76 



BULLETIN 71, U. S. DEPARTMENT OF AGRICULTURE. 



secured by speeding up the pump. The efficiency will be less when working 

 under an overload, but this increased capacity at times of heavy precipitation 

 is very desirable, even if secured at a sacrifice of efficiency. The rated capacity 

 of ordinary centrifugal pumps is based on a velocity of flow, through the dis- 

 charge opening on the pump, of from 10 to 12 feet per second. Pumps are now 

 being designed to increase this velocity to 14 or 15 feet per second when run 

 at high speeds, i. e., overloads. This will make possible the installation of 

 smaller pumps. 



Another point that must be considered in the design of both engine and pump 

 is the gradual increase in static lift that may be expected, due to the subsidence 

 of the muck soils. Of course the amount of this subsidence will depend upon 

 the nature and depth of the original muck, but it will often amount to 2 or 

 even 3 feet in the course of 10 or 15 years. Both the average and the maximum 

 lift will thus be increased by this amount. After a period of perhaps 15 years 

 further subsidence will amount to little. 



The total amount of water pumped in a year will affect the character of ma- 

 chinery to be used. The rainfall in this section is greater than in the northern 

 latitudes, and the run-off, consequently, is also greater. The following table 

 gives the yearly rainfall and run-off on three of the districts previously de- 

 scribed, and the number of days on which rainfall and pumping occurred for 

 the period from June, 1909, to May, 1912, inclusive. 



Yearly and average yearly rainfall, run-off, number of days on which pumps 

 were operated, and number of days on which rain fell on areas Nos. 1, 2, 

 and 8, June, 1909, to December, 1912. 





Area No. 1. 



Area No. 2. 



Area No. 8. 



Year. 



Rain- 

 fall. 



Run- 

 off. 



Num- 

 ber of 

 days, 

 pumps 

 ran. 



Num- 

 ber of 

 days 

 rain 

 fell. 



Rain- 

 fall. 



Run- 

 off. 



Num- 

 ber of 



days 

 pumps 



ran. 



Num- 

 ber of 

 days 

 rain 

 fell. 



Rain- 

 fall. 



Run- 

 off. 



Num- 

 ber of 

 days 

 rain 

 fell. 



19091 



1910 



Ins. 

 42.32 

 43. 08 

 52.32 

 M8.22 



Ins. 

 16.33 

 11.58 

 23.41 

 3 34. 42 



45 



45 



69 



3 8L 



66 

 83 

 75 



3 48 



Ins. 



37.21 



41.48 



54.56 



58.86 



Ins. 

 15.83 

 10.83 

 25.84 

 4 50..34 



41 

 55 

 102 

 131 



57 

 84 

 96 

 91 



Ins. 



27.74 



2 42.54 



62.22 



65.72 



Ins. 



9.98 



2 15.77 



32.69 



40.59 



62 

 2 68 



1911 



108 



1912 



120 







Total 



185.94 



85.74 



240 



272 



192.11 



102.84 



329 



328 



198.22 



99.03 



358 







Average, year. . .. 



57.21 



26.38 



74 



84 



53.36 



28.57 



91 



91 



59.53 



29.80 



108 



i June to December, inclusive. 



8 February, September, and October omitted. 



8 June to September, inclusive, omitted. 



* Large run-off due to excessive seepage through levees. 



The above table would indicate that an average run-off of about 28 inches 

 per year might be expected and that it would be necessary to operate the 

 pumps on from 70 to 90 days a year. During the 3 years and 7 months that the 

 records were kept the boilers of the plant on area No. 2 were fired up 278 

 times, and those on area No. 1 189 times in 3 years and 3 months. The average 

 number of times that these boilers were fired per year was 68. This yearly 

 average is higher than should be expected on districts with larger reservoir 

 capacities. 



Cost of Pumping Plants. 



The cost of drainage pumping plants per indicated horsepower varies widely 

 according to type of machinery, expense of transportation of machinery to site 

 of plant, character of foundation, and difficulty of erection. The last three 



