Park, Edward

In recent decades, global climate change has made natural hazards increasingly prevalent. Droughts, as a common natural hazard, have been a hot study topic for years. Most studies conducted drought monitoring in arid and semi-arid regions. In humid and sub-humid regions, due to climate change, seasonal droughts and seasonal water shortages were often observed too, but have not been well studied. This study, using a MODIS satellite-based aridity index (SbAI), investigated spatiotemporal changes in drought conditions in the subtropical Pearl River Basin. The study results indicated that the inter-annual SbAI exhibited a significant decreasing trend, illustrating a wetter trend observed in the basin in the past two decades. The decreasing trend in the SbAI was statistically significant in the dry season, but not in the monsoon season. The drought conditions displayed an insignificant expansion in the monsoon season, but exhibited statistically significant shrinking in the dry season. The Pearl River Basin has become wetter over past two decades, probably due to the results of natural impacts and human activities. The areas with increased drought conditions are more likely impacted by human activities such as water withdrawal for irrigation and industrial uses, and fast urbanization and increased impervious surfaces and resultant reduction in water storage capacity. This study provided a valuable reference for drought assessment across the Pearl River Basin.

The Mekong plume, which sustains the geomorphology and rich biodiversity along the coastal zone of the Vietnamese Mekong Delta (VMD), has been under intensifying threats from dams, riverbed mining and sea level rise. However, our understanding of how much the intensifying stressors have altered the long-term spatiotemporal dynamics of the plume remains limited. In this paper, we investigate the spatiotemporal dynamics of the Mekong plume over 1988–2022 based on remote sensing, in-situ hydrological data, and continuous wavelet transform. Specifically, we analyze (1) the variability of the long-term average seasonal plume extent, (2) the interannual trends of the seasonal suspended sediment discharge and alongshore drift, and (3) changes in the long-term exposure of coastal mangroves to suspended sediment. Field suspended sediment concentration (SSC) data from five major gauging stations representing the upstream, midstream, and downstream sections of the VMD between 2010–2021 were used to calibrate 101 Landsat surface reflectance observations to estimate SSC. The top-performing algorithm with band combination (NIR SWIR2) * (G / R) was selected for SSC retrieval (R2 = 0.72, SE = 40.6 mg L−1). Our results reveal a 63 % decline in plume extent in the dry season (Dec–May) and an 83 % decline in the wet season (Jun–Nov) between 1988–1999 and 2000–2022. The seasonal variation in suspended sediment discharge (SSD) remained relatively constant until 2014 but significantly weakened thereafter. Despite the decline in the plume, the surface SSC at the alongshore drift began to rise in 2008, marking a reversal in the trend after two decades of descent. The upswing in the surface SSC is associated with rising SSD in the dry season, which we attribute mainly to the release of water from dams upstream of the VMD. However, if the post-2014 decline of wet season SSD persists in the long term, it would deplete sediment deposition around the estuary and starve the alongshore drift. Along the coastline of the Mekong Delta, the annual exposure to suspended sediments declined by 37–54 % between 1988–1999 and 2000–2022, corresponding to a maximum annual mangrove loss of 15 % by coastal erosion. The decrease in plume and coastal exposure is primarily attributed to reduced terrestrial sediment flux, largely influenced by the construction and operation of transnational dams. To restore the Mekong plume, it is imperative to advocate for cross-border governance of dam operations.

This study proposes a new method to retrieve the bathymetry of turbid-water floodplains from the inundation frequency (IF) data derived from over 32 years of composite optical remote sensing data. The new method was tested and validated over the Curuai floodplain in the lower Amazon River, where the entire bathymetry was surveyed in 2004, and water level gauge data has been available since 1960. The depth was estimated based on the relationship derived from IF and surveyed depth data, and the results were compared to those retrieved from bare-Earth DEM. We further assessed the sensitivity of the approach by analyzing the deepest part of the lake (i. e., permanent water body ~ 8m) with high IF, as well as the effect of gradual sedimentation in the lake over time. The results showed that the model is highly accurate and sensitive to IF changes even in the permanent water body areas, suggesting that this model can be used in other seasonal lakes worldwide with turbid-waters, where large-scale bathymetry surveys are not feasible due to high operation costs.

Ecotourism in national parks of developing countries is increasingly recognised as a promising option to achieve sustainable development goals, regardless, might imply various paradoxical managerial challenges. This paper, therefore, seeks to contribute a methodological framework utilising ES-based social landscape metrics (SLM) to address the potential barriers in managing ecotourism-integrated multi-functional national parks. We present a mixed-method case study in Vietnam's Tram Chim National Park (TCNP), conducted via semi-structural interviews and PGIS with tourists and locals. Multiple key informants, i.e. TCNP's authorities were also interviewed to provide their managerial insights and assist in verifying the PGIS results obtained from the tourists and locals. Via the quantified and mapped SLMs, the study reveals the differences between tourists and locals in terms of how and where they perceive and appreciate the intangible values of TCNP. Through spatial statistics, we reported important spatial correlations (i) between different categories of Ecosystem Services (ES) and (ii) between ES richness and diversity on different TCNP's land covers. As a contribution to the decision-making outlook, we remarked potential areas to expand of ecotourism activities based on the spatial hot and cold spots. This study concludes by highlighting opportunities for future research in expanding on socio-geographical assessments of ES, especially in the fields of ecotourism.

Here we present a proof of concept evaluation of the impacts of riverbed-mining on river-wetland connectivity by analyzing the temporal trends of the flood frequencies in the Vietnamese Mekong Delta (VMD), whilst accounting for the effect of dyke constructions. We focus on the Long Xuyen Quadrangle (LXQ), which is significant in terms of biodiversity and economic contribution to the VMD as it is one of the most important food baskets of Southeast Asia that depends on seasonal flooding. Our results indicate that the flood frequency in LXQ has decreased significantly over the past 20 years (1995-2015). Time-series analyses of water level data at Chau Doc, Tan Chau, and Can Tho stations confirmed that the overall descending trend is statistically significant (p-value<0.001 and tau ~0.1). However, the river discharge at Kratie showed no significant trend (p-value=0.98) over the same period. This indicates that the flood frequency is associated with the lowering of the riverbed (incision) other than climatic factors. The connectivity analysis also revealed a remarkable drop in the inundation duration after early 2000, which corresponds to the previous observations of the shifting shoreline of the VMD from construction to shrinking. Finally, regression and principal component analyses underpinned the strong causality between the riverbed-mining and the decreased seasonal flooding patterns in LXQ, whilst accounting for the effect of the dyke system over the last decades (R2=0.75). This study offers compelling evidences on the relationship between sand-mining in the river and the disrupted flood regimes in VMD. The reduction in water and sediments that is necessary for sustaining current rates of agricultural production in the long term would endanger the livelihoods of millions of VMD inhabitants.

Global measurements of reservoir water levels are crucial for understanding Earth’s hydrological dynamics, especially in the context of global industrialization and climate change. Although radar altimetry has been used to measure the water level of some reservoirs with high accuracy, it is not yet feasible unless the water body is sufficiently large or directly located at the satellite’s nadir. This study proposes a gauging method applicable to a wide range of reservoirs using Sentinel–1 Synthetic Aperture Radar data and a digital elevation model (DEM). The method is straightforward to implement and involves estimating the mean slope–corrected elevation of points along the reservoir shoreline. We test the model on six case studies and show that the estimated water levels are accurate to around 10% error on average of independently verified values. This study represents a substantial step toward the global gauging of lakes and reservoirs of all sizes and in any location where a DEM is available.

In this paper, we address the impact of agricultural expansion on hydrological patterns of water and sediment budget in one of the largest floodplains along the Cambodian Mekong since the 1980s, using field (water level, discharge, sediment, rainfall, and groundwater level) and remote sensing (land use, surface suspended sediment) data, and numerical simulations. Specifically, while both the surface suspended sediment concentration and water level in the Mekong River around Kampong Cham and Neak Luong decreased, the floodplain seasonal water storage increased. In addition, the rate of sediment input from the river to the floodplain was almost constant throughout the studied period. The investigation of the floodplain’s annual sediment budget, however, reveals a significant drop during the analyzed period, mainly due to the decreased sediment trapping rate (66% in the 1980s to 46% in the 2010s). Currently, a good amount of sediment bypass the floodplain and return back to the river. The observed hydrological patterns in the floodplain could have been triggered by the agricultural expansion that has increased surface erodibility (due to removals of primary vegetation) and lowered the land surface elevation (due to groundwater extraction). Despite the well-known impacts of the hydropower dams on the Mekong Delta hydrological conditions, particularly the reduction of sediment reaching the delta due to sediment trapping by dam reservoirs, our observations point to new and more localized driving factors of sediment deficit in floodplain: agricultural expansion. Finally, we used 2D hydrodynamic simulation (Telemac-2D) to visualize the processes of water routing and sedimentation in the floodplain accounting for land cover change since the 1980s. The floodplain hydrology reported in this paper is an unexplored environmental consequence of agricultural expansion in the lower Mekong. Geomorphologically, this study presents a peculiar case of floodplain showing how agricultural expansion can diminish the role of a floodplain as a sediment sink through decreasing the trapping rate.