The name of the game is coverage.  

Air craft can get good coverage at 3 gpa, most ground rigs can not.     The aerial application uses the down draft air as the carrier, the water is just the means of getting the chemical into the air flow.  

 There have been some ground rigs over the years that have used air booms to try to accomplish the same thing that air craft naturally does.    “Miller Spray Air” is one recent example.  

http://www.millerstn.com/products/spray-air-technology/