LAND DRAINAGE BY MEANS OF PUMPS. 21 



starting the pump is less quickly seen in the morning, but the lower- 

 ing of the water is gradual throughout the day. After a few days of 

 such operation the supply of water diminishes and the pumps are 

 not run the whole day. 



Later, since it is troublesome to start up a big plant for only a few 

 hours of operation, the speed of the engine and pump is reduced 

 to diminish its discharge, or, if there is more than one pump in the 

 plant, only one may be run. At last, days are skipped and pumping 

 ceases entirely. After every rain of any importance, the pumps are 

 started and a similar cycle of operations transpires, extending over 

 a longer or shorter period, according to the amount of water to be 

 removed. In some districts pumping for the season ends in June, and 

 drainage is by gravity for the remainder of the summer. In others no 

 gravity drainage is available, and the pumps must be run later in the 

 season. Thus it is clear that, though the pumps must constantly be 

 in readiness for use, their operation is very intermittent. The total 

 time of running in a whole year never exceeds 60 to 90 days of 24 

 hours each. In some seasons it is not more than 15 to 20 such days. 



The size of pumping plant required for any particular district 

 depends upon a variety of conditions, among which are the size and 

 slope of the drainage district and of any other area from which drain- 

 age water is received; the amount of storage capacity available in 

 ditches and reservoirs; the system of interior drains used and the 

 method of construction adopted for the levee; the nature of the soil 

 and subsoil; the method of operation of the pumping plant; the 

 kind of crops raised and the degree of drainage required; and the 

 amount and distribution of the rainfall throughout the year. 



In the vicinity of Illinois the weight to be given to some of these 

 factors is very small, and in the present state of our knowledge no 

 definite allowance can be made for them. Yet, notwithstanding our 

 ignorance concerning some of them, it may be profitable to discuss 

 briefly the nature of the effect of each. This is a field in which much 

 careful study is still needed and must be carried out to secure the 

 complete knowledge necessary for the most satisfactory design and 

 management of the pumping machinery of drainage districts. 



Along the Illinois and upper Mississippi Rivers the majority of 

 pumping plants already installed have been planned to have a 

 maximum pumping capacity sufficient to remove in 24 hours one- 

 fourth inch in depth of water over the entire district. This is 

 equivalent to 6.74 cubic feet per second per square mile of drainage 

 area, or 0.0105 cubic feet per second per acre. 



1 1 is doubtful whether any change should bo made in the one- 

 quarter inch coefficient solely on account of change in the size of 

 the district; bowever, where some portions of the district are mate- 

 rially higher than others, the coefficient might be increased to pre- 



