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).
Therefore, we know that inflow to the lake right now is somewhat higher than 250 cfs because some of the inflow is being lost to evaporation in the lake itself. On average, Sebago Lake loses 18 billion gallons of water to evaporation each year — or nearly 50 million gallons a day — depending on the weather, of course. To put this number in perspective, the average withdrawal of water from Sebago Lake for drinking water is about 24 million gallons per day. So, on an average day, Sebago Lake loses twice as much water from evaporation than from withdrawals by the Portland Water District for drinking water.
Using PWD’s estimates we can construct a basic water budget for the lake on an average daily basis:
Inflow = 544 million gpd (gallons per day).
Evaporation = 49 million gpd.
PWD drinking water withdrawal = 24 million gpd.
As these numbers show, the amount of water which directly exits Sebago Lake via the Presumpscot River is nearly 8 times larger than what is removed by evaporation and for drinking water. This also shows that PWD’s drinking water withdrawals are inconsequential to the lake’s water budget, totalling less than 5 percent of average daily inflow. Or to put it another way, the average outflow of Sebago Lake into the Presumpscot River is about 600 cubic feet per second (cfs). The PWD’s daily withdrawal of water from Sebago Lake is equal to about 35 cfs. So even if the PWD stopped taking any water from Sebago for drinking water, the outflow to the Presumpscot River would only increase by about 5 percent.
So what does all this tell us? It tells us that inflow (ie. streamflow, groundwater inflow and precipitation) is what determines if Sebago Lake rises or falls. Evaporation and PWD’s water withdrawals are fairly negligible — and nothing can be done about evaporation anyways. Since late April, Sebago Lake has been close to equilibrium, with inflow and evaporation balancing the 250 cfs legal minimum outflow at the Eel Weir Dam. Absent a lot of precipitation in the next few weeks (ie. in the 8-10 inch range), Sebago Lake is not going to rise and will not hit its full pond ‘target’ of 266.65 feet. Is that a problem? No.
Natural lakes never hit the same ‘full pond’ height each and every year. The 266.65 msl ‘full pond’ height is an arbitrary, man-made number. This is for the same reason that rivers do not achieve the exact same flood height each and every year or the same low-water level each and every year. If S.D. Warren tried to raised the height of Sebago Lake now, its only option would be to drastically reduce outflows at the Eel Weir Dam, causing the Presumpscot River to go into an extreme and unnatural drought. This is because lakes do not naturally rise during dry weather any more than rivers do.