The study referenced below compared four irrigation control methods and evaluated each for water-savings.  The methods were 1) Schedule based, 2) Schedule with Soil Moisture Add-On, 3) Soil-Moisture Control and 4) Evapo-Transpiration.

The order of water savings among these methods were :

1) Soil-Moisture Control for On-Demand Irrigation – most water savings

2) Soil-Moisture Add-On to Scheduled Irrigation – second most

3) Evapo-Transpiration – third most

4) Scheduled Irrigation – this is really the baseline of water use with no specific opportunity for savings.

http://cedb.asce.org/cgi/WWWdisplay.cgi?165363