determination in a cycle of measurements. Otherwise, moisture 

 moving out of the sampled profile would be incorrectly included 

 as evapotranspiration in the soil moisture depletion measurement. 



During the growing season, the general procedure was to 

 sample approximately every seven days, except as modified by ir- 

 rigation, harvest, or other cultural (farming) operations. Durin: 

 the nongrowing season, measurements were made less frequently be- 

 cause of the lower rates of v/ater use. 



At the initiation of the evapotranspiration measuring 

 program in 1955, the gravimetric technique was accepted as the 

 best method available, and was the first technique employed in 

 the studies reported here. Moisture content of soil samples was 

 determined by weight change resulting from moisture loss during 

 oven drying. Soil samples were taken by means of a soil tube, in 

 1-foot increments of depth, from the soil surface to a depth of 

 7 or 9 feet. As the soil tube is difficult to handle at depth 

 below 9 feet, sampling below that depth was attempted only in 

 special cases where knowledge of the substratum conditions was 

 desired. 



The initial evapotranspiration measurements were made 

 in the mountain valley areas in the northern and northeastern part 

 of the State, and in the northern Sacramento Valley. The objectiv 

 at that time was to determine the irrigation requirements of only 

 those areas. Plots in the mountain valleys were located on typica 

 irrigated parcels of land. The irrigated lands in this area exist 

 as narrow and isolated "oases" separated by large areas of native 

 vegetation. 



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