rainfall of this immediate locality as 45 inches and deducting for the probable evaporation from a water surface 34 inches, leaves 11 inches net gain in depth during the year upon the reservoir surface, exclusive of the stream flow into it. The total loss by evaporation from the reservoir during the year taken at 34 inches in depth, is equivalent to a constant flow averaging for the year about 95 cubic feet per second, but during the months of July and August, in each of which a monthly loss of about 5.6 inches in depth of evaporation from a water surface may be expected, the equivalent constant uniform rate of loss in these months is about 156 cubic feet per second. The recent experiments of the U. S. Reclamation Service made in Nevada, show the loss from a broad lake is proportionally less than from the ordinary experimental tank or than from a small reservoir. The rate of flow from the entire Sacandaga watershed as measured near the end of the uncommonly severe drought of the past summer about September 1st, was 130 feet per second. We thus see that the evaporation loss from the lake surface may at times slightly exceed the inflow, so that the lake under these conditions of extreme drought would be lowered about onethirtieth of an inch per day due to evaporation, although its outlet gates were shut tight, and its inflow was 130 second feet. In other words there will seldom or never be a drought so severe that the inflow will not supply the evaporation loss. The net evaporation loss from the total watershed of 1,050 square miles, due to this water surface of nearly 40 square miles, is an extremely small portion of the total regulated flow, the 36 cubic feet per second of mean annual evaporation loss from the Scandaga reservoir as reckoned above being but 2 per cent. of the estimated yield of 1,700 cubic feet per second. We thus see that there need be no fear that the loss of water from so great a lake surface will be a serious feature in the problem. DEPLETION DIAGRAM. The table following and the diagram presented on plate No. 3, show the reservoir depletion that would have occurred with a constant twenty-four hour discharge for every day in the year at various rates from 1,700 cubic feet per second down to 1,475 cubic feet per second, the inflow to the reservoir being computed at the same rate per square mile as was actually measured at Mechanicville, and thus for the moment offsetting the 10 per cent. to 15 per cent. greater yield per square mile of the Sacandaga watershed to compensate for this evaporation and the possible margin of uncertainty in the Mechanicville gagings. It is of interest to note from diagram No. 3 that in all the years since the gagings began, excepting the remarkably dry years of 1895 and 1901. the depletion called for by this very large constant discharge of 1,700 cubic feet per second, corresponding to 22 inches of annual run-off, would have amounted on the average to only about ten billion gallons, corresponding to lowering the reservoir only about 10 feet below its spillway level. It will also be seen from this diagram that in most years, water would have wasted over the spillway most of the time during the months of April and May. This dagram of plate No. 4 is particularly interesting in showing how great the depletion of the reservoir below high water mark would have been from month to month throughout the past nineteen years, had constant quantities of 1,700 or 1,824 or 1.550 cubic feet per second, twenty-four hours per day been continuously drawn. It is, however, based on the Mechanicville #zing and therefore under estimates the Sarandaga yield. The average rainfall per square mile of the Sacandaga drainage is shown by our isobgetal map of the Adirondack region on plate 10 to be probably 15 per cent. larger than that of the entire Hudson drainage area above Mechanicville, and therefore if precise accuracy of Mechanicville gagings be granted, it is probable that after allowing 2 per cent. for evaporation loss, 13 per cent. more water could be drawn with about the same depletion shown on this diagram, or say, about 1,900 ̊ eubie feet per second instead of 1,700. |