46 THE DATE PALM. 



liter (0.35 quart) per minute, or 126 gallons per day, a trifle over 4 

 acre-feet would be required, of which nearly 3 acre-feet would be used 

 in the four hottest months, from June to September, inclusive. On 

 the basis of Holland's estimate, which is also given by Jus as the 

 optimum quantity, viz, one-half liter (0.53 quart) per minute, or 190 

 gallons per da}-, some 5i acre-feet a year would be required, of which 

 4 acre-feet would be used during the four summer months, or at the 

 rate of 16 acre-feet per annum. 



The amount of water needed per acre depends of course directly on 

 the number of date palms per acre, and in planting care should be 

 taken not to set out more than can be irrigated with the water supply 

 covering the land. 



It must be remembered that the figures given above are for the 

 western Sahara, a region noted for its extreme dryness, where the 

 evaporation from a free surface of water often averages nearly one- 

 half inch per day during the three summer months June, July, and 

 August. It is probable that a smaller amount of water would suffice 

 in regions where the air is not so dry and consequently where the 

 evaporation is less, as, for example, in the Salt River Valley and most 

 other parts of southern Arizona/ while in hotter, drier regions, such 

 as the Sal ton Basin, even more will be required. In the latter region 

 it will be well to allow only about 12 palms to each acre-foot of water 

 available, and this only if the water can be had whenever desired during 

 the summer. This would permit planting some 50 date palms to the 

 acre where 4 acre-feet of water are available whenever needed during 

 the year. 



It must be remembered in considering the needs of the date palm 

 that the water supply must be practically continuous; that is to sa}^ 

 that the ground must in some way be kept damp throughout the entire 

 year. It is probable, however, that the date palm does not require as 

 much water as do ordinary fruit trees. It is, indeed, probable that 

 owing to their having thick, leathery leaves, protected by a coating 

 of wax, they evaporate a considerably less quantity than would an ordi- 

 nary fruit tree having delicate leaves not adapted to withstand the hot, 

 dry air of deserts. It is nevertheless necessary for the roots to have 



o At Tucson, Ariz., the average of three years' records taken at the University gives 

 the annual evaporation from a free surface of water at 77.7 inches, and the average 

 rate during the three hottest months, June, July, and August, at one-third inch per 

 day. At Tempe, in the Salt River Valley, Arizona, a calculation by the United States 

 Geological Survey from imperfect data gives 91 inches as the probable annual evapo- 

 ration. At Biskra the careful records of M. Colombo show a mean annual evapora- 

 tion during the ten years from 1884 to 1893 of 2.8374 meters, or 111.7 inches, averaging 

 12.47 mm., or 0.4915 inch (very nearly one-half inch) per day during June, July, and 

 August. In the Oued Rirh country, where most of the observations relative to the 

 amount of water necessary for irrigating date palms have been made, the rainfall is 

 less than at Biskra and the temperature higher, so the evaporation is doubtless greater. 



