Assessment of three long-term gridded climate products for hydro-climatic simulations in tropical river basins

10.3390/w9030229Water (Switzerland)9322

Evaluation of Satellite-Based Rainfall Estimates in the Lower Mekong River Basin (Southeast Asia)

Satellite-based precipitation is an essential tool for regional water resource applications that requires frequent observations of meteorological forcing, particularly in areas that have sparse rain gauge networks. To fully realize the utility of remotely sensed precipitation products in watershed modeling and decision-making, a thorough evaluation of the accuracy of satellite-based rainfall and regional gauge network estimates is needed. In this study, Tropical Rainfall Measuring Mission (TRMM) Multi-Satellite Precipitation Analysis (TMPA) 3B42 v.7 and Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) daily rainfall estimates were compared with daily rain gauge observations from 2000 to 2014 in the Lower Mekong River Basin (LMRB) in Southeast Asia. Monthly, seasonal, and annual comparisons were performed, which included the calculations of correlation coefficient, coefficient of determination, bias, root mean square error (RMSE), and mean absolute error (MAE). Our validation test showed TMPA to correctly detect precipitation or no-precipitation 64.9% of all days and CHIRPS 66.8% of all days, compared to daily in-situ rainfall measurements. The accuracy of the satellite-based products varied greatly between the wet and dry seasons. Both TMPA and CHIRPS showed higher correlation with in-situ data during the wet season (JuneSeptember) as compared to the dry season (NovemberJanuary). Additionally, both performed better on a monthly than an annual time-scale when compared to in-situ data. The satellite-based products showed wet biases during months that received higher cumulative precipitation. Based on a spatial correlation analysis, the average r-value of CHIRPS was much higher than TMPA across the basin. CHIRPS correlated better than TMPA at lower elevations and for monthly rainfall accumulation less than 500 mm. While both satellite-based products performed well, as compared to rain gauge measurements, the present research shows that CHIRPS might be better at representing precipitation over the LMRB than TMPA

Dissémination des bactéries indicatrices de contamination fécale dans les hydrosystèmes tropicaux : transport et devenir d'Escherichia coli dans le bassin versant du Mékong au Laos

La contamination fécale des eaux de surface demeure une menace majeure pour la santé publique, en particulier dans les zones rurales des pays en développement. Les maladies diarrhéiques sont l'une des principales causes de décès notamment chez les enfants de moins de cinq ans, en raison de manque d'infrastructures sanitaires, et du faible accès aux ressources en eau salubre et aux soins médicaux. Plus de 70 millions de personnes dépendent de ressources en eau non améliorées dans le bassin inférieur du Mékong. Les progrès significatifs réalisés pour mieux comprendre la dynamique de la contamination fécale en milieu tempéré, de nombreuses lacunes subsistent en milieu tropical. Pour réduire la morbidité, il est nécessaire de mieux comprendre la dynamique des pathogènes fécaux surtout dans le contexte de changements globaux (croissance démographique, changements d'usage des terres, barrages hydroélectriques, et changement climatique). L'utilisation d'une approche multidisciplinaire est essentielle pour évaluer les risques contamination fécale à l'interface animal-homme-écosystème. L'objectif principal de la thèse est d'identifier les facteurs clés contrôlant le devenir et transport de la bactérie fécale indicatrice (FBI), Escherichia coli (E. coli), à différentes échelles spatiales des tributaires du Mekong au Laos. Cette thèse présente les résultats basés sur (i) les données in situ collectées dans les principaux tributaires du Mékong au Laos, afin d'identifier les facteurs (hydrologie et utilisation des terres) contrôlant les concentrations d'E. coli à l'échelle du bassin versant ; (ii) une approche expérimentale pour évaluer deux facteurs clés (rayonnement solaire et dépôt de particules en suspension) contrôlant la mortalié/survie d'E. coli dans une zone humide tropicale montagneuse; et (iii) des approches statistiques et de modélisation pour évaluer l'impact d'un barrage hydroélectrique sur la dynamique hydro-sédimentaire et d'E. coli dans un tributaire majeur du Mékong, la Nam Khan, au Laos. Les résultats des campagnes de mesures in situ ont révélé des variabilités saisonnières des concentrations d'E. coli dans les cours d'eau, plus élevées pendant la saison humide, et fortement corrélées aux concentrations en matières en suspension (MES) et aux pourcentages de forêts exploitées à l'échelle du bassin versant. Ces résultats soulignent le rôle des MES en tant que vecteurs pour le transport bactérien, ainsi que l'importance de des usages des terres comme l'un des facteurs clés ayant un impact sur la dissémination d'E. coli à l'échelle du bassin versant dans un contexte tropical érosif. La majorité des tributaires échantillonnés présentaient des concentrations d'E. coli pendant la saison des pluies, dépassant 500 colonies par 100 ml, seuil au-delà duquel l'OMS considère que le risque de maladie gastro-intestinale après une seule exposition est de 10 %. Le rôle des MES a été mis en évidence dans l'approche expérimentale, où les bactéries attachées à des particules étaient prédominantes (91%) et présentaient des taux de mortalité plus faibles que ceux des bactéries sous forme libre. Alors que le processus de dépôt était le principal facteur de réduction du stock d'E. coli dans la colonne d'eau, comparé aux radiations solaires, nous avons constaté que la remise en suspension temporaire des sédiments déposés suggérait un potentiel de survie ou même de croissance d'E. coli dans les sédiments en milieu tropical. Enfin, étant donné l'importance de la dynamique hydro-sédimentaire sur la dissémination bactérienne, nous avons évalué l'impact du barrage, reflété par des diminutions brutales en termes de débit (en moyenne de 42%), et les concentrations en MES et E. coli (en moyenne de 89%) mesurées en aval du barrage. Cette approche fournit de nouvelles preuves de l'atténuation de la contamination bactérienne induite par le barrage. Dans l'ensemble, ces résultats de thèse fournissent de nouvelles informations sur la dynamique des FBI dans le bassin inférieur du Mékong, qui pourraient être utiles dans l'établissement des stratégies efficaces de gestion des ressources en eau.Fecal contamination of surface water remains a major threat to public health especially in the rural areas of developing countries. Diarrheal diseases are a leading cause of death especially among children under age five, due to inadequate sanitation infrastructure, low access to safe water resources, and poor medical care in developing countries. Over 70 million people depend on unimproved water resources in the lower Mekong basin stretching from southern Chinese border to the delta in southern Vietnam. Despite the significant advances made towards a better understanding of the fecal contamination dynamics in temperate regions, yet many knowledge gaps exist in tropical conditions. Reducing the disease burden, requires a better understanding of fecal pathogens dynamics in the context of rapid global changes, e.g. population growth, land use changes, hydropower dam constructions, and climate change. Therefore, the use of a multi-disciplinary approach is essential to adress existing and potential risks of fecal contamination at the animal-human-ecosystems interface. The main objective here was to identify key factors controlling the fate and transport of the fecal indicator bacteria (FIB), Escherichia coli (E. coli), at different spatial scales of major Mekong tributaries in Lao PDR. This research work presents the results from (i) in-situ data collected from major Mekong tributaries from northern to southern Lao PDR aiming to identify main factors (hydrology and land use) controlling the in-stream E. coli concentrations at watershed-scale; (ii) experimental approach to assess two key factors (solar radiation exposition and suspended particles deposition) controlling E. coli decay/survival in a mountainous tropical headwater wetland; and (iii) statistical and modeling approaches to assess the impact of hydropower dam on hydro-sedimentary and E. coli dynamics in a major Mekong tributary, the Nam Khan in northern Lao PDR. Our spatial and temporal monitoring results reported seasonal variabilities of in-stream E. coli concentrations, significantly higher during the wet season, and strongly correlated to total suspended sediment (TSS) concentration, and unstocked forests percentage areas at watershed-scale. These results point out the role of TSS as an important vector for bacterial transport, as well as the importance of land use management as one of major factors affecting E. coli dissemination at watershed-scale in a tropical context prone to soil erosion. The majority of sampled tributaries had E. coli concentrations during the rainy season, exceeding 500 colonies per 100 mL, the threshold above which the WHO considers a 10% risk of gastrointestinal illness after one single exposure. The role of TSS in E. coli dynamics was further highlighted in the experimental approach, where particle-attached E. coli were predominant (91%) and showed lower decay rates as opposed to those of free-living E. coli. While deposition process was the main factor for E. coli stock reduction in the water column as opposed to solar radiation, we found that temporary resuspension of deposited sediments suggested a potential E. coli survival or even a regrowth in the sediment under tropical conditions. At last, given the importance of the hydro-sedimentary dynamics on bacterial dissemination, we assessed the dam impact reflected by abrupt decreases in terms of discharge (by an average of 42%), as well as TSS and E. coli concentrations (by an average of 89% for both) measured downstream of the dam. These statistical and modeling approaches provide new evidence of the attenuation of the bacterial contamination by the dam reservoir. Overall, this thesis work provides new insights on FIB dynamics in a tropical context that could be helpful in establishing effective strategies for water resource management

Effect of baseline meteorological data selection on hydrological modelling of climate change scenarios

This study evaluates how differences in hydrological model parameterisation resulting from the choice of gridded global precipitation data sets and reference evapotranspiration (ETo) equations affects simulated climate change impacts, using the north western Himalayan Beas river catchment as a case study. Six combinations of baseline precipitation data (the Tropical Rainfall Measuring Mission (TRMM) and the Asian Precipitation – Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE)) and Reference Evapotranspiration equations of differing complexity and data requirements (Penman-Monteith, Hargreaves –Samani and Priestley – Taylor) were used in the calibration of the HySim model. Although the six validated hydrological models had similar historical model performance (Nash–Sutcliffe model efficiency coefficient (NSE) from 0.64-0.70), impact response surfaces derived using a scenario neutral approach demonstrated significant deviations in the models’ responses to changes in future annual precipitation and temperature. For example, the change in Q10 varies between -6.5 % to -11.5% in the driest and coolest climate change simulation and +79% to +118% in the wettest and hottest climate change simulation among the six models. The results demonstrate that the baseline meteorological data choices made in model construction significantly condition the magnitude of simulated hydrological impacts of climate change, with important implications for impact study design.NER

Impact of human intervention and climate change on natural flow regime

According to the ‘natural flow paradigm’, any departure from the natural flow condition will alter the river ecosystem. River flow regimes have been modified by anthropogenic interventions and climate change is further expected to affect the biotic interactions and the distribution of stream biota by altering streamflow. This study aims to evaluate the hydrologic alteration caused by dam construction and climatic changes in a mesoscale river basin, which is prone to both droughts and monsoonal floods. To analyse the natural flow regime, 15 years of observed streamflow (1950–1965) prior to dam construction is used. Future flow regime is simulated by a calibrated hydrological model Soil and Water Assessment Tool (SWAT), using ensemble of four high resolution (~25 km) Regional Climate Model (RCM) simulations for the near future (2021–2050) based on the SRES A1B scenario. Finally, to quantify the hydrological alterations of different flow characteristics, the Indicators of Hydrological Alteration (IHA) program based on the Range of Variability Approach (RVA) is used. This approach enables the assessment of ecologically sensitive streamflow parameters for the pre- and post-impact periods in the regions where availability of long-term ecological data is a limiting factor. Results indicate that flow variability has been significantly reduced due to dam construction with high flows being absorbed and pre-monsoon low flows being enhanced by the reservoir. Climate change alone may reduce high peak flows while a combination of dam and climate change may significantly reduce variability by affecting both high and low flows, thereby further disrupting the functioning of riverine ecosystems. We find that, in the Kangsabati River basin, influence of dam is greater than that of the climate change, thereby emphasizing the significance of direct human intervention

Hydrological evaluation of open-access precipitation and air temperature datasets using SWAT in a poorly gauged basin in Ethiopia

Precipitation and air temperature are key drivers of watershed models. Currently there are many open-access gridded precipitation and air temperature datasets at different spatial and temporal resolutions over global or quasi-global scale. Motivated by the scarcity and substantial temporal and spatial gaps in ground measurements in Africa, this study evaluated the performance of three open-access precipitation datasets (i.e. CHIRPS (Climate Hazards Group InfraRed Precipitation with Station data), TRMM (Tropical Rainfall Measuring Mission) and CFSR (Climate Forecast System Reanalysis)) and one air temperature dataset (CFSR) in driving Soil and Water Assessment Tool (SWAT) model in simulation of daily and monthly streamflow in the upper Gilgel Abay Basin, Ethiopia. The “best” available measurements of precipitation and air temperature from sparse gauge stations were also used to drive SWAT model and the results were compared with those using open-access datasets. After a comprehensive comparison of a total of eight model scenarios with different combinations of precipitation and air temperature inputs, we draw the following conclusions: (1) using measured precipitation from even sparse available stations consistently yielded better performance in streamflow simulation than using all three open-access precipitation datasets; (2) using CFSR air temperature yielded almost identical performance in streamflow simulation to using measured air temperature from gauge stations; (3) among the three open-access precipitation, overall CHIRPS yielded best performance. These results suggested that the CHIRPS precipitation available at high spatial resolution (0.05°) together with CFSR air temperature can be a promising alternative open-access data source for streamflow simulation in this data-scarce area in the case of limited access to desirable gauge data

The role of reservoirs under the impacts of climate change on the Srepok River basin, Central Highlands of Vietnam

Forecasting streamflow is important for managing future water resources and environmental needs under the impacts of climate change. Moreover, quantifying the combined effects of future climate variations and human-made infrastructures, e.g., dams and reservoirs, poses a significant challenge. In this study, we used the Soil and Water Assessment Tool (SWAT) for a case study in the Srepok River Basin—a tributary of the Mekong River Basin. Here, we aim to reveal the impacts of various climate change scenarios and the effects of reservoir operations in this region. Our findings indicate that 1) the projected annual streamflow is anticipated to increase by a minimum of 9.2% (2046–2065) and could peak at an increase of 14.9% (2080–2099) under the highest greenhouse gas emissions, 2) Srepok 4, Srepok 3, and Buon Kuop demonstrate a higher capability for mitigating flood peaks and managing seasonal flow in the downstream floodplain, whereas Buon Tua Srah shows the least performance, and 3) reservoirs operated with annual regulation have more pronounced impacts than those regulated on a daily schedule. Our work provides i) a scientific foundation for regional stakeholders and decision-makers to develop sustainable strategies that address the combined effects of reservoir operation and future climate, and ii) it supports national authorities and officials in resolving conflicts related to transboundary rivers within the Mekong River Basin

Advancing understanding of development policy impacts on transboundary river basins: Integrated watershed modelling of the Lower Mekong Basin.

The management of transboundary river basins across developing countries, such as the Lower Mekong River Basin (LMB), is frequently challenging given the development and conservation divergences of the basin countries. Driven by needs to sustain economic performance and reduce poverty, the LMB countries are embarking on significant land use changes in the form hydropower dams, to fulfill their energy requirements. This pathway could lead to irreversible changes to the ecosystem of the Mekong River, if not properly managed. This thesis aims to explore the potential effects of changes in land use —with a focus on current and projected hydropower operations— on the Lower Mekong River network streamflow and instream water quality. To achieve this aim, this thesis first examined the relationships between the basin land use/land cover attributes, and streamflow and instream water quality dynamics of the Mekong River, using total suspended solids and nitrate as proxies for water quality. Findings from this allowed framing challenges of integrated water management of transboundary river basins. These were used as criteria for selecting eWater’s Source modelling framework as a management tool that can support decision-making in the socio-ecological context of the LMB. Against a combination of predictive performance metrics and hydrologic signatures, the model’s application in the LMB was found to robustly simulate streamflow, TSS and nitrate time series. The model was then used for analysing four plausible future hydropower development scenarios, under extreme climate conditions and operational alternatives. This revealed that hydropower operations on either tributary or mainstream could result in annual and wet season flow reduction while increasing dry season flows compared to a baseline scenario. Conversely, hydropower operation on both tributary and mainstream could result in dry season flow reduction. Both instream TSS and nitrate loads were predicted to reduce under all three scenarios compared to the baseline. These effects were found to magnify under extreme climate conditions, but were less severe under improved operational alternatives. In the LMB where hydropower development is inevitable, findings from this thesis provide an enhanced understanding on the importance of operational alternatives as an effective transboundary cooperation and management pathway for balancing electricity generation and protection of riverine ecology, water and food security, and people livelihoods