46 BULLETIN 652, U. S. DEPARTMENT OF AGRICULTURE. 



the long growing season various kinds of water plants grow in the water and 

 float on the surface. Such plants will have to be removed about twice a year, 

 or they will impede the flow of the water seriously. 



The influence of evaporation at different seasons of the year causes a great 

 variation in the manner of operating the pumps. A heavy rainfall in summer 

 necessitates continuous operation of the pumps for a period sufficient to empty 

 the canals and ditches. The water that will continue to run out of the lateral 

 ditches will often be more than balanced by the evaporation, so that it will not 

 be necessary to start the plant again until another period of heavy precipitation 

 occurs. Small local rains in summer in all likelihood will pass unnoticed. 

 During the winter months a heavy precipitation necessitates a relatively longer 

 period of pumping than in summer. Two or three days after the canals have 

 been emptied the ground-water drainage entering through the lateral ditches 

 will make it necessary, owing to lack of evaporation, to operate the plant for 

 a few hours, and after an interval of about 10 days it will be necessary to do 

 some more pumping, although no precipitation may have occurred in the inter- 

 vening period. If the reservoir capacity of the canals be small, the operation 

 of the pumping plant will be still more intermittent. If the plant be divided 

 into two or more units, one unit only may be operated for the dry-weather run- 

 off. The total time of pump operation during the year rarely exceeds 45 days 

 of 24 hours each, and often drops to as low as 15 days. The total number of 

 days on which the pumps are operated averages about 70. 



In southern Louisiana most of the pumping plants so far installed have a 

 theoretical capacity of at least 1 inch, and many of them \\ inches, in depth 

 of water over the inclosed area in 24 hours. The 1-inch run-off is equivalent 

 to approximately 27 second-feet per square mile of area, or 0.042 second-foot 

 per acre. 



The necessary capacity of a pumping plant depends on the size and slope of 

 the district to be drained, the depth and nature of the muck, the available 

 storage capacity of canals and ditches, the system of lateral drains used, the 

 method of operation of the plant, the character of the crops raised, and the 

 amount and distribution of the rainfall. The proper allowance to be made for 

 each of these factors can be determined only as the result of careful and com- 

 plete observations in the field. Not only should the results for each district 

 examined be worked out carefully, but the investigations should include a suffi- 

 ciently large number of typical districts and should continue for such a length 

 of time as to make the results of general application. Some of the above fac- 

 tors have been quite closely investigated over a few districts, and all of them 

 have been covered in a general way. While the results obtained are not final, 

 and the investigations still are being carried on, these details will be discussed 

 in the light of such investigations as have been made. 



In planning gravity drainage districts it is customary gradually to decrease 

 the run-off coefficient as the size of the district increases. With one exception 

 the variation in size of the district in this section is as yet not great ; therefore 

 not much attention need be given this feature. In the summer, when rains are 

 almost purely local, the larger district is not so likely to receive rains over its 

 whole surface as is the smaller. However, the rains that tax the pumping 

 plant most heavily occur in the spring of the year and are general in character. 



The variation in surface elevation on the average district is slight, but where 

 the district fronts on a ridge having an elevation above the prairie land of from 

 8 to 12 feet it has been noticed that the lower lands become flooded at times of 

 heavy rainfall, even though the pumping plant capacity on the district in ques- 

 tion be larger than that suilicient for the flat lands. This flooding of the lower 





