22 BULLETIN 304, U. S. DEPARTMENT OP AGRICULTURE. 



vent danger to the lower portions from the run-off from the higher 

 areas. Where drainage from higher land flows into the district a 

 larger run-off coefficient must be used for the higher area. The 

 proper amount depends upon the nature of the country. For rolling 

 pasture land free from undergrowth, or for cultivated corn land, 

 probably an increase of 50 per cent in the coefficient, or a total of 

 three-eighths inch, for such upland areas is none too great. Where 

 feasible, the drainage from such higher areas should be diverted 

 around the district, as previously explained, instead of being brought 

 to the pumping plant. 



With a considerable reservoir storage area within the district, the 

 size of the pumping plant may be materially reduced. If the stor- 

 age area equals 5 per cent of the total area, a 2-inch rainfall, all run- 

 ning into the reservoir, would raise its level only 40 inches. There- 

 fore, if a rise of 3 feet in the reservoir can be permitted it would 

 seem that the capacity of the pumping plant could certainly be re- 

 duced by one-third and possibly by one-half of what would otherwise 

 be required. 



If tile drainage at a depth of 3 feet or more is extensively used, the 

 demand upon the pumping plant will be somewhat reduced, for the 

 tiled land will act like a reservoir in storing a large amount of water 

 and will deliver it only slowly to the ditches. On the other hand, an 

 extensive system of shallow open field ditches will deliver the rainfall 

 rapidly and may thus damage any especially low areas unless the 

 capacity of the pumping plant is increased. If the levees have been 

 properly constructed, the amount of seepage through them can never 

 be sufficient to affect the pumping plant. Cases have been known, 

 however, of levees so poorly built that the quantity of water coming 

 through them was sufficient not only to endanger the stability and 

 safety of the levees themselves, but also to increase appreciably the 

 amount of water to be pumped. 



If the soil is very stiff and tenacious the surface run-off after a 

 heavy summer shower will be greater than from a porous open soil. 

 During a time of flood in the river, a sandy subsoil may develop 

 springs and boils, not only in the bottoms of the ditches, but some- 

 times even in the fields. The quantity of water that may be received 

 in this way is very uncertain, but it is not probable that it is often 

 sufficient in amount to affect the pumping plant. 



A pumping plant that is designed to operate night and day may, 

 of course, be smaller than one intended to operate only in the day- 

 time. Practically all plants are expected to run nights occasionally, 

 when needed. Pumping plants are operated chiefly during the 

 months from March to June. One of the times of longest continuous 

 operation during the whole year is always at the beginning of pump- 

 ing in the spring, because if there has been no pumping during the 



