Unidentified oasis in Chad, photographed c. 1930
Inecology, an oasis (/oʊˈeɪsɪs/; pl.: oases /oʊˈeɪsiːz/) is a fertile area of a desert or semi-desert environment[1] that sustains plant life and provides habitat for animals. Surface water and land may be present, or water may only be accessible from wells or underground channels created by humans. In geography, an oasis may be a current or past rest stop on a transportation route, or less-than-verdant location that nonetheless provides access to underground water through deep wells created and maintained by humans.
The word oasis came into English from Latin: oasis, from Ancient Greek: ὄασις, óasis, which in turn is a direct borrowing from Demotic Egyptian. The word for oasis in the latter-attested Coptic language (the descendant of Demotic Egyptian) is waheorouahe which means a "dwelling place".[2] Oasis in Arabic is wāḥa (Arabic: واحة).
Oases develop in "hydrologically favored" locations that have attributes such as a high water table, seasonal lakes, or blockaded wadis.[3] Oases are made when sources of freshwater, such as underground rivers or aquifers, irrigate the surface naturally or via man-made wells.[4] The presence of water on the surface or underground is necessary and the local or regional management of this essential resource is strategic, but not sufficient to create such areas: continuous human work and know-how (a technical and social culture) are essential to maintain such ecosystems.[5][6] Some of the possible human contributions to maintaining an oasis include digging and maintaining wells, digging and maintaining canals, and continuously removing opportunistic plants that threaten to gorge themselves on water and fertility needed to maintain human and animal food supplies.[7] Stereotypically, an oasis has a "central pool of open water surrounded by a ring of water-dependent shrubs and trees…which are in turn encircled by an outlying transition zone to desert plants."[8]
Rain showers provide subterranean water to sustain natural oases, such as the Tuat. Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to the surface. Any incidence of water is then used by migrating birds, which also pass seeds with their droppings which will grow at the water's edge forming an oasis. It can also be used to plant crops.
Oases in the Middle East and North Africa cover about 1,000,000 hectares (10,000 km2), however, they support the livelihood of about 10 million inhabitants.[9] The stark ratio of oasis to desert land in the world means that the oasis ecosystem is "relatively minute, rare and precious."[8]
There are 90 “major oases” within the Sahara Desert.[4] Some of their fertility may derive from irrigation systems called foggaras, khettaras, lkhttarts, or a variety of other regional names.[10][11]
In some oases systems, there is "a geometrical system of raised channels that release controlled amounts of the water into individual plots, soaking the soil."[11]
Oases often have human histories that are measured in millennia. Archeological digs at Ein Gedi in the Dead Sea Valley have found evidence of settlement dating to 6,000 BC.[12] Al-Ahsa on the Arabian Peninsula shows evidence of human residence dating to the Neolithic.[13]
Anthropologically, the oasis is "an area of sedentary life, which associates the city [medina] or village [ksar] with its surrounding feeding source, the palm grove, within a relational and circulatory nomadic system."[14]
The location of oases has been of critical importance for trade and transportation routes in desert areas; caravans must travel via oases so that supplies of water and food can be replenished. Thus, political or military control of an oasis has in many cases meant control of trade on a particular route. For example, the oases of Awjila, Ghadames and Kufra, situated in modern-day Libya, have at various times been vital to both north–south and east–west trade in the Sahara Desert. The location of oases also informed the Darb El Arba'īn trade route from Sudan to Egypt, as well as the caravan route from the Niger RivertoTangier, Morocco.[8] The Silk Road "traced its course from water hole to water hole, relying on oasis communities such as Turpan in China and Samarkand in Uzbekistan."[8]
According to the United Nations, "Oases are at the very heart of the overall development of peri-Saharan countries due to their geographical location and the fact they are preferred migration routes in times of famine or insecurity in the region."[10]
Oases in Oman, on the Arabian Peninsula near the Persian Gulf, vary somewhat from the Saharan form. While still located in an arid or semi-arid zone with a date palm overstory, these oases are usually located below plateaus and "watered either by springs or by aflaj, tunnel systems dug into the ground or carved into the rock to tap underground aquifers." This rainwater harvesting system "never developed a serious salinity problem."[9]
In the drylands of southwestern North America, there is a habitat form called Palm Oasis (alternately Palm Series or Oasis Scrub Woodland) that has the native California fan palm as the overstory species.[15] These Palm Oases can be found in California, Arizona, Baja California, and Sonora.[15]
People who live in an oasis must manage land and water use carefully. The most important plant in an oasis is the date palm (Phoenix dactylifera L.), which forms the upper layer. These palm trees provide shade for smaller understory trees like apricots, dates, figs, olives, and peach trees, which form the middle layer. Market-garden vegetables, some cereals (such as sorghum, barley, millet, and wheat), and/or mixed animal fodder, are grown in the bottom layer where there is more moisture.[16] The oasis is integrated into its desert environment through an often close association with nomadic transhumant livestock farming (very often pastoral and sedentary populations are clearly distinguished). The fertility of the oasis soil is restored by "cyclic organic inputs of animal origin."[14] In summary, an oasis palm grove is a highly anthropized and irrigated area that supports a traditionally intensive and polyculture-based agriculture.[1]
Responding to environmental constraints, the three strata create what is called the "oasis effect".[1] The three layers and all their interaction points create a variety of combinations of "horizontal wind speed, relative air temperature and relative air humidity."[9] The plantings—through a virtuous cycle of wind reduction, increased shade and evapotranspiration—create a microclimate favorable to crops; "measurements taken in different oases have showed that the potential evapotranspiration of the areas was reduced by 30 to 50 percent within the oasis."[14]
The keystone date palm trees are "a main income source and staple food for local populations in many countries in which they are cultivated, and have played significant roles in the economy, society, and environment of those countries."[17] Challenges for date palm oasis polycultures include "low rainfall, high temperatures, water resources often high in salt content, and high incidence of pests."[18]
The oases consist of almost unbroken forests of date palms, divided up into many gardens that are separated by mud walls and intersected by innumerable irrigation and drainage ditches… In the shade of the palms are grown many other kinds of fruit trees—oranges, olives, figs, apricots, peaches, pomegranates, and jujubes—interlaced with large grape vines that often hang in festoons from the palm trunks. Beneath the trees are small plots of garden vegetables, barley, and alfalfa. Neither date palms nor other trees are planted with any regularity, and the growth is often so dense that the garden resembles a tropical jungle. Very beautiful are these gardens in the spring, when the apricot and peach trees are in blossom here and there among the palms and the figs and vines are putting forth their leaves. In autumn, when the dates are ripening, the color effects, especially when the tops of the palms are lit up by the last rays of the setting sun, are something that once seen can never be forgotten. The great clusters of fruit, displaying every tint from bright yellow, through orange, vermilion, and maroon, to plum purple and chestnut brown, with their brilliant yellow or rich orange ivory-like stalks, contrast superbly with the dull bluish or gray green of the feathery crowns of foliage. It is small wonder that a whole folklore of poetic legends and proverbs has grown up around the date palm in the regions where it flourishes.
— Kearney, Thomas H. (1906-09-06). Bulletin: Date Varieties and Date Culture in Tunis.
Many historic oases have struggled with drought and inadequate maintenance.
According to a United Nations report on the future of oases in the Sahara and Sahel, "Increasingly ... oases are subject to various pressures, heavily influenced by the effects of climate change, decreasing groundwater levels and a gradual loss of cultural heritage due to a fading historical memory concerning traditional water management techniques. These natural pressures are compounded by demographic pressures and the introduction of modern water pumping techniques that can disrupt traditional resource management schemes, particularly in the North Saharan oases."[10]
For example, five historic oases in the Western Desert of Egypt (Kharga, Dakhla, Farafra, Baharyia, and Siwa) once had "flowing spring and wells" but due to the decline of groundwater heads because of overuse for land reclamation projects those water sources are no more and the oases suffer as a result.[19]
Morocco has lost two-thirds of its oasis habitat over the last 100 years due to heat, drought, and water scarcity.[11] The Ferkla Oases in Morocco once drew on water from the Ferkla, Sat and Tangarfa Rivers but they are now dry but for a few days a year.[11]
Old World Oases
Country
Administrative Division
Name
Elevation
Typology
Ref.
272
355
759
663
405
851
778
705
1017
782
503
306
569
480
95
287
522
177
-8
503
160
105
301
93
62
189
50
209
204
50
156
0
56
34
-11
79
64
58
48
71
52
101
90
6
54
44
21
7
66
14
1022
983
851
753
936
868
1022
106
104
91
107
127
110
99
69
126
135
53
99
114
81
72
497
533
494
532
465
303
514
399
511
354
420
459
399
847
508
379
331
506
350
347
521
602
356
360
210
244
367
221
185
294
276
275
252
198
267
230
351
224
234
240
244
287
233
243
256
233
204
359
225
307
373
247
342
236
259
316
234
252
259
264
288
358
226
233
171
255
361
268
254
268
203
249
160
262
327
272
377
312
373
163
244
226
376
216
220
360
246
225
243
224
249
196
223
251
248
252
232
Zaouia Sidi Abd El Kader Oasis
222
241
354
195
376
558
1032
1118
300
274
256
?
?
-41
-11
118
123
32
117
66
54
111
85
316
?
?
-2
421
338
213
35
31
39
47
539
270
247
216
366
350
380
419
413
656
675
466
451
465
466
382
292
381
255
472
474
453
494
152
248
252
223
250
494
139
229
478
162
263
166
358
354
446
149
180
213
415
276
173
313
195
235
261
190
250
259
240
368
230
410
416
400
237
106
126
111
113
109
408
134
352
112
443
269
252
199
188
396
89
170
174
146
194
173
208
80
969
769
860
886
787
827
874
878
928
1191
1000
1261
1003
1368
1187
942
1613
1115
1390
1421
797
873
719
1021
965
893
931
755
872
858
775
886
817
1201
808
1028
771
835
1180
1024
713
1161
1306
940
1207
1348
958
978
696
927
949
792
599
914
1064
737
898
763
1249
549
770
1045
1120
917
564
828
858
621
567
760
760
1224
1107
1370
1178
1317
905
910
1083
685
959
842
879
914
862
1406
884
1257
1095
877
852
899
785
941
892
996
897
1282
949
799
1350
988
837
1554
724
595
945
1078
1255
599
870
778
1558
636
461
1256
474
801
538
779
1048
819
1549
1130
1145
1481
767
606
664
478
847
647
1049
500
766
600
934
783
1115
561
1655
790
758
700
550
605
280
1157
558
969
1170
1233
?
?
695
955
696
771
871
305
620
768
531
573
646
287
305
669
822
906
393
793
1477
418
1537
364
577
447
1458
371
660
487
377
415
858
954
775
857
1530
507
877
315
325
338
322
403
614
623
510
658
549
259
827
802
742
669
430
645
451
504
598
470
605
701
362
317
655
657
541
309
659
373
373
398
415
403
528
524
589
229
215
497
466
1121
686
137
343
399
616
208
877
257
812
559
200
457
976
168
189
622
242
748
598
187
671
515
445
323
348
648
725
766
553
486
82
50
298
639
867
541
689
529
678
719
596
913
776
725
343
399
464
360
692
490
702
647
565
283
585
549
647
766
561
569
218
632
565
169
594
550
583
422
523
710
915
630
439
522
568
368
260
495
1867
616
605
546
401
429
434
484
420
358
402
428
432
607
373
395
315
406
393
424
453
657
446
438
722
293
900
600
547
479
521
490
389
591
409
652
339
572
506
357
306
327
348
271
394
311
669
160
?
108
260
8
584
812
149
180
173
564
533
692
493
714
701
584
583
554
658
675
594
575
635
1083
1307
?
?
224
281
115
48
40
367
35
276
?
36
32
63
51
57
27
46
35
210
32
112
40
63
24
58
18
37
49
15
28
43
24
25
450
276
298
232
300
307
Daşoguz Province
Karakalpakstan
?
?
292
130
281
95
663
1068
310
1043
1039
678
672
651
738
593
642
563
769
917
881
736
423
496
950
590
303
346
270
314
700
830
95
973
1025
989
93
976
92
418
1075
376
586
586
821
924
6
410
360
80
826
909
963
533
648
233
667
986
885
1136
88
628
614
1186
101
801
216
270
6
1109
1303
1402
1450
8
25
15
6
A 1920 USGS publication about watering holes in the deserts of California and Arizona gave this advice for travelers seeking oases:[40]
The usual watering places are springs or wells. Springs are frequently clogged with gravel or rubbish or sometimes even with the bodies of dead animals, and it may be necessary to clean them out. For this work a shovel is needed. Wells may or may not be equipped with pumps. Frequently the pumps are broken and useless, and a rope and bucket are then necessary to obtain water. Most of the wells in this region are less than 100 feet deep, but some are deeper, and 100 feet of rope is not too much to provide. As a rule the rope and bucket at a well, if they were ever provided, soon disappear, and one should never trust the chance of finding them there. Open wells are sometimes contaminated in the same way as springs and need to be cleaned out, particularly in little-frequented places where they are unused for months at a time.
{{cite book}}: CS1 maint: location missing publisher (link)
The dictionary definition of oasis at Wiktionary