April 30, 1920] 



SCIENCE 



439 



freely used follows. Attention is next turned 

 to the theory of dilute solutions, disperse 

 systems, some additional elements, the periodic 

 classification, and radio-activity. 



In the discussion of the topics noted many 

 chemical facts are brought before the stu- 

 dent but stress is laid on principles and little 

 space devoted to facts of general interest un- 

 less they serve as examples of these principles. 

 For example, the only reference to the prep- 

 aration of iron from its ores is a paragraph on 

 carbon as a reducing agent, in which the 

 statement is made that metallic iron is made 

 from the mineral hematite by reduction with 

 coke at white heat. 



The book is clearly written. It will be of 

 interest to teachers to see how rather diffi- 

 cult subjects can be handled effectively in a 

 simple manner. It will be looked upon with 

 favor as a text for beginners by those who 

 desire to teach facts only through the use of 

 laws and theories and do not think it ad- 

 visable to unduly emphasize the applications 

 of the science. 



James F. Noeris 



NOTES ON METEOROLOGY AND 

 CLIMATOLOGY 



RAINFALL INTERCEPTION BY TREES AND CROPS 



For several years Mr. Eobert E. Horton, 

 consulting hydraulic engineer, Voorheesville, 

 N. Y., has carried on investigations of the 

 various aspects of rainfall in relation to run- 

 off.^ In such studies what the hydraulic engi- 

 neer needs to know first is how much rainfall 

 reaches the ground, over a watershed. Is it 

 the amount of precipitation that as shown by 

 well-exposed gages ?^ N'o. Much rain and 

 snow is intercepted by trees, and evaporated. 



1 See "Additional Meteorological Data Needed 

 by Engineers," by E. E. Horton, Engineering 

 N&ws Becord, March 27, 1919, pp. 614-616; re- 

 printed in Monthly Weather Meview, May, 1919, 

 Vol. 47, p.p. 305-307. 



2 See ' ' The Measurement of Rainfall and 

 Snow, " by E. E. Horton, Jmir. New England 

 Water Worl's Assoc, 1919, Vol. 33, pp. 14-71, 21 

 figs., 12 tables; reviewed in Monthly Weather Eev., 

 May, 1919, Vol. 47, pp. 294-296. 



Thus the hydraulic engineer, unlike the 

 meteorologist, needs to study the catches of 

 rain-gages under trees as well as in the open. 

 [Some cooperative observers seem to have an- 

 ticipated this need.] Mr. Horton has made a 

 careful study of the amount of precipitation 

 which falls through different kinds of trees 

 and of that portion of the intercepted rain- 

 fall which runs down the trunks. Also, in 

 order to enable him to form an estimate of the 

 water which reaches the groixnd over a varied 

 watershed he has determined the amount of 

 rainfall intercepted by different gTowing crops 

 in various stages. The results of his investi- 

 gations have been published in the Monthly 

 Weather Review^ 

 Mr. Horton concludes that 



Rainfall interception represents a loss of precipi- 

 tation which would otherwise be available to the 

 soil. The loss takes place through evaporative 

 processes, but may, for convenience be subdivided 

 into (a) interception storage, and (6) evapora- 

 tion during rain. 



The amount of interception loss is primarily a 

 function of the storage capacity of the plant sur- 

 face, the duration of precipitation, and the evapo- 

 ration rate diu-ing precipitation. Since there is 

 generally a fairly close correlation 'between shower 

 duration and amount of precipitation, estimates of 

 interception loss can, for practical purposes, be ex- 

 pressed in terms of precipitation amount per 

 shower. 



The interception storage loss for trees varies 

 from 0.02 to 0.07 inch per shower, and approaches 

 these values for well-developed crops. . . . The . . . 

 loss is greater in light than in heavy showers, 

 ranging from nearly 100 per cent, where the total 

 rainfall does not exceed the interception storage 

 capacity to about 25 per cent, as an average con- 

 stant rate for most trees in heavy rains of long 

 duration. [Of this] the amount of water reaching 

 the ground by running down the trunks of trees 

 . . . is . . . commonly 1 to 5 per cent, of the total 

 precipitation. The percentage increases from zero 

 in light showers to a maximum constant percentage 

 in heavy showers of long duration. Light showers 

 are much more frequent than hea^'y ones, and the 

 interception loss- for a given precipitation in a 

 month or season varies largely according to the 

 rainfall distribution. 



3 September, 1919, Vol. 47, pp. 603-623, 17 figs. 



