268 REPORTS ON THE STATE OF SCIENCE.—1914. 
in the central part of North Dakota. Such differences have a profound 
influence upon the water-requirement of plants. 
Shantz * has shown that under practically uniform soil conditions 
a pure short-grass formation is found in Northern Texas with an 
annual rainfall of about 21 inches; in Eastern Colorado with an 
annual rainfall of about 17 inches; and in Montana with an annual 
rainfall of approximately 14 inches. The region throughout has a 
summer rainfall. The same plant formation then requires 50 per 
cent. more rainfall in Northern Texas than in Montana. The explana- 
tion of this is to be found in the difference in the evaporation-rate 
in the two sections. Reference to fig. 2 will show that the evapora- 
tion in Northern Texas is approximately 60 per cent. higher than in 
Central Montana. A similar comparison between Northern Texas and 
North-Eastern Colorado shows that short-grass requires about approx- 
imately 27 per cent. more rainfall in Northern Texas, where the 
evaporation is 23 per cent. higher than in North-Eastern Colorado. 
The effectiveness of rainfall depends of course upon its penetration 
iuto the soil, so that any relationship which may be developed 
between evaporation and precipitation will necessarily be an approxi- 
mate one. The above figures indicate, however, a rather close 
parallelism between the evaporation and the rainfall required to 
maintain a given plant formation, and emphasise the necessity of 
knowing the evaporation as well as the rainfall in judging the dry- 
farming possibilities of a region.® 
A direct relationship between evaporation and water-requirement— 
i.e., the pounds of water required by a plant in the production of a 
pound of dry matter—is shown in the following measurements by 
Briggs and Shantz of the water-requirement of the same strain of 
alfalfa when grown in different parts of the Great Plains (Table I.). 
TABLE I.— Water-requirement of Grimm alfalfa (second cutting) at different 
Stations in the Great Plains, 1912. 
} ] | 
| Water-re- : 
| | quirement | Evap.| Daily Set ef 
Location | Growth period | Days |(toproduce| in | Evap. in Dak 
1 lb. dry |inches; inches “nad 
| | matter) P 
| | | | Ab, 20x21 | 
| Williston, N.D.. July 29-Sept. 16 47 518 12] 7-5 | 0-159 33 | 
| Newell, S.D. .| Aug. 9Sept. 24, 46 630 8) 86 0-187 34 
| Akron, Col. .| July 26-Sept. 6 42 | 853 13 9:5 | 0-226 38 
Dalhart, Tex. .| July 26-Aug. 31| 36 | 1005 8/ 110 | 0306 | 34 | 
{ Ward 2 |! / | | 
Tt will be seen that the water-requirement increases steadily as 
one proceeds southward through the Great Plains, being twice as 
great in Northern Texas as in North Dakota, The daily evaporation 
4 Shantz, H. L., Natural Vegetation as an Indicator of the Capabilities of Land 
for Crop Production in the Great Plains Area, U.S. Department of Agriculture, Bureau 
of Plant Industry, Bulletin 201, 1911, p. 12. 
nee L. J., and Belz, J. O., Bureau of Plant Industry, Bulletin 188, 1911, 
Pp: . 
