Vepth of Drains. 



297 



water surface in a tile drained field where the lines are 33 

 feet apart, 3 to 4 feet below the surface and where the sub- 

 soil at 3 to 4 feet and below is sand. The slope of the sur- 

 face was obtained by boring wells with a 4-inch auger be- 

 tween the lines of tile and the measurements were made 48 



FIG. 116. Showing the observed conformation of the ground water sur- 

 face in a tile drained field 48 hours after a rainfall of .87 inch. 



hours after a rainfall of .87 inch May 13, when the soil 

 was already well saturated. On this date the highest level 

 above the top of 3 inch tile between any two lines was 1 

 foot and the lowest .3 foot. 



369. Rate of Change in the Contour of the Ground Water 

 Surface Between Lines of Tile. At the time the data for the 

 last section were taken observations were also made to de- 

 termine the rate of change in the level of the ground water 

 after the rain, and Fig. 117 represents the differences in the 

 level of the water at and between the tile drains on three 

 different dates. It will be seen that the water fell fastest 

 under the highest ground and on the 16th was below the 

 tile in the upper part of the field. 



FIQ. 117. Showing changes in the level of the ground water surface in 

 a tile drained' field. 



This illustration makes it clear also how the tile in the 

 lower portion of the field make their influence felt in the 



