280 National Oeogra^hic Magazine. 



20 and 35 inches — for instance, in the valley of the Po, the classic 

 land of irrigation, the annual precipitation is from 25 to 35 inches. 

 There are none of these European irrigation regions where the 

 rainfall is less than 10, and generally it is over 20 inches. But 

 you will see that the most of the Californian irrigation regions 

 have less than 15 inches, some less than 10, and the greatest rain- 

 fall of any large irrigable region in California is 18 inches, or, 

 exceptionally, for smaller regions, 25 inches ; while in Europe, 

 the maxima are from 25 to 40 inches in countries where irriga- 

 tion has long been practiced. It follows, then, that there is no 

 place in Europe where it is so much needed as over a large part 

 of California. Another reason why the necessity is felt in our 

 Pacific Coast State, is found in the character of our soils ; and 

 not alone the surface soils, but the base of the soil — the deep 

 subsoils. We have soils exceptionally deep ; soils which extend 

 below the surface to 50 feet, underlaid by loose sand and open 

 gravels, so that the rainfall of winter is lost in them. The annual 

 rain seldom runs from the surface. It follows that these lands 

 ai-e generally barren of vegetation without the artificial applica- 

 tion of water. 



Considering now the sources of water-supply : we have in the 

 southern pai't of the State many streams which flow only for a 

 few weeks after rainfall, and other streams which run two or 

 three months after the rainy season. But there is not a stream 

 in all California south of the Sierra Madre (except the Colorado, 

 which has it sources of supply outside of the State) which flows 

 during the summer with a greater volume than about 70 to 80 

 cubic feet per second — a stream 15 feet in width, 2 feet deep, and 

 flowing at the rate of 2^ to 3 feet per second— a little stream that, 

 in the eastern part of the continent, would be thought insignifi- 

 cant. The largest stream for six months in the year, in all south- 

 ern California, is the Los Angeles river. The Santa Ana river, 

 the next largest, flows from two sevenths to one third as much ; 

 the San Gabriel, the next largest, has perhaps two thirds or three 

 fourths as much as the Santa Ana ; and so, a stream which will 

 deliver as much water as will flow in a box 4. feet wide and 1^ 

 feet deep, at a moderate speed, during summer months, would be 

 regarded as a good-sized irrigation feeder in that southern country. 

 In the greater interior basin or central valley, we find other con- 

 ditions. Here we have a different class of streams. The great 

 Sierra Nevada receives snow upon its summits, which does not 



