drains were also monitored, but on a less frequent basis. 

 The monitoring program was greatly reduced In 1969; samples 

 were collected on a monthly basis from only 20 tile systems. 



Selection of Tile Drainage Systems for Monitoring 



Large, well-designed tile drainage systems having good 

 agricultural management were quite easy to find during the 

 initial investigations when only a few systems were required 

 for study. However, selection becajne increasingly difficult 

 when a number of representative tile drainage systems were 

 sought with regard to specific soils, physiographic posi- 

 tions, and certain agricultural practices. These and other 

 factors were considered peiramount in selecting an individual 

 tile drainage system to be monitored. These are listed in 

 their order of desirability. 



1. Location . Several systems were chosen to represent 

 the four major tiled areas. Isolated systems and 

 satellite systems were chosen for supplemental data. 



2. Size. Larger systems were preferred to smaller ones 

 in hopes of reflecting drainage from a large area of 

 soils . 



3. Design . "Total relief" systems were chosen for 

 greater representation of soil conditions in prefer- 

 ence to "interceptor" type drains. 



^. Agricultural Practices . Tile systems draining fields 

 having an active irrigation and fertilization program 

 were selected. 



5- Physiographic Positions and Soils . Tile drainage 

 systems occupying different physiographic positions 

 and representing various soils were also selected. 



6. Sumps and Outlets . Tile systems with sumps were 

 selected for convenience of sampling and flow 

 measurement . 



7. Age. A range of ages from 1955 to 196? was selected. 

 Very few tile drains were installed in the Valley prior 

 to 1950. 



Selecting tile drains was at times difficult, because very 

 few had all the desirable features. Generally, as many tile 

 drains with the foregoing factors were chosen as possible 

 within the major areas of interest. 



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