LAND DBAIJSTAGE BY MEANS OF PUMPS. 17 



below the surface of the ground in the lowest spots that are to be 

 cultivated. This is necessary, not only for the purpose of providing 

 room for the root systems of growing crops, but also as a margin of 

 safety by furnishing a reservoir capacity in the drained soil for the 

 temporary storage of water after an unusually heavy period of rain. 



In some localities there are sloughs so low that to drain them would 

 require undue expense. In such a case it is better to reserve these 

 areas for use as reservoirs in connection with the ditch system, with 

 the expectation that they will fill with water during heavy rains and 

 store it until it is gradually removed by the operation of the pumps. 

 This tends to reduce the maximum capacity required of the pumping 

 plant of the district concerned. Some districts on the Mississippi 

 River have as much as 7 per cent of their total area occupied by such 

 storage basins, but other districts have none at all, and such storage 

 areas do not seem to be essential in regions where the natural topog- 

 raphy of the ground surface is not favorable for their use. 



The main ditch and its branches should traverse the lowest areas 

 in the district, following the land subdivisional fines where practicable. 

 This will secure surface drainage in the lowest areas, where it is most 

 needed, and also prove cheapest because the amount of excavation 

 will be a minimum. Cutting ditches through the ridges should be 

 avoided as much as possible, especially where these are sandy, on 

 account of the great difficulty of making and maintaining a ditch in 

 sandy soil owing to the trouble caused by the caving banks. At 

 times the previously existing natural drainage channels, when suit- 

 ably cleaned, may be made to form a considerable part of the ditch 

 system. 



Sometimes the district as a whole has very little surface slope, and 

 it becomes necessary to dig the main canal deeper as it nears the 

 pumping plant in order to keep the water surface parallel with the 

 ditch bottom. Having determined the point above which the water 

 should not rise in the portion of the district remote from the pumping 

 plant, and knowing the capacity of the plant, the slope necessary in 

 a ditch of given size to bring the water to the pumping plant fast 

 enough to keep it operating continuously can be calculated. A small 

 ditch will deliver the water at a lower point at the pumping plant 

 than will a larger one, as the slope will have to be greater for the 

 same discharge. This will mean increased power for driving the 

 pumps. The size of ditch and of pumping plant should be mutually 

 adjusted to secure a minimum operating cost, including interest on 

 investment, depreciation, and expense of operation for the pumping 

 plant, and interest and depreciation on the ditch. The foregoing is 

 based on the fact that the capacity of pumping plant used in this 

 section i, nearly uniform. At times of heavy rainfall the ditch will 

 be filled with water and for a short time will deliver more water than 

 5393°— Bull. 304—16 3 



