discharged from rice fields were possibly due to a combina- 

 tion of two interrelated factors. Dilution, which is a 

 physical lowering of nutrient levels resulting from irriga- 

 tion, and denitrification, which involves nitrogen losses 

 that are promoted by anaerobic soil conditions, are probably 

 the main reasons for the low nitrogen levels . In some cases 

 tile drainage from flooded rice fields in the Gustine- 

 Mendota area showed mid-summer decreases in nitrogen to less 

 than one-third their wintertime concentrations. One tiled 

 field in peirticular dropped from a January level of 63 mg/l 

 to 7 mg/l for the months of May, June, and July. 



The average annual nitrogen concentration in the combined 

 drainage from all rice fields, which for this study was 

 limited to the Gustine-Mendota area, was 10.9 mg/l. The 

 average concentration for all other tile-drained crops 

 investigated in the same area was 43.7 mg/l. 



Irrigation Influence on Discharges 



The influence of crops upon tile drainage flow without 

 regard to the quantities of irrigation water applied has 

 already been discussed in the foregoing subsection. Actual 

 applied water was determined only for a few tile systems 

 during the initial investigations. Because of the large 

 number of tile systems being sampled and the complexity of 

 irrigation practices on many large tiled fields, measurements 

 were not made during the more intensive investigations. 

 However, headgate irrigation requirements (lO), the quanti- 

 ties of water required in the growth of crops exclusive of 

 rainfall, are very nearly the same for many crops in the 

 different major tiled areas. The quantity of tile discharge 

 compared to the irrigation intensity (determined from head- 

 gate irrigation requirements) within the major tiled areas 

 is presented in Table 13. 



Irrigation Influence on Nitrogen Concentrations 



Although the type of crop grown usually determines the 

 relative amounts of water required, overirrigation and, to 

 a lesser extent, under irrigation can greatly affect the 

 quantity and nutrient quality of tile drainage. Low 

 nutrient concentrations seemed to be associated with flooded 

 soil conditions; denitrification and dilution were suspected. 

 Heavy irrigations may also add nitrogen to the soil. 



I ndividual Systems . The influence of irrigation upon 

 nutrient concentrations in individual systems varied greatly. 

 In many but not all systems, nitrogen concentrations varied 



55 



