DRAINAGE. 51 



less vigorous than those receiving good water, nevertheless produce 

 moderate crops of fruit (see p. 98). 



In most date plantations made by the French in the Sahara, drainage 

 is provided by means of open ditches from 2 to 6 feet deep, running 

 between alternate rows of palms, or at distances of about 50 feet 

 apart (see PL XVII, tig. 1). Very unusual conditions of drainage are 

 found at the oasis Fougala, Algeria (see PL XV, fig. 1), as will be 

 explained in treating of the alkali soils collected at that place (pp. 78 

 and 84). The superfluous water there runs off through holes in an 

 impervious hardpan, and the downward flow of water through the 

 holes, induced by surface irrigation, has washed the alkali out of the 

 surface soil, has aerated the subsoil, and has had marvelous effects in 

 promoting the growth and increasing the yield of the date palms, 

 which had managed to live for years before surface irrigation was 

 begun with the supply of water absorbed by the roots from below the 

 hardpan layer. 



It will doubtless be found necessary to irrigate date palms about 

 Tempe, Ariz. 9 even where their roots penetrate to the subsoil con- 

 stantly wetted by the water that seeps down from the irrigated fields 

 located at higher levels. Unless this is done the palms are likely to 

 become stunted and sterile, as they were at Fougala before surface 

 irrigation by artesian water was commenced. 



The presence of a hardpan layer, as at Fougala, may be advanta- 

 geous in providing a means of drainage through holes made under each 

 tree, while at the same time confining the drainage water below the 

 hardpan, thereby preventing its rising to the surface by capillarity 

 and carrying with it the alkali of the subsoil. When no hardpan 

 exists, as at Tempe, a certain amount of drainage can nevertheless be 

 accomplished, since the water applied at the surface drains into the 

 great body of ground water, which has a practically constant level 

 unless raised by excessive irrigation. In case the subsoil is too 

 impervious to permit quick seepage from the surface to the ground 

 water, outlets for drainage water can sometimes be provided advan- 

 tageously by putting down wells. 



In most parts of the Salt River Valley the natural drainage is good 

 and no ditches or tile will be needed. In the Salton Basin drainage is 

 impeded by the impervious nature of the clay, which occurs in many 

 places as surface soil and nearly everywhere as subsoil. Drainage is 

 especially desirable here, for the subsoil is often laden with alkali 

 even where the surface soil is free from harmful quantities of salts. 

 Natural drainage, nevertheless, will probably suffice for the date palm 

 in many parts of this region, provided the level of the ground water 

 is not raised too high by excessive and ill-timed irrigation. In some 

 places, where natural drainage is insufficient, occasional open ditches 

 will provide adequate drainage, especially where the soil is a sandy 



