Hydrological interactions between surface water and groundwater in ancient manmade village tank cascade systems (VTCSs) in the dry zone of Sri Lanka.

Sri Lanka, a tropical island, confronts climate-driven water scarcity and is of great concern to building climate-resilient water management to achieve UN SDGs 6 and 13. This study explores the dynamic interaction between surface water (SW) and groundwater (GW) in the dry zone of Sri Lanka, employing a multi-tracer of dual stable isotopes (O and H) and chloride. Two basins, Mahakanadarawa (MK) basin with numerous village tanks and tank cascade systems (VTCSs/TCSs) and Kawudulla (KW) basin serving as a control with a lack of tanks, were selected in this study. Results show that the presence or absence of TCSs emerges as a pivotal factor influencing regional water dynamics. Water quality (TDS and Cl) in natural waters between the two basins was significantly different (ANOVA: p < 0.05) in both seasons. Statistically uniform dissolved mineral content in tanks and shallow groundwater in the MK basin contrasted with significant regional deviation in the KW basin. ANOVA and Tukey tests showed significant seasonal differences (p < 0.05) in mean values of ẟH and ẟO compositions of shallow groundwater and tank water within the MK basin, while non-significance in the KW basin. Isotopic mass balance calculations revealed that tank water mixing in the shallow groundwater (f: mixed Tank water mass fraction) ranged from 0.9 % to 77.8 % across the MK basin depending on the regional soil characteristics. Lack of tank-aquifer interconnection observed in the shallow soil in rock knob plain results in groundwater depletion while enhanced interconnections within alluvial sediment regions encourage stable and pollution-resistant shallow groundwater bodies. The consistency of the groundwater flow system in TCS regions throughout the year ensures a stable water supply, highlighting the vital synergy between TCSs and regional groundwaters in these arid regions. These results shed insights for policymakers and water managers to implement effective conservation strategies for rehabilitation and restoring these ancient VTCSs/TCSs.