TIMING OF WATER STORAGE AND RELEASE OF THE TONLE SAP LAKE BASED ON WEEKLY DISCHARGE ANALYSIS AT KRATIE, TAN CHAU AND CHAU DOC STATIONS

Abstract

This study analyzes the weekly discharge variations (ΔQ) during 1980 - 2024 using observed data from the Kratie, Tan Chau and Chau Doc stations hydrological stations to identify the timing and dynamics of water storage and release of the Tonle Sap lake. The discharge difference was defined as ΔQ = (Q_TanChau + Q_ChauDoc) - Q_Kratie and examined for two periods: 1980 - 2009 (pre-mainstream dams) and 2010 - 2024 (post-dam construction). Results reveal that the Tonle Sap continues to maintain its natural dual-phase regulation, yet the storage-release cycle has become phase-shifted and flattened. The storage phase starts about half a month earlier, while the release phase occurs 1 - 2 weeks later, and the amplitude of ΔQ has decreased from 13.5 × 10³ to 12.0 × 10³ m³/s. The Mann - Kendall trend test indicates a statistically significant (p < 0.05) decline in both ΔQ minima and maxima, evidencing the progressive weakening of the lake’s natural regulation capacity. These changes reflect the combined impacts of upstream dam operations, altered monsoon timing and climate change, leading to reduced dry-season baseflow and earlier salinity intrusion in the Mekong delta. The weekly ΔQ approach proves effective in detecting short-term hydrological fluctuations and phase shifts, providing a robust scientific basis for transboundary water management and adaptive flow regulation in the lower Mekong basin.