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isohyetals, or lines of equal precipitation, run through the State in a general north-south direction, as shown by the map, fig. 1.

The customary definition of "arid region" " is one having less than 20 inches of mean annual rainfall. This, however, is not sufficiently complete, for it does not take into account the distribution of the rain throughout the year. In certain parts of the country, where the greater portion of the precipitation occurs during the crop season, wheat and other cereals are successfully raised when the rainfall is far less than 20 inches; while in other parts of the United States, as,

for example, near the Pacific coast, where the greatest precipitation occurs during the winter months and the summers are practically rainless, irrigation is necessary during a part, at least, of the crop season. Thus the distribution by months is almost as important an element as the total quantity occurring during the year.

The accompanying diagram, fig. 2, shows the quantity and average distribution of rainfall by months at a number of selected stations fairly typical of the State as a whole. The first of these is Galveston, on the coast, in the eastern part of the State. Here the average