Helgesen, John O. <br/><br/>
Hydrology of the Great Plains aquifer system in Nebraska, Colorado, Kansas, and adjacent areas / 
by John O. Helgesen, Robert B. Leonard and Ronald J. Wolf 
 - Washington :  U.S. Government Printing Office,  1993 
 - 79 p. :  ill. ;  27 cm. 
 - U.S. Geological Survey professional paper ;  1414-E .