The Irrigation Association
The IA promotes modern water management skills and techniques for irrigated agriculture, turf, landscape and golf.
http://www.irrigation.org
CIMIS - California Irrigation Management Information System
Since the beginning of the CIMIS weather station network in 1982, the primary purpose of CIMIS was to make available to the public, free of charge, information useful in estimating crop water use for irrigation scheduling
http://wwwcimis.water.ca.gov/cimis/welcome.jsp