NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 67 



MEASUREMENT OF WATER OVER WEIRS 



Many nurserymen prefer to know the volume of water used through- 

 out the season, especially where water rights must be paid for. Such 

 measurement is usually made by means of a weir or water meter. The 

 water meter can be used only where water is transported under pres- 

 sure through pipes; in ditch irrigation, weirs are commonly used. 



A weir consists of a bulkhead or wall built across a stream or ditch 

 at right angles to the flow of water with an opening cut in the top of the 



Vertical Section Om Ditch Center Line 



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Figure 19. — Details of construction of a weir and weir pond. 



wall through which the water is measured as it flows. The opening 

 is called a weir notch. The bottom edge of the notch over which the 

 water flows is called the crest. That portion of the ditch immedi- 

 ately upstream from the weir is called the weir pond. The height of 

 the water surface in the weir pond above the level of the weir crest is 

 the head. This is measured a few feet upstream from the weir. If 

 the flow of water over the weir crest discharges into free air before 

 striking the level of the water on the downstream side, the weir is said 

 to have free discharge. A weir with a sharp upstream corner or edge 

 so formed that the water springs clear of the crest is called a sharp- 

 crested weir. 



A weir notch 2 feet wide and 1 foot deep will meet the requirements 

 of all anticipated flows used in the irrigation of most nurseries. The 

 details of the construction of such a weir are given in figure 19. Care 



