flows of only a few measurements at different times of the 

 year prior to 1966. Time-weighted average flows were 

 computed from data obtained during the more intensified 

 studies. Average TDS concentrations were determined from 

 available mineral data prior to I963 and flow-weighted from 

 electrical conductivity values thereafter. 



The data show evident variability of nitrogen concentrations 

 in drainage between years for nearly all the systems. The 

 changes in nitrogen concentrations can possibly be explained 

 in that many of these stations were sampled at different 

 times of the year and are represented by changes in agricul- 

 tural practices (i.e., crops and irrigation). 



By making comparisons of two distant periods of most complete 

 data (1961 and I968), the following observations were made: 



1. Nitrogen concentrations increased in ten out of eleven 

 systems over a seven-year period. 



2. TDS concentrations decreased in seven out of eleven 

 systems for the same period . 



3. Nearly as many tile systems decreased in flow as 

 increased . 



Increases of nitrogen were also noticed during the periods 

 of more intensive sampling (1967 through 1969 inclusive). 

 Six out of eight systems showed increases in nitrogen concen- 

 trations and, again, these increases were not particularly 

 related to changes in flow or concentrations of TDS. 

 Although limited data were available for the earlier years 

 and nitrogen concentrations varied greatly between years for 

 some of the drains studied, two facts remain clear: 

 (1) there is no apparent trend that might indicate leaching 

 or an accumulation of nitrogen in the soil over long periods, 

 and (2) tile systems having rather high initial nitrogen 

 concentrations remain rather high. 



Short-term Nitrogen Variability 



Nitrogen and phosphorous concentrations in drainage from all 

 systems investigated varied according to the season. 

 Frequent irrigation tended to lower the nutrient concentra- 

 tions in most of systems monitored, and highly erratic varia- 

 tions of nutrient concentrations were observed during 

 irrigation of many systems . Nitrogen concentrations as 

 observed in the field and from graphical analyses of individ- 

 ual systems would increase, decrease, or remain about the 

 same during irrigation at a particular site. No matter what 



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