78 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
Ganges Canal irrigates 1,600,000 acres in an area having an average an¬ 
nual rainfall of 33 inches. The Lower Ganges Canal irrigates 1,187,000 
acres, with a rainfall of 31.1 inches over the area which it controls. On 
the lands watered by the Agra Canal, the precipitation averages 27 inches 
per annum. Movements have been made to bring the very humid lands 
of Louisiana and Mississippi under some system of irrigation. As irri¬ 
gation is more appreciated it will be more practiced all over the world in 
what are now considered veiy humid lands, to increase the yield and 
offset the disastrous consequences of drouth. 
Where the precipitation is small and evaporation great, particular at¬ 
tention must be given to the size of the catchment area, and selection of 
the reservoir site. After determining the percentage of evaporation and 
percolation from a given catchment basin, the amount that can be col¬ 
lected in the reservoir can then be estimated. This is ordinarily esti¬ 
mated at 40 per cent of the total rainfall. The reservoir should be in 
some narrow place between hills, so that the cost of the dam will be 
minimized and the sides of the reservoir steep, in order that a small sur¬ 
face will be exposed to evaporating influences. 
The “ storm-water” storage system of irrigation can not be considered 
impracticable or impossible, and when it is seen that for such a large part 
of the fertile lands of Texas it is the only system practicable, some cre¬ 
dence will be given to its importance. The necessity of making land 
available for agriculture is not yet felt or appreciated by the people to 
the extent it will be. 
To encourage this system of irrigation, and to demonstrate its practi¬ 
cability, Major Powell, of the United States Geological Survey, at the 
the urgent request of the Texas Survey, caused several catchment basins 
to be surveyed in West Texas, and the favorable sites for reservoirs lo¬ 
cated. All data pertaining to rainfall, etc., have been collected, on 
which the following estimates are based. 
One nine miles south of Marfa, in Presidio county: 
Approximate area of catchment basin.. 440 square miles. 
Surface area of reservoir. 1.43 square miles. 
Height of dam. 100 feet. - 
It is estimated that a rainfall of one and one-third inches from the en¬ 
tire basin would fill the reservoir and be sufficient to irrigate 20,000 
acres. 
One seven miles south of Alpine: 
Approximate area of catchment basin. 203 square miles. 
Surface of reservoir. 1.43 square miles. 
Height of dam. 50 feet. 
