The Tonle Sap is the most fertile and diverse freshwater ecosystem in Southeast Asia, receiving nurturing water flows from the Mekong and its immediate basin. In addition to rapid development in the Tonle Sap basin, climate change may threaten natural flow patterns that sustain its diversity. The impacts of climate change on river flows in 11 sub-basins contributing to the Tonle Sap Lake were assessed using the Soil and Water Assessment Tool (SWAT) model to quantify the potential magnitude of future hydrological alterations. Projected river flows from three General Circulation Models (GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR) for three time horizons (2030s, 2060s and 2090s) indicate a likely decrease in both the wet and dry season flows. The mean annual projected flow reductions range from 9 to 29%, 10 to 35% and 7 to 41% for the 2030s, 2060s and 2090s projections, respectively. Moreover, a decrease in extreme river flows (Q₅ and Q₉₅) was also found, which implies there could be a decline in flood magnitudes and an increase in drought occurrences throughout the basin. The results of this study provide insight for water resources planning and adaptation strategies for the river ecosystems during the dry season, when water flows are projected to decrease.

Water infrastructure development is considered necessary to drive economic growth in the Mekong region of mainland Southeast Asia. Yet the current understanding of hydrological and flood pattern changes associated with infrastructural development still contains several knowledge gaps, such as the interactions between multiple drivers, which may have serious implications for water management, agricultural production, and ecosystem services. This research attempts to conduct a cumulative assessment of basin-wide hydropower dam construction and irrigation expansion, as well as climate change, implications on discharge, and flood changes in the Cambodian Mekong floodplain. These floodplains offer important livelihoods for a considerable part of the 6.4 million people living on them, as they are among the most productive ecosystems in the world – driven by the annual flood pulse. To assess the potential future impacts, we used an innovative combination of three models: Mekong basin-wide distributed hydrological model IWRM-VMod, with the Mekong delta 1D flood propagation model MIKE-11 and 2D flood duration and extent model IWRM-Sub enabling detail floodplain modelling. We then ran scenarios to approximate possible conditions expected by around 2050. Our results show that the monthly and seasonal hydrological regimes (discharges, water levels, and flood dynamics) will be subject to substantial alterations under future development scenarios. Projected climate change impacts are expected to decrease dry season flows and increase wet season flows, which is in opposition to the expected alterations under development scenarios that consider both hydropower and irrigation. The likely impact of decreasing water discharge in the early wet season (up to −30 %) will pose a critical challenge to rice production, whereas the likely increase in water discharge in the mid-dry season (up to +140 %) indicates improved water availability for coping with drought stresses and sustaining environmental flows. At the same time, these changes would have drastic impacts on total flood extent, which is projected to decline by around 20 %, having potentially negative impacts on floodplain productivity and aquaculture, whilst reducing the flood risk to more densely populated areas. Our findings demonstrate the substantial changes that planned infrastructural development will have on the area, potentially impacting important ecosystems and people's livelihoods, calling for actions to mitigate these changes as well as planning potential adaptation strategies.

Water infrastructure development is crucial for driving economic growth in the developing countries of the Mekong. Yet it may also alter existing hydrological and flood conditions, with serious implications for water management, agricultural production and ecosystem services, especially in the floodplain regions. Our current understanding of the hydrological and flood pattern changes associated with infrastructural development still contain several knowledge gaps, such as the consideration of overlooked prospective drivers, and the interactions between multiple drivers. This research attempts to conduct a cumulative impact assessment of flood changes in the Cambodian part of the Mekong floodplains. The developmental activity of six central sectors (hydropower, irrigation, navigation, flood protection, agricultural land use and water use) as well as climate change were considered in our modelling analysis. Our results show that the monthly, sub-seasonal, and seasonal hydrological regimes will be subject to substantial alterations under the 2020 planned development scenario, and even larger alterations under the 2040 planned development scenario. The degree of hydrological alteration under the 2040 planned development is somewhat counteracted by the effect of climate change, as well as the removal of mainstream dams in the Lower Mekong Basin and hydropower mitigation investments. The likely impact of decreasing water discharge in the early wet season (up to −34 %) will pose a critical challenge to rice production, whereas the likely increase in water discharge in the mid-dry season (up to +54 %) indicates improved water availability for coping with drought stresses and sustaining environmental flow. At the same time, these changes would have drastic impacts on total flood extent, which is projected to decline up to −18 %, having potentially negative impacts on floodplain productivity whilst at the same time reducing the flood risk to the area. Our findings urge the timely establishment of adaptation and mitigation strategies to manage such future environmental alterations in a sustainable manner.

Water infrastructure development is crucial for driving economic growth in the developing countries of the Mekong. Yet it may also alter existing hydrological and flood conditions, with serious implications for water management, agricultural production and ecosystem services, especially in the floodplain regions. Our current understanding of the hydrological and flood pattern changes associated with infrastructural development still contain several knowledge gaps, such as the consideration of overlooked prospective drivers, and the interactions between multiple drivers. This research attempts to conduct a cumulative impact assessment of flood changes in the Cambodian part of the Mekong floodplains. The developmental activity of six central sectors (hydropower, irrigation, navigation, flood protection, agricultural land use and water use) as well as climate change were considered in our modelling analysis. Our results show that the monthly, sub-seasonal, and seasonal hydrological regimes will be subject to substantial alterations under the 2020 planned development scenario, and even larger alterations under the 2040 planned development scenario. The degree of hydrological alteration under the 2040 planned development is somewhat counteracted by the effect of climate change, as well as the removal of mainstream dams in the Lower Mekong Basin and hydropower mitigation investments. The likely impact of decreasing water discharge in the early wet season (up to −34 %) will pose a critical challenge to rice production, whereas the likely increase in water discharge in the mid-dry season (up to +54 %) indicates improved water availability for coping with drought stresses and sustaining environmental flow. At the same time, these changes would have drastic impacts on total flood extent, which is projected to decline up to −18 %, having potentially negative impacts on floodplain productivity whilst at the same time reducing the flood risk to the area. Our findings urge the timely establishment of adaptation and mitigation strategies to manage such future environmental alterations in a sustainable manner.