152 CONSERVATION OF GROUND WATER 



diverted from the river. The flood plain of the middle Rio 

 Grande, between White Rock Canyon and San Marcial in 

 New Mexico, averages 2 miles in width and comprises 120,- 

 000 acres, of which about half is irrigated by surface water. 



Ground water occurs at shallow depths everywhere under 

 the flood plain. In 1936 the water table was less than 8 feet 

 deep under 90 per cent of the irrigated area of the flood 

 plain, and less than 5 feet deep under 40 per cent of the area 

 — high enough to be detrimental to the growing of many 

 crops. The high water table is evidently due chiefly to ir- 

 rigation. Many drainage structures have been constructed 

 in the valley; these have been largely instrumental in lower- 

 ing the water table as much as 3 feet in the 10 years pre- 

 ceding 1937. During- 1938, however, the water table rose 

 nearly 0.2 foot under the entire flood plain, probably due 

 to increased rainfall and floods, and to clogfginsr of some 

 drains. 



Another important reason for the rise in water table is 

 the raising of the bed of the Rio Grande by sedimentation. 

 Between 1927 and 1936 the river bed at San Marcial rose 

 5.4 feet, because of lessened gradient following construc- 

 tion of Elephant Butte Reservoir downstream, depletion 

 of stream flow by diversions for irrigation, and increased 

 rate of erosion in the tributary area. 



Some of the water in the ground-water reservoir is dis- 

 charged into drains and returns to the river for further use. 

 Underflow from the valley is of small magnitude, perhaps 

 750 acre-feet a year. Most of the discharge is to the atmos- 

 phere, for the water table is high enough to permit large 

 quantities to be discharged by transpiration and evapora- 

 tion. According to the report by the National Resources 

 Committee, there was 100,000 acres of water-loving vegeta- 

 tion in the valley above Elephant Butte Dam in 1936, which 

 consumed more than 380,000 acre-feet of water a year. Some 

 of the native vegetation used about twice as much water 

 per acre as the irrigated crops in the valley. Since 1936 both 



