We had freezing temperatures at night on January 6 and 8th. Winds did not compound the problem. Humidity dropped to single digits twice during the month. No major rainfall events. ET was about 2. 4 inches during the month of January. Generally speaking, irrigation should have added about 2 inches of water in January, 2014.
The UNCE Orchard is located at 36 degrees North Latitude and 2000 ft (666 meters) elevation in the Mojave Desert.
The soil temperature was reset from 6 inch depth to 2 inch depth on January 21. This is why the range became so scattered starting January 21. If this scattering continues we will reset it a bit deeper. This temperature is used to estimate seeding dates for direct seeding into the raised beds.
The National Weather Service used to measure Phoenix's soil temp at 2" and 4" depths. After 15 years of observation they noticed the air temps and the 2" depth temps correlated too closely. They measured at 1, 2, 3, 4 and 5" tests and found the first 3" were too identical to air temps—makes a great case for thick organic mulch. Starting in 2000, they now measure at 4" and 20" depths.
http://ag.arizona.edu/azmet/15.htm