When the rainsreturn the lake level will climb back up again– but by how much? To answer this, we obtained data from our electronic rain gauge and lake level gauges for 16 rain events in 2011 and 2012. We graphed the data using a Microsoft Excel XY scatter graph. Each point is a storm’s rainfall (X axis) and the increase in the height of the lake level (Y Axis). We used Excel’s trend line feature to draw a line through the plotted points. By eliminating two outliers, large storms after a long dry period (plotted as square points), the trend’s correlation is very strong (0.94 where 1.0 is perfect).
Excel assigned a formula for the trend: Y=5.24X – 2.27. The formula tells us that for every inch of rain (x), the lake level increases by 5.24 inches (Y). Of course one of those inches is rain falling directly on the lake. So the contribution from the watershed is 4.24 inches per inch of rain. The formula also tells us that unless there is at least a half inch of rain there will be no observable increase in lake height. The rain is absorbed at the beginning of the storm,.This is an estimate.There are other factors not considered, such as discharge rates and time. You can read more about our lake’s hydrology, and ecology in our study, A Study of Phosphorous Loading of Ballston Lake by Tributary