76 
TRANSACTIONS OF TIIE TEXAS ACADEMY OF SCIENCE. 
For any system of irrigation the question of rainfall is of first import¬ 
ance, and especially so with the “storm water” storage system. It is 
particularly necessary that the minimum annual precipitation, ascertained 
from observations extending over a number of years, be known. This 
will indicate the maximum amount of water that will be required to be 
stored to irrigate any given area. In the sub-humid regions exceptional 
seasons will come, when the precipitation is sufficient and distribution 
even, which will make unnecessary the using of an}' stored water. A 
rainfall of 28 inches per annum is considered sufficient for the successful 
growth of any crop, if evenly distributed throughout the cropping sea¬ 
son; even a less amount is considered ample by a great many authorities 
—some as low as 20 inches; but stress is laid on the fact that it must be 
evenly distributed. Such a season as the latter must be preceded by a 
wet period, that the ground be already in prime condition. 
Lands over which the annual rainfall is from 20 to 28 inches are classed 
as sub-humid or semi-arid; though more properly semi-arid lands are 
those over which the rainfall is from 18 to 24 inches per annum. In 
Texas the rainfall decreases almost gradually along an east and west line, 
from very humid climate in the east to a very arid one in the west. The 
line of 28-inch rainfall may be considered as the dividing line between 
the humid and the arid areas. This line follows approximately the 98th 
meridian from the north as far south as San Antonio, where it swings 
slightly to the east, and follows the 97tli meridian to the coast. More 
properly speaking, the arid region is separated from the humid, not by 
any well defined, closely cut line , but by an area or belt , over which this 
line of 28-inch rainfall fluctuates each year, passing eastward or westward, 
according as the rainfall is greater or less than the average. For any 
point within this belt it will be seen that the rainfall throughout a num¬ 
ber of years will suffer some extreme variations; for instance, at Abilene, 
in 1888, the rainfall was 29 inches, while in 1891, it was only 17; at San 
Angelo, in 1884, the precipitation was 35 inches, and in 1886 it was 11. 
In these regions, where the annual rainfall suffers such extreme variation, 
and when sufficient, such uneven distribution, it will be seen immediately 
that some means of irrigation must be provided, if it is intended to make 
them a successful agricultural country. Some steps should be taken, 
properly by the State government, towards the establishment of observ¬ 
ing stations, particularly in Western Texas, so that a complete and ac¬ 
curate knowledge of the rainfall may be obtained. The data at present 
is very inaccurate and insufficient; and where there have been stations, 
they have been so far apart that no definite information has been obtained 
regarding particular localities. No system of irrigation should be pro¬ 
jected unless a correct knowledge is had of the amount and distribution 
of the annual rainfall. 
