The approach taken in these studies has been to 

 study at a few locations the relationship between measured 

 evapotranspiration, under specified crop conditions, and certain 

 climatic indices. Concurrently with the measurement and corre- 

 lation of evapotranspiration and climatic factors, a netv;ork of 

 asroclimatic stations was established and observed throughout 

 the several major inland agricultural areas in northern and central 

 California Having determined evaporation at these stations, esti- 

 mates of evapotranspiration can be extrapolated into these areas 

 by using the relationship between evapotranspiration and evapora- 

 tion data measured at the key evapotranspiration stations. 



In the early years of these studies, available knowl- 

 edge on climatic station environmental requirements v/as very 

 meager. However, as data v/ere collected and analyzed, the im- 

 portance of certain environmental effects became apparent. 

 Stations were relocated to sites where they were surrounded by 

 an extensive area of vigorous, low-growing vegetation at full 

 ground cover. Large, well -managed pastures best meet these 

 requirements. At such sites, the confounding effects of micro- 

 environment differences are minimized. 



The techniques used for the determination of evapo- 

 transpiration were the best available methods for the task, at 

 the time they were employed Hov/ever, as the study progressed, 

 techniques were modified to take advantage of new and better 

 tools as they became available. The initial soil moisture 

 measureme.its to determine evapotranspiration xvere made by the 



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