24

June, July, August, and September, at the time when most needed by many crops. The diagram for El Paso, from thirty-six years' averages, shows a relatively uniform but small precipitation throughout the year, with the exception of the months of July, August, and September.

CLIMATIC AND GEOGRAPHIC DIVISIONS.

In order to discuss the present condition and development of irrigation in Texas it is essential to pursue some systematic order of arrangement based upon climatic or geographic factors. The simplest arrangement is that of taking first the humid region, or that having 40 inches or over of rainfall; next the subhumid, with from 30 to 40 inches; then the semiarid, with from 20 to 30 inches of rain; and last the more arid portions, having less than 20 inches of annual precipitation, this being in the western part of the State, and including the Panhandle and Trans-Pecos regions. - The greater part of the irrigation works are, however, in the southern half of the State, or near the coast, and these divisions, especially those of subhumid and semi-arid, extending, as they do, in a north-south direction, though simple, do not afford a wholly satisfactory grouping. In the central part of the State the irrigation works are small, and there is no essential difference between those in the more humid and those in the semiarid areas. In fact, the actual amount of annual precipitation has less to do with the necessity for artificially applying water than the local conditions of soil and character of crops.

From the above-mentioned conditions it has been found desirable to adopt a somewhat arbitrary classification, based partly upon the distribution of rainfall and partly upon geographic position. In the following pages, therefore, a description will first be given of the irrigation plants in the humid area near the coast, particularly those lying within 50 miles or more of Galveston. Next in order are the small irrigation projects scattered about the center of the State from the humid areas east of Brazos River westward nearly to the borders of the arid region and southward to the vicinity of Austin and San Marcos. The third division is taken to include the old irrigation works at San Antonio, with small irrigation projects found at intervals down to the coast. Next in order to these are the irrigation works planned or constructed along the Nueces River and the lower Rio Grande. The fifth division to be considered is that of the Llano Estacado and adjacent areas, including the greater part of Panhandle Texas; then the lands watered by the Pecos River, and finally the irrigation works in Trans-Pecos Texas, extending to the extreme westerly end at El Paso.