The increasing frequency and intensity of flooding, driven by climate change, are having a detrimental impact on Phnom Penh’s urban ecosystem. Disaster managers and decision-makers increasingly recognize the value of tools that can directly translate flood hazards—specifically, the extent of inundation during flood events—into estimates of expected socio-economic impacts, including affected populations and economic losses. While previous studies have often focused on broad-scale flood impacts at local or regional levels, this paper presents a straightforward approach for estimating impacts at a finer, more localized scale within the affected area, using flood hazard data and damage functions. This study aims to assess flood inundation areas and the direct economic losses resulting from flood events in the southern and southwestern parts of Phnom Penh, the capital city of Cambodia. The methodology involves applying Google Earth Engine (GEE) with Sentinel-1 synthetic aperture radar (SAR) data to assess flooding, including the generation of flood extent and depth maps. These maps are then used as input for an impact assessment module, which translates the estimated flood extents and depths into quantitative assessments of socio-economic impacts. The results indicate that the inundation areas in the southern and southwestern urban areas of Phnom Penh reached 1,638 ha, with flood depths ranging from 0.5 to 6 m, during the October 2020 flood event. The total estimated economic damage was approximately USD 20 million. Agricultural areas were the most affected, followed by residential and commercial areas. This study provides a valuable tool for rapid assessment of urban flood impacts, offering crucial data on flood extent, depth, and economic consequences for urban policymakers and planners.

A modified piano key weir with a rounded nose and a parapet wall (MPKW) can improve the discharge capacity significantly compared to a standard piano key weir. However, the optimum of the inlet/outlet width ratio (Wi/Wo) on the discharge efficiency of MPKW is still not investigated numerically. The present work utilized the numerical modeling to investigate and analyze the effects of the inlet/outlet key width ratios on the hydraulic characteristics and discharge capacity of the MPKW. To validate the numerical model with the experimental data, the results indicate that the average relative error is 2.96%, which confirms that the numerical model is fairly well to predictthe specifications of flow over on the MPKW. Numerical simulation results indicated that the discharge capacity of the MPKW can be improved up to 8.5% by optimizing the Wi/Wo ratio ranging from 1.53 to 1.67 even if the other parameters of the MPKW keep unchanged. A big Wi/Wo ratio generally leads to an increase in discharge capacity at low heads and a little effect on the discharge efficiency at high heads. The discharge efficiency of the inlet and outlet crests increases up to 9.6% for high heads, while discharge efficiency of the lateral crest decreases up to 23.5% compared with the reference model. The findings of the study revealed that the intrinsic influencing mechanism of the Wi/Wo ratio on the discharge performance of MPKWs.

The Tonle Sap Lake (TSL) Basins of the Lower Mekong are one of the world’s most productive ecosystems and have recently been disturbed by climate change. The SWAT (Soil & Water Assessment Tool) hydrological model is utilized to investigate the effect of future climate scenarios. This study focused on two climate scenarios (RCP2.6 and RCP8.5) with three GCMs (GFDL-CM3, GISS-E2-R-CC, and IPSL-CM5A-MR) and their impact on the hydrological process and extremes in the Sen River Basin, the largest tributary of the TSL basin. The annual precipitation, surface runoff, lateral flow, groundwater flow, and total water yield are projected to decrease in both the near-future (2020–2040) and mid-future period (2050–2070), while actual evapotranspiration is projected to increase by 3.3% and 5.3%. Monthly precipitation is projected to increase by 11.2% during the rainy season and decrease by 7.5% during the dry season. Two climate models (GISS and IPSL model) lead to decreases in 1-day, 3-day, 7-day, 30-day, and 90-day maximum flows and minimum flows flow. Thus, the prediction results depend on the climate model used.

Climate change alters hydrological cycles and streamflow regimes at the local, regional and global levels. In this study, we aimed to assess the change in water balance change and hydrological extremes in the Prek Thnot River Basin of the Lower Mekong in Cambodia through a hydrological model (SWAT) under the two climate change scenarios (RCP2.6 and RCP8.5) following three different GCMs. An ensemble of 3 GCMs included GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR models and was applied to a well-calibrated SWAT model through climate change factors. Annual precipitation under RCP2.6 likely decreases by 0.1–0.5% for the near future (2021–2040) and mid-future (2051–2070) and decreases by 0.2–1.3% under RCP8.5. The decrease in precipitation will lead to reductions in water yield by 1–4% (RCP2.6) and 2–5% (RCP8.5). However, peak flow is expected to increase, while the low flow was projected to decrease (1–2% for RCP2.6 and 8–9% for RCP8.5). The study further found that high flow events will increase in both magnitude and frequency. The finding highlights water resources management issues in the Prek Thnot River Basin, including the frequency of future flood events.

The watershed assessment provides information about the condition of water quality and biological integrity to identify the source of stressors and their impacts. In the present decades, different watershed assessment method has been established to evaluate the cumulative impacts of human activities on watershed health and aquatic systems. This study proposes a new approach for assessing watershed vulnerability to contamination based on spatial analysis using the Geographic Information System (GIS). and Analytic Hierarchy Process (AHP) methods. This new procedure designed to identify vulnerable zones depends on seven basic factors representing watershed characteristics: land use/land cover, sediment load, nitrate load, phosphorus load, soil type, average annual precipitation, and slope. The new watershed vulnerability assessment technique was used to create a map showing the relative vulnerabilities of specific sub-watersheds in the Sen River Basin, the largest sub-basin of the Tonle Sap Lake. The results showed a remarkable difference in watershed susceptibility between the sub-watersheds in their vulnerability to pollution. The approximate area of 10,846 km2 (76%) in the Northwest part and the long distance from the river of the study area were categorized in a range from moderate, low, and very low watershed vulnerability. However, consisting 3,341 km2 (24%) located downstream and near distance from the river were displayed as a very high and high vulnerability to pollution in the watershed. Furthermore, the results of the evaluation of the predictive reliability of the watershed vulnerability assessment method revealed that the proposed approach is suitable as a decision-making tool to predict watershed health. The process of this study indeed provides an application performance for the Sen River Basin and calls for action to sustain the water ecosystem and use.

The exacerbation of flood inundation has been highlighted by revolving around climate change, given that this global phenomenon aggravates the flooding status quo and poses a multitude of collateral damages to humans and societies. Climate change leads to changes in precipitation patterns and thereby changes in patterns of discharge and flood inundation. Consequently, estimating these changes using any numerical models is deemed essential to develop countermeasures. There is growing anticipation of climate change over Cambodia and the lower Prek Thnot River Basin, yet it is unknown how serious climate change is on the basin's hydrological and especially flood characteristics. Thus, this study aims to assess the change impacts by integrating the Rainfall-Runoff-Inundation (RRI) model and the Soil and Water Assessment Tool (SWAT) model to apply in the lower Prek Thnot River Basin, Cambodia, whose susceptibility to climate change is relatively high. The RRI model was integrated with the discharge outputs from the SWAT model to simulate four flood events: 2000, the biggest flood event, 2001, 2010, and 2020 flood events. Hydrographs, water levels, and flood extent maps for these events were generated. The biggest flood event of 2000 was set as the baseline period for a possible flood event in the future projected climate in the 2030s and 2060s using three general circulation models (GCMs) [Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL-CM3), Goddard Institute for Space Studies Model E2, coupled with Russell and interactive terrestrial Carbon Cycle (GISS-E2-R-CC), and Institut Pierre-Simon Laplace Coupled Model, version 5A, Medium Resolution (IPSL-CM5A-MR)] under two representative concentration pathways (RCPs) including RCP2.6 and RCP8.5 emission scenarios. The results of climate change suggested that flood magnitude in the lower Prek Thnot River Basin will vary slightly in the 2030s and greatly in the 2060s. In the 2030s, relative changes in precipitation between -8.2% and 21.6% could lead to relative changes in extreme flow (Q(5), flow exceeding 5% of the time) of -21.4% - 8.4%, changes in peak water level of -6.3% - 1.3%, and changes in inundated area of -12% - 7.4%. In the 2060s, relative changes in precipitation between -17.7% and 46.4% could lead to relative changes in Q(5) of -39.3% - 13.5%, in peak water level of -19.6% - 5.2%, and in inundated area from -31.4% to 20.7%. These relative changes imply the increases and decreases due to climate change compared with the present climate. Overall, the resultant outputs of this study revealed how impactful climate change is on the basin's hydrological and flood characteristics, which have long been in question. The results can thus be deployed to underpin decision making and offset the impending negative impacts of climate change on flood inundation within the lower Prek Thnot River Basin. (C) 2022 American Society of Civil Engineers.

The Mekong River Basin (MRB) in Southeast Asia is among the world’s ten largest rivers, both in terms of its discharge and sediment load. The spatial and temporal resolution to accurately determine the sediment load/yield from tributaries and sub-basin that enters the Mekong mainstream still lacks from the large-scale model. In this study, the SWAT model was applied to the MRB to assess long-term basin hydrology and to quantify the sediment load and spatial sediment yield in the MRB. The model was calibrated and validated (1985–2016) at a monthly time step. The overall proportions of streamflow in the Mekong River were 34% from surface runoff, 21% from lateral flow, 45% from groundwater contribution. The average annual sediments yield presented 1295 t/km2/year in the upper part of the basin, 218 t/km2/year in the middle, 78 t/km2/year in the intensive agricultural area and 138 t/km2/year in the highland area in the lower part. The annual average sediment yield for the Mekong River was 310 t/km2/year from upper 80% of the total MRB before entering the delta. The derived sediment yield and a spatial soil erosion map can explicitly illustrate the identification and prioritization of the critical soil erosion-prone areas of the MR sub-basins.

Methane constitutes a minor part in entire greenhouses emission, nevertheless its global warming potential is 20-30 times higher than carbon dioxide. Moreover, methane production and emission increases with eutrophication, and it has been demonstrated to serve as a carbon source for pelagic food webs. The methane oxidising bacteria (MOB) are the only group, which is able to utilize methane, as their sole source of carbon and energy and make a link between methane producers and higher food chain levels. However, the environmental controls of this alternative carbon source for plankton community in lakes with different trophic statuses are still poorly understood. Here we evaluated differences in fluxes of CH4 and its potential as a carbon source for zooplankton in relation to the trophy state of two lakes. For this purpose two morphologically similar lakes with different trophy levels were investigated to determine methane efflux as well as zooplankton community structure and carbon stable isotope ratios in order to determine sources (C/C). The results revealed that C-CH4 is incorporated into zooplankton through MOB community. Based on literature C-CH4 has lower share of total carbon built into zooplankton in environments with higher trophy. However, our result showed that this process is more sophisticated and depends on food selectivity and vertical distribution of zooplankton members. This research was funded by National Science Centre in Poland, project no. UMO-2015-18/M/NZ8/00119.