i8o 
Journal of Agricultural Research 
Vol. VII, No. 4 
Similar measurements for other crops during a later io-day period in 
1915 (Table XII) showed a marked reduction in the daily transpiration 
rate compared with the preceding year. The evaporation rate during 
the 1915 period was only about one-half that in 1914. The transpiration 
rate of the alfalfas during the 1915 period was also approximately one- 
half that observed in 1914. 
Table XII .—Loss of water per unit of dry matter harvested during the maximum trans¬ 
piration period July 2J to August 5, IQ15 
Crop. 
Dry 
Mean daily transpira¬ 
tion July 27-Aug. 5, 
1915- 
Hourly 
transpira¬ 
tion during 
midday 
per gram 
of dry 
matter. 
Daily loss 
of water 
per ton of 
dry matter 
per acre. 
matter. 
Actual. 
Per gram 
of dry 
matter. 
Cowpea. 
Grams. 
128 
Kilograms. 
I- 15 
Grams. 
9-0 
Grams. 
0.9 
Acre-inches . 
0. 08 
Millet. 
256 
I- 15 
4.5 
• 5 
.04 
Sorghum. 
203 
.84 
4. I 
• 4 
.04 
Com. 
112 
.56 
5 -o 
*5 
.04 
Potato. 
67 
.41 
6.1 
.6 
• 05 
Amaranthus.. 
129 
.90 
7.0 
• 7 
. 06 
Sudan grass.. 
I76 
I. 18 
6. 7 
• 7 
. 06 
Alfalfa, E23.. 
133 
2. 52 
18. 9 
1.9 
• 17 
Alfalfa, 162-98A1. 
”5 
2. 38 
20. 7 
2. 1 
. 18 
°Mean daily evaporation, 6.2 kgm. per square meter. 
HOURLY TRANSPIRATION DURING MIDDAY PER GRAM OR DRY MATTER 
HARVESTED 
An examination of the graphs of hourly transpiration on clear days 1 
shows that the transpiration during one hour at or near midday in mid¬ 
summer is approximately one-tenth of the total transpiration for the 
day. The hourly transpiration of different crops has been calculated 
on this basis for the midday hours during the io-day maximum trans¬ 
piration periods considered above (Tables IX to XII). In 1914 the 
midday transpiration of the small grains ranged from 1.2 gm. per hour 
per gram of dry matter for barley to 1.6 gm. for wheat, oat, and rye 
(Table IX). In other words, a crop of oat yielding 1 ton of dry matter 
per acre would have lost 1.6 tons of water per acre per hour during the 
midday hours of its maximum transpiration period. 
Cowpea and lupine lost each hour during midday an amount of water 
equal to 1.9 times the dry weight of the crop (Table IX). . Millet, com, 
and sorghum transpired at a much slower rate than the other crops 
here considered, the hourly loss of sorghum being 0.6, millet 0.8, and com 
0.7 to 0.9 that of its dry weight. 
1 Briggs, L. J., and Shantz, H. L. Hourly transpiration rate on clear days as determined by cyclic 
environmental factors. In Jour. Agr. Research, v. 5, no. 14, p. 583-650, 22 fig., pi. 53-55. 1916, Literature 
cited, p. 648-649. 
