and hydrological drought severity. The numerical model demonstrates high accuracy in simulating both hydrodynamic and salinity behaviors, validating the cross-wavelet analysis and offering a reliable approach for modeling salinity in this complex estuarine system. We revise and suggest strategies to mitigate the salinity intrusion for emergent drought periods (e.g., optimal redistribution of the diverting freshwater) and proactive/long-term solutions, e.g., using impacts of urban runoff from tributary rivers and developing a robust prediction system. These findings offer essential insights to guide management strategies and the development of prediction tools for the LCPYR and surrounding regions.