242 



SCIENCE. 



[Vol. XII. No. 303 



THE INFLUENCE OF FORESTS ON THE QUANTITY 

 AND FREQUENCY OF RAINFALL. 

 The effect of the growth of forests on rainfall is receiving fresh 

 attention in the Philosophical Society of Washington. At a recent 

 meeting of that society Professor Fernow of the Department of 

 Agriculture gave an historical review of the experiments made in 

 Europe during the present century to determine the influence of 

 forests on the quantity and frequency of rainfall. When the read- 

 ing of Professor Fernow's paper was concluded, there was no time 

 for the discussion of it, and a symposium on the subject was ar- 

 ranged for the ne.xt meeting. Mr. Henry Gannett opened the dis- 

 cussion, and his address was the most notable of the evening. He 

 was followed by others, notably by Gen. A. W. Greely, who has 

 made this subject a special study. That part of Professor Fer- 

 now's paper relating to the experiments in Europe are given here, 

 followed by Mr. Gannett's remarks in full. 



Professor Fernow's Paper. 



I wish to call attention to the latest, most important, in fact the 

 only scientific work, it seems to me, which has been done to estab- 

 lish the important but difficult question of the influence of forests 

 on precipitation. I refer to the work of Dr. F. J. Studnika, profes- 

 sor of mathematics at the University of Prague, published under 

 the title 'Basis for a Hyetography of Bohemia,' in which the re- 

 sults of many years of observation at 700 ombrometric stations 

 are embodied, critically sifted, and scientifically considered. 



The work of ombrometric observations, although begun in Bo- 

 hemia during the last century, was newly organized in 1879 or iSSo, 

 when a systematic net of ombrometric stations was instituted ; and 

 in 1885 and 1886 it was extended over 700 stations, for the purpose 

 of obtaining accurate data of the quantity and distribution of pre- 

 cipitation over the kingdom. Uniform ombrometers were used and 

 very carefully placed. As at present organized, there is one sta- 

 tion for every seventy-five square kilometres (about thirty square 

 miles). No other country, I believe, can boast of such a service. 

 Although the time of observation at most stations has been 

 short, and the average would have been more accurately repre- 

 sented by an extension of observations for ten or twelve years, yet 

 the last four years of observation, for which all stations furnish 

 data, according to the author, represent two extreme and two aver- 

 age years, and are therefore quite useful. 



The very large mass of material permitted a sifting-out of doubt- 

 ful observations without impairing the number available for the 

 construction of a rain-map of Bohemia, showing by isohyetal lines 

 seven rain-belts or zones. The zones are so arranged that the 

 lowest shows less than 500 millimetres rainfall, the three follow- 

 ing differ by 100 millimetres each, the fifth and sixth by 200 milli- 

 metres, and the seventh by 300 millimetres ; showing, therefore, a 

 rainfall of 1,200 to 1,500 millimetres. 



The central basin divides itself into halves by a line from north 

 to south, running somewhat east of the middle Moldau, crossing 

 the Elbe near the mouth of the Iser, and following the latter river ; 

 the western half showing the smaller amount of precipitation, 

 namely, 500 to 600 milhmetres ; the eastern, with 600 to 700 milli- 

 metres, continuing in a small belt along the foot of the Erzebirge 

 and the Boehmer-wald, encircling the first zone. 



The other isohyetal lines do not embrace continuous areas, but 

 follow in small belts the trend of the mountains. The larger 

 amounts of precipitation are found in belts or islands in the higher 

 altitudes of the mountains which surround this great basin. The 

 continuity of the zones is much interrupted, so that it would be 

 difficult to describe it without a map. 



The maximum rainfall with over 1,200 millimetres is observed in 

 the'south near the sources of the Moldau and Wotawa ; in the 

 north, near the sources of the Elbe, Iser, and Aupa. on the range of 

 the Schneekoppe. In regard to the distribution through the months, 

 the experience has confirmed, that, with increasing absolute height, 

 the winter precipitation increases in greater proportion than that 

 of the summer, while those of spring and autumn are nearly equal. 



Sufficient material was on hand from which to calculate the in- 

 fluence of altitude on the increase of precipitation, although for 

 altitudes above 500 metres the material is not considered sufficiently 

 accurate. Yet the general law is well shown, that with the altitude 



the quantity of precipitation increases in a retarded progression. 

 This progression is calculated by forming altitude zones from 100 

 to 100 metres, grouping the stations in each, calculating the 

 mean elevation and also the mean annual precipitation as observed 

 for each class ; then, by dividing the difference of precipitation in 

 two neighboring zones by the difference of altitude, the amount of 

 precipitation which corresponds to each one metre of elevation 

 within that class is found. With this figure, the average amount of 

 rainfall which theoretically belongs to each station according to 

 its absolute elevation can be approximated by addmg to or sub- 

 tracting from the mean precipitation of the class as many times 

 this amount as the actual altitude differs from the mean. 



A single example will make this clear. Tetschen, for instance, 

 is situated 150 metres above the sea-level. According to the table, 

 the average elevation of 13 stations of the lowest zone, to which 

 Tetschen belongs, is 182 metres, with an average precipitation of 

 506 millimetres. Now, as Tetschen has an elevation of 32 metres 

 lower than the average, the rainfall should be 32 x .79 = 25.4 

 millimetres less than the mean of the class ; hence, theoretically, 

 according to its altitude, the quantity of rainfall for Tetschen 

 should be 506 — 25.4= 480.6 millimetres ; that is, 248 millimetres 

 less than that actually found in an eight-years' average. 



By using the figures for the two extreme zones and dividing by 

 100, the mean increase of precipitation for every 100 metres eleva- 

 tion is found to be 69 millimetres. 



And now comes the application of this method to our proposi- 

 tion. The author argues that if the actually observed rainfall dif- 

 fers considerably from the theoretical, this is an indication that 

 special agencies are at work. 



He finds now, that, of 186 stations which he subjects to scrutiny 

 (those offering the longest and most trustworthy observations), 48 

 show a considerable difference between the observed and the 

 theoretically expected rainfall, and he finds also that these stations 

 are situated in the most densely wooded portions of the kingdom. 



The increased rainfall at the 48 stations is so considerable, that 

 sufficient quantity may be ascribed to other local causes, as, for in- 

 stance, height and form of a mountain-range in front or back, etc., 

 without losing significance. Besides, the greater amounts of rainfall 

 at these stations have been used in calculating the. averages for the 

 altitude zones, magnifying, therefore, these averages so that the 

 actual difference between the calculated quantity and the actually 

 observed one appears really smaller than if the quantities from de- 

 forested and forest areas are compared. 



Expressed in percentages of the height of precipitation, an in- 

 creased rainfall is shown for several localities in very large quanti- 

 ties, which will allow considerable reductions for other influences 

 without losing their significance for the main proposition. 



Especially important appears the fact relating to two stations 

 near the rain minimum, which also shows this influence of the 

 forest. 



Lastly, as a matter of interest, I may state that the water bal- 

 ance is drawn for the whole kingdom, which is of special value, 

 because the political boundaries coincide with those of the upper 

 Elbe watershed ; therefore it is easy to determine how much of the 

 yearly rainfall is removed by the natural water-courses. Accord- 

 ing to the calculations made for the-various zones by addition, the 

 total precipitation upon the area of 51,955.98 square kilometres 

 (about 20,000 square miles) of the kingdom is found to be 35.398,- 

 670,000 cubic metres, of which the Elbe carries about one-quarter, 

 or ten cubic kilometres, to the sea. This figure represents a mean 

 rainfall for the whole country of 6S1 millimetres, while the mean 

 observation is 693 millimetres. 



In conclusion, allow me to say that I believe neither of the 

 methods employed will alone be sufficient to investigate such a 

 complicated relation in its generality as that which they try to 

 establish or refute. All of them, modified and provided with such 

 safeguards as will exclude the many disturbing influences, will have 

 to work together towards a solution of the question. 



Mr. Gannett's Paper. 



At its last meeting, the society was favored with a very interest- 

 ing and important paper by Dr. Fernow, in which there was pre- 

 sented a resiling of certain investigations made in this country and 



