TABLE 10 

 HOURLY VS. WEEKLY MITROGEll CONCENTRATIOSS 



?/ Tventy-four hour period. 



sampling frequency could be established. In samples 

 collected dally, nitrogen concentrations varied slightly 

 from day to day but tended to remain generally constant, 

 unless the field in question was being irrigated. Intensive 

 irrigations effectively reduced the nitrogen for short 

 periods in many cases. The longest period of consecutive 

 sampling covered five days at one station. Nitrogen concen- 

 trations in the drainage ranged from 30. 5 to 45.1 mg/l . The 

 mean value was 35.5 mg/l, with a standard deviation of 

 6.3 mg/l (18 percent). 



Samples collected at three-to-four-day intervals showed a 

 greater variation than those collected daily. In a random 

 example which was sampled 11 times during the month, the 

 standard deviation was 9.7 mg/l, or 30 percent of the mean 

 concentration of 30.6 mg/l. Similctr variations were 

 observed in the data collected from the other systems 

 sampled at three- and four-day intervals during a period of 

 irrigation. 



The foregoing data led to the decision that weekly sampling 

 was adequate for the investigation. This determination was 

 based on the following findings: (l) mean hourly nitrogen 

 concentrations show a correlation to mean weekly values over 

 a two-month period; (2) large variations in nitrogen due to 

 the immediate effects of irrigation usually occur rapidly 

 and tend to return to the original concentration after irri- 

 gation; (3) because irrigation is generally repeated every 

 10 to 15 days for most crops, weekly samples tend to show 

 nutrient levels before, after, and during irrigation; and 



49 



