PRECIPITATION IN THE DRAINAGE AREA OF THE GREAT LAKES, 1875–1924

Abstract

SYNOPSIS Owing to extensive discussion of lake levels and the causes controlling them, the precipitation data of the Weather Bureau and the Canadian Meteorological Service for the drainage basin of the Great Lakes have been examined with care. From 1875 trustworthy determinations are possible. The precipitation averages about 32 inches per year, and in general increases from north to south. As a rule the stations considerably above the levels of the lake surfaces indicate more precipitation than those close to the lake levels. The lake surfaces probably receive about as great precipitation as do stations at or near the shore, the deficiency in catch in gages exposed on islands or peninsulas being considered as due to increased Rind effect. The precipitation was unusually heavy, on the average, from 1875 to 1885, and the period 1875 to 1899 shows more precipitation at almost every station than the period 1900 to 1924. However, there were groups of a few successive years with scanty precipitation before 1900, one, indeed, before 1890. From 1917 to the present nearly the whole area has had decidedly scanty precipitation. It is not believed that removal of the forest cover has materially affected the amount of water reaching the lakes. The water levels seem to be closely related to the quantity of precipitation, delays of a year or more often appearing in the response of the levels, since the run-off is not immediate. It is highly improbable that the deficient falls of recent years indicate permanent or semi-permanent establishment of scanty supplies of precipitation. A return over several years to the normal quantity of precipitation or to even greater amount may be expected to end the present prevalence of unusually low levels in the lakes, just as various periods of low water during the nineteenth century were terminated by generous rainfall.