Moisture balance map of the north central United States from
"A Zone Map for Mean Annual Moisture Balance in the North Central United States"
by Mark P. Widrlechner, Landscape Plant News, Vol. 10, No. 2 (1999): 10-14.

Im = 100((mean annual precipitation/potential evapotranspiration)-1).

For more information please contact Dr. Widrlechner at 515-294-3511, 
      or send e-mail messages to Mark.Widrlechner@ars.usda.gov.