These are some of the email reports and communications from Scot Evans from Truesdale Lake, James Gorman from Pond & Lake Connection, and others involved with the project. Putting selected parts up here for all to see what some of the discussion and learning is looking like.
October 7, 2019
Rob and Harold:
One of the assumptions you provided for our lake to refill is 12 x Precip (rain or Snow Water Equivalent – SWE). I want to determine the best average multiplier for our lake to refill when the siphons are not operating.
The dry warm weather weâ€™ve had is allowing the ground to absorb most of the rainfall and maintain a high evaporation rate of the lake.
Hereâ€™s my first weekly report for the drawdown where NO siphons have been operating and no water is departing over the spillway.
The level gauge in this photo shows the top white tape as summer level when the water was at the top of the weir boards. The bottom white tape is the winter level just when the water no longer flowed over the spillway.
- Monday 9/30/19 3.3 feet below spillway
- Monday 10/07/19 3.1 feet below spillway
Rain we received during that week was 0.48 inches:
Lake level increased about 2.5 inches in one week with about 0.5 inch of rain. This is a multiplier of 5. When the ground becomes saturated and evaporation diminishes, weâ€™ll see how close to 12 the multiplier reaches.
Thanks, Scott, this is very interesting. It’s really important to get a reality check from actual measurements!
The Watershed tab of the latest spreadsheet indicates that, at 50% runoff, we should expect a rise of 14.8 inches per inch of rain (up from 12 because I had a low estimate of the watershed area initially). That does not take account of evaporation, and also would take some time after the rain for the water to make its way to the lake.
Rob told me that 50% is the “normal” runoff rate for our terrain, but obviously very dry or very wet seasons are not normal, and the rate changes accordingly. Plugging your measurements into the sheet shows a runoff rate of about 15%, which seems very reasonable in these circumstances.
Of course, since evaporation is not considered, the actual runoff rate is somewhat higher, but this number reflects the net effect of (rain – evaporation). And this is all a somewhat gross estimate anyway, because the area of the lake right now is considerably less than the 83 acres used in the calculations, but I think it’s good enough for us to work with.
And thanks, too, for your amazing rescue of the swan!