Intensifying saline water intrusion and drought in the Mekong Delta: From physical evidence to policy outlooks

Highlights

• Recent intensification of salinity intrusion in the Mekong Delta is assessed.

• Strengthened backwater effect is indicated by the wavelet and rating curve analyses, and numerical modeling.

• Riverbed incision is identified as the main driver of the saline water intrusion.

• Novel approach is introduced to decouple compound effect of salinity intrusion from multiple drivers.

• Political reform is recommended based on the new findings.

Abstract

This paper assesses the recently intensified saline water intrusion (SI) and drought in the Vietnamese Mekong Delta (VMD). While the existing literature predominantly points the cause of drought to the hydropower dams in the upstream of the Mekong Basin, we contribute new physical evidence of the intensification of saline water intrusion (through backwater effect) in the VMD caused by three anthropogenic drivers: riverbed incision (due to both riverbed mining and dam construction), sea level rise and land subsidence. Thereupon, we highlight that it is critical to not underestimate the impacts from the localized factors, especially the riverbed-mining which can incise the channel by up to 15 cm/year and amplify the salinity intrusion. Our analysis is based on the extensive sets of hourly-to-daily hydrological time series from 11 gauge stations across the VMD. First, several signs of significantly increased tidal amplification (up to 66%) were revealed through the spectral analysis of the hourly water level data. This trend was further validated through the changes in slopes of the rating curves at the tidal zones, implying the relationships between the shift of the backwater effects on the rivers in VMD and the lowered water levels caused by the riverbed incision. Finally, we introduce a novel approach using the annual incision rates of the riverbed to compare four SI driving factors in terms of their relative contributions to the balance between fresh and saline water in the VMD.

Introduction

The Mekong River originates from the Tibetan-Qinghai Plateau, runs across China and five other countries, and forms the Vietnamese Mekong Delta (VMD) at the downstream course before draining into the East Sea. With a total area of more than 4 million ha, VMD houses 17 million people and accounts for half of the country's food supplies. With the current productivity of ~25 million tons of rice every year, VMD not only safeguards the country's food security but also contributes about 18.7% of the national GDP through agricultural products exports (Dung et al., 2019; Eslami et al., 2019; GSO, 2016; Piesse, 2019). The sustainable development of this important food basket, however, is under several internal and external environmental pressures (e.g. Arias et al., 2019; Trung and Thanh, 2013; Triet et al., 2017). Upstream, there is the looming threat from the accelerated construction of hydropower dams which have drastically modified the hydro-sedimentary regimes of the river (e.g. Anthony et al., 2015; Dinh et al., 2012; Eslami et al., 2019; Van et al., 2012; Yang et al., 2019). Operation of these dam reservoirs, combined with the downstream sea level rise (SLR) has resulted in the retreat of approximately 70% of the Delta's shorelines (Li et al., 2017). Internally in VMD, Park et al. (2020a) reported on the constant decrease of flood frequency over the past two decades due to sand mining and dike construction in both the wet and dry seasons (1995–2015). Several authors, including Brunier et al. (2014); Hackney et al. (2020); and Binh et al. (2020b) have also reported significant geomorphic impacts due to extensive mining in the lower reach of the river such as dramatic riverbed incision and bank erosion. Likewise, due to the groundwater extraction, VMD is currently experiencing land subsidence (LS) at an alarming rate; in the worst-case scenario, this can be as much as a meter by 2050 (Erban et al., 2014).

Within the long list of environmental perils that VMD is facing, the intrusion of saline water is the main threat towards agricultural production as it salinizes the essential freshwater resources, while simultaneously degrading the soil on its penetration path. Annually, approximately 2 million ha of agricultural land are prone to the risks of salinity intrusion (SI) (Smajgl et al., 2015; Toan, 2014). In addition, the intrusion of saline water can disrupt the supply of freshwater for both domestic (e.g. household daily use) and industrial consumers, especially in the coastal areas. Carew-Reid (2008) remarked that during dry seasons, strong northwestward winds combined with flood tides and sea water, can penetrate anywhere inland between 40 and 60 km from the estuaries on both east and west coasts of the delta. While these numbers seem significant, they pale in comparison to observations from the 2016 drought, which is the most severe event over the past 90 years (Kantoush et al., 2017). The event affected 12 out of 13 provinces in VMD (>2 million ha in area); of which were forced to declare a state of natural disaster (Toan, 2017). The total economic loss was estimated to be around 650 million USD, including 800,000 tons of rice completely lost. In addition, the lives of 17 million VMD inhabitants were adversely affected. In 2020, the VMD experienced yet another catastrophic drought which possibly surpasses any historical records, including the ones observed in 2016 (From the local news:https://vtv.vn). The preliminary assessment has shown that the SI, with a salinity rate (i.e. salt concentration) of 4 g/L, has entered 50–130 km deep into the main rivers since February in 2020, a substantial increase compared to the same period in 2016 (United Nations, 2020). In the last 30 years, the interval between the drought events has decreased by at least 2 years: 1992, 1998, 2005, 2010, 2016, and 2020; the close occurrence of the last two historical events constitutes the gravest warning signs regarding the ever-increasing severity of SI.

In principle, SI is the outcome of the competition between upstream fluvial discharges and downstream tidal mixing forces (Pritchard, 1952; Savenije, 2012). However, this might not be entirely true in the case of the VMD, whereby the discharge upstream at Kratie station in Cambodia has not shown a significant trend in the last 20 years, even during the dry period (Park et al., 2020b). Given the stationary discharge trend into the VMD, we argue that the dynamics of water level between the river and the sea is the main control of SI, by continuously changing the intensity of the backwater effects (BWE). Naturally, stronger BWE is imposed on the river when the sea level offshore is relatively higher; the tidal amplication in VMD has risen by 1.5–2 cm/year from 1.2–2 mm/year, from offshore to the inland (Eslami et al., 2019). Therefore, monitoring water levels in rivers in relation to the tidal gauges, can be used for assessment of the BWE. In the VMD, the four causes of the recent increase in SI have been identified by Eslami et al. (2019) and Minderhoud et al. (2018) as upstream dams, riverbed mining, SLR and LS. Even though the upstream dams are predominantly in the spotlight, we consider the localized riverbed mining activities at least comparable regarding the impacts on SI despite having substantially fewer associated studies. However, Binh et al. (2020a) remarked that while it might take more than a decade to realize the impacts at the VMD from the upstream dams, sand mining can cause immediately visible consequences, such as a drop of more than 1 m in less than 15 years (Park et al., 2020a). Climate change-induced SLR also contributes to the increased BWE in the VMD. However, the magnitude of impacts is much slower (1–2 cm/year rise) compared to the sand mining in rivers (10–15 cm/year drop along Tien River) (Erban et al., 2014). Even though there is a general understanding that the intensity and extent of the saline water intrusion are largely determined by fluvial (discharges) and marine (tidal mixing) processes, the roles of localized factors, such as riverbed mining should not be overlooked. In fact, if left unmonitored, these localized factors can even cause far more tangible impacts than the upstream reservoir construction, in terms of the sediment losses downstream (Bravard et al., 2013). Yet the mining-induced morphodynamics in the VMD is spatially complicated (Brunier et al., 2014), hence have only been measured through indirect inferences, such as the developments of tidal signals. More specifically, riverbed incision increases in-land tidal amplitudes and velocities, both in turn enhance the intrusion of saline water (Eslami et al., 2019).

In this paper, we present multiple novel evidence of the impacts of riverbed sand mining, upstream hydropower dams, SLR and LS on the recent intensification of SI in the VMD, focusing on the post-2000 period. We employ four different analytical methods, including time series trend analysis (Mann-Kendall), Fast Fourier Transform (FFT) based power spectral analysis, rating curve analysis and numerical modeling of saline water intrusion in the VMD. In addition to confirming the ascending trend of SI, this paper clarifies the pivotal role of riverbed incision in intensifying the SI related droughts in the VMD. Therefore, this scope of this paper expands into introducing a conceptual approach to estimate the relative contributions to SI from upstream hydropower dams and riverbed mining. Finally, based on the novel biophysical evidence presented, we recommend potential measures for the VMD policy makers to deal with the ever-intensifying intrusion of saline water, incorporating both technical and political aspects.