The thing necessary to find out is how much of the yearly precipitation sinks down into the water table and then seeps into Walden. There is no way to measure this amount directly, but we can estimate it reasonably accurately by a round-about method. Of the 43 inches of precipitation in this area, about 21 inches, on the average, remain after evaporation and transpiration by plants have taken their customary toll -- this is the average amount that flows off to the sea in streams as shown by many years of measurements of stream flow. So, it is likely that on the average about 21 inches of the precipitation on Walden's watershed remains to sink to the water table and seep to Walden pond.
A little multiplication now shows that 21 inches of water per year over 110 acres amounts of 63 million gallons of water per year, and presumably this is the amount that seeps into Walden from the surrounding slopes. This figure of 63 million gallons per year is an estimate, not an exact figure, of course, because it is impossible to determine the size of the watershed and the yield of water exactly. but even if the annual seepage were 25% less, it would equal a flow of 100 gallons per minute year round. A brook that flows 200 gallons per minute is not insignificant in New England but is larger than many of the brooks that turned the wheels of the small mills once found in every valley.
Walden also receives an average of 43 inches of water per year from rain and snow. These 43 inches of water over the 58 acres of Walden come to about 68 million gallons of water per year.
We have now estimated that Walden receives a total of about 131 million gallons of water per year, 63 million from seepage from its watershed and 68 million from rain and snow. This represents also the total outflow from Walden per year, on the average, because over the long run, outflow has to equal inflow, or else the pond would get higher and higher or lower and lower.
Studies made by engineers who need the information to design reservoirs show that evaporation takes about 26 inches of water per year from bodies of water in this part of New England. These 26 inches over the 58 acres of Walden come to around 41 million gallons of water per year -- about 60% of the average precipitation.
After subtracting the 41 million gallons of water that evaporate from Walden from the total inflow of 131 gallons per year, we still have 90 million gallons of water per year left. This is the amount that probably seeps out of Walden southward under the railroad tracks into the head of Well Meadow Swamp and southeastward into Andromeda Swamp, the directions in which water level drops most steeply away from Walden and, therefore, the most likely routes for seepage to take. A seepage of 102 million gallons per year represents a steady flow of 171 gallons per minute, and one often sees at least that much water flowing in the brooks that drain the Andromeda and Well Meadow Swamps down into Fairhaven Bay on the Sudbury River.
Now, getting back to the fluctuations of Walden, it is fairly obvious that the decline in water level from 1963 until late 1967 was due to the drought of those years. The diagrams at the bottom of figure 1 show that the precipitation was near normal from 1959 through 1962, and the water level at Walden in those years shows only the usual seasonal changes. Precipitation was eight inches below normal in 1963, and Walden sank about a foot below the previous year. This was the first of four years of the great New England drought, which meteorologists recall as "an extreme event" because it was the worst since records of precipitation began in Boston in 18181. Total precipitation in 1965 was only 23.7 inches, setting a record low. Total precipitation in the four years 1963-66 was 40 inches less than normal. Ground water continued to leak away into streams during these years, and in 1966, as a result of four years of below-normal replenishment, the water table sank to a record low level and wells dried up that had never failed within the memory of man. The water table fell about six feet in this vicinity, and Walden fell accordingly.
Though the drought that caused Walden to sink ended early in 1967, Walden still has not yet quite returned to its normal level. Some people say this shows how Walden behaves in a strange and unpredictable way, while others wonder if pumping of the town well almost a mile away on Walden Street is depleting Walden's supply. The bar graphs showing the amounts of precipitation below average during the drought and above average since 1967 give the true answer -- that there has not been enough extra precipitation since the drought to make up for the overdraft during the four drought years. In the dry years the water level fell six feet in the ground-water reservoir, and this means that at least 14 inches of water drained out of the ground because that is the amount that it takes to fill the pores of a column of sand six feet high. If the average precipitation of 43 inches just keeps Walden balanced and prevents it from sinking, how could we expect an overdraft of 14 inches to be satisfied and the water level to rise rapidly when the total precipitation in the four years 1967-70 has been only nine inches above average? probably we are lucky that Walden has risen as nearly back to normal as it has.