(4) changes in flow and resulting changes in nutrient 

 concentrations due to Irrigation are not always abrupt 

 because of the number of days required to Irrigate a given 

 field. 



Monthly Variations In Individual S ystems . Monthly vairla- 

 tions of nutrient concentrations were observed in every tile 

 system investigated. The direction, magnitude, and 

 frequency of nutrient variability differed between tile 

 systems and seemed to depend mainly upon irrigation manage- 

 ment and field location. Many tile drain flows fluctuated 

 widely, which grossly affected the nutrient concentrations; 

 others had sustained flows (flooded conditions) that caused 

 a prolonged reduction in the nutrient concentrations. 



The many variable conditions affecting flows and nutrient 

 concentrations meant that results for an individual system 

 cannot be considered typical of a given number of tile 

 systems or representative of a large area. However, 

 examples presented in Figures 10 and 11 do serve to illus- 

 trate the monthly and seasonal variability of tile discharge, 

 nutrients and TDS concentrations in Individual systems where 

 irrigation practices differ. 



Monthly Variations in Composited Drainage . Monthly varla- 

 tions of nutrients in the drainage composited from the 

 entire study area show a definite trend In relation to the 

 seasons . 



Nutrient concentrations in drainage varied greatly between 

 individual tile systems in the study area and appeared to 

 decline in many tile systems during the summer months. In 

 drainage composited from the entire Valley, nutrient concen- 

 trations declined appreciably during the peak irrigation 

 season. The magnitude of seasonal variability is illustrated 

 in Figure 12. Nutrient and TDS concentrations are flow- 

 weighted monthly averages based on a two-year period of 

 study (1967-68). Figure 12 shows that concentrations of 

 nitrogen, phosphorus and total dissolved solids varied 

 monthly and inversely with the tile discharge. Twofold 

 variations of both nitrogen and phosphorus were apparent in 

 the drainage from winter to summer. The highest concentra- 

 tion of total dissolved solids occurred in April and the 

 lowest in August. The average TDS varied about 1,000 mg/l 

 from spring to summer, a change that was not as spectacular 

 as the changes in the nutrients. 



A fourfold Increase was noted in the average discharge from 

 winter to summer. 



50 



