[Sidenote:  Irrigation and horticulture.]

The few and limited spots where the desert or steppe affords water for cultivation require artificial irrigation, the importation of plants, and careful tillage, to make the limited area support even a small social group.  Hence they could have been utilized by man only after he had made considerable progress in civilization.[1123] Oasis agriculture is predominantly intensive.  Gardens and orchards tend to prevail over field tillage.  The restricted soil and water must be forced to yield their utmost.  While on the rainy or northern slope of the Atlas in Algiers and Tunis farms abound, on the Saharan piedmont are chiefly plantations of vegetables, orchards and palm groves.[1124] In Fezzan at the oasis of Ghat, Barth found kitchen gardens of considerable extent, large palm groves, but limited fields of grain, all raised by irrigation; and in the flat hollow basin forming the oasis of Murzuk, he found also fig and peach trees, vegetables, besides fields of wheat and barley cultivated with much labor.[1125] In northern Fezzan, where the mountains back of Tripoli provide a supply of water, saffron and olive trees are the staple articles of tillage.  The slopes are terraced and irrigated, laid out in orchards of figs, pomegranates, almonds and grapes, while fields of wheat and barley border the lower courses of the wadis.[1126] In the “cup oases” or depressions of the Sahara, the village is always built on the slope, because the alluvial soil in the basin is too precious to be used for house sites.[1127]

[Sidenote:  Effect of diminishing water supply.]

The water supply in deserts and steppes, on which permanent agriculture depends, is so scant that even a slight diminution causes the area of tillage to shrink.  Here a fluctuation of snowfall or rainfall that in a moist region would be negligible, has conspicuous or even tragic results.  English engineers who examined the utilization of the Afghan streams for irrigation reported that the natives had exploited their water supply to the last drop; that irrigation converted the Kabul River and the Heri-rud at certain seasons of the year into dry channels.[1128] In the Turkoman steppes it has been observed that expanding tillage, by the multiplication of irrigation canals, increased the loss of water by evaporation, and hence diminished the supply.  Facts like these reveal the narrow margin between food and famine, which makes the uncertain basis of life for the steppe agriculturist.  Even slight desiccation contracts the volume and shortens the course of interior drainage streams; therefore it narrows the piedmont zone of vegetation and the hem of tillage along the river banks.  The previous frontier of field and garden is marked by abandoned hamlets and sand-buried cities, like those which border the dry beds of the shrunken Khotan rivers of the Tarim basin.[1129] The steppe regions in the New World as well as the Old show great numbers of these ruins.  Barth found them in the northern Sahara, dating from Roman days.[1130] They occur in such numbers in the Syrian Desert, in the Sistan of Persia, in Baluchistan, the Gobi, Takla Makan Desert, Turfan and the Lop Nor basin, that they indicate a marked but irregular desiccation of central and western Asia during the historical period.[1131]