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A Regional Public Water Supply Scheme for the Mekong Delta—Design in Phases of a Transmission-distribution Network

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Public water supply in the Mekong delta is at risk due to the impact of climate change (i.e. sea level rise; changing rainfall pattern), over-extraction of groundwater, and developments in the catchment area of the Mekong river upstream from Vietnam.  These factors contribute to increasing salinity of groundwater and —primarily in the dry season— surface water.  In a substantial part of the Mekong delta, surface water cannot be relied upon during the dry season as a water source for conventional treatment plants due to high salinity. The dire conditions in the Mekong delta during the dry season of 2016 made this point more than sufficiently clear. The Government of Vietnam considers the current level of groundwater extraction in the Mekong delta to be environmentally unsustainable.  The related induced land subsidence is endangering life in the Mekong delta —largely located at elevations of not more than few meters above sea level— even more than the sea level rise due to climate change (Minderhoud, 2017). In the long-term, business as usual, i.e. relying on groundwater for public water supply, is considered not sustainable in the Mekong delta Southwest of the Hau or Bassac river, which is one of the main branches of the Mekong river in its estuary Moreover, a shift from reliance on groundwater to surface water will not be a viable and sustainable alternative for the provinces bordering the Vietnamese East and West Seas, as mentioned especially not during the dry season.  Hence, the Ministry of Construction (MoC) of Vietnam has been investigating the possibility to establish a regional piped water supply scheme that would meet the medium and long term demand for safe and clean water in the indicated area. MoC sought the support of the World Bank for this initiative via a prospective “Mekong regional water security project”. Moreover, targets for per capita water consumption were expected to be met within a period of 5 to 10 years, even if historical data indicated that the actual water consumption was far less.  The workshop concluded that this approach was expected to overestimate actual water consumption in the future —especially in the short- to medium-term— and may result in unnecessary large and expensive investments in public water supply infrastructure. The approach of the workshop with respect to water demand projections was different.  Historical trends in water demand were considered good predictors of developments in the short-term, i.e. the next five years.  Standards can be used to determine demand for water in the long-term, 25 – 40 years from now.  As indicated in Figure 2, water consumption in the medium-term can be based on interpolation between the long-term target for water use and the actual water use of today.  The implementation of a project can accelerate —fast track— the journey from where one is now to the envisaged situation as reflected by the long-term target. While there is clearly a risk of over-dimensioning the system due to exaggerated water demand projections, there is equally a risk of under-dimensioning, as the available finances for implementation are limited.  Hence, not only proper demand prognosis is important, but also the timing and the (possibility of) phasing of the infrastructural investments. An approach identifying adaptive pathways for investments in water transmission and distribution infrastructure was advocated to ensure that eventual investments can be considered as no-regret measures. In addition, an enhanced level of water security and technical redundancy of critical elements of the system were recommended. An approach based on separation of transmission and distribution of water was advocated by the workshop participants, as it reduces investment and operational costs.  Figure 3 depicts this approach schematically.  It implies that there will be a limited number of take-offs from the transmission mains, and that those points of take-off will generally be combined with the presence of distribution reservoirs.  Daily fluctuations of demand for water are to be covered by the distribution reservoirs. Due to the likelihood of limited available investment funding —the total project is estimated to cost 2 billion USD, with the World Bank having pledged 400 million USD and the German development bank KfW, 200 million EUR— phasing of the scheme realization by prioritizing supply of water to the most acute areas is not only a necessity, but will also enhance its financial viability and its affordability, which remains for the time being a crucial matter of uncertainty.  In this respect, a certain degree of solidarity —cross-subsidies— among provinces in the envisaged supply area is considered a condition for success, one that definitely cannot be taken for granted. Leadership from central government is a prerequisite.