150 Bulletin 86 



pipe line systems 



A key map of a 540-acre field at Continental is shown in Fig. 33. 

 The arrows indicate the direction of irrigation. The 10-foot con- 

 tours exhibit the steepness and the rolling character of the land. 

 The field is composed of a succession of fans built by the intermit- 

 tently flooded side washes. The pipe-line system is laid out so as 

 to divide the land into fields of quite uniform slopes. The possi- 

 bility of running the main supply line over fans and through the 

 intermediate low districts is of great advantage over open gravity 

 ditches. The supply line shown in Fig. 33 continues a total length 

 of over six miles, the entire water supply, 7.6 second-feet, being 

 derived from the two wells shown. 



On important pipe-line systems, where many men are permitted 

 to adjust the gates and riser valves, a set of rules for the operation 

 of the system will be useful. The three following rules may be 

 placed at the head of the list. 



1. Always provide some place for the water to go. Begin the 

 irrigation with too many valves open and regulate by closing one 

 valve at a time until the desired head is flowing from each of the 

 open valves. 



2. Open or close each gate or valve slowly. 



3. Do not shut oflf one gate or set of valves until another has 

 been opened. 



SPECIAL STRUCTURES 



Special structures are often required. A division and measuring 

 gate pit is shown in Fig. 34. In this case it is necessary to divide 

 the flow from the 20-inch supply line into two equal parts. Two 

 equal weirs are provided. The certainty of equal division is as- 

 sured, for if it is assumed that the irrigators on Line A do not take 

 their full half of the water and that the water in their line is backed 

 up onto the weir, then the hydraulic gradient in the pit at the head 

 of Line A will be higher than in Pit B and will also be flatter than 

 the normal gradient, while the gradient in Line B will be steeper; 

 hence the gate pit with closed gate on Line A will be the first to 

 overflow. Therefore, the irrigators on Line A will be warned by 

 the overflowing gate pit and will open more valves. The irrigators 

 are instructed to watch their gate pit and keep water level at nor- 

 mal elevation. 



Another special structure at Continental is a gate pit in the 

 corner of the nursery. The nursery lateral is on a 1.66 percent 



