Headgate irrigation requirements (quantities of water 

 required in the production of various crops, exclusive of 

 precipitation) were used in lieu of actual applied water 

 data for determining irrigation requirements in major tiled 

 areas during 196? and 1968. 



Samples of applied water were collected during irrigation to 

 determine the relative amounts of nitrogen being applied in 

 the irrigation water. These samples were analyzed for 

 nitrate, phosphates, and electrical conductivity. Laboratory 

 results showed very low concentrations of nitrogen in forms 

 other than nitrates. Nitrate-nitrogen (NO3-N) in the irriga- 

 tion waters was found in concentrations of ^ mg/l, which 

 amounts to about 11 pounds of nitrogen per acre-foot of 

 water applied. 



Phosphate -phosphorous (PO4-P) concentrations in Irrigation 

 water were nearly always less than O.5 rag/l . Electrical 

 conductivity usually ranged from 25O to 350 micromhos. 



Irrigation Runoff 



Although irrigation runoff was only measured during the 

 initial investigations, samples were collected to determine 

 the nitrogen concentration. Nitrogen (NO3-N) averaged about 

 6 mg/l for all of the samples collected. 



Fertilization 



Fertilization has been considered by many to be one of the 

 most important factors contributing to nutrients in tile 

 drainage systems. To investigate the quantities of nitrogen 

 available for leaching and possible accumulation in the 

 subsurface waters, historic and current fertilization rates 

 were obtained. These are discussed in this subsection. 

 Certain aspects of nitrogen gains and losses are also 

 discussed. However, a nitrogen-balance study was not the 

 intent of this investigation. 



Historical Use of Fertilizers . Commercial nitrogen ferti- 

 lizer has been used in the areas around Gustine, Mendota 

 and Tulare Lake area for the last 30 years . In the area 

 from Byron to Gustine, fertilization has been limited by 

 dairy farming and raising of lima beans, a legume which 

 does not require fertilization. However, orchards between 

 Byron and Gustine have been fertilized regularly with 

 ammonium-sulfate fertilizer at the rate of ^ to 8 pounds 

 per tree (about 80 to 100 trees per acre). Although ferti- 

 lizer use is still somewhat limited by the type of crop 



39 



