Investigating Hydrological Connectivity inthe Yamuna River System, India

Vertical connectivity between groundwater and river discharge is fundamental in maintaining river health. Groundwater sustains baseflow in rivers during lean period, helps in maintaining e-flow, and facilitates the transfer of nutrients and organisms across the hyporheic zone. The present study has been conducted in a ~300 km long alluvial reach of the Yamuna river from the Himalayan mountain front to Delhi National Capital Region (NCR), India. The Yamuna river is the largest tributary of the Ganga River system. It originates from the Higher Himalaya at an elevation of 6387m with an average annual rainfall of 906 mm. Seasonal and downstream variability in vertical hydrological connectivity along long profile was assessed using field measurements and laboratory analysis of stable isotopes.

A total of 71 samples from both groundwater and river water were analyzed for δd, δ¹⁸O, and d-excess values during both pre-monsoon and post-monsoon seasons of the year 2021. The average δ¹⁸O isotopic composition of most of the post-monsoon river water samples exhibit depletion in heavy isotopes in comparison to pre-monsoon samples. Depleted δ¹⁸O values indicate increase in rainfall contribution to river water during monsoon period. The low slope of the pre-monsoon groundwater in the graph plotted between δ¹⁸O and δd indicate lack of groundwater recharge during pre-monsoon period. This is responsible for higher values of δ¹⁸O isotopic composition of the groundwater, which is especially significant in the upstream reaches of Delhi NCR. Seasonal changes are evident only in the reaches upstream of Delhi NCR with similar pattern in groundwater and river water isotopic values. Data indicates a well-developed vertical hydrological connectivity in the upstream reaches. Our field based measurement of river and groundwater head validates the existence of hydrological connectivity in this reach. However, vertical connectivity is not apparent in the downstream reaches around Delhi NCR, as the seasonal changes in the isotopic composition of both river and groundwater are not prominent. Integrating these findings with water depth measurements in the field facilitated that out of 4 sub-reaches of our study area loose water to the groundwater, while 3 sub-reaches are gaining streams. This study will help to identify areas of concern and develop an effective management and conservation strategies to protect and restore the health of the river.