Abstract
Study RegionVietnamese Mekong Delta. Study focusThis study investigates the trends of groundwater levels (GWLs), the memory effect of alluvial aquifers, and the response times between surface water and groundwater across the Vietnamese Mekong Delta (VMD). Trend analysis, auto- and cross-correlation, and time-series decomposition were applied within a moving window approach to examine non-stationary behavior. New hydrological insightsOur study revealed an effective connection between the shallowest aquifer unit (Holocene) and surface water, and a high potential for shallow groundwater recharge. However, low-permeable aquicludes separating the aquifers behave as low-pass filters that reduce the high‐frequency signals in the GWL variations, and limit the recharge to the deep groundwater. Declining GWLs (0.01−0.55 m/year) were detected for all aquifers throughout the 22 years of observation, indicating that the groundwater abstraction exceeds groundwater recharge. Stronger declining trends were detected for deeper groundwater. The dynamic trend analysis indicates that the decrease of GWLs accelerated continuously. The groundwater memory effect varied according to the geographical location, being shorter in shallow aquifers and flood-prone areas and longer in deep aquifers and coastal areas. Variation of the response time between the river and alluvial aquifers was controlled by groundwater depth and season. The response time was shorter during the flood season, indicating that the bulk of groundwater recharge occurred in the late flood season, particularly in the deep aquifers.
Highlights
Alluvial aquifers play an important role in sustaining agricultural activities and the livelihood of the population in river deltas
The relative variability of the groundwater levels (GWLs) was assessed based on the variances of the trend, seasonal, and remainder components along with the variance of the original time series
Our results indicate that groundwater abstractions highly exceeded groundwater recharge, resulting in a considerable decrease of GWLs and groundwater storage in the Vietnamese Mekong Delta (VMD) over the last 22 years
Summary
Alluvial aquifers play an important role in sustaining agricultural activities and the livelihood of the population in river deltas. In areas where rainfall is not uniformly distributed throughout the year (e.g., tropical or arid regions), they are primary sources for good quality freshwater, as they are less vulnerable to contamination or climate variability than surface water bodies. Journal of Hydrology: Regional Studies 33 (2021) 100746 estimates of groundwater recharge and the response between surface water and alluvial aquifers can be difficult to obtain, and sig nificant uncertainties exist in groundwater storage in alluvial settings. Under impacts of climate change and human activities (Syvitski et al, 2009; Vorosmarty et al, 2009; Hirabayashi et al, 2013), which challenge national to global food security (Kummu et al, 2012), understanding the mechanisms of alluvial aquifers is fundamental
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