WET LANDS OF SOUTHERN LOUISIANA. 47 



lands can be partly overcome by a proper design of the various parts of the col- 

 lecting system. By careful location of gravity outlet ditches tliis drainage water 

 from the higher lands can be entirely diverted in some cases. 



The character and depth of the layer of muck ..overlying the subsoil on these 

 lands have a large influence on the run-off. The muck absorbs water very 

 readily, and if well drained to a depth of 3 feet its storage capacity is about 8 

 inches. When the land is first drained this muck will absorb water nearly as 

 fast as the heaviest rate of precipitation, but as it decays and compacts both 

 the storage capacity and the rate of absorption decrease very rapidly. A gradual 

 increase in the rate of run-off then must be expected. 



The effect of reservoir capacity has already been discussed. However, it 

 might be well to point out that increase of reservoir capacity does not decrease 

 the amount of pumping to be done, but simply acts to decrease the time of 

 flooding in case the run-off overtaxes the capacity of the plant. 



Deep lateral drainage acts to decrease the intensity of run-off. If such lat- 

 erals be lines of tile, the rate of run-off will be decreased still further, for prac- 

 tically all water must then pass downward through the soil and out through 

 the tile before it can reach the canal, while in the case of open field ditches most 

 of the water can flow over the surface to the ditch and thus directly into the 

 canal. Especially will this effect be noticed as the muck gradually loses its 

 oower of rapid absorption. 



If the pumping plant be designed to operate continuously, its capacity may 

 be less than that of a plant intended for day use only. As mentioned previ- 

 ously, there is need of the plant at all times of the year. The fact that the 

 water is always pumped out promptly and all reservoir capacity is quickly avail- 

 able makes a smaller plant capacity practicable. 



While a knowledge of the total yearly and monthly amounts of rainfall, either 

 maximum or average, is important in determining the probable total amount of 

 water to be pumped each year or month, the intensity of rainfall during storm 

 periods extending over only a few clays is the factor that fixes the necessary 

 capacity. As the run-off from a given rainfall will depend largely on the con- 

 dition of the soil before the rain occurred, a determination has been made of the 

 proportion of storms that occur when the land is wet, by examining the records 

 of rainfall and pumping kept since June, 1909, on a number of typical districts 

 in southern Louisiana. Of all the storms exceeding 2 inches and under 4, in 

 24 hours, 64 per cent occurred on a wet and 36 per cent on a dry surface ; for 

 the storms exceeding 4 inches in 24 hours the percentages are 54 and 46 per cent, 

 respectively. Of course, the storage capacity of the land influences the rate of 

 run-off from the small storms relatively much more than it does that of the large 

 ones. However, an examination of the daily rainfall and pumping records shows 

 that the heavy rains on a dry soil do not make a very heavy demand on the 

 pumping plant. It is believed that on this account a reduction of about 30 per 

 cent in the average frequency of storms as tabulated on page — could safely be 

 made and that the resulting figure would be the proper one to use when esti- 

 mating how often a storm would occur on a wet soil. 1 



Knowing the character of the operations to be conducted on the land of a 

 given district, a decision can then be reached as to the heaviest storm for which 

 provision must be made. On a district where staple crops are to be raised it 

 would be a matter of economy to allow a certain amount of flooding oftener 

 than would be permissible on land where high-priced truck crops are to be 

 raised, while in residence districts it would be desirable to prevent all sur- 



1 For a full discussion of the relation between rainfall and run-off in southern Louisi- 

 ana, see Journal of Agricultural Research, Vol. XI, No. 6, Nov. 5, 1917. 



