LAND DRAINAGE 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 if 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 lines 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 is 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—15 3 
