22 BULLETIN 652, I T . S. DEPARTMENT OF AGRICULTURE. 



EXPLANATION OF TERMS. 



Before proceeding with the discussion of the reclamation districts a brief 

 explanation of some of the technical terms hereinafter used will be given. 



By run-off is meant the water that flows over or through the ground to 

 drainage outlets. All run-off originates in precipitation ; therefore the latter 

 is the most important of all the factors that influence the rate of run-off, some 

 other controlling features being size, shape, topography, and geological struc- 

 ture of the watershed, climatic conditions, and the character of the vegetation. 



In so-called gravity drainage districts the run-off is removed by gravity 

 through the main outlets. In pumping districts the run-off is collected at some 

 point within the district and pumped over or through the levee. The rate of 

 run-off is expressed as a quantity of water removed in a unit of time. This 

 quantity of water can be conceived as a certain depth distributed uniformly 

 over the entire drainage area. Similarly, the capacity of a pumping plant is 

 the quantity of water, expressed in a depth distributed uniformly over the 

 drainage area, that the pump can dispose of in a given time. As used in the 

 following discussion, the rate of run-off is expressed by the depth of water, 

 in inches, distributed uniformly over the drainage area, that passes from the 

 area in a period of 24 hours. Likewise, pumping-plant capacity is expressed 

 in terms of a depth of water, distributed uniformly over the drainage area, that 

 can be removed by the plant in 24 hours of continuous operation. Reservoir 

 capacity, also, is represented by a uniform depth distributed over the drainage 

 area. The object of expressing the rate of run-off and the pump and reservoir 

 capacities in depths rather than in volumes per unit of time is to arrive at a 

 basis of comparison that is independent of the drainage areas. On any par- 

 ticular tract, the area being known, these rates can easily be reduced to cubic 

 feet per second or to any other convenient unit. 



In a long period the total amount pumped would equal approximately the 

 run-off for that period ; but for short intervals, as, say, 24 hours, the run-off 

 from an area might be much greater than the quantity pumped, the excess 

 being stored in the reservoir for subsequent pumping. 



WILLSWOOD PLANTATION, WAGGAMAN, JEFFERSON PARISH, LA. 



This plantation is selected for discussion because it is typical of the character 

 of pumping district that was drained first in Louisiana. Conditions prevailing 

 in this plantation are also typical of those on all river and bayou front lands, 

 and as most of the land at present in cultivation in this section of the State is 

 of this character, a rather detailed description of conditions will be given. 



As shown in figure 3. this plantation, of 2,600 acres, fronts on the Mississippi 

 River and extends back into the prairie swamp lands in the rear. About one- 

 third of the area of the present plantation originally was prairie land and was 

 included by an extension of the levees in 1896. A pumpiug plant was in- 

 stalled, and cultivation has been continuous since that date. The area of prairie 

 land included was typical of such land in this part of the State and was 

 covered with a scattering growth of willows and the usual rank growth of 

 grass. Originally the muck was about 4 feet deep and was overlain by 

 layer of river silt about 4 to 6 inches thick, although at present, after 22 years 

 of cultivation and decay, it is well compacted and has subsided or shrunk until 

 it now averages from 2 to 3 feel lower than it was originally. The total fall 

 in the surface of the ground from the front to the rear of the plantation is) 

 about 10 feet. 



