THE WATER SUPPLIES IN THE ARID REGION. 



59 



the rainfall is less the proportions are changed. With 

 30 inches of rainfall 18 inches will go to the air and only 

 12 inches to the stream, two-fifths runoff and three- 

 fifths flyoff ; but the amount will be variable in different 

 districts, because of topographic conditions. With 20 

 inches of rain the amount of runoff will be 5 inches, 

 one-fourth runoff and three-fourths flyoff; but the pro- 

 portion will vary by reason of topographic conditions. 

 Where the rainfall is 10 inches the runoff is a little 

 less than 1 inch, one-tenth runoff and nine-tenths fly- 

 off, but variable by reason of topographic features. 

 As rainfall diminishes, topographic conditions have 

 greater control. At 10 inches and below, topography 

 almost wholly controls the runoff. Where the rain- 

 fall is the same the streams may be few and small 

 or many and great. There are large tracts of 

 country in Arizona, southern California and Nevada 

 where 10 inches of rainfall never gives a permanent 

 stream and rarely a storm stream. There are other 

 districts of country where 10 inches of rainfall gives 

 birth to many living waters. If the lands are com- 

 paratively level the sands drink all the water; if the 

 lands are traversed by canons carved by rivers that 

 have their origin in the mountains, a labyrinth of 

 lateral canons is formed and the rainfall is promptly 

 gathered into streams which roll into salt lakes or into 

 the sea. The rain in the desert is gulped down by the 

 sand; the rain in the canon is gathered into a creek. 



WHAT BECOMES OF THE RAINFALL. 



We must now get a clear understanding of what is 

 meant by runoff. Most of the streams of the United 

 States ultimately discharge into the ocean; all of 

 the water thus carried to the sea is runoff. Some of 

 the streams of the arid region empty into salt lakes; 

 all the water thus discharged is runoff. A very large 

 number of small perennial streams of the arid re- 

 gion are discharged into what are usually called 

 sinks; that is, into sand valleys, where their waters 

 are t evaporated; all this water is runoff. There is 

 still a great multitude of small storm-water streams 

 that live only a short time after a rain and whose 

 waters are gathered into sand valleys and evaporated 

 or into perennial streams; all such waters are runoff. 

 Much rainfall sinks into the soil; a part slowly 

 evaporates and becomes flyoff, but another part issues 

 again as springs, and spring water is here considered 

 as runoff. Rivers, creeks, brooks, storm-water streams 

 and springs constitute this available water which we 

 call runoff. 



The water supply for irrigation in the arid region 

 must mainly come from the runoff where the rainfall 

 is 20 inches or less, for with some exceptions it is the 

 runoff water which is used in irrigation. Crops are 

 not raised throughout the entire season, but during 

 a period varying in different portions of the country, 

 and with different crops, from 60 to 150 days. If the 



rainfall of the entire year would come during the 

 growing season, with a fair distribution throughout 

 the days, a large part of the arid lands could be cul- 

 tivated without irrigation, but in fact the rainfall is 

 unequally distributed throughout the year. 



INEQUALITIES OF THE RAINFALL. 



The inequalities of rainfall through the season 

 are very great. Everywhere throughout the arid re- 

 gion it often occurs, now here and now there that no 

 rainfall comes during the growing season; so that it 

 is necessary for the farmer to provide by irrigation 

 water for the entire crop. He may have rain- 

 fall and he may not. If therefore his agricultu- 

 ral operations are to be successful from year to year 

 he must provide all the water necessary for the crop. 

 The water which can be utilized for this purpose 

 must come from the runoff, with exceptions hereafter 

 to be mentioned. 



We have given certain laws relating to runoff for 

 definite amounts of rainfall of 40, 30, 20 and 10 

 inches. Now it is proposed to apply these laws to the 

 arid region by district of country, and to show the 

 average runoff by such districts; and for this purpose 

 we shall consider runoff in zones or regions where the 

 runoff varies from 20 to 30 inches, from 10 to 20 

 inches and from to 10 inches. Of course, these 

 zones everywhere run into one another; definite lines 

 of boundary are not made in nature, and only ap- 

 proximate lines of boundary can be laid down. 



THE MEANING OF THE MAPS. 



It is not practicable to describe areas of country in 

 words; the mind fails to properly conceive that 

 which is told. But where speech fails map representa- 

 tion succeeds. For this reason a map of the United 

 States is presented, exhibiting the several districts as 

 above defined in comparison with other districts of 

 the eastern portion of the United States where the 

 runoff is greater. This map, entitled "Mean Annual 

 Runoff," will help to make my statements clear and 

 show to what districts of country they apply. It will 

 be found especially instructive when compared with 

 the rainfall map which precedes it. In the construc- 

 tion of the runoff map the laws heretofore explained 

 have been used, and topographic features have re- 

 ceived consideration. It will be seen that the runoff 

 map does not wholly coincide with the rainfall map, 

 from the fact that topographic features play a more 

 important part in runoff. Altitude affects rainfall, 

 and altitude and character of surface affect runoff; 

 and as rainfall becomes less, runoff is affected by 

 character of surface in a steadily increasing ratio". 



Turning to the districts represented in the map, the 

 following statements may be made: It has been seen 

 that the practical duty of water is 24 inches. Now, 

 where the runoff is from 5 to 10 inches, if all the run- 



