AVAILABLE WATER SUPPLIES. 



BY F. H. NEWELL, OF THE UNITED STATES GEOLOGICAL SURVEY. 



IN the February number of THE IRRIGATION AGE, 

 Major Powell has discussed the water supplies of 

 the arid region, and has drawn some general con- 

 clusions as to the amount of water available and its 

 distribution. His article is illustrated by two large 

 maps of the United States, which show in graphic 

 form the average distribution of rainfall, and the 

 average distribution of that portion of the rainfall 

 which flows off of the ground to form streams of 

 comparatively permanent character. The matter is 

 of such general importance that it may be of interest 

 to enter more fully into a discussion of the data upon 

 which these maps are based, and to answer certain 

 queries that have arisen regarding them. 



THE RAINFALL MAP. 



The rainfall map, as shown by its title, was pre- 

 pared by Henry Gannett, Geographer of the U. S. 

 Geological Survey and of the Eleventh Census, from 

 observations made and published by the Weather 

 Bureau. This map differs somewhat in general ap- 

 pearance from similar maps issued by the Weather 

 Bureau, in that more weight is given to the topography 

 of the country and its probable effect upon the quan- 

 tity of precipitation. In the earlier maps, at least, 

 prepared by the Weather Bureau, little if any account 

 was placed upon the existence of mountain ranges or 

 inequalities of the ground. By drawing general lines 

 connecting individual observations, these maps 

 showed simply the areas within which stations 

 giving a certain result were located, as it was prob- 

 ably considered that this would be less misleading 

 than the attempt to introduce facts which were not 

 clearly known by the compilers. This method has 

 its defenders, and is undoubtedly entitled to careful 

 consideration; but where the observations are very 

 widely scattered, as they are in many parts of the 

 West, it has sometimes happened that the maps 

 apparently show similar conditions of precipitation 

 prevailing across a great mountain range as that of 

 the Sierra Nevadas, whereas persons acquainted with 

 the region know well that this is impossible, and that 

 there is a far greater precipitation near the summits 

 than on the lower grounds. In the preparation of the 

 present map, therefore, while all the observed facts 

 have been used, allowance has been made for those 

 areas where there are no records but where, from a 

 consideration of the topography, it is safe to assume 

 that the precipitation is large or small. In other 

 words, reasoning from known data of a given locality, 

 assumptions are made for the probable conditions in 

 localities of similar altitude and position. 



A glance at this map shows that the heaviest rain 

 falls of the United States are on opposite sides of the 

 continent, being along the southeastern coast and the 

 northwestern border, the quantity varying on one side 

 from 60 to 70 inches near the Gulf of Mexico and in 

 the highest parts of the Appalachian range, and on 

 the other side from 70 inches upwards, this being 

 along the coast of Oregon and Washington. There 

 is a wide difference, however, in the areal extent of 

 heavy precipitation in these extreme regions. In the 

 southwest the rainfall of 50 inches and over covers an 

 area of many thousand square miles, extending from 

 Louisiana northeasterly along the Appalachians and 

 the Atlantic coast nearly to Pennsylvania; while in 

 the northwest the area of heavy precipitation, or of 

 50 inches and upwards, is confined to a narrow belt 

 along the ocean and to an equally narrow strip along 

 the summits of the Cascades, also extending south- 

 erly along the Sierra Nevadas. 



On the printed map to which reference is made the 

 rainfall figures cannot be clearly distinguished except 

 after patient search. The shading of the map indi- 

 cates, however, that inland from the Atlantic ocean 

 and Gulf of Mexico the rainfall decreases gradually, 

 being from 40 to 50 inches in the lower part of the 

 Ohio valley and the adjacent regions, from 30 to 40 

 inches in the upper Mississippi and lower Missouri 

 basins, and from 20 to 30 inches in the strip of country 

 extending from Minnesota to southern Texas. Taking 

 the 97th meridian as dividing the United States into 

 about equal portions, it may be said that the country 

 to the east of this receives 30 inches and upwards of 

 rainfall, while on the west it receives far less except 

 where high mountain summits or the coasts introduce 

 modifying conditions. As shown by the map the 

 greater part of the area west of the 97th meridian 

 receives from 10 to 20 inches in depth of rain, a few 

 mountain masses receiving from 20 to 30 inches, 

 while the greater part of Nevada and portions of 

 Arizona and Southern California receive less than 10 

 inches of precipitation annually. 



DATA FOR RAINFALL MAP. 



For the construction of a rainfall map of the east- 

 ern half of the United States there is abundance of 

 data, there being, as a rule, one or two places in every 

 county where the depth of rainfall has been observed 

 for a number of years. But in the western half of the 

 United States, with the exception of California, there 

 are comparatively few stations which have a record 

 of more than two or three years. There has been 

 lately a great increase in the number of stations 



99 



