126 Kansas Academy of Science. 



Evaporation in Kansas. 



Charles A. Siiull, University of Kansas. 



The state of Kansas, with its eastern boundary touching the Missouri 

 river valley, and its western boundary lying in the midst of an area once 

 popularly known as the great American desert, lies in an important 

 climatic tension zone of the United States. 



Eastern Kansas has an average annual rainfall of 35.25 inches, based 

 upon thirty-one years of observation. During the same period the 

 western division of the state has had an average rainfall of only 19.19 

 inches. And the central portion is just between, with an average of 

 26.24 inches per year. This difference in rainfall, connected directly, as 

 it must be, with differences in relative humidity and evaporation in- 

 tensity of the atmosphere, is one of the dominant climatic factors which 

 determine the character of the native vegetation of the state; and the 

 native plants of eastern and western Kansas are as widely different as 

 one might expect from the difference in annual precipitation. 



Measured in terms of crop production, a 19 to 35 inch rainfall, if 

 properly distributed through the growing season, and utilized only by the 

 crop growing on the land (a condition impossible of realization, of 

 course), is sufficient for the production of from 60 to 105 bushels of 

 wheat per acre. This estimate is based upon a water requirement for 

 wheat of 500 pounds of water for each pound of dry matter produced, 

 and assumes that grain and straw are produced in the proportion of two 

 pounds of grain to three pounds of dry straw. 



What becomes of the rainfall is therefore a problem of great interest 

 in this great agricultural center of North America, where we think we 

 do well if we average 25 bushels of wheat to the acre. Very frequently 

 it happens that distribution of the rainfall is not favorable for maximum 

 plant growth, and lack of moisture more often than any other condition 

 becomes a limiting factor in crop production. 



Of course, everyone knows that part of the rainfall is lost as run-off 

 water before it has a chance to penetrate the soil. But a considerable 

 part of the water does soak into the ground and becomes a part of the 

 great subsurface reservoir of water which fills our wells, forms the source 

 of the waters found in our permanent drainage systems, and furnishes 

 the supply of moisture to the roots of growing plants. A third portion 

 of the water evaporates from the soil surface, and from the leaf surfaces 

 of vegetation. It is this loss which becomes serious when dry weather is 

 prolonged, and the efforts of the farmer must be directed toward reduc- 

 ing direct soil evaporation to a minimum. Whatever loss occurs from 

 the soil should always be confined, so far as possible, to the transpira- . 

 tion of living plants, if maximum yield is to be obtained. 



Just how much water evaporates from the soil itself during a growing 

 season is not known. It must depend on many factors, and vary greatly 

 with soil type, configuration of the surface, methods of culture, climatic 

 conditions, etc. But it would be of vital interest to know how much 

 water goes out of our fields on days when atmospheric evaporation in- 



