Weekly discharge analysis at the Kratie, Tan Chau, and Chau Doc stations for 1980–2024 indicates that Tonle Sap Lake continues to exhibit natural dual-phase regulation, although the timing of water storage and release has shifted. The findings of Nguyen Trinh Chung, Nguyen Dang Tinh, and Phan Khanh Khanh contribute to the scientific evidence base for monitoring changes in river discharge and informing water-resource management across the Mekong River Basin.
Weekly discharge differences as an indicator of flow regulation
Located within the Mekong River system, Tonle Sap Lake in Cambodia plays an important role in regulating seasonal flows. When water levels in the Mekong River rise during the wet season, water flows in reverse through the Tonle Sap River into the lake, expanding the inundated area and increasing water storage. As water levels in the mainstem decline, water flows back from the lake into the Mekong River, helping sustain downstream flows during the dry season.
This flow-reversal mechanism establishes a natural regulation cycle that is important for ecosystems, fisheries, and downstream areas, including Vietnam’s Mekong Delta. Assessing changes in this mechanism requires identifying when water storage begins, when the system transitions to water release, and how these timings vary over time.
In their study, “Timing of water storage and release of the Tonle Sap lake based on weekly discharge analysis at Kratie, Tan Chau and Chau Doc stations”, Nguyen Trinh Chung, Nguyen Dang Tinh, and Phan Khanh Khanh adopted an approach based on long-term hydrological observations. Daily mean discharge data from the three stations for 1980–2024 were aggregated into weekly means to identify changes in river discharge during seasonal transitions.
A key feature of the method is the use of the discharge difference between upstream and downstream locations, denoted by ΔQ. This indicator is calculated by subtracting the discharge at Kratie from the combined discharge at Tan Chau and Chau Doc. In the study, a negative ΔQ is interpreted as indicating the water-storage phase, whereas a positive value indicates the water-release phase.
Rather than examining variations only at monthly or seasonal scales, the researchers analyzed weekly discharge variations and required ΔQ to retain the same sign for two consecutive weeks to identify a phase transition. This approach helps reduce the influence of short-term fluctuations while allowing more precise identification of shifts in the storage–release cycle.
To assess changes over time, the researchers divided the data series into two periods: 1980–2009 and 2010–2024. Comparing these periods provides a basis for examining changes in the lake’s regulation cycle under different hydrological conditions, as upstream dam operations have become an increasingly important consideration in analyses of Mekong River flows.
An earlier storage phase and a delayed release phase
The analysis shows that Tonle Sap Lake continues to exhibit two relatively distinct phases: water storage and water release. However, the timing of the transition between these phases has changed, with water storage tending to begin earlier and water release beginning later in the more recent period.
During 1980–2009, ΔQ became consistently negative in the first week of May, marking the onset of the water-storage phase according to the study’s criteria. The indicator then gradually increased and became consistently positive in the final week of September, signaling the transition to the water-release phase. This pattern of seasonal transition was relatively clear in the discharge series analyzed.
During 2010–2024, the water-storage phase tended to begin approximately half a month earlier, while the onset of water release shifted to the first week of October, about one to two weeks later than in the preceding period. The key finding is therefore not that the storage–release cycle has disappeared, but that the timing of this continuing cycle has shifted.
The Mann–Kendall trend test also showed statistically significant downward trends in both the minimum and maximum values of ΔQ (p < 0.05). This provides quantitative evidence that discharge characteristics associated with both phases of the regulation cycle have changed over time, rather than fluctuating anomalously in only a few individual years.
From a hydrological management perspective, the timing of phase transitions may be as important as discharge magnitude. If water begins to accumulate in the lake earlier but its release back into the Mekong River starts later, the timing of water replenishment downstream may also change. Weekly discharge analysis therefore helps identify an important dimension of hydrological variability: shifts in the seasonal timing of water availability.
However, patterns identified in discharge records should be distinguished from direct measurements of the actual volume of water stored in or released from the lake. ΔQ is a composite indicator used in the study to infer the system’s regulation state. The findings must therefore be interpreted within the scope of the methods and data employed.
Multiple drivers and the limits of causal attribution
The authors examine shifts in the storage–release cycle in the context of the combined effects of upstream dam operations, changes in monsoon timing, climate change, and potential changes in channel morphology and sediment dynamics. These factors are considered in the study’s interpretation of hydrological changes across the Tonle Sap–Mekong system.
Upstream dam operations can alter the timing and characteristics of seasonal flows. Changes in the timing and distribution of rainfall can affect runoff generation, the onset of rising water levels, and the replenishment of the lake. Changes in channel morphology and sediment dynamics may also influence flow transmission through the river system.
Nevertheless, identifying changes in a discharge time series does not establish the separate contribution of each potential driver. Comparisons between the two periods and statistical trend tests indicate systematic changes in discharge characteristics, but these results alone cannot determine the extent to which the observed shifts are attributable to hydropower operations, climate variability, or changes in river morphology.
The authors also note that the study did not incorporate hydrodynamic and morphodynamic modeling to isolate the effects of sediment deposition and channel changes. Further research could combine long-term discharge records with morphological data and hydrodynamic and sediment-transport models to clarify the mechanisms underlying these changes.
This cautious approach is important for management of the Mekong River Basin, where flow variations may arise from multiple interacting factors. Identifying a hydrological trend provides an important basis for further research and management decisions, but additional evidence is needed to distinguish associations from causal relationships and assess the influence of individual drivers.
The value of the study therefore lies not only in identifying shifts in the timing of water storage and release, but also in providing a quantitative approach for tracking these changes over a long-term data series. This offers a basis for further investigation into the relationships among dam operations, climate variability, and the natural flow-regulation functions of Tonle Sap Lake.
Implications for downstream flow forecasting and water-resource management
Changes in the timing of water storage and release at Tonle Sap Lake have implications for monitoring downstream discharge in the Mekong River, particularly during the transition between the wet and dry seasons. As the onset of water release shifts, forecasts of downstream flows also need to account for changes in the system’s regulation cycle.
For Viet Nam’s Mekong Delta, dry-season flows are essential for water supply, agricultural production, and efforts to manage salinity intrusion. If supplemental flows decline or arrive later, pressure on water-resource management may increase. The actual extent of these effects, however, depends on multiple conditions across the basin and requires assessment using appropriate data and models. The present study does not directly quantify changes in salinity intrusion attributable to shifts in the lake’s storage–release cycle.
One practical contribution of the study is the potential use of weekly ΔQ as an indicator for monitoring flow dynamics. When updated regularly and integrated into a broader hydrological data system, this indicator could provide additional information for identifying flow-reversal timing, assessing seasonal shifts, and supporting downstream discharge-forecasting models. The authors particularly highlight the September–November transition period, when monitoring changes in river discharge is relevant to water-management operations.
To improve practical application, continued hydrological monitoring and discharge data sharing among countries in the basin are essential. Weekly analysis could also be developed further by incorporating water-level observations, tidal effects, and hydrodynamic models to distinguish seasonal flow-regulation signals from short-term fluctuations. These are also among the directions proposed by the authors for refining the method used to identify the storage–release cycle.
In the long term, water-resource management in the Mekong River Basin requires an approach that extends beyond individual gauging stations or infrastructure projects. Changes at Tonle Sap Lake need to be assessed in relation to upstream flows, downstream river branches, and water demands across the basin. Improved data coordination, more reliable forecasting, and consideration of natural flow-regulation patterns in infrastructure operations could help strengthen water-management capacity.
Drawing on a 45-year data series and weekly discharge analysis, the research by Nguyen Trinh Chung, Nguyen Dang Tinh, and Phan Khanh Khanh adds evidence of shifts in Tonle Sap Lake’s regulation cycle. Although the findings do not replace the need for more detailed investigations into causes and impacts, they provide a useful quantitative basis for monitoring hydrological variability, identifying further research questions, and supporting transboundary water management and adaptive flow regulation in the Lower Mekong Basin.
Sources: This article is based on the study “Timing of water storage and release of the Tonle Sap lake based on weekly discharge analysis at Kratie, Tan Chau and Chau Doc stations” by Nguyen Trinh Chung, Nguyen Dang Tinh, and Phan Khanh Khanh, published in the Science Journal of Agriculture and Environment, Issue 3 + 4, February 2026.
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