Soil Temperature Definition:

The temperature measured at a given soil depth, typically at 2, 4, 8, and sometimes 20 and 40 in. Many biological processes, including seed germination, plant emergence, microbial activity, and soil respiration are a function of soil temperature. (from The Glossary of Meteorology)

Evapotranspiration Definition:

The process known as evapotranspiration (ET) is of great importance in many disciplines, including irrigation system design, irrigation scheduling, and hydrologic and drainage studies. In a broad definition, the evapotranspiration is a combined process of both evaporation from soil and plant surfaces and transpiration through plant canopies. In the evapotranspiration process, the water is transferred from the soil and plant surfaces into the atmosphere in the form of water vapor. (from the University of Florida)

Soil Temperatures for the Southern Sacramento and Norther San Joaquin Valley, monthly averages

(based on 2003-2006 readings, averaging the reporting stations at Fair Oaks,
Davis, Nicolaus, Lodi):

January: 46-52
February: 48-53
March: 51-58
April: 56-62
May: 62-69
June: 71-77
July: 76-82
August: 73-79
September: 69-75
October: 64-69
November: 57-61
December: 49-54

SOIL TEMPERATURE RANGES FOR SEED GERMINATION

Vegetables (Optimal Range and Workable Range, in degrees F)
Arugula   50-55   40-70
Beans   75-85   60-95
Beets   65-85   45-95
Carrots   65-85   45-95
Chard   65-85   50-90
Corn   75-85   55-95
Cucumber   80-90   60-95
Gourd   80-90   70-100
Lettuce   60-65   45-75
Melons   85-95   70-100
Mustard   60-70   50-75
Okra   80-95   70-100
Peas   65-75   40-85
Radish   75-85   45-95
Spinach   65-80   50-85
Squash   80-95   60-100
Radicchio/Chicory   65-75   45-85

Herbs (Optimal only)
Cilantro 55-65
Dill 60-70
Parsley 60-65
Chamomile 65-75
Fennel 65-75
Basil 75-85

Flowers (Optimal only)
Hardy Annual 50-70
Tender Annual 65-85