Estimation of subsurface components in the water balance of Lake Nainital (Kumaun Himalaya, India) using environmental isotopes

Abstract

Lake Nainital, located in the Kumaun Himalayan region in northern India, is a major drinking water source to the people living in and around the lake basin. The water balance of the lake has been computed for the first time. The subsurface outflow components are estimated by indirect methods and then the subsurface inflow calculated by means of the water balance equation. The results are verified using the environmental isotope mass balance method. Further, the chloride mass balance method is also employed for comparison of the results with two other methods. The slope of the δ18O-δD water line of the lake (7.1) is very close to that of the local meteoric water line (7.5) indicating that the effect of evaporation in the lake is not manifested in the isotope characteristics of the lake. The mass balance results indicate that the groundwater contribution is about 50% of the total annual inflow to the lake. The subsurface outflow is about 55% of the total annual outflow from the lake. This shows that the lake is a “flow-through” type with substantial groundwater inflow and lake seepage. The results of both chloride and isotope mass balance methods corroborate the results of the water balance method. The water retention time for Lake Nainital, estimated by isotopic mass balance, chloride mass balance and conventional water balance methods, is about 1.93, 1.77 and 1.92 years, respectively.