mLLETIX 1233, U. S. DEPARTMENT OF AGRICULTURE. 



Table 23 contains a number of surprises and brings out certain 

 relationships of the types and* localities studied, the knowledge of 

 which could hardly have been obtained through the examination of 

 the several conditions affecting evaporation — sunshine, humidity, 

 wind, and temperature. The desirability of a direct measure of 

 evaporation is therefore apparent, and the importance of a method 

 which gives the most probable weight to each of the component 

 factors in evaporation must not be minimized. 



One of the first points to be noted is that, with the exception of 

 one or two of the very open situations, the winter ratios for the 

 outlying stations are much lower than the summer ratios to the 

 control station. This is partly due to the effect of cover and to the 

 fact that there is some slope, which may cut off insolation much more 

 completely in winter than in summer. A still more important 

 point is that when the air temperatures are below freezing there can 

 be practically no evaporation without sunlight to thaw the in- 

 strument (or, for that matter, the leaf), but. when the air is every- 

 where warm, insolation is by no means so important a factor. It 

 is well to bear in mind that under very arid conditions shading is by 

 no means a preventive of water losses. 



The following observations may be made with regard to conditions 

 of the several forest types: 



(1) It is readily seen that, of the yellow pine sites examined, the 

 most severe is by no means the planting site in the Nebraska sand- 

 hills, but is found at the foot of the Pikes Peak region (M-l) , where the 

 low humidity of the mountains is combined with the relatively high 

 temperatures of the low elevations. The ratio for Station M-l is 

 uniformly high throughout the year, the depression in November and 

 December not being accounted for by any of the factors which have 

 been more extensively studied. On the other hand, because of the 

 low wind movement and higher humidity, the Nebraska sandhills 

 present a far less severe condition, except during the summer when 

 the temperature contrast is greatest. It is possibly of some signifi- 

 cance that the highest relative rate occurs here about September 1. 

 Of* tin 1 several yellow pine sites examined, that which approaches 

 a condition favorable lor Douglas fir (F-4) is seen to be most mod- 

 erate in its evaporation stresses. The low values denote the influence 

 of shade more than anything else. 



(2) The contrasl between open and shaded Douglas fir sites is 

 brought out very clearly. Thus the four local north-slope stations 

 show a marked grading down from the clear-cut to the virgin stand, 

 the hitter, for the year as a whole, having only two-fifths the evapora- 

 tion rate of the former. Whether or not the high evaporation rate 

 in the open area is a prohibitive factor for any of the species, there is 

 presented, at a single glance, a striking picture of the changed con- 

 dition- for reproduction which necessarily follow cutting of the 

 parent forest. Again, in this group a surprise is met witli in the high 

 evaporal iod rate for the Wagon Wheel Chip stations, where, as already 

 -down, the temperatures and wind movements are generally lower 

 than at Fremont, and the vapor pressures markedly Lower. Thus 

 at Station W \1. which probably receives les- than' half the direct 

 insolatiop received at the control station, evaporation goes appre- 

 ciably higher during the early pint of the Bummer but dwindles 

 almost to nothing in the coldest winter month-. Where atmospheric 



