XURSERY PRACTICE FOR PRAIRIE-PLAIXS PLANTING 



71 



Table 18 gives, for 2-inch diameter pipe and discharge of 50 gallons 

 per minute, a loss in head of 9.9 feet per 100 feet of pipe due to friction. 



For 1,000 feet of pipe the loss of head will be 10 times this amount, or 

 99 feet. Since the total head in feet at D is 79 feet, it is readily seen 

 that the 2-inch pipe is too small. For 2 1 2 -inch pipe the loss per 100 

 feet is 3.32 feet, or 33.2 for 1.000 feet of pipe. The nurseryman should 

 therefore use 2^-inch pipe, which will deliver the desired 50 gallons per 

 minute at D with only 33.2 feet friction loss in the total available 79 

 feet of head. If 3-inch pipe were used the loss in head would be only 

 13.8 feet. If D were the same elevation as E and the total available 

 head only 69 feet, the nurseryman could still use the 2 1 2 -inch pipe. 

 If the water were received at E from a gravity-flow ditch, however, 

 the total head at D would have been 10 feet, requiring 4-inch pipe, of 

 which the friction head loss for 1.000 feet is 10X. 34 = 3.4 feet. 



Discharge in gallons per minute from ditches of sizes commonly 

 used — 12 inch and 16 inch ffig. 13) — computed on the basis of a water 

 depth of 6 inches, are given in table 19. Depth may be increased some- 

 what if more capacity is necessary, but there is danger of breaks in 

 ditch bank- if the water level is carried too high. 



Table 19. — Computed discharges from 12-inch and 16-inch ditches, with water 

 depth of 6 inches, down different gradients 



Ditch 12 inches wide 



Ditch 16 inches wide 



Slope (inches per 100 feet l Approximate discharge Slope (inches per 100 feet) Approximate discharge 



1H- 



2H- 



3 



3H- 



4, 



Cubic feet 



per second 

 0.22 

 .33 

 .41 

 .47 

 .57 

 .68 

 .74 

 .82 

 .89 

 .94 



Gallons per 

 minute 



99 

 148 

 184 

 212 

 256 



X- 



X- 



333 

 369 

 400 

 423 

 522 



m. 



2 ... 



2H. 



3... 



4___ 



Cubic feet 



Gallons per 



per second 



minute 



0.28 



126 



.42 



189 



.52 



234 



.60 



270 



.73 



329 



.88 



396 



.96 



432 



1.05 



472 



1.15 



51 x 



1.21 



•>44 



1.49 



670 



Overhead Irrigation 



Since deciduous nursery seedlings can. under normal conditions, 

 be grown satisfactorily under ditch or row irrigation, no general need 

 exists to install the more expensive overhead sprinkling systems. 

 Such systems, however, afford a convenient method of irrigation and 

 are especially helpful during the germination period of small-seeded 

 species such as elms and mulberry. Since the initial investment per 

 acre is large, and the number of deciduous seedlings that can be grown 

 on an acre is limited, an overhead system can be justified only as a 

 long-time investment in permanent nurseries. In the Forest Service 

 nurseries in the plains region, overhead sprinkling lias proved satis- 

 factory only on sandy soils where the water penetrates easily. Ob- 

 servations on relative effectiveness at different times of day in the 

 warm summer months indicate that watering with the overhead sys- 

 tem is most effective in the evening or at night. In the hot part of 

 the day. less than 50 percent of the water may reach the ground owing 

 to vaporization in midair, and some of the water that does reach the 

 ground evaporates before it can penetrate to an effective depth. 

 Nozzles with comparatively large orifices will reduce excessive loss 

 due to vaporization. 



