32 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



Table V shows a comparison of yields obtained on summer-tilled 

 land and on land cropped the preceding year. With the exception of 

 the cotton crop, the increased }deld resulting from summer tillage 

 is very marked. The cotton on cropped land was greatly helped by 

 seasonal rains during July and August, a critical period with this crop. 

 The yield of corn on the cropped land was only one-sixth and of 

 sorghum one -third that on summer -tilled land. Oats grown as a 

 hay crop during the winter months were a failure except on the 

 summer-tilled land. These results were obtained during an exceed- 

 ingly dry year. Usually the rainfall of this region is sufficient to 

 make crop rotations practicable, although cultivated crops should 

 be used as far as possible. 



NORMAL PRECIPITATION OF THE WESTERN UNITED STATES. 



Table VI has been specially compiled for this bulletin, principally 

 from the numerous reports of the United States Weather Bureau, 

 and includes the rainfall records available to January, 1910. The 

 table gives the station, the county in which the station is located, 

 the elevation of the station above sea level, the normal precipitation — 

 that is, the average annual precipitation for a number,of years — and 

 the number of years of records upon which the normal is based, ending 

 with the year given in the table. 



In the state rainfall maps (figs. 6-23) each number on the map 

 represents the normal rainfall at the station located at that point. 

 These maps, which were prepared from the figures given in the tables, 

 show the distribution of the rainfall in each State. The heavy curved 

 lines drawn through the maps pass through points having approxi- 

 mately the same rainfall. The amount of rainfall in inches repre- 

 sented by each line is shown by the numbers at the ends of the line. 

 These lines have been drawn when possible to represent differences 

 of 5 inches in the rainfall. For example, in the Kansas map one line 

 passes through points having an annual rainfall of 20 inches, another 

 line through points where the rainfall is 25 inches, and so on. In 

 some States there are found separated regions which have the same 

 rainfall, as in Colorado. These lines have been located by means of 

 the figures given on the map, so that the figures are to be considered 

 rather than the lines. The county boundaries in each State have 

 been revised to date from the maps of the Post-Ofnee Department. 

 The precipitation tables and maps will be found arranged alphabetic- 

 ally according to States. 



°The term "precipitation " is used so as to include both rain and snow. The snow 

 that falls into the gauge in the winter is melted, and the water is measured and treated 

 in the records as rain. The records, then, include the total precipitation for the year, 

 whether rain or snow, but measured always as rain. 

 188 



