Modelling water quality dynamics with remote sensing images: Tonle Sap Lake, Cambodia, the largest freshwater lake in South East Asia

Abstract

Eutrophication is one of the problems the world needs to tackle to achieve one of the targets of the United Nations’ Sustainable Development Goal 6: to improve water quality by 2030. Albeit a global problem that has been studied for many years, there is a huge gap in the reporting of eutrophication incidences and associated research work between the Global South and Global North countries. However, the available evidence suggests that nutrient loads and hence eutrophication may be increasing in the Global South. Nevertheless, assessing these trends with confidence can be challenging, especially in the Global South, because long-term (i.e. multidecadal) water quality monitoring data, which is important for the formulation of science-based environmental policy, is very scarce. The utilisation of freely accessible satellite images to monitor long-term (i.e. multi-decadal) trends in water quality indicators is a potential solution to this challenge, as has already been used by the Great Lakes in North America and UNESCO. This thesis aims to explore the potential of employing satellite-based methods to evaluate water quality trends in large water bodies in the Global South, using the Tonle Sap Lake, Cambodia, as a case study. The work focuses on retrieving spatial and multi-decadal (1990-2019) temporal trends of two water quality parameters: (i) Total Suspended Sediment (TSS) and (ii) chlorophyll-a (Chl-a) concentration of the Tonle Sap Lake using satellite-based techniques. Tonle Sap Lake is the largest freshwater lake in South East Asia, whose hydrology is an integral part of the Mekong River, an international river that flows through 6 countries. The lake experiences an unusual flow reversal from and to the Mekong River every six months, supporting a wide range of valuable ecosystem functions. Specifically, the Tonle Sap Lake and its flow reversal play an important role in the economic, ecological, and socio-cultural life of Cambodia and, indeed, the broader South East Asia region. With the Mekong Basin undergoing rapid development and with the threat posed by ongoing climate change, there is a real concern that alterations of the Tonle Sap’s flow regime, changes in annual water level, open water area, and suspended solid delivery from the Mekong River may adversely impact these critical ecosystem services. However, little is known about the current and recent historical eutrophication status of Tonle Sap Lake, and water quality monitoring efforts are woefully inadequate. For this study, a field campaign undertaken in February 2019 collected new ground-based measurements of TSS concentrations, chl-a concentrations, and surface reflectances of land covers/water surfaces, which have allowed regression relationships to be developed that link the surface reflectances from Landsat OLI imagery to these critical water quality parameters. These regression relationships were then used to estimate trends in TSS and chl-a during the period 1990-2020 at each of five locations across the lake, these locations representing discrete environments within the lake that may be affected by drivers of change in different ways. The satellite-based retrievals of TSS and chl-a concentrations were analysed, together with water level data from a gauging station located at Kampong Luong on the shores of the lake, to develop data-driven conceptual models of the response of TSS and chl-a to seasonal and inter-annual variations in the lake hydrology during the study period. These models highlight that TSS concentrations vary in opposite phase with the seasonal water cycle, with maximum values of TSS concentration induced by wind-generated waves resuspending fine-grained sediments from the bed of the lake during the dry season. In contrast, seasonal and inter-annual variations in chl-a have a more complex response, being influenced not only by water levels but likely also by the nutrients associated with suspended sediments. Thus the lowest values of chl-a concentration typically occur during the low water period, while the highest chl-a concentrations occur during the period of high water as the lake begins to flow out towards the Mekong River (October to December).The data-driven conceptual models also highlight variability in the TSS and chl-a concentrations driven by inter-annual variability in the magnitude of the Tonle Sap Lake’s annual flood. Overall, the new conceptual models based on the satellite retrievals cast new light on the seasonal and inter-annual variations in TSS and chl-a, and their inter-relationship, during the last 30 years. An important conclusion of the research presented herein is that, contrary to some studies that have speculated on the risks of deteriorating water quality, the Tonle Sap Lake is in a mesotrophic condition, with little evidence of any statistically significant long-term (i.e. multidecadal) trend in chl-a concentrations.

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Identifiers

ORCID for Mukarramah Yusuf: orcid.org/0000-0002-4369-0721

ORCID for Stephen Darby: orcid.org/0000-0001-8778-4394

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