24 BULLETIN" 721^ U. S, DEPARTMENT OF AGRICULTURE. 



Seepage from ditches. — It is sometimes necessary to construct irri- 

 gating ditches higher than the surrounding area in- order to carry 

 the water across low places or to carry it long distances. It is very 

 common for the water to seep through the ditch bank in such cases 

 and to saturate the surrounding fields, especially if the soil in those 

 fields has a high water-holding capacity. Ditches constructed of clay 

 or other close-textured material seep to a less extent than ditches con- 

 structed of sandy or other porous material. In the case of sandy 

 ditches the holding capacity may be increased very greatly by oiling 

 the surface of the ditch with crude oil ; this can be applied economi- 

 cally only in those localities not far removed from the source of oil 

 supply. Where the oil is available it may be spread over the inner 

 surface of the ditch when the ditch is empty and should be allowed 

 to soak into the soil. A second or even a third application of the oil 

 will improve the condition of the ditch. Ditches treated in this man- 

 ner will carry water with almost no loss, and when the work is done 

 properly the oiled surface will frequently last for several years. If 

 the oil is spread over the ditch bank, weed growth will be prevented, 

 or at least decidedly retarded. There is always some seepage from 

 the ordinary untreated earth ditch, and the amount of damage done 

 by this seepage will depend upon the nature of the ditch and the 

 soil of the adjacent fields. The seeped areas may sometimes be ren- 

 dered tillable by drainage. 



Seepage from earth reservoirs. — The area of land damaged by 

 seepage from earth reservoirs is less than that from ditches, but the 

 total is considerable and is usually progressive ; that is, a small area 

 first appears to be water-logged and this gradually increases from 

 year to year until large areas involving fields and sometimes entire 

 farms are destroyed. Seepage from reservoirs will depend upon the 

 construction of the reservoir and upon the nature of the surrounding 

 country. Reservoirs are sometimes constructed in mountainous areas 

 for the purpose of catching and storing flood waters due to melting 

 snows. Generally the location of these reservoirs is such that no 

 damage is done to the soil through seepage; the only loss in such 

 cases is caused by the quantity of water which seeps away and be- 

 comes unavailable for irrigation purposes. In other instances reser- 

 voirs are constructed of cement, which is nearly impervious to water, 

 and consequently little or no loss from seepage results. The serious 

 injury due to seepage from reservoirs takes place in those localities 

 in which the reservoirs are constructed entirely or in part of earth 

 and in which the surrounding country is composed of tillable land : 

 in such cases the loss due to seepage is sometimes of considerable 

 importance. 



Alkali. — As already indicated, one of the serious results of seepage 

 is the accumulation of alkali in the surface soil. In such cases 



