SALINITY OF IRRIGATION WATER — SCOFIELD 277 



and boron, to name only a few, occur in gaseous form with hot water 

 vapor deep in the earth's crust. These gases reach the surface 

 through rock faults or fumaroles. In some situations these gases 

 may escape into the atmosphere to return to the earth dissolved in 

 rain water, but more often they encounter and are dissolved in 

 superficial percolating water and appear at the surface as springs, 

 often warm or even very hot. 



Many of the so-called " mineral springs " originate in this way, 

 and although it is probable that only a small proportion of the total 

 quantity of salt that is carried by irrigation water comes directly 

 from magmatic sources, these sources are important in some instances. 

 The rains that fall in the occasional torrential storms of desert re- 

 gions contribute a part of the dissolved salts to the streams that 

 drain those regions and that are most extensively used for irriga- 

 tion. Another source of salts exists in the irrigated lands that lie 

 adjacent to these streams. The water that is diverted from streams 

 or drawn up by pumps from the underground supplies that are 

 replenished from streams contains dissolved salts. A large part of 

 the water used in irrigation is absorbed and transpired by crop 

 plants or evaporates directly from the soil. Of the dissolved salt it 

 contains, very little is absorbed by the crop plants, and none is dissi- 

 pated by evaporation. Consequently, it accumulates in the soil to 

 which the water is applied. 



The salts thus transported to and left in the soil by irrigation 

 water are of many different kinds. Some of them, such as calcium 

 carbonate and calcium sulphate, have very low limits of solubility, 

 and as the water is absorbed by plants or evaporated from the 

 soil these salts are precipitated from solution so that they become 

 inactive and not injurious to plants. Many of the other salts have 

 higher limits of solubility and remain dissolved in the soil solution 

 until concentrations are reached that make this solution unsuited for 

 use by the roots of crop plants. In order to avoid crop injury from 

 this cause it is necessary, in many situations, to provide artificial 

 drainage for irrigated land to remove the salts of high solubility. 

 The drainage from such land, whether natural or artificial, usually 

 finds its way or is discharged into the same stream from which the 

 irrigation water is diverted. The total quantity of salt returned to 

 the stream by drainage from irrigated land may be less than the 

 quantity brought in by the irrigation water, but, because the volume 

 of drainage or return flow is less, the concentration of the dissolved 

 salts is higher in the stream below the diversion point than above it. 

 An example to illustrate this effect of diversion and drainage on the 

 salt content of a stream may be taken from the Eio Grande in New 

 Mexico and Texas. Water is diverted from this stream at Leas- 

 burg, N. Mex. Between that point and Fort Quitman, Tex., there 



