278 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1935 



is an irrigated area of some 150,000 acres contiguous to the stream. 

 The drainage from this irrigated land returns to the stream above 

 Fort Quitman. During the calendar year 1933 the volume of the 

 discharge of the river at Leasburg, including the water diverted for 

 irrigation, was 824,000 acre-feet containing 651,000 tons of dissolved 

 salts or 0.79 ton per acre-foot. During the same year the volume of 

 water passing Fort Quitman, including returned drainage water, was 

 214,000 acre-feet containing 582,000 tons of dissolved salts or 2.72 

 tons per acre-foot. Consequently, the effect of irrigation and drain- 

 age on the salinity of the stream in this section of its course was to 

 increase its concentration from 0.79 to 2.72 tons of dissolved salt 

 per acre-foot of water. 



It is obvious that the net effect of the irrigation of this land is to 

 increase the salt concentration, if not the total salt burden, of the 

 stream that serves it. Thus it seems proper to include irrigated land 

 as one of the sources of the salinity found in irrigation water. The 

 principal sources, then, are: (1) The soluble products of the forma- 

 tion of soil from rock weathering; (2) gases of magmatic origin from 

 deep in the earth's crust; (3) the erosion by rainfall of older sedi- 

 mentary deposits and the solution of salts deposited at the time of 

 their formation ; and (4) the drainage of irrigated land. 



IRRIGATION WATER AND THE SOIL SOLUTION 



The water required by crop plants in the process of growth is ab- 

 sorbed by tlieir roots from the solution suspended in the soil within 

 the zone occupied by the roots. This root zone varies in depth with 

 different crops and with different soil conditions. In general it ranges 

 from 2 to 6 feet or more in depth. The capacity of the soil of the 

 root zone to hold water against the force of gravity is limited to what 

 is known as its field capacity, and, on the other hand, the crop plant 

 is not able to utilize all of the water contained in the root zone because 

 of the tenacity with which the soil holds a portion of that water. The 

 quantity of water held in the root zone at any one time and available 

 for use by crop plants must lie between these limits. The relationship 

 between this quantity of water and the current dail}^ water require- 

 ments of crop plants is such that during the active growing season 

 the supply of water must be replenished from time to time. In an 

 arid climate this is done by irrigation, and experience has shown that 

 during the season of active growth the periods between irrigations 

 should rarely exceed 40 days, and with some crops and some soil con- 

 ditions the period should not exceed 14 days. 



In dealing with the subject of salinity it must be understood that 

 when the water supply of the root zone of the soil, known as the soil 

 solution, is replenished by irrigation, there is added to that solution 



