AVAILABLE WATER SUPPLIES. 



101 



ity where observations were maintained continuously 

 for a year. A study of the size of these dots shows a 

 wide divergence in the recorded rainfall for adjacent 

 localities, this difference being due, in many instances, 

 to difference in altitude or location of the gauge. The 

 small amount of rainfall through Colorado, New 

 Mexico, Arizona, and other divisions of the arid re- 

 gions, is indicated by the small size of the dots, these 

 being scarcely perceptible on this scale. 



In consulting any map of rainfall similar to the one 

 first mentioned, it should not be forgotten that this 

 represents only the average conditions for the past few 

 years or decades, and cannot in the nature of things 

 tell the whole story of the great fluctuations which 

 take place, not only from month to month, but from 

 year to year. To illustrate these would require a vol- 

 ume. At this time, however, it may be allowable to 

 introduce a few general figures showing the varia- 

 tions which have taken place for ten years. The fol- 

 lowing table gives the average rainfall at all the 

 stations in the western part of the United States, at 

 which full sets of observations were made contin- 

 uously. 



Average precipitation in western United States, in- 

 cluding both arid and humid areas: 



Year. 



1880 

 1881 

 1882 

 1883 

 1884 

 1885 

 1886 

 1887 



1889 



Depth in 

 inches. 



20.6 

 18.1 

 18.0 

 17.2 

 25.2 

 19.2 

 18.7 

 19.5 

 20.0 

 23.9 



Average 20.0 



Per cent, in 

 average. 



103.0 



90.0 



90.0 



86.0 



126.0 



96.0 



93.0 



97.0 



100.0 



119.0 



100.0 



From the above table it appears that the ten years' 

 average rainfall at all of the important stations in the 

 western half of the United States was 20.0 inches, and 

 that there has been a variation of from 86 to 126 per 

 cent. At any one station this variation would have 

 been far greater, but the point to be emphasized is 

 that even taking all stations there are wide differences, 

 one year with another. Thus a map of average con- 

 ditions must be used very cautiously in any but gen- 

 eral discussions. To illustrate, it may be said that 

 any proposition of irrigation, or storage, based upon 

 considerations of average rainfall, is apt to be mis- 

 leading, as the vital points in such matters are not 

 the averages but the extremes, the greatest and the 

 least rainfalls and their times of occurrence. 



The amount of rainfall is a matter, of great impor- 

 tance to farmers in all parts of the country, as well as 



to those of the arid region, but to the irrigatorsandto 

 engineers, or others concerned with the development 

 of the water resources, by canals or by storage, the 

 amount of rain has less immediate interest than the 

 quantity of run off. It is only that part of the rain 

 which flows upon the ground which is of possible 

 value in irrigation. 



CONSTRUCTION OF RUNOFF MAP. 



The runoff, that is to say the quantity of water 

 which flows from the land into the rivers, may be ex- 

 pressed in .many different ways, but of these there are 

 two in use, viz: The average quantity per unit of time 

 flowing from a given area ; for example, a cubic foot per 

 second per square mile, or by an equivalent depth of 

 water over the drainage basin. This latter has been 

 chosen as the most useful in general discussion, be- 

 cause of the possibility of making immediate com- 

 parison with the depth of rainfall. As depth of rain- 

 fall is usually expressed in inches, the computation 

 of runoff has been reduced to the same unit. This unit 

 has also another advantage that comparison can be 

 drawn with data of water supply, which is sometimes 

 also expressed in depth of water over an acre, that 

 is, in acre-inches or acre-feet. 



For constructing a map showing runoff, there is far 

 less data available than in the case of a map of the 

 mean annual rainfall, and therefore it has been nec- 

 essary to make more general assumptions, and to 

 assume wider limits as to the probable runoff from 

 various areas. In the case of the rainfall map seven 

 divisions were made that is, from to 10 inches, 

 from 10 to 20 inches, from 20 to 30 and so on up to 70 

 inches and over. In the case of the runoff map only 

 five divisions were taken, viz: to 2 inches, 2 to 5 

 inches, 5 to 10 inches, 10 to 20 inches and 20 inches 

 and upwards. These figures have been placed upon 

 the map in the February number of THE IRRIGATION 

 AGE, but owing to the large reduction they are to be 

 seen with difficulty. In general it may be said that 

 this map resembles that of the mean annual rainfall 

 in that, as might be expected, the heaviest runoff is 

 in the southeastern and eastern portions of the 

 United States and near the northwestern coast, where 

 the rainfall is greatest. The decrease of runoff, how- 

 ever, is by no means proportional to the decrease of 

 rainfall, as will be discussed later. 



The facts upon which this runoff map is based 

 were obtained from all possible sources, measure- 

 ments "made by the Geological Survey being tabu- 

 lated with those had from the reports of the army 

 engineers of the Hayden, Wheeler and other surveys 

 and of various State Engineers and investigators of 

 water sources and water powers. The work of as- 

 sembling these upon a common basis has been a 

 matter of many years' patient labor on the part 



