Application of the cumulative rainfall departure method in determination of deep groundwater recharge in Soc Trang Province, Vietnam

Groundwater (GW) is a vital freshwater resource extensively exploited in the Vietnamese Mekong Delta, especially during the dry seasons. This study applies the Cumulative Rainfall Departure (CRD) method to estimate GW recharge in deep aquifers of Soc Trang Province, located in the southernmost region of Vietnam under a tropical climate. Monthly rainfall records and daily GW level data of the aquifers from 2010 to 2020 were used. The Pearson correlation between observed GW levels and CDR model GW levels exceeds 0.995, indicating high model accuracy. The analysis reveals that the CRD fractions for the Upper Pleistocene (qp3), Middle Pleistocene (qp2-3), Lower Pleistocene (qp1), and Middle Pliocene (n22) aquifers are 0.085%, 0.104%, 0.130%, and 0.180%, respectively, totaling approximately 0.5% of the annual rainfall. This corresponds to an annual GW recharge of 25.86 million m3, or 70,850 m3/day, equivalent to 70% of the current GW abstraction rate of 101,000 m3/day. Given the critical role of GW as a freshwater source, implementing an enhanced GW recharge program using surface water and rainwater is strongly recommended. Additionally, the analysis suggests that the decline in GW levels due to abstraction corresponds to 0.85 times the mean annual precipitation, a finding that warrants further investigation.