66 BULLETIN 71, U. S. DEPAKTMENT OF AGKICULTUKE. 



the run-off at any time of the year must be taken out promptly. 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 con- 

 tinuous operation of the pumps for a period suflScient 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 again be 

 necessiiry to start the plant until another period of heavy precipitation occurs. 

 Small local rains in summer will in all likelihood 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, 

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

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

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

 vening periods. If the reservoir capacity of the canals be small the operation 

 of the pumping plant will be still more intermittent. If the plant is 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 average 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 1* 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 draine<J, 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 only be determined as the result of careful and com- 

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

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

 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 factors 

 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, the 

 investigations still 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 coefl3cient 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 district. However, the rains that most heavily 

 tax the pumping plant occur in the spring of the year and are general in 

 character. 



The variation in surface elevation on the average district is usually 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 and that the required capacity of pumping plant is nearly 50 per cent 

 greater than on the flat lands. This flooding of the lower lands can be partly 



