Long-term veLriability of nitrogen was, however, apparent In 

 the drainage from individual tile systems. Though the 

 overall percentage of change was small, the absolute changes 

 were on the order of 6 to 8 mg/l over a decade. This degree 

 of variability is due primarily to changes in agricultural 

 management which includes the type of crop grown and the 

 irrigation frequency and duration. 



Nitrogen accumulation in the subsoil due to fertilization or 

 leaching of nitrogen from the soil due to Irrigation was not 

 apparent in the drainage observed from the individual 

 systems studied over several years. High concentrations of 

 nitrogen are still found in the drainage from fields tiled 

 since 1950 and irrigated for forty or more years in the San 

 Joaquin Valley. 



Nitrogen also varies on a short-term basis in drainage from 

 individual systems. Hourly, daily, and weekly variability 

 was studied for several drainage systems. During a 24-hour 

 sampling period, nitrogen ranged from 4 mg/l to 12 mg/l in a 

 particular drain that had a fixed discharge. The standard 

 deviation was 1.8 mg/l. Another system, considered rather 

 typical of many along the west side of the Valley, had hourly 

 concentrations that ranged from 92 mg/l to l82 mg/l and a 

 standard deviation of 19.8 mg/l. For the most part these 

 hourly vaj?iations were con^arable to those observed from 

 week to week during the tile monitoring investigations. 

 These short-term changes of nitrogen in drainage can be 

 attributed to dynamic moisture conditions — caused by 

 varying irrigation patterns and soil texture combinations. 



Nitrogen vaj'ies monthly in drainage composited from the San 

 Joaquin Valley tile systems . Monthly changes were gradual 

 but rather consistent from year to year. Two-fold seasonal 

 variations were observed; nitrogen concentrations in the 

 composited drainage were about 3^ mg/l in March and l4 mg/l 

 in August . 



Nitrogen concentrations vary in drainage from certain 

 geographical areas in the San Joaquin Valley. Drainage was 

 composited separately for the four heavily tiled aj'eas in 

 the Valley. The systems sampled represent about one-third 

 of present total tiled acreage. These major tiled areas are: 

 two northern areas (Byron to Westley and Westley to Gustine), 

 the central area (Gustine to Mendota) and the southern area 

 (Tulare lakebed). Nitrogen concentrations averaged 33 mg/l 

 in drainage from the central area, which was more than three 

 times higher than the drainage from either of the two 

 northern areas or the southern eirea. At first these varia- 

 tions were thought to be due to differences in fertilizer 

 application. Immediate or prolonged fertilization had no 



