Staged and monitored groundwater management in the Mekong Delta under deep uncertainty

Abstract Groundwater in Vietnam's Mekong Delta is being depleted and salinised, yet aquifer conditions are only partly observed, and the rates of degradation and the effects of management are deeply uncertain. We model annual management as a manager acting on a belief about the aquifer's condition and choosing among six measures from the region's four-pillar strategy. Degradation is anchored to published head-decline rates and initial salinity to an open salinity model; unobserved inputs are varied in 1000 scenarios. Business as usual leaves aquifers saline in 17\,\% of years; active management reduces this to about 4\,\%. The efficient strategy is staged: wait while the aquifer is in good condition, recharge when it is stressed, and apply the full package only when it is critical. Applying the full package every year forgoes 19\,\% of net benefit. A monitoring survey that reaches managers before the annual decision raises net benefit by about 6\,\%, more than a more accurate later survey (4\,\%). Caution pays in 18\,\% of scenarios, mostly where degradation exceeds 2.5 times the published rates. Rules that look ahead with a belief have the lowest regret, and perfect knowledge of the dynamics is worth a third of a timely survey.