60 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



PRECIPITATION. 



Precipitation should be measured at as many special stations as 

 possible, but only at those which are fairly permanent. In general, 

 the regular Weather Bureau data collected at a large number of sta- 

 tions will suffice for the purposes of forest investigators. Because 

 of the difficulty of obtaining an average exposure under canopies, 

 precipitation should always be measured in more or less open situa- 

 tions, or above the crowns, except of course when it is desired to de- 

 termine the amount intercepted by crowns (89). 



Since precipitation has no important action on plants until it is 

 added to the moisture of the soil, there can be no object, in a biologi- 

 cal study, and especially in a study of forests, in analyzing precipi- 

 tation data very closely. For this reason there is no need of hourly 

 precipitation records except possibly in a few localities to study the 

 general character of the storms, which, of course, will vary only 

 slightly with the forest types. For this purpose the tipping-bucket 

 rain gauge (93) should be used. Standard eight-inch rain gauges 

 (93), if properly exposed, will serve in most cases, though more 

 valuable results will be secured where it is possible to install shielded 

 gauges. On the whole, however, the gain in catch through the use 

 of the Marvin shielded gauge 9 is hardly of enough significance to 

 justify the additional expense of the installation, at least for any 

 practical benefit to ecology. The methods of measuring precipitation 

 are too well known to need description. 



Under certain circumstances, as in situations which can not be 

 conveniently visited every day, it is possible to increase considerably 

 the value of the record by keeping some kerosene in the rain gauge, 

 which will cover the water and in large measure prevent its loss by 

 evaporation. In this event it will be desirable either to pour off the 

 kerosene before attempting to measure the water or to pour both 

 into a glass graduate in which the amount of water can be seen in 

 a few moments, after which as much of the kerosene as possible may 

 be replaced in the gauge. This method needs little modification for 

 the winter period, if the snow is melted before measuring, as ordi- 

 narily it would be. It is, however, very desirable to have the snow, 

 as it melts naturally, drop into a seamless basin containing some 

 kerosene. This may be accomplished by placing a loose funnel near 

 the bottom of the gauge. 



Exposure of Gauges. 



While the measurement of precipitation in gauges is very simple, 

 the securing of a true " catch " is much more difficult, and for this 

 reason the greatest care should be used to install gauges in such 



9 Designed by the present Chief of the United States Weather Bureau. 



