The study undertakes hydrological investigations to develop a conceptual model for the water balance of Lake Nainital in Kumaon Himalayas. All the components of lake water balance, except the sub-surface ones, have been measured or estimated using standard methods like isotope mass balance method in conjunction with the conventional water balance method. The sub surface outflow from the lake was assessed by studying the hydraulic interconnection of the lake and downstream springs using environmental tracers.
The analysis of the hydro-chemical data indicates that Lake Nainital and a few downstream springs such as Sariyatal and Balia ravine springs are hydraulically interconnected. At the same time, the other downstream springs such as those located on the west of the lake catchment and in Kailakhan area are not connected to the lake. In order to match the information obtained through the hydrochemical analysis, stable isotopic investigations were carried out.
In order to assess the proportion of the lake water being tapped in the wells located in the northern bank of the lake, a two-component mixing model was employed using the stable isotope data. The results of the model indicate that the proportion of the lake water in the pumpage varies from 33 % to 100 % in different seasons.
By combining the discharge data on the spring that are interconnected to the lake and the proportion of the lake water in the pumpage, the total sub-surface outflow from the lake has been estimated (56 % of the total outflow). The sub-surface inflow has been estimated as the residual or remainder of the water balance equation, as all other components have been computed or estimated using standard methods.
The results shows that the estimation of sub-surface inflow to the lake and outflow from the lake obtained through the isotopic and chemical balance compares very well with those obtained through conventional water balance method. The water retention time of the lake, computed using isotopic mass balance approach is about 2 % lower and that computed using chlorine mass balance results is about 10 % lower than the water retention time using conventional water balance results.
The value that reflects the true water retention time is the one computed using the isotopic mass balance approach as it was arrived independent of the estimates of pumpage and outflow through springs. Further, isotope mass balance has the advantage over chlorine mass balance as chlorine may be introduced into the lake and groundwater systems through anthropogenic activity.
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