24S 



THE FORESTER. 



November, 



The fluctuations to which the average 

 annual rainfall is subject have been very 

 exhaustively discussed by Mr. Binnie, M. 

 Inst. C.E., in a paper read before the In- 

 stitution of Civil Engineers, London, in 

 1S92. 



From close analysis of records, of forty- 

 two stations in various parts of the world, 

 covering periods of from fifty to ninety- 

 seven years, he drew the following conclu- 

 sions : 



That for records of five years the probable 

 error in averages ranged from minus 16 per 

 cent, to plus i7tV P er cent., the limits of 

 error decreasing to minus 2 per cent, and 

 plus 2 percent, for periods of 30 years and 

 over. And that the least number of years 

 the continuous records of which would 

 give an average annual fall that would not 

 be materially altered by extending the rec- 

 ord, would be thirty-five years. He also 

 concluded that dependence could be placed 

 on any good record of that duration to give 

 an average rainfall correct within two per 

 cent. 



These examples will serve to illustrate 

 the uncertainty attached to any deductions 

 based on rainfall records of short duration. 

 As a case directly to the point we have 

 certain French observations made about 

 sixty miles south of Paris. The observa- 

 tions for' one year gave the precipitation 

 over woods as 33 per cent, in excess of 

 that over open ground. Three years con- 

 tinuous observations changed this to 2 per 

 cent. 



Long records for forest purposes are 

 rare. The necessity for long records is but 

 one of the many obstacles in the way of ar- 

 riving at absolute comparisons between 

 precipitation over woods and open grounds. 

 Even after sufficient time shall have elapsed 

 to reduce this particular trouble to a mini- 

 mum, there will still remain the errors in- 

 herent to measuring rainfall and the diffi- 

 culty of obtaining two locations, the one 

 wooded and the other bare, whose condi- 

 tions are absolutely comparable. 



Rain-gauges are not instruments of pre- 

 cision, yet no conclusions- can be more ac- 

 curate than the data upon which they are 

 based. The gauges record all that falls 

 within them, but, except in still weather 



and with gentle rains, they do not inter- 

 cept all that they should. Some very in- 

 teresting and instructive data upon this 

 subject has been published by the U. S. 

 Department of Agriculture. It is shown 

 that with the common unprotected gauge 

 very large errors sometimes occur. It has 

 also been shown that the decrease in catch 

 of gauges raised above the ground form- 

 erly believed to have been clue to height, 

 is in reality due to increased freedom of 

 wind action. 



There is also another and perhaps even 

 greater element of uncertainty attendant 

 upon rain-gauging, viz. : The smallness 

 of the actual collecting area of a gauge 

 and the comparative immensity of the area 

 of the country to which its readings are 

 applied. With a ten-inch gauge to every 

 four square (and this is a distribution far 

 above the average), the ratio of area 

 would be as about 200,000,000 to 1 . At 

 Rothamsted, in England, there is a rain 

 gauge with an area of one thousandth of an 

 acre. The catch on this gauge from 1853 

 to 1SS0 was about nine and eight-tenth 

 per cent, more than the catch in an adja- 

 cent five-inch gauge. The ratio of their 

 respective catchment areas is as 320 to 1. 

 Now, if this ratio of area gave a variation 

 of nearly ten per cent., we must not rely 

 too implicitly on the results shown by ap- 

 plying the readings of a gauge to an area 

 of country several hundred millions times 

 the area of the gauge. The rain-gauge is 

 an invaluable instrument, but to fully 

 profit by its readings we must recognize, 

 as with other instruments, its limitations 

 and surroundings. 



All these various difficulties considered, 

 we are not warranted in hoping for any 

 decisive direct quantitative comparison be- 

 tween the rainfall over wooded and open 

 grounds similarly situated and exposed. 

 However, while lacking in direct proof of 

 this point, we do not know from the 

 records of the various forest stations that 

 woods reduce temperature and increase 

 the humidity of the air, and, therefore, 

 must to some extent increase precipitation. 



The efficiency of foliage in mechan- 

 ically arresting and condensing moisture is 

 well known. If there are doubts, a walk 



