Variation of δD, and δ18O in springs and precipitation of Takoli gad watershed Uttarakhand in the lesser Himalayas

The environmental isotopes (δD and δ¹⁸O) in natural water play a crucial role as indicators for hydrological processes, serving as a significant method to track the moisture sources in mountainous watersheds. The current study presents the isotopic compositions (δD, and δ¹⁸O) of the springs, streams, and rainwater samples from the Takoli Gad catchment, Uttarakhand in the Lesser Himalayas. Our results show that the spring, and stream samples, with an average δD and δ¹⁸O values of (-60‰ ± 4.11‰) and (-8.81‰ ± 0.55‰), respectively, represent the most depleted isotopic compositions during the monsoon season. During post-monsoon and pre-monsoon seasons, isotopic compositions are enriched with an average of (δ¹⁸O = -8.44 ± 0.43‰, δD: -57.79 ± 2.43‰) and (δ¹⁸O = -8.10 ± 0.42‰, δD: -55.7 ± 3.07‰), respectively. The depleted isotopic compositions during the monsoon period suggest the impact of monsoon precipitation on spring waters. Additionally, evaporation from the spring water has led to an enrichment of isotopic compositions during the pre-monsoon season. This conclusion is reinforced by the highest d excess values observed in spring water during the monsoon (10.27‰ ± 1.47‰) and the lowest during the pre-monsoon (9.12‰ ± 1.75‰). Furthermore, The rainwater samples collected during the winter season have the highest d excess values (13.7‰ ± 5.4‰) in comparison to the same during pre-monsoon (9.9‰ ± 4‰) and monsoon period (9.2‰ ± 2‰). These highest d values of the precipitation during winter mostly correspond to the westerlies' effect. The mass balance equation, including δ¹⁸O and d-excess values, estimates that approximately 83% of the spring water budget is contributed by monsoon precipitation. Similarly, the δ¹⁸O-enabled altitude effect (0.06‰/100m) is found to be within range of other Himalayan catchments. However, the same is ~5 times lower in comparison to the altitude effect estimated using the precipitation of the region (0.3‰/100m). Also, our study suggests a significant role of evaporation in altering the δ¹⁸O-associated altitude effect in precipitation. Finally, the rainout percentage approximation (using both δ¹⁸O and δD compositions of the rainfall) estimates that ~32% ± 4% of the moisture is being removed from the cloud as the same is traversing in the region.