EVAPORATION. 13 



Plains region all these conditions but one, moisture, are favorable for 

 crop production. Thus it is that the amount and distribution of 

 rainfall become the question preeminent, and moisture conservation 

 becomes the vital ])r()blem to all fanners. 



As has been said, there is a fairly uniform decrease in })recipilation 

 from east to west across the Plains to some distance into Colorado 

 and to about the Wyoming boundary line. (See maj), fig. 1.) This 

 decrease is 1 inch to about 17 miles along the south line of Kansas, 

 1 inch to about 21 miles along the north line, and 1 inch to about 

 40 miles along the north line of Nebraska from (lie Missouri Iliver 

 west. Over most of the region 70 per cent or more of the precipita- 

 tion falls during the growing season. This, it is often argued, makes 

 a much smaller annual precipitation necessary than if much of it 

 came during the wdnter. The truth of this supposition may at first 

 seem self-evident, but there is grave doubt whether our small-grain 

 crops may not mth proper tillage succeed better with a small amount 

 of precipitation wliich comes in the winter than with the same 

 amount of rain coming in the summer. At least, the regions which 

 are producing satisfactory crops on the least rainfall are regions of 

 winter rain, and there summer rain (after July) is considered a 

 misfortune, except when falling on fallow land.^ 



It seems, however, fairly well established that late-maturing cro})s, 

 such as corn, must have considerable rain during the middle and 

 latter part of the growing season.^ 



The rainfall of the region is very uneven in distribution, a large 

 part of it falling in the form of local showers which cover but limited 

 areas and are often torrential in character. Tliis makes the rainfall 

 extremely variable, both as to annual precipitation and distri])uti<)n 

 through the season. Instead of calling the region "semiarid'' it would 

 be more ]H-()])erly descril)ed as varying from year to year between 

 arid and humid. This variability is the most serious feature of the 

 climate. If dry seasons came with any regularity the settler could 

 be prepared for them, but coming as they do with no regularity and 

 without warning they are the constant dread and often the ruin of 

 the homesteader. If the precipitation were fairly luiifoiin and favor- 

 ably distri})uted the conditions might be easily met, but this varia- 

 bility has always been the limiting factor of success. It is this, 

 more than the scarcity of moisture, that must be overcome. 



EVAPORATION. 



From an agricultural standj)oint evaporation is of equal imjior- 

 tance with precipitation, although few ])eo])le api)ieciate this fact. 

 It is tliis factor wliich determines the amount of water needed to 



1 Thatcher, R. W., Director of the Washington Agricultural Experiment Station, in address at Com 

 Exposition, Onialia, 190y. 



2For a further discussion of this subject, see Bulletin S.'i, V. S. Weather Bureau; Yearbook, V. S. 

 Dept. of Agriculture for 1903, p. 215; Annual Report, Nelira-ska State Hoard of Agricviliure, 1909, p. 312. 

 215 



