16 BULLETIN 854, U. S. DEPARTMENT OF AGRICULTURE. 



for the same depth of flow was run again. About 20 minutes were 

 required to obtain the data for each depth of flow, the amount of time 

 depending upon the grade of the flume. 



The readings for the first two and the last piezometers were not 

 included to obtain the depth of flow in the drain. It should be 

 remembered that water as it enters the drain has not the velocity it 

 will acquire after traveling some distance; therefore the first two 

 piezometers usually recorded a depth slightly different from that of 

 the piezometer 60 feet from the tile entrance or those of the succeed- 

 ing piezometers. Even with a gradual, conical entrance to the tile 

 drain (PI. Ill, figs. 1 and 2), the entrance velocity could not be easily 

 regulated to be the same as the uniform velocity through the main 

 portion of the tile. The average of the readings of the intermediate 

 nine piezometers, less 0.09 foot, usually was taken as the true depth 

 of flow, although at times very erratic individual piezometer readings 

 were obtained which were not used in obtaining the average. 



Only the upper weir readings were used in the final computations. 

 It was found in the earlier experiments that after waiting some time 

 for the lower weir box to fill to a steady height, the lower weir would 

 read practically the same as the upper weir, proving that there was 

 no measurable loss of flow in passing through the tile line. Hence, to 

 save time in performing the experiments, it was decided not to wait 

 for the lower weir to reach a steady reading. It may appear that, 

 in using only the upper weir readings to obtain the carrying capacity 

 of the drain, too great a quantity of water was recorded due to 

 seepage into the earth adjacent to the drain, which would credit the 

 tile with carrying more water than it actually did carry. However, 

 observation of the condition of the soil indicated clearly that the 

 soil became sufficiently saturated by the time steady flow was 

 obtained in the tile, that there was no such loss, at least not in 

 quantity that could have affected the results of this investigation. 



The use of the dam at the tile outlet did not affect the carrying 

 capacity of the drain, for special care was taken not to allow any 

 piezometer readings at the lower end of the flume to exceed the 

 readings near the upper end. The dam merely assisted in obtaining 

 a uniform depth of flow throughout the length of the drain. Thus 

 the necessity of corrections for changing velocity heads due to 

 decrease in the water cross-sectional area at succeeding piezometers 

 near the outlet was eliminated. Without the dam and with a con- 

 stant flow over the upper weir, the successive piezometers showed a 

 continuous decrease in depth, and therefore increase in velocity, 

 toward the outlet of the tile line. In other words, the hydraulic 

 gradient or water slope was greater than the grade of the tile. With 

 no change in the amount of water passing over the weir, the height 

 of the dam could be raised until the piezometer near the outlet re- 



