An unusually dry April and May has caused Sebago Lake to remain more than a foot below its ‘full pond’ target of approx. 266.65 feet mean sea level (msl). From late April to early May the lake flattened out at 265.35 msl despite S.D. Warren allowing only the legally minimum flow from the Eel Weir Dam (15,000 cubic feet per minute or 250 cubic feet per second). Rainy weather on the weekend of May 11-12 has caused the lake to creep up about 1 inch but now has appeared to flatten out again. This rise corresponds closely to the amount of rain received over the weekend.
The lake’s behavior this spring illustrates a useful and basic lesson in lake hydrology. Simple logic dictates that when a lake is neither rising or falling, the amount of water entering the lake is about equal to the amount leaving the lake. But an additional factor is evaporation. Since we know that S.D. Warren is now releasing 250 cubic feet per second from the lake, if the lake level is not rising or falling, then we know that inflow to the lake minus evaporation from the lake’s surface must equal its outflow. So we know that:
Inflow (I) – evaporation (E) = 250 cfs (measured outflow).