Khosro Morovati

Sand mining, dam construction, and climate change have profoundly altered the hydrological dynamics of the Mekong River-Tonle Sap Lake floodplain system, one of the world's most ecologically productive river-lake complexes. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with the Tonle Sap Lake across three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight mainstream stations, we quantify shifts in long-term flow memory, short-term variability, and flow amplitude. We use a hydrodynamic response-time model to link upstream regime change to the Mekong–Tonle Sap flow exchange. During the post-dam period, flow memory increased in the upper mainstream but decreased slightly downstream, indicating growing upstream–downstream divergence in flow-regime characteristics and increasing hydrological fragmentation. Upstream stations, particularly Chiang Saen, exhibit a 33.3 % increase in minimum flows and a 40 % decrease in maximum flows during the post-dam period compared to the pre-dam period, alongside higher flow memory (+0.11), reflecting flow smoothing by reservoir operations. At the most downstream station at Kratie, a key control point for Tonle Sap inflow, maximum flows decreased by 9.4 %, while minimum flows increased by 117 % relative to the pre-dam baseline; the seasonal timing of peak discharge was also delayed by approximately two to three weeks. Critically, the combined effects of the altered mainstream flow regime and sand-mining-driven riverbed lowering have shifted the conditions under which Tonle Sap flow reversal initiates. Using the lag-adjusted Kratie discharge at onset as a proxy for the Mekong-Tonle Sap confluence stage/head condition, we find that the median onset discharge increased from ∼ 3200 m ³ s ⁻¹ (pre-dam) to ∼ 4300 m ³ s ⁻¹ under the post-dam fixed-bed scenario (2010–2024 hydrology with reference morphology), and to ∼ 7200 m ³ s ⁻¹ once sand-mining-driven channel incision is added. This indicates that post-2010 hydrological alteration reduced the probability of exceeding the discharge threshold for initiating reverse flow, while sand-mining-driven riverbed lowering substantially amplified this shift by modifying local stage–discharge relationships. Together, these interacting drivers shorten the reverse-flow season by approximately 28 d relative to the pre-dam baseline. Sustaining the Mekong–Tonle Sap flood pulse will therefore require basin-wide monitoring and transboundary governance that address flow timing and variability, not volume alone.

Rice, a principal global staple crop, is increasingly threatened by flooding under climate change, placing rice‐dependent economies at risk. Yet the capacity of human activities to mitigate inundation impacts on rice remains insufficiently quantified. Here we develop an integrated agro‐hydrological framework that links climate forcing, hydrological regulation, and farmer decision‐making from a systems perspective. We apply the framework to the rapidly changing Lancang‐Mekong River Basin, focusing on Tonle Sap Lake, where flood‐pulse dynamics strongly shape rice production. Relative to the baseline period (1980–2014), with mean annual inundation‐induced rice losses of US$78 million, average losses decline in the near future (2021–2060) across all SSP scenarios, mainly because inundated area and rice exposure decrease. In the far future (2061–2100), losses increase relative to the near future; they remain below the baseline under SSP1‐2.6 and SSP2‐4.5 but slightly exceed the baseline under SSP5‐8.5. Extreme losses remain possible, with the maximum projected loss reaching US$160 million under far‐future SSP5‐8.5. Future losses are controlled not only by inundation extent and duration, but also by the timing of inundation relative to rice phenology. Losses shift from the reproductive and maturity stages in the baseline period toward the vegetative and reproductive stages in future periods, reflecting earlier flood recession and delayed planting. Reservoir operation substantially reduces inundation‐induced losses, particularly during the reproductive and maturity stages, while planting date adjustment provides additional benefits, especially in the far future. This framework supports climate‐resilient assessment of coupled water‐agriculture systems and adaptation pathways in flood‐dependent rice regions.

In transboundary river basins, water resource pressure results from the combined effects of internal water use growth, external transboundary withdrawal, and climate change, yet the relative contributions of these drivers to both water pressure and upstream–downstream interactions remain poorly quantified. To address this issue, this study adopts the Pressure–State–Response analytical framework to explicitly disentangle and characterize the spatiotemporal patterns and evolution of irrigation water withdrawal pressure in the Lancang–Mekong River Basin under climate change. Results indicate that the proportion of irrigation water withdrawal relative to available water exhibits a persistent increasing trend. Under the SSP5–8.5 scenario, this proportion is projected to rise to 19 % annually and 59 % during the dry season by 2040. Irrigation water withdrawal exerts a substantially greater reduction in water availability in downstream subregions than in upstream subregions, particularly during the dry season. In the historical period (1980–2020), internal irrigation water withdrawal pressure dominates in Subregions 1 (China), 8 (primarily in Thailand), and 13 (primarily in Vietnam), exceeding external pressure from upstream irrigation water withdrawal, whereas external irrigation-induced water appropriation is the primary driver in the remaining subregions. Under the SSP1–2.6, SSP2–4.5, and SSP5–8.5 scenarios during 2021–2040, both internal and external irrigation pressures intensify across the basin, exhibiting pronounced nonlinear dynamics and spatial heterogeneity. Notably, Subregion 8 undergoes a structural shift in dominant pressure, transitioning from internally driven irrigation pressure in the historical period to externally driven irrigation appropriation in the future. Meanwhile, the growth rates of irrigation water withdrawal pressure are redistributed spatially: compared to the historical period, the growth of external irrigation pressure slows in downstream subregions (9–13), while it continues to increase in midstream and upstream subregions (2–8). The analysis identifies vulnerable components of the basin system, clarifies the spatial distribution of transboundary water pressures, and provides a basis for differentiated water governance strategies across the basin.

Managing salinity in arid rivers is impeded by sparse monitoring, relying on low-frequency grab samples that miss hydrological event dynamics. Here, interpretable machine learning is applied to a 50-year monthly archive (1968–2018; Discharge, major ions, pH) from three stations on Iran’s Karkheh River. Gradient Boosting Regression achieves high predictive skill for Total Dissolved Solids (TDS)/Electrical Conductivity (EC) (test-set R ² = 0.94/0.97; RMSE = 55 mg L ⁻¹ /56 µS cm ⁻¹ ), validated via time-aware cross-validation. SHAP-based feature attribution reveals that Na ⁺ and SO ₄ ^–2 are the strongest contributors to TDS, while Na ⁺ and Cl ⁻ dominate EC, consistent with conservative salinity sources under baseflow conditions. A reduced-input decision tree (four predictors) retains R ² = 0.81–0.87, enabling minimal-sensor monitoring. Flow-regime partitioning and STL (Seasonal-Trend decomposition using Locally estimated scatterplot smoothing)-detrended event composites reveal low-flow salinization and ion-specific post-flood recovery (Cl ⁻ : 1–2 months; Na ⁺ : 2–3 months), guiding targeted sampling. This framework extracts predictive power, process associations, and operational guidance from legacy grab-sample archives, scalable to data-limited basins worldwide.

The Mekong–Tonle Sap Lake–Delta system supports highly productive floodplain ecosystems and regional food security, in part through the delivery of nutrient-rich suspended sediment. However, this sediment pathway is strongly modulated by the flood-pulse–driven “reverse flow” at the Mekong–Tonle Sap confluence: during the wet season, high Mekong stages drive net inflow from the mainstream into Tonle Sap Lake, whereas during the dry season the lake releases stored water back to the mainstream and onward to the delta. How this bidirectional exchange reshapes sediment delivery—specifically whether Tonle Sap acts as a net sink or a net source of suspended sediment for the downstream Mekong—remains poorly quantified. Most existing assessments approximate delta sediment supply using upstream stations and do not resolve the river–lake exchange, largely because near-confluence discharge and continuous sediment observations are limited.Here we develop an integrated modelling framework that couples a physically based, spatially distributed hydrological model with Delft3D-Flow hydrodynamics to reconstruct daily discharge and river–lake exchange over the last ~35 years, including the reversal period. We then estimate suspended sediment fluxes using seasonally stratified, hysteresis-aware rating curves that account for distinct sediment–discharge relationships on rising versus falling limbs of the hydrograph. Combining daily exchange discharge with the corresponding rating-curve sediment concentrations enables a bidirectional suspended-sediment budget across the Tonle Sap River, separating wet-season import to the lake from dry-season export back to the mainstream.During the historical baseline (1980–2000), we estimate that the Mekong mainstream delivers ~4 Mt yr⁻¹ of suspended sediment into the lake on average, and a comparable magnitude is returned to the mainstream during the dry season, indicating that Tonle Sap primarily acts as a transient store rather than a sustained additional sediment source to the downstream system. In the mega-dam period (2010–2025), despite substantially reduced upstream sediment supply, the river–lake exchange continues to route similar volumes of water into the lake, but the suspended-sediment contribution released from the lake does not compensate for the mainstream deficit. These results suggest that reverse-flow dynamics alone do not sustain suspended-sediment delivery to the Mekong Delta under contemporary sediment scarcity, with implications for nutrient replenishment, recession agriculture, and floodplain productivity.

Rice, a principal global staple crop, is increasingly threatened by intensified flooding under climate change, placing rice-dependent economies at risk. Yet the potential for human activities to mitigate inundation impacts on rice remains essential and largely overlooked. Here we develop an interdisciplinary agro-hydrological framework that links climate forcing, hydrological regulation, and farmer decision-making from a systems perspective. We apply the framework to the rapidly changing Lancang-Mekong River Basin, focusing on Tonle Sap Lake – a biodiversity hotspot and major floodplain rice-production region-where shifting flood dynamics interact with rainfall-driven planting practices. We project that farmers' rainfall-driven planting decisions will progeressively delay planting, particularly as climate change intensifies, while the hydrological regime will produce more consistently high but less damaging water levels. Relative to a baseline period (1980-2014) with mean annual rice losses of US$78 million, losses are projected to inthe near future (2021-2060), and then increase in the far future (2061-2100). A Joint adaptation strategy combining reservoir operation (dam regulation) with farmer-led shifts towards earlier planting substantially reduces inundation damages and improves climate resilience. We further find that the dominant phenological window of inundation-induced loss shifts from the reproductive and maturity stages (baseline) to the vegetative and reproductive stages in both the near and far future. Reservoir operation primarily constrains losses during the reproductive and maturity stages, thereby limiting late-season damage. This framework enables investigation of coupled water-agriculture dynamics and their growing interdependencies under climate change, supporting robust assessment of climate-resilient adaptation pathways.

Dam construction and climate change have profoundly disrupted the hydrological dynamics of the Mekong River and its floodplain–lake system. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with Tonle Sap Lake for three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight stations, we quantify changes in long-term flow memory, short-term variability, and flow amplitude. These analyses are complemented by a hydrodynamic response-time model that simulates how upstream regime shifts reshape the Tonle Sap–Mekong flow exchange. We show that dam regulation and climate change have fragmented both spatial synchrony and temporal persistence, disrupting the Mekong’s behavior as a coherent hydrological continuum. Upstream stations, particularly Chiang Saen, exhibit a 33.3 % increase in minimum flows and a 40 % decrease in maximum flows during the post-dam period compared to the pre-dam period, alongside higher flow memory (+0.13), reflecting flow smoothing by reservoir operations. Downstream variability remains more pronounced due to the influence of monsoonal tributaries. At the most downstream station–Kratie, a key control point for Tonle Sap inflow–maximum flows decreased by 9.4 %, while minimum flows increased by 117 % in the post-dam period relative to the pre-dam baseline, also delaying the seasonal timing of peak discharge by two weeks. Critically, these regime shifts–driven by the compounding impacts of dam regulations and climate change, together with observed riverbed lowering from sand mining, caused the discharge threshold for initiating reverse flow to rise significantly: the median onset flow increased from ~3,000 m³ s⁻¹ (pre-dam) to ~7,000 m³ s⁻¹ (post-dam), marking a >130 % increase. Collectively, these alterations have shortened the reverse flow period by 24 days compared to the historical baseline. Our findings demonstrate that cascading dam operations across the mainstream and tributaries, amplified by climate change and other anthropogenic stressors, have triggered multi-scalar hydrological fragmentation in one of the world's most ecologically productive river–lake systems. Preserving the Mekong's natural flood pulse and its interaction with Tonle Sap lake will require basin-wide hydrological monitoring and transboundary governance frameworks that account not only for volume but also for flow timing, variability, and ecological function.

Dam construction and climate change have profoundly disrupted the hydrological dynamics of the Mekong River and its floodplain–lake system. This study provides an integrated, multi-scale assessment of flow regime alteration across the Mekong mainstream and its coupling with Tonle Sap Lake for three periods: pre-dam (1976–1991), transition (1992–2009), and post-dam (2010–2024). Using daily and sub-daily data from eight stations, we quantify changes in long-term flow memory, short-term variability, and flow amplitude. These analyses are complemented by a hydrodynamic response-time model that simulates how upstream regime shifts reshape the Tonle Sap–Mekong flow exchange. We show that dam regulation and climate change have fragmented both spatial synchrony and temporal persistence, disrupting the Mekong’s behavior as a coherent hydrological continuum. Upstream stations, particularly Chiang Saen, exhibit a 33.3 % increase in minimum flows and a 40 % decrease in maximum flows during the post-dam period compared to the pre-dam period, alongside higher flow memory (+0.13), reflecting flow smoothing by reservoir operations. Downstream variability remains more pronounced due to the influence of monsoonal tributaries. At the most downstream station–Kratie, a key control point for Tonle Sap inflow–maximum flows decreased by 9.4 %, while minimum flows increased by 117 % in the post-dam period relative to the pre-dam baseline, also delaying the seasonal timing of peak discharge by two weeks. Critically, these regime shifts–driven by the compounding impacts of dam regulations and climate change, together with observed riverbed lowering from sand mining, caused the discharge threshold for initiating reverse flow to rise significantly: the median onset flow increased from ~3,000 m³ s⁻¹ (pre-dam) to ~7,000 m³ s⁻¹ (post-dam), marking a >130 % increase. Collectively, these alterations have shortened the reverse flow period by 24 days compared to the historical baseline. Our findings demonstrate that cascading dam operations across the mainstream and tributaries, amplified by climate change and other anthropogenic stressors, have triggered multi-scalar hydrological fragmentation in one of the world's most ecologically productive river–lake systems. Preserving the Mekong's natural flood pulse and its interaction with Tonle Sap lake will require basin-wide hydrological monitoring and transboundary governance frameworks that account not only for volume but also for flow timing, variability, and ecological function.