Theme 2-15-Estimation of sub-surface components in the water balance of lake Nainital (Kuamun Himalaya, India) using environmental isotopes

Abstract

Nainital Lake 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. Unfortunately, detailed computations of the lake water balance have not been carried out in the past. In this present investigation, the sub-surface outflow component has been computed by indirect means and used in the water balance equation to estimate the sub-surface inflow. The results have been verified by using the environmental isotope mass balance method. Further, chloride mass balance method has also been employed for comparison of results. The results indicate that the groundwater contributes about 50% of total annual inflow to Nainital lake. The sub-surface outflow is about 55% of the total annual outflow from the lake. It 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 water balance method. Water retention time - WRT (volume/outflow) as computed for Nainital lake by isotopic mass balance, chloride mass balance and conventional water balance methods is about 1.93y, 1.77y and 1.92 y respectively. The slope of the 18O - D 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 its isotope characteristics. This fact substantiates the shorter water residence time of Lake Nainital.