The hydrological study develops a water balance of the Lake Nainital to facilitate the utilization of its water in a planned and systematic manner. The lack of knowledge of input and output parameters such as subsurface inflow, use of lake water for domestic and industrial purposes, evaporation loss and leakage from lake including the outflow through sluice gates had created uncertainty in determining the availability of water in the lake. The study attempts to understand the behavior of the input and output parameters in detail by using both conventional and isotopic techniques.

The lake meets the local drinking and domestic water needs and is for the most part fed by rainfall during monsoon and by inflow from perennial springs and subsurface inflow during the non-monsoon season.

The rainfall data collected from the basin was used to estimate the average aerial rainfall in the lake basin by adopting the Theissen Polygon method. The direct precipitation over the lake surface accounted for about sixteen percent of the total inflow into the lake during 1995. The volume of the lake has been computed as 8.58 mcum at full supply level (FSL) using bathymetric data.

The study presents equations for estimating changes in storage for different water levels in the shallow zones. Surface inflow to the lake has been estimated by using two different techniques viz. the Lake Level Trend Analysis (LLTA) method and the Soil Conservation Services-Curve Number (SCS-CN) method. The difference can be attributed to the errors in arriving at true lake level trend values during monsoon as the trend is masked by the large subsurface inflow and outflow.

The surface inflow into the lake as rainfall-runoff accounts for about thirty percent of the total inflow. The inflow through the perennial drains that are sustained by spring discharges and domestic waste accounts for nearly 15 %. The sub-surface inflow estimated as a residual of the solved water balance equation is nearly 39 % of the total inflow components.

The surface outflow from the lake has been computed using an empirical formula for submerged triangular sluices and accounts for 41 % of the total outflow in 1995. Stable isotope data was used to distinguish the differential contribution from the lake and groundwater to the water being pumped. The open surface evaporation from the lake has been estimated from the discharges interpolated from observed and historic discharges of the downstream springs located along the Balia river. The subsurface outflow as springs accounts for nearly 16 % of the total outflow from the lake.

The report provides the results and the detailed methodology adopted for estimating different components of the water balance of the lake. An effort has also been made to understand the implications of sedimentation on the lake water balance and on the discharge of downstream springs that are sustained by the lake.

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