74 



BULLETIN 71, U. S. DEPARTMENT OF AGRICULTURE. 



It will be noted from the foregoing table that the smallest reservoir capacity 

 that is indicated is 0.4 inch in depth of water on the whole area. The size of 

 canal necessary to bring the water to the plant rapidly enough to secure con- 

 tinuous operation and to keep the slope of water surface reasonably small will 

 give a storage capacity of 0.4 inch. In the following summaries the estimated 

 costs of pumping plants and reservoirs are given. The head on pump has been 

 assumed at 6 feet, with 60 per cent efficiency of pump and 90 per cent me- 

 chanical efficiency of engine. The figures for cost of plant are for simple slide- 

 valve noncondensing engines and have been taken from the curve in figure 18, 

 which shows the average cost of pumping plants in this State under condi- 

 tions similar to those on the tract in question. The cost of the reservoir has 

 been calculated at 7 cents per cubic yard and includes only that part of the 

 canal prism between the surface and a depth of 4 feet. It is assumed that the 

 reservoir canal would not be widened below the 4-foot level. 



Costs for necessary capacities. 

 STORM OF MAR. 21-28, 1912. 



Capacity 



Cost of 



Capacity 



Cost of 





of reservoir. 



reservoir. 



of plant. 



plant. 





Inches. 





Inches. 







0.4 



84, 100 



1.4 



87,000 



811, 100 



.6 



6,100 



1.2 



6,100 



12,200 



.8 



8,200 



1.0 



5,400 



13, 600 



1.0 



10, 220 



.9 



4,900 



15, 120 



1.3 



13, 600 



.8 



4,700 



18, 300 



STORM OF APR. 11-21, 1912. 



0.4 



84, 100 



1.4 



87,000 



811,100 



.4 



4,100 



1.2 



6, 100 



10, 200 



.5 



5,100 



1.0 



5,400 



10, 500 



.7 



7,150 



.9 



4,900 



12, 050 



.9 



9,200 



.8 



4,700 



13, 900 



It appears from the above estimate that the cheapest improvements to take 

 care of the storm of March 21-28, 1912, would be a combination of a plant 

 capacity of 1.4 inches and a reservoir capacity of 0.4 inch, and that for the 

 storm of April 11-21, 1912, there should be a plant capacity of 1.2 inches and 

 a reservoir capacity of 0.4 inch. Excepting in the first case, the cost of provid- 

 ing for the second storm is less in each capacity of plant than for the first 

 storm. While the run-offs from these two storms were nearly the same, the 

 time over which the second was distributed was greater, thus allowing smaller 

 capacity of reservoir. 



Although as regards first cost alone it appears that the larger plant and 

 small reservoir should be used, there are other factors that enter into the 

 problem. The larger canals would decrease the lift of the plant, as the slope in 

 water surface during operation would not be so great as in the small canals. 

 The operation of the plant, in pumpiug from a large canal, would not be so 

 intermittent as from a small canal ; this would make for better fuel economies. 

 The rate of interest on plant and on reservoir would be the same, but the per- 

 centage to be allowed for depreciation and repairs probably would differ some- 

 what. In removing the run-off from a given storm the smaller plant would 

 have to operate longer, thus increasing the labor charges over those of the 

 larger plant. To determine the proper weight to be given these various factors, 

 continued and detailed records on a number of typical districts are needed, but 

 they are here mentioned to make clear the fact that there are other features 



