386 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1912. 



almost every ruin. Accordingly, in the following discussion we shall 

 follow the archeologists in assuming that the ancient Hohokam 

 were an agricultural people. We shall depart from them, however, 

 in assuming that, in the absence of any evidence to the contrary, the 

 Hohokam were like the rest of mankind, and their ruins are to be 

 interpreted by the same criteria as those universally employed in the 

 study of the archeology of other parts of the world. 



With these assumptions in mind, we are prepared to investigate 

 the relation of the ancient population to rainfall. Let us first con- 

 centrate our attention upon a single region, the Santa Cruz Valley 

 of southern Arizona. I select this valley, not because it is particu- 

 larly remarkable, but because it happens to contain Tucson, the site 

 of the Desert Laboratory. This town is the largest in the two 

 States of New Mexico and Arizona, although it has only 16,000 

 people, including all its suburbs. 



The reason for the scantiness of population is found m the climate. 

 The average rainfall at Tucson amounts to about 12 inches. This is 

 distributed between two rainy seasons; one of them comes m the 

 winter from November to March, and has an average of about 5 

 inches of rain, while the other, with 7 inches of ram, begins at the 

 end of June and lasts until early in September. The months of 

 April, May, and June, or the foresummer, as MacDougal has called 

 them, are practically ramless and very hot, and the same is true of 

 the interval from the end of the summer rains to the beginning of 

 those of winter. Nothing can be raised without irrigation of some 

 sort. Since the coming of the white man, winter crops, such as 

 barley, alfalfa, and the hke, have become important. The Indians, 

 however, cultivated practically nothing except corn and beans, 

 which they irrigated by means of the summer floods. 



The Santa Cruz Valley has a length of at least 200 miles, but most 

 of it is well-nigh a desert, and can be utiHzed only for cattle raismg. 

 According to Prof. R. H. Forbes, director of the Arizona Experiment 

 Station, the entire drainage area of the Santa Cruz River contains 

 only about 6,000 acres of land under cultivation. Part of the 6,000 

 acres is under full irrigation and produces four or five crops of alfalfa 

 per year, while a considerable portion is only under flood irrigation 

 and produces but one crop each year, when the heavy rains of Jul}' 

 and August redeem the desert for a brief space. Under the best 

 system of irrigation available in modern times, Prof. Forbes estimates 

 that for every 2 acres brought under full cultivation one person is 

 added to the population of Arizona. In other words, if the Santa 

 Cruz Valley were cut off from all the rest of the world and left to its 

 own resources %vithout railways, muies, health-seekers, or other 

 extraneous sources of wealth, the population would be limited to 

 the 3,000 who could be supported on the 6,000 acres of irrigated or 



