Long Pine,
The drainage area of Lake Winnipesaukee does not lend itself very well to efficient inflow gaging. Though there are a few significant feeder streams - like the Merrymeeting River - the basin is so large that trying to generalize conditions for the whole basin by what is happening in the Alton/New Durham area doesn't work well. We've tried using gages at Poorfarm (Gunstock) and Shannon (Ossipee Mountains) brooks, but again we found that we were really only measuring conditions in a very small area.
What we do is use the change in lake elevation as measured at the Weirs Beach gage and the discharge at Lakeport Dam to estimate basin inflow.
inflow = (change in lake level (in hundredths of feet) X 230 cfs) + discharge at Lakeport
Example for a rising lake - yesterday to today:
level on day1 = 503.33
level on day2 = 503.43
ave. outflow at Lakeport between 7AM yesterday and 7AM today = 1017 cfs
Estimated average inflow between day1 and day2 is:
(+10x230) + 1017 = 3317 cfs
If the lake had fallen by 3 hundredths then inflow is about:
(-3x230) + 1017 = 327 cfs
If the lake is steady from one day to the next then inflow = outflow.
Though this method is approximate and doesn't provide us with "real-time" inflow values it is very useful, when combined with forecast information, in establishing inflow trends and guiding management decisions. As you state, other variables like snow melt, precip. and soil moisture can make a huge impact on inflow trends. The basin is ripe at this time of year, in general, and the water content in the current snow pack this spring is above normal.
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