474 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1915. 



SEEPAGE AND DRAINAGE. 



Wlien work in the nature of supplemental construction, already 

 referred to, becomes desirable, it is usually for the purpose of. drain- 

 ing the irrigated tracts of excess water. 



One of the important things to which attention must be given 

 after an irrigation system is completed and put in operation is the 

 protection of the irrigated lands from becoming seeped and water- 

 logged, destroying their productivity. 



In the use of the irrigation water, there is more or less loss due 

 to the application of a larger quantity than can be retained by the 

 soil and given up to the plants. The amount of water which sinks 

 below the zone of plant growth and is wasted in this manner depends 

 upon the perfection with which the land is graded and ditched and 

 the care and skill used by the irrigator in handling the water. Until 

 the processes have been sufficiently perfected to distribute the exact 

 quantity of water required for the growing crop, some underground 

 wastes are to be expected from this cause. Wliere, as is ordinarily 

 the case, earthen canals are used for carrying or distributing the 

 water, there is more or less loss by seepage from these channels. 

 This loss, together with the underground waste from irrigation, 

 tends to fill the soil and raise the water plane or height of free 

 water. T^Hien this is raised above certain limits the irrigability of 

 the land is destroyed, since plants can not thrive in a soil the inter- 

 stices of which are filled with water. 



To protect lands from becoming thus seeped and water-logged, 

 it is essential that the position of the ground waters be known in 

 order to prevent their rise to or near the surface, rendering the lands 

 unfit for cultivation. Occasional observations must be made at 

 various points to determine the elevation of the ground water and 

 whether or not it is rising. This is done through the medium of 

 wells. Wlien a rise in the water plane is noted and there is danger 

 of it coming too near the surface, action must be taken to prevent 

 it. The means employed to lower and control the ground-water aim 

 at the prevention of losses from canals, the reduction of the amount 

 of water applied to the soil in irrigation, or the construction of 

 drains for carrying out excess waters. 



Drainage works have become necessary on a number of the Gov- 

 ernment projects, and a total of 500 miles of drains have been built 

 to relieve or protect about 100,000 acres. The types of drain used 

 include both covered tile lines (pi. 3, fig. 1) and open ditches (pi. 3, 

 fig. 2). The latter largely predominate, as they have been found 

 through experience to give very satisfactory service, while the effi- 

 ciency of the tile lines may be seriously impaired by accumulations 

 of sand or by moving out of line. 



