Jan. 3, 1916 
Hourly Transpiration Rate on Clear Days 
607 
A striking feature of the radiation curve is the rapid rise in radiation 
intensity during the early morning hours. Reference to the graphs will 
show that the radiation has attained approximately one-half its maxi¬ 
mum value two hours after sunrise, and a corresponding decrease occurs 
in the late afternoon. 
The mean air temperature during the rye transpiration period ranged 
from 54 0 F. at 4.30 a. m. to about 83° F. at 4.30 p. m. The maximum air 
temperature thus occurs four hours later than the solar-radiation maxi¬ 
mum. The wet-bulb-depression graph is similar in form to the air- 
temperature curve, and its maximum occurs at approximately the same 
time. The maximum of the evaporation curve, on the other hand, cor¬ 
responds with that of solar radiation, but the slope of the evaporation 
graph is more nearly uniform during the morning and afternoon than 
that of the radiation graph. 
The transpiration graph of rye shows the same flattening during the 
middle part of the day that was observed with wheat and oats in 1912. 
With rye this flattening begins at 8.30 a. m., and continues until 1 p. m., 
the slope being nearly uniform during this period. During the late 
afternoon the transpiration falls rapidly and the night transpiration is 
seen to be very low. 
The mean wind velocity in miles per hour is plotted at the bottom of 
figure 6. The maximum rate of about 9 miles per hour occurs from 8 to 
10 o'clock in the morning. During the night the rate is less than 5 miles 
per hour. There is little indication from the graphs that differences in 
the velocity of the wind had much influence on either the transpiration 
or the evaporation rate. 
alfalfa 
The transpiration measurements upon alfalfa (Medicago sativa) 1 are 
the most extensive of the series and include 52 day records taken during 
26 days, embracing late-season as well as midsummer measurements. 
The transpiration data are given in detail in Table XXI and the physical 
measurements in Table XXII to XXVI, inclusive. The hourly means 
will be found plotted in figure 7. Since the period covered by the meas¬ 
urements is so extended, it has seemed advisable also to separate the 
measurements into shorter periods for comparison. Summaries covering 
a short transpiration period in June and another period in October are 
accordingly presented in Tables XXVII and XXVIII, and are plotted in 
figure 15, to which reference will be made later. 
1 Grimm alfalfa, A. D, I. (Alkali and Drought Resistant Plant Investigations) No. 23. 
