Closer correlations between annual nitrogen concentrations 

 occurred during the more intensive Investigations conducted 

 from 1966 to 1969. Average nitrogen concentrations for 1966 

 and 1967 were identical. However, the data for 1966 were 

 based on just seven tile systems in the central area, whereas 

 the data for 1967 were based on forty systems located through- 

 out the San Joaquin Valley. 



The small difference between I967 and I968 may have occurred 

 because only seven months' data were obtained in the 

 northern area during I967 and the acreage sampled was 

 slightly less. The I969 value of 19.^ mg/l, which also 

 compares favorably, is based on a monthly sampling frequency 

 of fewer than half the systems sampled in I968. 



Average discharge (based on the total acre-feet discharged 

 per estimated areal tiled acres) was nearly the same for all 

 years, except during the studies conducted by the University 

 of California, Los Angeles, when only four tile systems were 

 investigated. 



Phosphorous concentrations in composited drainage were 

 nearly the same for I967 and I968 but were much lower in 

 1969^ when the Tulare Lake area could not be sampled because 

 it was flooded. Although phosphorous concentrations appear 

 to change little from year to year, the percent of change is 

 higher for phosphorus than it is for nitrogen. 



The high TDS value for 1959-63 is based on "salt routing" 

 investigations conducted by the Department of Water Resources 

 (1). The average value (flow-weighted) of 4,550 mg/l was 

 determined from seven systems in the Gustine-Mendota area. 

 This is a significantly higher value than the values obtained 

 for 1967 and I968. 



I ndividual Systems . Although average nitrogen concentrations 

 changed little from year to year in drainage composited from 

 the study area., leaching and fertilization were thought to 

 affect the nitrogen concentrations in individual systems on 

 a long-terra basis. Therefore, in an attempt to evaluate the 

 possible long-term variability of nitrogen, all historic 

 sampling records available for individual tile systems were 

 analyzed. Twelve tile systems were selected for which 

 records showed occasional sampling prior to I966. These 

 data are presented in Table 8. Data for individual stations 

 from 1966 to 1969 are based on weekly samples collected over 

 a full yegir and monthly samples collected in I969. 



Flow values in gallons per minute (GPM) and concentrations 

 of total dissolved solids (TDS) are shown for comparative 

 purposes. The values given for flows represent the average 



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