58 



THE IRRIGATION AGE. 



ration gives the best results for dry-land plants. 

 This is the reason why over- irrigation is injurious. 



Again, it is found by experiment that the growth of 

 the plant will be checked by checking the water sup- 

 ply, which must be continuous to the most success- 

 ful; that if checked the plant adapts itself to the new 

 conditions, and if afterward the normal supply is 

 given, the plant will not avail itself of the better con- 

 dition. It is also found that if the plant is urged to 

 its greatest capacity by the best conditions during 

 the early part of its growth and until the grain or 

 fruit has nearly attained its full size and the water 

 supply is then diminished, the yield will usually be 

 increased and improved in quality; if the leaf 

 growth is then checked the fruitage is increased. 



II. THE SOURCES OF WATER SUPPLY. 



The preliminary question of the duty of water 

 having been examined, it is proposed to indicate the 

 source of water supply for the regions of the United 

 States where irrigation is practiced. The rainfall of 

 the arid region is variable, ranging from 3 to more 

 than 20 acre-inches per acre of surface. The facts 

 relating to the distribution of this unequal rainfall 

 are graphically set forth in map No. 1, entitled 

 "Mean Annual Rainfall of United States." On care- 

 ful examination it will reveal all of the important 

 facts as they have been generalized from the rain- 

 gauge records of the United States. Both official 

 and unofficial records have been used, so far as they 

 have been discovered. It is to be carefully noted 

 that the rainfall is variable not only from district to 

 district, but also from year to year and month to 

 month, and that the yearly and seasonal variations 

 are very great. 



PECULIAR CONDITIONS. 



There are large districts in the arid region where, in 

 extreme cases, not a drop of rain falls for an entire 

 year, while in other years the very same regions ex- 

 perience terrific storms, and utterly arid deserts are 

 suddenly transformed by the creation of storm- 

 water streams, and rivers roll as floods, creeks as tor- 

 rents and brooks as leaping waters. Scattered 

 throughout the arid region are many mountains tow- 

 ering above the valleys and performing the beneficent 

 function of condensing the waters from the heavens 

 and gathering them into lakes as natural reservoirs 

 for perennial streams. These mountain-gathered 

 waters constitute the most important supply for the 

 fertilization of the land. Throughout the arid region 

 there are many comparatively large districts which 

 have no perennial streams, and these districts in- 

 crease in size from north to south, until districts as 

 large as any one of a number of the eastern states 

 are found without a single living stream. But all of 

 these districts without permanent rivers and creeks 



have storm-water streams that are sometimes of great 

 volume. Throughout the arid region streams rise in 

 the mountains and flow into valleys so arid that the 

 waters are all consumed by the thirsty soil and evap- 

 orate into the wind-vexed air. Sometimes the sands 

 do not drink up all of these waters, and salt lakes 

 are formed, from whose noxious surfaces the waters 

 are discharged by evaporation. 



Having determined the mean rainfall of the arid 

 region with a reasonable degree of accuracy, we have 

 next to determine what becomes of the rain. 



"RUNOFF" AND "FLYOFF." 



When it falls upon the earth a part of the water is 

 gathered into streams and is carried away into sinks, 

 lakes and the ocean; let us call all this stream water 

 runoff. Another part is carried away by the air and this 

 air-borne portion is in part evaporated from the sur- 

 face of the land and from the leaves of plants, while 

 another part is delivered to the air by transpiration. 

 All this air-gathered water is drifted away by the 

 winds, and therefore let us call \lflyoff. The rainfall, 

 then, is divided between runoff and flyoff. 



The Geological Survey has been engaged for several 

 years in an investigation designed to determine the 

 relative proportions of flyoff and runoff, in order to 

 properly account for the disposition of the rainfall 

 made by nature. It is proposed to give the general 

 results of this investigation. 



Knowing the rainfall, we must then determine the 

 runoff, and this is done by gauging the streams. All 

 the streams have not been gauged, but many have 

 been, and these have been selected as typical cases. 



AN ANALYSIS OF RUNOFF. 



It has been found by observation that the runoff is 

 variable in three ways. 1. It varies with the amount 

 of rainfall. If the rainfall is greater the runoff is 

 greater. 2. It also varies with the character of the 

 rains. When the rains come in great storms a large 

 proportion runs off. A gentle shower is found to be 

 almost wholly evaporated. If a year's rainfall is con- 

 centrated into two or three great storms it will largely 

 go into the streams, but if distributed through many 

 showers it will be returned to the air. 3. Again, 

 topographic conditions greatly influence the runoff. 

 In a region of steep hills, mountains and canons, 

 with many naked rocks, the runoff is very great; in a 

 level district, where loose sands and soils prevail, the 

 runoff is small. Thus the rainfall becomes runoff in 

 an unequal degree by reason of the inequality of 

 storms and also by reason of the inequality of topo- 

 graphic features. 



In gauging the rivers of the United States results 

 have been reached as follows: 



Where the rainfall is 40 inches the runoff will be 20 

 inches; one-half is runoff and one-half flyoff. Where 



