The above percentages, while showing the averages for two entire years, and so of decided value, are not to be depended upon for special years or months. For instance: in the Ohio valley in 1881, the outflow was 33 per cent., while in 1882 it was 50 per cent., and as the rainfall in 1882 was 180 cubic miles against 151 cubic miles in 1881, it appears evident that a much greater proportional quantity of water reaches the rivers during seasons of heavy rainfalls than when the precipitation is moderate or scanty.
Evaporation is also a very potent cause in diminishing river outflow, and as this depends largely on the temperature of the air and the velocity of the wind, any marked deviation of these meteorological elements from the normal, must exercise an important influence on the ratio of outflow to rainfall.
In connection with Professor Russell's work it is desirable to note that Professor F. E. Nipher has lately made a report on the Missouri rainfall based on observations for the ten years ending December, 1887, in which he points out as an interesting coincidence that the average annual discharge of the Missouri river closely corresponds in amount to the rainfall which falls over the State of Missouri. From Professor Nipher's figures it appears that the discharge of the Missouri river in the ten years ending 1887, was greatest in 1881 and next greatest in 1882, so that the averages deduced from Professor Russell's report of the outflow of the Missouri are too large, and should be somewhat reduced to conform to the average conditions. In different years the average of the discharge in the outflow of the Missouri varies largely, as is evidenced by the fact reported by Professor Nipher, that the discharge in 1879 was only 56 per cent. of the outflow in 1881.
In New South Wales, under the supervision of H. C. Russell, Esq., government astronomer, the question of rainfall and river discharge has also received careful attention, especially in connection with evaporation. The observations at Lake George are important, owing to the shallowness of the lake (particularly at the margin); its considerable surface area (eighty square miles), its moderate elevation (2,200 feet), and the fact that it is quite surrounded by high lands. Observations of the fluctuations of this lake have been made from 1885 to 1888, inclusive. In the latter year the evaporation was enormous, being 47.7 inches against a rainfall of 23.9 and an in-drainage of 5.3 inches, so that the total loss in depth was 18.5 inches for the year. It appears that the evaporation in different years on this lake varies as much as 50 per centum of the minimum amount. According to Russell the amount of evaporation depends largely on the state of the soil, going on much faster from a wet surface of the ground than from water; with dry ground the conditions are reversed. In 1887, the outflow from the basin of Lake George, the drainage from which is not subject to loss by long river channels, was only 3.12 per centum of the rainfall.
In the Darling river, above Bourke, says Russell, the rainfall is measured by 219 gauges. The average river discharge, deduced from observations covering seven years, is only 1.45 per centum of the rainfall, and in the wettest year known the discharge amounted only to 2.33 per centum of the rainfall, and has been as low as 0.09 per centum in a very dry year. In the Murray basin the average discharge relative to the rainfall is estimated to be about 27 per centum from a record of seven years, and has risen as high as 36 per centum in a flood year.
In connection with the regimen of rivers, it appears a proper occasion to again refute the popular opinion that the spring and summer floods of the Missouri and Mississippi valleys result from the melting of the winter snows. This is an erroneous impression which I have combatted since 1873, when my duties required a study of the floods of the entire Mississippi catchment basin. It is only within the last two years, however, that the meteorological data has been in such condition that the opinion put forth by me could be verified, namely: that the floods of the late spring and early summer owe their origin almost entirely to the heavy rains immediately before and during the flood period. Occasionally a very heavy fall of snow precedes extended general rains; but in this case the snow is lately fallen and is not the winter precipitation.
Referring to the Missouri valley, the section of the country where the winter snowfall has been thought to exercise a dominating influence in floods, it has elsewhere been shown by me that about one-third of the annual precipitation falls over that valley during the months of May and June. In either of the months named the average precipitation over the Missouri valley is greater than the entire average precipitation for the winter months of December, January and February.
Woiekoff thinks that the anomalies of temperatures shown in forest regions, particularly in Brazil—with its abnormally low temperatures, are due to heavy forests promoting evaporation, and by causing the prevalence of accompanying fogs thus prevent more intense insolation. He considers this an argument for the maintenance of forests to sustain humidity and distribute rain over adjacent cultivated land, as well as to maintain the fertility of the soil, which diminishes rapidly by washing away of the soil after deforestation.
W. Koppen has devised a formula for deriving the true daily temperature from 8 A.M., 2 P.M. and 8 P.M. observations in connection with the minimum temperature, in which the minimum has a variable weight dependent on place and month. The results of Koppen's formula tested on six stations in widely different latitudes, indicate that it is of value.
Paulsen's discussion of the warm winter winds of Greenland is interesting. These unusual storm conditions last three or four days, or even longer, the temperature being at times from 35° to 40° Fahr. above the normal, and they appear principally with winds from northeast to southeast, which Hoffmeyer believes to be foehn winds. Paulsen contends that the extensive region over which these winds occur make the foehn theory untenable, and that a more reasonable explanation of these winds is to be found in the course of low areas passing along the coast or over Greenland. This appears evident from the fact that not the easterly winds only but the southerly winds share this high temperature, and that as low areas approach from the west, at first the regions of the Greenland coast within its influence have south to southwest winds.