776 EXPERIMENT STATION EECOED. 



the nearest regular station. In case rainfall and temperature data are meager, 

 charts showing isotherms and isohyetals for the portion of the State in which 

 the watershed is situated are of asssistauce. (B) Data relating to wind ve- 

 locity, relative humidity, and any other prominent weather characteristics. 

 (C) Data relating to topography, vegetal cover, soil, and subsoil as affecting 

 evaporation. (D) Data relating to character and density of vegetation and 

 length of growing season with reference to temperature and hours of sunshine. 

 (E) Data relating to area of open- water surfaces, swamps, and marshes. 



"II. Determination of losses. — (A) Evaporation from water area: (1) 

 Monthly evaporation corresponding to given temperature and season, and 

 multiplied by percentage of water surface, based on data under I-E, and coeffi- 

 cient based on data under I-B. (B) Evaporation from land area: (1) Deter- 

 mination of coefficient for given watershed, based principally on physical data 

 under I-C and I-B. (2) Determination of evaporation, in inches depth per 

 month, corresponding to given monthly temperature and rainfall for given 

 season of year, from curve of evaporation from land areas, and multiplication 

 of the same by percentage of land area and coefficient determined under II-B-1. 

 (C) Transpiration from land area: (1) Determination of normal seasonal 

 transpiration, based on physical data under I-D. (2) Determination of tran- 

 spiration coefficient by finding ratio between seasonal transpiration determined 

 from base curve of transpiration for the normal monthly temperatures for the 

 given watershed, and the normal seasonal transpiration determined under 

 ll-G-1. (3) Determination of monthly transpiration by applying transpiration 

 coefficient to monthly values talien off transpiration curve for given monthly 

 temperatures, and modification of these monthly values on basis of rainfall, 

 percolation, and storage. 



" III. Determination of total loss by summation of monthly losses from land 

 and water areas, the deduction of these monthly losses from the monthly pre- 

 cipitation, and summation of these monthly residuals to give the annual yield 

 of the given watershed, with or without correction of this annual total for fall 

 surface run-off or changes in ground and surface storage. 



"IV. Where the annual yield and its distribution throughout the year are 

 both desired, additional curves . . . and computations for the same watershed 

 must be made. When the more detailed computations, as here indicated, are 

 carried out, it is possible to make more accurate estimates of transpiration 

 during months of deficient rainfall, because more accurate values of soil and 

 subsoil storage are available." 



Other special applications of the author's method are discussed in detail with 

 data from observations. 



Snow survey provides basis for close forecast of watershed's yield, J. E. 

 Chukch, Jr. {Engin. Rec, 11 {1915), No. 16, pp. m, 495, figs. 3).— Rapid and 

 economical methods of measuring large areas of snow at high altitudes are 

 described which have proved useful at Lake Tahoe, Nev. 



"The general method pursued was to determine the water content of the 

 snow on typical slopes and under characteristic forest covers, making each topo- 

 graphic unit as large as possible. Measurements were made in sufficient sec- 

 tions of the basin to determine the local difference in snowfall, and enough 

 courses at high levels were measured to determine the relation of the snowfall 

 on the higher slopes of the watershed to that on the floor of the basin. The 

 courses did not always follow contour lines but were frequently diagonal and 

 sometimes vertical to them, being so laid as to determine the water content of 

 the slope in question." 



It is stated that the method of snow surveying is a practical substitute for 

 the expensive method of contour line surveying. 



