The ‘gizu’: Winter grazing in the South Libyan desert

Journal of Arid Environments (1978) 1,327-344

The 'gizu': Winter grazing in the South Libyan desert R. T. Wilson* 'Gizu' is a Sudanese term for an ephemeral type of succulent winter grazingwhich occurs on the southern fringes of the Libyan desert in approximately two years out of five. The occurrence is apparently associatedwith a combination of climaticfactors. The paper is a review of the literature of the gizu, both published work and government records. It describes the gizu environment, provides a calendar of its occurrence, an outline of its vegetative composition and ecology and finally draws attention to its importance in the livestock economyof the area.

Introduction The 'gizu' (also spelt 'gizzu', 'jizu' and 'jizzu') is a vegetation community which occurs Sporadically in the dry winter months in Northern Darfur, and to some extent in Northern Rordofan, in the northwest of the Republic of Sudan. It is not confined within the geographical boundaries of the Sudan, however, and continues westwards into Tchad, in the region of the Ennedi Massif. Indeed, it has been said that the main area of gizu is outside Sudan between the Wadi Howar and the Wadi Bao in Tchad (Harrison & Jackson, 1956). The intermittent occurrence of the gizu-occasionally appearing for two or three years in succession,then not for two, three or more years; or appearing in alternate years-is attributed to combinations of climatic conditions. Insufficient evidence is availableto define precisely these climatic phenomena. The derivation of the word 'gizu' has caused some discussion, and its transliteration in this form from the Arabic is by no means universal. It was considered by Newbold (1924b) to be derived from a root meaning 'he satisfied, or contented, the camels with green or fresh Pasture or herbage (so that they were in no need of water)'. In the Arabian Peninsula, Doughty (1936) said that when the owners referred to stock as 'jezzin' they meant they were not drinking free water. It is not the purpose of this review to enter into polemics on the origins or meanings of the word. Whether it is to do with contentment or thirst, one of its major attractions to the Sudanese herder is that it is green enough and succulent enough for camels (and to a lesser extent other stock) to maintain themselves on it without need of free Water, occasionally for periods of several months. During these periods the herders themselvesobtain their liquid requirements mainly from the milk of the camels. In the Sudan Republic the word 'gizu' is almost universally used to denote the vegetation type or the area in which it occurs. It has also been used in a soil classification system (Worrall, 1961) and, apparently due to a misunderstanding of the primary source (which was: Asad, 1964), has been used to denote the actual trek to the grazing (Johnson, 1969). • Hunting Technical Services, Ltd, Elstree Way, Boreham Wood, Herts, U.K. Present address: 21 Westfield Grove, Wakefield, West Yorkshire, U.K. 0140-1963/78/040327 + 18 $02.00/0

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1978 Academic Press Inc. (London) Limited

328

R. T. WILSON

Outside the Sudan and Tchad a gizu-type vegetation occurs over much of the Sahara, generally under the name of 'asheb' which is the common Arabic vernacular for Cyperus conglomeratus: it has been noted specificallyin the Fezzan (le Houerou, pers. comm.) and in the north of Niger to the west of Alr, where the Tuareg and their camels survive without water from the start of the cold season to the end of February (Bemus, 1969). Analogous vegetation is recorded from Arabia (Philby, 1922; Doughty, 1936) and in the Kalahari, where a melon known as 'tsama' is said to be similar to the 'handal' of the gizu (Newbold, 1924a).

Literature and sources of information This paper attempts to synthesize what is known about the gizu. While a comprehensive review of the literature is presented, it is not claimed that it is exhaustive. It is confined mainly to sources in English relating mainly to the Sudan. During two long periods in the country the opportunity was taken to abstract information from non-conventional and archival material, particularly from the Sudan Monthly Record (SMR), the Darfur Monthly Diary (DMD) and the Kordofan Monthly Diary (KMD): the first of these was a continuation of the Sudan Intelligence Reports, which were a collation of matter relating to a wide range of subjects relevant to the military conquest and occupation of a country, and which later became a confidential monthly newsletter for the military and civilian administrators; the second and third were the official monthly reports of the Governors of Darfur and Kordofan to the Governor-General in Khartoum, much of which eventually found its way into the SMR. A number of gizu references are also to be found in the 1 : 250,000 topegraphical maps of the Sudan Survey Department, most of which are now about 40 years old and are still based on route traverses, usually made by camel, carried out during the period 1903 to 1915. Most of the information on the gizu has been a by-product of exploration in the general area of occurrence. Most of these visits took place during the third and fourth decades of this century, at a time when the camel was in the process of being succeeded by the motor vehicle as the mode of transport-s-although the latter for long depended on the former to supply remote petrol dumps. Thus we have visits to Northern Darfur by civilian administrators. entomologists, military personnel and patrols, commoners, nobles and royal princes: all with some more or less legitimate reason for being there, whether it be testing the ability of the internal combustion engine to withstand extremes of climate and cleanliness, the mapping of new routes, the study of the physics of the sand, the delineation of international boundaries or just plain geographical exploration in its widest sense (Tilho, 1920; Maydon, 1923; Boulnois, 1924; Newbold, 1924a; Ball, 1927; Shaw, 1928; Bagnold, 1933; Bourbon, 1933; Hutchinson, 1934; Maxwell-Darling, 1934, 1936, 1938; Sandford, 1935; Shaw, 1936). The GeographicalJournal of the RGS and the earlier volumes of Sudan Notes and Records are the richest sources of published data. Recent issues of Sudan Notes and Records occasionally contain trivial references to the gizu (Lebon, 1961; Quezel, 1970). In view of the importance of the gizu to the livestock economy of a number of pastoral tribes, and particularly of the Kababish, socio-anthropological studies appear to have paid far too little attention to it (MacMichael, 1912; Seligman & Seligman, 1918; Asad, 1964, 1970).

The environment of the gizu

Location and land use The general location of the gizu area in northern Sudan is shown in Fig. 1. The figure, which also attempts to show the relative frequency of occurrence in a particular area, has been compiled from literature references. It can be seen that within the Sudan the most favourable gizu area is bounded by 15°30' N to 17°00' N and 25°00' E to 27°00' E, an area about 30,000 kroll • Gizu also occurs fairly commonly to the west of this area, but is much less

THE 'GIZU'

329

Common to the east, where it is also confined to a narrower latitudinal belt. The area of occurrence as shown in the figure is to a large extent coincident with the area of 'gizzu sand' mapped by Worrall (1961). The most southerly occurrence appears to be at 14°30' N in the vicinity of Jabal Meidob (Lampen, 1928); it apparently occurs only very occasionallyto the eastof 29° E, beyond the Wadi el Milk. In the north it rarely occurs beyond 17° N (Newbold, 1924a), whereas it fairly regularly extends beyond the present Sudan-Tchad border west of 24°E.

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Figure 1. The location and tribal use of the gizu, The area is traversed by two ancient north-south caravan routes: one from the Mediterranean via Kufra oasis in Libya; the other by way of the 'Darb el Arba'in', the fabled Forty Days' Road from Assiut in Egypt. Both lead to the former capital of the old sultanate of Darfur at Kobe. There is usually no free water in the gizu area itself, although there are a nUmber of oases along both routes. One of these, at EI 'Atrun, has for ages been a source of salt which is still occasionally extracted. Water can also be obtained from wells in the bed of the Wadi Howar in some areas. The big advantge of the gizu, however, is its ability to maintain domestic animals in the absence of free water. Thus it can be grazed for long periods when other areas are not available. As it is usually the dry season carrying capacity of an area-whether carrying capacity is limited by the availability of either forage or waterWhich limits total livestock numbers, the gizu plays an important role in the economy of several pastoral tribes. The gizu is to the north of the general zone of transition between what is usually considered to be semi-desert and true desert, as for instance it is delineated by Harrison & J.ackson (1956). Some sporadic use may be possible outside the main gizu along old drainage lines such as those of the Wadi Magrur and the Wadi el Milk. Lebon (1965) classifies the

R. T. WILSON

330

area as ' "6a"-9 Used Sparse Semi-Desert Grazing, alternating with Desert' and supports this classification with data, reproduced here as Table 1, from the 1 : 250,000 topographiC maps. Table 1. Data from Sudan topographic maps relating to gizu area

Map Sheet No.

Edition

Co-ordinates N E 0

Remarks on map"

0

44M 44B 44N

16 07 1939 16 10 1939 16 10

25 10 26 08 26 43

44N 44N 44N

1933 16 17 1939 16 21 1939 16 40

26 49 26 26 25 40

44N

16 50

26 14

44N

16 50

26 03

Flat open desert with patches of thorn trees 'Handal' and 'Haskanit' Many gazelle and bustard in Wadi Magrur: many acacias Rolling grassy plain, no trees Fairly hard sand. 'Tumam' grass Undulating featureless desert of reddish sand; very poor gizzu grazing Rolling qozes, with no trees except very occasional 'tundub'; all gizu grazing after rainfall Depression among qozes with 'seiyal', 'heglig' and 'tundub'

• The quotation marks enclosing the vernacular names of plants are the author's: for botanical nomenclature see Table 4.

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Physicalfeatures

The nature of the soils is known in general terms only. Worrall (1961) assigns the area to his category 2, which he describes as stabilized sand. Category 2a is 'Qoz', dunes fixed by vegetation; these cover large parts of Darfur and Kordofan. To his category 2b he gives the specific epithet for 'Gizzu Sand'; his description, which appears to be based primarily on that of Newbold (1924a), classifiesthe soil as lithesolie, slightly stabilized, soft reddish sand with good water-holding capacity and with a probably discontinuous distribution. His map, which also has thin, lithosolic, true desert soils to the south of the gizu, shows outcrops of Nubian Sandstone, from which the soils are mainly derived, and some outcroppings of granite. The soils are described by Barbour (1961) as sandy, overlying the Nubian series, and of Mesozoic origin. Shaw, who travelled in the area on several occasions and (being Government Forester) was perhaps best qualified to comment on plant/soil relations, found-in January-sufficient moisture at 15in (380 mm) depth to ball the sand momentarily in his hand. He estimated that he was 300-400 ft (90-120 m) above the static water table and suggested that the loose sandy surface played an important part in preventing evaporation and so conserving moisture (Shaw & Hutchison, 1934). On apparently similar soils in central Arabia it has been postu· lated that heavy rainstorms, in which moisture penetrates more than 100 mm, are able to achieve a 'sealing effect' thus making water available to plants over a long period of time: conversely light rainfall tends to dry out the surface (Vesey-FitzGerald, 1957). In Arabia, however, it was noted that growth is virtually confined to the inter-dune hollows, which was attributed to seepage from the slopes and crests.

Geomorphology The gizu area is generally at a medium altitude, varying between 500 and 700 m. The topography is gently undulating to rolling, with numerous areas of dunes and occasional isolated hills. These latter are usually of the underlying Nubian series but are sometimes, and

THE 'GIZU'

331

Particularly in the south, of granite. The most conspicuous of these hill features are Jabal Meidob (15°13' N, 26°30' E) at the southern limit of the gizu, the Teiga Plateau centred on 16° N, 26° E and Jabal Tageru, a long ridge running north-south (16° N-17° N) on the Darfur-Kordofan boundary at about 27° E. Surface drainage is scanty and intermittent. The four major wadis from west to east are the lIowar, Magrur, Urn 'Afarit and el Milk. These all discharge northwards towards the Nile ~though surface flow, with the exception of the el Milk in its southern reaches, is an exceptional event. Considerable underground movement of water probably takes place along these drainage lines and occasionally pools in the bed last for a few weeks. The wadis are marked by their relatively abundant and varied vegetation. Micro-relief and topography play an important part in the ability of plants to establish themselves and survive in desert environments, facts which have been amply demonstrated for the Sudan by Kassas (1956) in the Omdurman area.

Climate Few measurements of climatic parameters have been taken within the gizu area, and those Which are available relate to short periods only and are spatially and temporally widely separated. With the exception of Kutum (14°21' N, 24°40' E) to the south of our area and at a slightly higher elevation, the only comparable stations are along the Nile, 800 km to the east. These include Khartoum (15°36' N, 32°33' E), Atbara (17°42' N, 33°58' E), and possibly Karima (18°33' N, 31°51' E). All of these maintain a full range of meteorological records. :Mellitt (14° 08' N, 25°33' E), Sodiri (14°25' N, 29°05' E) and Malha Wells (15°08' N, 26°12' E) have shorter term, often interrupted and probably to some extent unreliable records of rainfall only.

General climate Synthetic thermal and aridity indices constructed by a variety of methods, using data for Khartoum, give extreme values. For example, the indices of Thornthwaite (1948) give an annual moisture index (1m) of -54 and a total PE ratio of 189. The monthly index of aridity (PIT + 10) is zero in five months of the year and is 0,02, 0,09, 0,20, 1,24, 1,84, 0·42 and 0·09 f?r the months of April to October. So in Khartoum only July and August cannot be conSIdereddry months. The amount of cloud cover is generally low which, coupled with the very high daytime temperatures, leads to high rates of evaporation and high albedo values. :Radiation at Khartoum averages 547·1 cal/cm 2 over the year, being highest in April at 627·3 and lowest in December at 469·5. In the dry winter season generally strong winds also affect the rate of evaporation. By extrapolating from Hurst & Philips (1931), some climatic parameters for the area to the south of the Wadi Howar were deduced by Sandford (1935) and are reproduced in Table 2.

Table 2. Some deduced climatic parameters for the gizu area Period Parameter Mean temperature eC) MeanRH(%) Rainfall (mm)

July-August

January

34 31

18 23

50-100

Nil

Mter Sandford, 1935.

1?ainjall lsohyetal maps prepared by the Sudan Meteorological Department for the period 1931-60 show rainfall varying from 200 mm in the south of the area to less than 75 mm in the north.

332

R. T. WILSON

Quezel (1970rsays that rainfall 'only' amounts to 350 mm near the Wadi Howar, which is almost certainly an error and should probably read 35·0 rnm. Rainfall at Sodiri averaged 202 mm over a 26-year period; at Mellit, during the 16 years for which records are complete, the average was 242 mm; and at Malha Wells the average for 16 years was 157 mm. In common with other areas of low rainfall, temporal and spatial variability are very high and the variability can be expected to increase northwards. At Malha Wells rainfall varied between 53 mm and 326 mm, with a standard deviation equal to more than 100 per cent of the mean: in the year of highest rainfall two storms, respectively of 150 mm and 60 mill, three days apart, accounted for more than 65 per cent of total rain in that year. At all stations about 90 per cent of annual rainfall occurs in a six-week period during July and August. The average numbers of days on which precipitation at Khartoum (167 mm) exceeded 1·0 and 10·0 mm were 18·3 and 5,1, and at Atbara (69 mm) were 8·5 and 2·0. It is probable that some dew forms on occasions, but none was recorded by Newbold (1924a).

Temperature A noon shade temperature of 42·4°C (108,0 OF) was recorded by Newbold (1924a), on 18 September, which is comparable with Khartoum temperatures for that month. In January 1928 maximum temperatures varied between 21·7 and 36·1 °C, while minima varied between -1,7 and 22·2 °C, with diurnal variations from 10 to 27° C; temperatures were apparently higher when the wind was not in the northeast (Newbold & Shaw, 1928). Mean daily temperatures at Khartoum (average of maximum and minimum) are 29.75 °C for the year, being 34·35 "C in June and 23·85 °c in January.

Wind Newbold (1924a) during September and October recorded 23 out of 26 days as 'blowing gale', all within 15° of north, and quoted figures for Kharga oasis in 1907 of 337 days windy and 29 days not windy (sic.). Of the 337 windy days 281 were northerly and on 103 dayS there were either sandstorms or the wind was visibly charged with sand; this sand, as we shall see, considerably affects the physical composition of the gizu vegetation. At Khartoum the wind is northerly for all months except June to September inclusive and, during the period the wind is in the north, has a mean scalar speed of about 4·2 m/s; at Atbara, it is northerly in all months except July and August, with a mean speed of 2·9 m/s; at Karima it is northerly for more than 11 months and has a mean scalar speed of 3·7 mfs.

Other climatic parameters The relative humidity at the Wadi Howar towards the end of October 1932 was between 60 and 70 per cent at 6.30 a.m, local time and between 30 and 40 per cent at 2.00 p.m. (Bagnold, 1933). These values are rather higher than those for Khartoum in October, t~e values there being 37 and 21 per cent. Values comparable to Bagnold's are achieved 1n Khartoum during July and August and are not achieved at all at Atbara and Karima. The Wadi Howar values may possibly be higher than the general area of gizu, particularly if they were taken in the wadi bed. The bright sunshine duration at all the riverine stations averages about 85 per cent of total possible sunshine hours, equivalent to about 10·5 h per day throughout the year. Piche evaporation is equivalent to 6241, 6935 and 7701 mm per year for Khartoulll, Atbara and Karima,

Calender of occurrence Although most casual references to the gizu give the impression that it is an annual, or even continuous, occurrence, this is far from being the case. Nor-an even more unlikely eventualf ity-does it occur every other year (with camels achieving 100 per cent calving in the years 0

THE 'GIZU'

333

OCCurrence and only 50 per cent in other years I) as has also been stated (Harrison & Jackson, 1956). In view of the vast area over which the gizu is found it is more than likely that some growth does occur somewhere in every year. It appears from a collation of the available eVidence, however-even though this is sometimes contradictory-that over a long-term period a significant contribution to livestock production is made in roughly two years out of five. Writing approximately 40 years after the event, and making the first firm chronological r~ference to a gizu occurrence, a one-time Local Government Inspector (District CommisSiOner) for Dar Kababish remarked on a large herd of camels on 11 May 1917 'just back on its Wells and having its first drink since the rainpools dried up' (Davies, 1957). There is afterWards a fairly continuous record of the gizu from 1920 to 1956, both from published sources and from government material, the available records being summarized in Table 3. Some anomalies are apparent, even where the primary source is probably the same. No information on OCcurrenceappears to be available for the immediate post-independence period from 1956 to 1964. A gizu occurred in 1964-5 and was seen and reported on by a Sudanese government officer (Lamprey, 1975). Informants were adamant that there were no gizu years from 1965 ~o 1975, a period concurrent with low rainfall throughout the Sahel zone and in particular In much of Darfur and Kordofan.

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Year

Table 3. A gizu calendar for 1920-77 Remarks

20/21 Exceptional year 21/22 Exceptional year 22/23 At 16°30' N, 26°45' E, 90 miles (145 km) WSW of Umm Gereinat with ostriches everywhere Did not see this but learnt from Sheikh el Tom that extended much farther north than usual, to el Jihaf north of lat. 17° N Saw much green grass east of Ennedi 23/24 Entered gizu at Wadi Urn 'Afarit at 15°30' N exactly on 27th meridian. Finished by north end of Jebel Tageru at 17°01' N 27/28 Nogizu 28/29 Probably none. This source states that 36/37 was first for seven or eight years, but other sources do not confirm this 29/30 Probably none. See 28/29 30/31 Probably none. See 28/29 31/32 Probably none. See 28/29 32/33 Gizu grazing at 16°30' N and between 24° and 25° E Very green in January 1933 in Hawawir country east of Wadi el Milk. Many patches of green gizu at 17°30'N, 24°15' E (17°30' possibly a misprint for 16°30' N) Less gizu farther east at 25°45' E 34/35 Very lush in September north of Teiga with many plants just springing up Gizu failed: Shaw travelled later than MaxwellDarling (in February/March 1935) but still perhaps a rather puzzling observation as he remarked on the very northerly rains at Jebel Uweinat (21°55' N) in June 1934 35/36 Nogizu 36/37 Exceptionally good with large numbers of Kawahla herds accompanying Kababish for the first time. The latter were expecting to stay until the rains

Source Newbold (1924a) Newbold (1924a) Maydon (1923) Newbold (1924a) Hassanein (1924) Newbold (1924a) Newbold & Shaw (1928) SMR NS 96, Jan. 1937 SMR NS 96, Jan. 1937 SMR NS 96, Jan. 1937 SMR NS 96, Jan. 1937 Bagnold, 1933 Maxwell-Darling (1934, 1938)

Shaw & Hutchinson (1934) Maxwell-Darling (1936) Shaw (1936)

SMR NS 83, Dec. 1935 SMR NS 96, Jan. 1937

334

R. T. WILSON

Table 3 (cont.) Year

Remarks

37/38 Not very good with only odd patches in the west 39/40 Fair grazing on Wadi Howar around Zarat, 16°30' N, 24°45' E. Kababish, Meidob Zaghawa and Zeyadia grazing together with Gura'an to north 'The gizuhas failed and the Kababish herds have returned' 40/41 Probably none: Sheikh el Tom at Hamrat es Sheikh where grazing very poor 42/43 Very good in northern areas. Where already grazed over a second crop was appearing in November 44/45 Very good on French frontier 45/46 Heaviest Iizzu (sic.) invasion of nomads that has ever occurred 46/47 Kababish reported a shooting addax in north Good year and probably widespread. Kababish returned with large herds in Marchand in May reported on an incident involving theft by the Gura'an while they were grazing over the border in French territory 47/48 Total failure of gizu this year 48/49 Failed for second year running. Kababish had returned to Umm Qozein by beginning of October Apparently none as most camels to east of Urn Qozein Gizu lacking for two years 49/50 Prospects of gizu disappearing rapidly as no late rains fallen Lack of gizu confirmed: as many Kababish in Western Kordofan (i.e, the central western area) as normally in April Implies no gizu: Rizayqat area (south Darfur) full of camels driven westward by lack of grazing No gizu 50/51 Gizu grazing best for 15 years and most camel herds expected to remain in northern desert until early rains Confirms widespread gizu-Kababish arrested in French territory 51/52 There will be no gizu this year 52/53 At least six months gizu expected in the north 53/54 Latest reports indicate good gizu grazing has sprung up 55/56 Prospects of gizu disappearing Probably none as northern Kordofan tribes given permission to enter eastern Darfur early Confirms none probability as Umm Qozein pool dry with Kababish· at Hamrat es Sheikh and KawahIa atUmmBadr 64/65 Reported by Sudanese government official 65/66 None to 74/75 75/76 Fair gizu 76/77 None 77/78 Good gizu

Source SMR NS 105, Feb. 1938 SMRNS 128, Jan. 1940

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KMD January, 1940 KMD February, 1941 SMRNS 164, Jan. 1943 DMD December, 1974 DMD March, 1946 DMD December, 1946 KMD March, 1947 KMD May, 1947 KMD January, 1948 SMR NS 215, Nov. 1948 KMD January, 1949 KMD March, 1949 KMD September, 1949 KMD November, 1949 DMD January, 1950 Game Pres. Br. 1951 KMD January, 1951 KMD March, 1951 DMD September, 1951 DMD November, 1952 KMD December, 1953 KMD September, 1955 SMR NS 300, Dec. 1955 SMR NS 301, Jan. 1956 Lamprey (1975) Personal information Personal knowledge Personal knowledge Personal knowledge

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THE 'GIZU'

335

Floral composition and ecology

A Summary of the floral composition of the gizu, as given by various authors, is shown in

~able 4. The various lists have only a few plants in common, this being hardly surprising in YIew of the different areas and the different years in which the records were collected. The botanical nomenclature is confused, but attempts are made to standardize this in Table 4. The list of Harrison & Jackson (1956) is probably largely culled from the earlier records of NeWbold (1924a), Shaw and Hutchinson (1934) and Maxwell-Darling (1934, 1936). Other authors (e.g. Lebon, 1965) quote largely from the earlier sources and several refer simply to the 'prized gizu grazing' or some other such term of approbation without attempting to give any composition data (Seligman & Seligman, 1918; Maydon, 1923; Sandford, 1935; Barbour, 1961; Asad, 1964; Bari, 1968). A comprehensive list of the commonest herbs and grasses of the 1975-6 gizu has been provided for the only recent collection of flora (Payne, pers.

comm.).

The lists in Table 4 contain only perennial and ephemeral species of the gizu proper. There are numerous associated shrubs and small trees. The commonest of these are: Acacia ehrenbergiana, Capparis decidua, Acacia mellifera, Saloadora persica, Commiphora africana, Cordia rothii and, in the southern areas and in Kordofan, Leptadenia pyrotechnica. Other ,,:oody species occur in especially favourable sites including Tamarix sp., Balanties aegyptraca, Tamarindus indica and Salix subserrata among others • .The occurrence of the gizu is sporadic and said to be associated with combinations of clunatic factors, the most important of these being the occurrence of late rains followed by colder nights as a result of the northerly wind. However, the actual conditions for the ~Illergence of the gizu must remain a matter for some speculation. Much of the published ~ormation is gleaned from the hearsay evidence of the pastoralists who make use of the gtZU-knowledge which is based on generations of intimate contact with the area and which is probably broadly correct. Thus, Newbold (1924a) follows the late rains/cold nights theory; lIarrison & Jackson (1956) say that it occurs every other year, appearing three months after ~he rains-which would put its emergence in late November or December-and is better on Qoz'. The physiognomy of the gizu presents a low profile, occasionally 300-600 mm high, with COnsiderable areas of bare ground even in the best years. It has been described as '••• grasses mostly low lying •.. and well spread so that the actual gizzu appears to be not a Prairieof waving grass, but very nearly bare desert, and it is only on closer inspection that the network of stalks and blades of tiny flowers is seen sprawling over the surface' (Newbold, 1924a). and it can be seen from

,

.•. many miles away a darker patch ... which is an area of gizzu vegetation. Although some of the procumbent plants straggle over a fairly large area of sand, the vegetation is not dense, and there is bare sand between the plants' (Maxwell-Darling, 1936).

In Northern Kordofan the gizu was described as growing in patches, with each plant being from its neighbour by two to four yards of bare sand (Maxwell-Darling, 1934); a \'isual impression of the Kordofan gizu of 1932-3 is given in Fig. 2, the actual ground cover being estimated from this at about 20 per cent. None of the species recorded for the gizu is peculiar to it alone. Many have wide geographical and ecological distributions. Studies on the vegetation in the same latitude within the Sudan as the gizu have been carried out by Obeid and Mahmoud (1969), and detailed studies in the vicinity of Omdurman by Kassas (1956) give a useful insight into the reasons for the apparent lack of uniformity in the lists of gizu vegetation, even when these are provided by one person (e.g, Maxwell-Darling in 1934and 1936). Other vegetational studies of relevance to the gizu have been undertaken in the Sudan by Baumer (1964) and Quezel ~arated

Gisekia phan\Q.coides 'L.

Mo\1ugl.naceae

Zygophyllaceae Fagonia indica Burro. f. (syn. F. bruguieri DC., F. cretica L.) Cruciferae Farsetia aegyptiaca Turra Farsetia longisiliqua Decne Farsetia ramosissima Hochst. Morettiaphilaeana (Del.) DC. Euphorbiaceae Euphorbia aegyptiaca Boiss. Compositae Geiger~a alata (DC.) Oliv. & Hiern Pulicatia undulata (L.) C. A. Meg.

Amaranthaceae Aervajavanica (Burro. f.) Juss. ex Schult. Amaranthus graecizans L. Acanthaceae Blepharis persica (Burro. f.) Kuntze (syn. B. edulis (Forsk.) Pers.) Cucurbitaceae Colocynthis vulgaris Schrad. .(syn. Citrullus colocynthus (L.) Schrad. Chenopodiaceae Comacula monocantha Del. Salsola barysoma (Schult.) Dandy (syn. S. vermiculata L.) LeguminosaejPapilionoideae Crotalaria thebaica (Del.) DC Crotalaria ? saltiana lndigofera ? hochstetteri Bak.t lndigofera argentea Burro. f. lndigofera bracteolata DC.

Nomenclature and synonymy

V V V V

hadh natash khoshaina

p*

V

V

V V

V

V V

(1977)

-r

V V

V

V V

V

V

V

H&J (1956)

V

V

M-D (1936), (D)

rihan

V V

V

M-D (1934), (K)

V

V

V V

V

S&H (1934)

ummlaban

tagha

dhayan dhayan

'aqul

V

V

handal

dirmy, argan, ummibeis

V

N (1924a) N&S (1928)

silih

khimat tammalayka

Vernacular names

Table 4. The floral composition of the gizu according to various authors

For footnotes to Table 4 see overleaf

Cistaceae Helianthum lipii Pers. Boraginaceae Heliotropium bacciferum Forsk. (syn. H. undulatum Vahl) Lithospermum callosum Vah1 Neuradaceae Neuradaprocumbens L.t Rubiaceae Oldenlandia capensis L.f. Oldenlandia lancijolia (Schumach.) DC. Zygophyllaceae Tribulus longipetalus V. v. (syn. T. alatus Del.) Tribulus sp. Graminae Andropogon and/or Cymbopogon sp, Aristida mutabilis Trin. & Rupr. Aristida papposa Trin. & Rupr. Aristida sp. Cenchrus biflorus Roxb. (syn. C. barbatus Schum.) Cenchrus catharticus Del. Danthonia forskalei Trin. Desmostachya bipinnata (L.) Stapf (syn. D. cynosuroides (Retz.) Stapf & Massey) Panicum turgidum Forsk, Stipagrostis plumosa (L.) Munro ex T. Anders. (syn, Aristida plumosa L.) Triraphis pumilio R. Br. §, Cyperaceae Cyperus compressus L. Cyperus congolomeratus Rottb. Cyperussp:

Nomenclature and synonymy

V V V V V V

tuman silayan 'ashub

V

V

V

V

V

N(1924a) N&S (1928)

zobat

haskanit

nissa; bayda

gutub

tagtaga

sa'adan

Vernacular names

Table 4. (cone.)

V

V V

V

V

V

S&H (1934)

V

V

(K)

M-D (1934),

V

V

V

V

V

M-D (1936), (D)

V

V

V

H&] (1956)

p.

V

V

V V

(1977)

338

R. T. WILSON

Figure 2. A visual impression of the gizu of 1932/33. (From a photograph, Maxwell-Darling, 1934.)

(1970) and in Egypt by Kassas (1953). Baumer gives the southernmost limits of one of the most important gizu constituents, Indigo/era hochstetteri, as about 1+°30' N. As an example of the temporal variability of the gizu vegetation-which can also be rel~te.d to spatial variability due to the variation of rainfall from place to place in the same year-It IS possible to draw on the work of Kassas (1956) in the Omdurman desert. In August 1954 he recorded a total of 78 species of which SO were ephemerals. Six months later only four ephemerals-Geigeria alata, Achyranthes aspera, Morettia philaeana and Solanum dubium,-remained and after a further five months, at the end of the dry season, there were only nine species of shrubs and perennial grasses remaining. In the next year, in which rainfall was less, only 44 species could be found and 34 of the ephemerals recorded in the wetter yeat failed to appear; in addition to the numbers of species the numbers of individual plantS present and the percentage of ground cover differed considerably. Indigo/era hochstetteri, one of the most frequently recorded gizu constituents, was found only in the wetter year; four other species of Indigofera-i-I, sessiliftora, I. cordifolia, I. colutea (syn. I. viscosa) and l • stenophylla-of which none has been recorded for the gizu, were also found only in t~e wetter year. Other species found in both years were Trianthema pentrandra, Euphorbia aegyptiaca, Cleome scaposa and Gisekia pharnacoides, while those found only in the wet yeat included Amaranthusgraecizans, Cleome viscosa and Cyperus rotundus. In Kassas' study fiV'e species of Aristida and three of Cenchrus were identified. Other species noted which have not • N (1924a): Newbold (1924a): N & S (1928): Newbold & Shaw (1928): S & H (1934): ShaW 8&). Hutchinson (1934); M-D (1934) (K): Maxwell-Darling (1934), (Kordofan); M-D (1936) (D . Maxwell-Darling (1936) (Darfur); H & J (1956): Harrison & Jackson (1956): P (1977): Payne

(1977).

.~

t There is someconfusion regarding the application of I. hochstetteri Bak, Most references areI:; I. arenaria A. Rich. which, in addition to being a synonym of the former, has also been univers a . misapplied to 1. disjuncta Gillett. It is possible that 1. hochstetteri and 1. disjuncta both occur it1 ~~

: Monotypic, formerly Rosaceae. Harrison & Jackson (1956) say that this is confined to north-Wes Darfur. § Aristida sp. in Newbold (1924a).

t

THE 'GIZU'

339

been recorded for the gizu but may be expected to be found there include Farsetia ramosissima, Tephrosia spp., Cucumis spp., Tribulus terrestris, Chrozophora brocchiana, Heliotropium zeylanicum, Melhania denhamii, Euphorbia scordifolia, Polygala irregularis and Indigo/era

SUaveolens.

One Saharo-Sindian species recorded for the Darfur gizu, Neuradaprocumbens, occurs also on the Red Sea coast of Egypt (Kassas, 1953) as well as in central Arabia, from where its special ecological adaptation has been described (Vesey-FitzGerald, 1957). The fruit Contains several seeds; when light rain falls only one of these germinates and rapidly sends a fine tap root deep into the sand. If no further rain falls the seedling dies but subsequent showers cause further seedlings to germinate, one at a time, until growth is successful. This arrangement, from the plant's point of view, allows the longest possible growing period in Conditions of low and variable rainfall; it also gives to the ecologist or climatologist an indication of the number of showers which have fallen-shown by the number of dead tap roots?efore the rains become generalized. In Egypt. N. procumbens together with Monsonia nivea IS found most frequently on young dunes (Kassas, 1953). As the dunes age, a basal water storing layer builds up with the upper boundary of the layer gradually rising. Perennials such as Panicum turgidum, which need deeper sand, establish themselves once the soil moisture can attain a continuity for the duration of the dry season. According to Smith (1949), only about two-thirds ofthe rainfall is required to maintain a species on sandy soils as is required on clay soils. The importance of mound-building species of the genera Panicum, Fagonia and Zygophyl!um cannot be overemphasized in gizu-type situations. One disadvantage of P. turgidum is that in areas where it occurs it might provide a site SUitable for the breeding of the locust Schistocerca gregaria (Maxwell-Darling, 1934). The ability to fix sand, or to trap it in the aerial parts of the vegetation, does affect the physical composition of that vegetation, as shown in Table 5, and probably also its feeding Value and digestibility. The data in Table 5 and the chemical analyses given in Table 6 are of the three numerically most important species on the 1975 gizu. The levels of both calcium and magnesium are high (unfortunately no analyses of phosphorus levels were undertaken) but, along with the high ash levels, may reflect some residual contamination with sand. The tnetabolizable energy values are comparable with those of moderate to poor quality grass. As

-

Table 5. Fresh weight, weight 0/ sand, and air dry weight 0/ three species 0/ gizu plants Weight (g) Fresh material

Plant species

-

Y;j;j{go/era hochstetteri Indigo/era bracteolata 'I'rianthema pentandra

Plant species

Indigo/era hochstetteri Indigo/era

21 ~racteolata

--

"anthema Pentandra

~3

Corrected fresh weight

Sand

89·27 144·57 60·80

24·23 12·54

113'5 157'1 60·8

Air dry material

Air dry \ matter as % of fresh weight

30·62 53·94 12·44

26·97 34·33 20·46

Table 6. Chemical analysis 0/ three species 0/ gizu plants Dry matter

.

Percentage by weight Fibre

Ash

Ca

Mg

Metabolizable energy

(%)

Oil

Crude protein

95·1

1·28

16'00

30·4

18·1

2'96

0'41

8'63

95·2

0·76

14·25

35·5

26·1

3·38

0·48

7·00

95·3

2·10

15·30

34·3

20·5

2·96

0·97

8·10

340

R. T. WILSON

far as I am aware, these are the only objective data available on the value of the gizu as livestock fodder. There are no data on digestibility or palatability but almost 60 years ago at Faya oasis-in Tchad, at about 18°N and 19°E-it was observed that one of the most characteristic and least bad plants of the area, Fagonia cretica, was 'a little bush of sharp thorns which the camels would eat but not without making a funny grimace at every mouthful' (Tilho, 1920).

Use and socio-economic value In arid zones limits on carrying capacity are almost invariably a result of dry season problems due to a lack either of water or of fodder, or to a combination of both these factors. AnY amelioration of these dry season constraints therefore enables greater numbers of livesto~ to be maintained, ensuring a better livelihood for stock owners and providing a larger margtn of security for eventualities such as prolonged drought or severe outbreaks of disease. -r:he gizu is probably unique in that it provides not only fodder but also enables the animals which make the most use of it-eamels-to survive without access to free water, on occasions for several months. Of course, because the gizu occurs in the cold winter season the physiological stress is not so great. The cooler winter must also be a consideration to the herders. Very soon after the reconquest of the Sudan the value of the gizu was recognized by the Anglo-Egyptian administrators-pastoral tribes whose livestock thrive and reproduce are lesS likely to create trouble-and one of the first references to the gizu, although not by t~at name, appears in an intelligence handbook (Intelligence Department, 1912). In view of 1tS importance in the total pastoral economy it is rather surprising that socio-anthropological studies of the tribes making use of the gizu, and particularly of the Kababish, have placed little emphasis on it. MacMichael (1912) contents himself by saying that in the winter months much of the tribe moves northwestwards to the desert grazing and the SeligmanS (1918) dismiss it in four lines: 'During the early part of the dry season the camel herds are driven to the waterless wastes far away in the northwest where the coarse gizza grass remains green until the end of FebruarY' The camels stay there for two or three months, roughly from November to February, during which time they are entirely without water.' In a later and very detailed study of the organization of the Kababish, the dry season limitations on the numbers of stock that can be carried are realized (Asad, 1964, 1970). The organization of the herds into 'main' and 'household' groups is described with the greater mobility of the former, unencumbered by family and chattels, enabling them to move much greater distances in search of grazing. The annual cycle is summarized, reproduced here as Table 7, but even so there is apparently no real appreciation of the value of the gizu. While the Kababish are numerically the most important users of the gizu, many other tribes or sections of tribes also make use of it, either regularly or intermittently. In Fig. 1 an attempt is made to show relative use by the principal tribes, in addition to the distances travelled by them. As well as the Kababish, the other tribes travelling regularly to the gizU are the Hawawir, Hasaniya, Zeiyadia, Kawahla, Maganin, Ma'aliya and Ma'agla (Lebon, 1961), while the Shenablaand Dar Hamid are probably less frequent users (Lebon, 1965). The Zaghawa, more or less equal numerically in both the Sudan and Tchad, and the Gura'an and Bedayat, mainly resident on the Tchad side, are also regular users (Barbour, 1961) as ar~ the Meidob (Lampen, 1928). A small camel-owning section of the Rizayqat, an offshoo: 0 the principally cattle-owning tribe of Southern Darfur, also use the gizu and were pres~nt there in 1975 (Lamprey, 1975). Minor sections of the Kawahla from as far east as the Wh1te Nile also occasionally travel to the gizu (Reid, 1930). Most references in government records refer to the Kababish, probably reflecting, in addition to the importance of the actual grazin to the tribe, the concern that the government had that this largest and most powerful 0 Sudanese pastoral groups should have sufficient fodder for its livestock.

1

THE 'GIZU'

341

Table 7. The annual migration cycle of the main 'herds of theKababish tribe

Season:

Climate:

Period:

Sayf

Hot/dry

Feb.-Jun.

Kharif

Cool/wet

Jul.-Sep.

Darad

Warm/dry

Oct.

Shita

Cold/dry

Nove-Ian.

Movements: Return from NW in early summer to dry season wellcentres; move to S or SW in late summer to exploit early pastures in central Kordofan and Darfur Rapid move N. to exploit pasture in Dar Kababish Separation from households and move NW slowly Move farther NW to exploit winter grazing in desert

While it is mainly camels that make use of the gizu (and the Kababish occasionally trek their beasts 600 km in each direction for it), other species of domestic livestock, as well as Wildlife, benefit from it. In the very good gizu conditions of 1921-2 goats as well as sheep accompanied the camels (Maydon, 1923); Sheep travel fairly regularly to the gizu although Sometimes for shorter periods than the camels and probably over only the shorter distances. Borses have been said never to be taken and donkeys only to areas where the melon, Colotynthis vulgaris, grows (MacMichael, 1912). In 1932 there were 'enormous' herds of antelope on the gizu (Sandford, 1935), but the value of the grazing to domestic stock detracts from its value as a resource for wildlife and in late 1942, for example, 'the Addax and the Oryx •.• moved further north to avoid the unwelcome incursion of man' and were 'said to be so fat as to be easily run down on camel back' (SMR New Series 164, December 1942/ January 1943). During September and early October of 1923 about 80 per cent (20,000 out of 26,000) of the mature camels of the Kababish were estimated to be north of the Wadi el Milk, that is in the general area of the gizu (Newbold, 1924a). This estimate of the number of mature camels owned by the Kababish may have been very much on the low side-although an enumeration of them had been carried out during 1920 and 1921 after an earlier attempt had had to be abandoned because of the effect on manpower of the First World War (Davies, 1957). By 1964 the Kababish had 34,690 camels listed for taxation (i.e. mature camels), although Asad (1970) estimated that their true holdings were at least fivetimes this numberequivalent probably to more than 300,000 head in all, including young animals. A recent estimate based on low-level aerial surveys gives a figure of more than 800,000 camels for the Whole of Northern Kordofan (Watson et al., 1977). In addition to the ecologicalstresses, one can appreciate the sociological problems which must arise when this number of animals has to be maintained through dry seasons following good years and bad. Metaphorically, as many entries in official sources demonstrate, the authorities heaved sighs of relief in the years in which a good gizu occurred. In years in which there was none there would be intense political and tribal diplomacy as attempts were made to secure alternate sources of grazing (see Table 3 for some examples). Usually, within the Sudan, peace reigned in gizu years with the otherwise often fractious tribes grazing their animals together in relative harmony. For example, in 1936 a large number of Kawahla herds accompanied the Kababish, reportedly for the first time (SMR New Series 96, November 1936/January 1937) and in 1939 there was 'room for everybody, which is only fitting in a grazing area whose root meaning is contentment and the Kababish, Meidob, Zaghawa and Zeyadia are grazing together, with Goran to the north' (SMR New Series 128, December 1939JJanuary 1940).

342

R. T. WILSON

The good years were not always free of trouble, however, and particularly when international as opposed to national or tribal politics were involved. In early 1951 eight Kababish were arrested in Tchad (KMD, March 1951) and later that year a meeting was held to discuss the 'problem' of frontier crossing, at which it was decided that laissez-passers would be issued to the heads of camps and that police would patrol with the herds to reduce the number of thefts (KMD, November 1951). Later a 'dia' (blood money) of £E100 was paid to the Gura'an by the Kababish (KMD, October 1952). Although the gizu is welcomed by the owners on account of its value to their stock and because the area is apparently virtually free from disease, it does pose problems for the herdsmen. While they can (and do) go without water for long periods they do not disdain it if it is available (Newbold, 1924a), and for this reason they stay in the vicinity of Jabal Tageru or the Teiga plateau while the water lasts there. But always, by December, they have to move north if they wish to make best use of the grazing. At this time the nights are very cold and the herdsmen suffer from rheumatism, bronchitis and pneumonia. But they put up with these personal hardships for the sake of their stock and their philosophy is admirably summarized in a quote to Asad (1964) by a Kabashi shaykh: 'A camel has no real price. The buying and selling of camels merely represents a rough equivalent of their value, and it goeson because people need things, not because it represents the true value of camels. One can drink camel's milk, use its hair, make it carry a load-and even eat it. And with the blessing of God it multiplies under your hands.'

Discussion and conclusions Little is known, and even less is understood, of the gizu, A number of the recorded species have normal requirements of 100-200 mm of rainfall, which would appear to be rather more than could be expected over much of the gizu area. There are, however, other parts of Africa at similar latitudes which do receive this amount of precipitation, notably Air in Niger, Adrar des Iforhas in Mali and parts of Mauretania, so it is not inconceivable that rainfall in some parts of Northern Darfur and Northern Kordofan could be higher than purely latitudinal parameters might lead one to believe. In addition to the total amount of rainfall, there is also the question of its seasonality. There appear to be two distinct elements in the recorded flora: the one usually associated with the winter rainfall of the northern and central Sahara, including Aristidaplumosa, Colocynthis vulgaris, Cornacula monocantha, Danthonia forskalei, the three species of Farsetia, Helianthum lipii, Lithospermum callosum, Neurada procumbens, Pulicaria undulata and Salsola vermiculata; the other associated with the summer rainfall of the southern Sahara/Sahel including Aerva javenica, Aristida mutabilis, A. papposa, Blepharis edulis, Cenchrus spp., Desmostachya cynosuroides, Geigeria alata, Gisekia pharnacoides, Indigo/era and Oldenlandia species, Trianthem a pentandra and Tribulus alatus. Fagonia cretica, which appears in two of the earlier lists and then in that of Harrison and Jackson (1956), is also usually considered to be a Mediterranean species: the absence of Schouwia thebaica, which is the commonest species in the gizu of Niger and parts of the Hoggar is also surprising, especially since it also occurs in eastern Sudan. It has been suggested (le Houerou, pers. cornm.) that these two groups of species occur in the same area as a result of two different rainfall regimes-the summer rains of the northern limit of the Inter-tropical Convergence Zone, and some winter rains occasionally occurring as far south as 17° N to 16° N as a result of disturbances linked with southern incursions of the polar front. Much of this review has been written as a narrative, as though the gizu was a current and continuing phenomenon. In spite of the facts of the general southerly shifts of rainfall isohyets, the advance of 'desertization' and the lack of gizu for a period of about ten years in the late sixties and early seventies, there is no real reason for supposing that the gizu will not continue to occur. The 1975gizu was small in area and at the southern end of the usual limits, but this is not the most southerly recorded occurrence, and its appearance there may be

THE 'GIZU'

343

unconnected with the southern isohyetal shift, as is implied by Lamprey (1975). The 1977 gizu was again a very substantial one and further to the north, although it has to be said that rains in general in Darfur were better than for some years. In view of the ecological interest of the gizu, as well as its socio-economic value, it is obviously worthy of further study. This could be expected to extend over a number of years (in particular in view of the irregular occurrence of the gizu). Such a study might best fall within the framework of the Desert Encroachment Control and Rehabilitation Project at present being mounted jointly by the Sudan Government and the United Nations Environmental Programme. I am most grateful to Dr H. N.le Houerou for reading and commenting. on the draft manuscript and for providing much of the information on which the final section of this paper is based; to Dr W. J. A. Payne for allowing me to use his unpublished data on the 1975 gizu and particularly for that incorporated in Tables 5 and 6; and to Dr G. E. Wickens for his constructive criticism and help with the nomenclature. My thanks also to Anne Phillips, who drew the figures. The literature survey on which this paper is based was carried out during the course of my dUties as a Livestock Production/Range Management specialist with Hunting Technical Services, Ltd and while that company was under contract to the Ministry of Overseas Development and the United Nations Development Programme.

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R. T. WILSON

Kassas, M. (1953). Landform and plant cover in the Egyptian desert. Bulletin de la Sociltl de Geographied' Egypte, 26: 193-206. Kassas, M. (1956). Landform and plant cover in the Omdurman Desert, Sudan. Bulletin de la Societe de Geographied'Egypte, 29: 43-58. Lampen, E. (1928). A short account of the Meidob, Sudan Notes and Records, 11: 55-67. Lamprey, H. F. (1975). Report on the desert encroachment reconnaissance in Northern Sudan. United Nations Environmental Programme, Nairobi (mimeo). 14 pp. Lebon, J. H. G. (1961). Some concepts of modern geography applied to the Sudan. Sudan Notes and Records, 42: 3-28. Lebon, J. H. G. (1965). Land Use in Sudan. The World Land Use Survey, Monograph No.4, Bude: Geographical Publ. xiii + 191 pp, MacMichael, H. A. (1912), The Tribes of Northern and Central Kordofan. Cambridge: Cambridge University Press: xv + 259 pp. Maxwell-Darling, R. C. (1934). The solitary phase of Schistocerca gregaria, Forsk., in Northeastern Kordofan (Anglo-Egyptian Sudan). Bulletin of Entomological Research, 25: 63-83. Maxwell-Darling, R. C. (1936). A short reconnaissance of Northern Darfur (Anglo-Egyptian Sudan) with respect to Schistocerca gregaria, Forsk. Bulletin of Entomological Research, 27: 71-76. Maxwell-Darling, R. C. (1938) Notes on the food of camels on the Red Sea coast and in Northeastern Kordofan. Sudan Notes and Records, 21: 189-195. Maydon, H. C. (1923). North Kordofan to South Dongola. GeographicalJournal, 61: 34-41. Newbold, D. (1924a) A desert Odyssey of a thousand miles. Sudan Notes and Records, 7: 43-92. Newbold, D. (1924b) A note on the 'gizzu' or jizzu. Sudan Notes and Records, 7: 129-130. Newbold, D. & Shaw, W. B. K. (1928). An exploration in the South Libyan desert. Sudan Notes and Records, 11: 103-194. Obeid, M. and Mahmoud, A. (1969). The vegetation of Khartoum Province. Sudan Notes and Records, 50: 134-159. Philby, H. St. S. (1922) The Heart of Arabia: A Record of Travel and Exploration. London: Constable. 2 Vols, xxiii + 386; vii+354 pp. Quezel, P. (1970) A preliminary description of the vegetation in the Sahel region of North Darfur. Sudan Notes and Records, 51: 119-125. Reid, J. A. (1930). Some notes on the tribes of the White Nile Province. Sudan Notes and Records, 13: 149-208. Sandford, K. S. (1935). Sources of water in the North-western Sudan. GeographicalJournal, 85: 412431. Seligman, C. G. & Seligman, Brenda Z. (1918). The Kababish, a Sudan Arab tribe. Harvard African Studies (Varia Africana), 2: 105-184. Shaw, W. B. K. (1936). An expedition in the South Libyan desert. GeographicalJournal, 87: 193-221. Shaw, W. B. K. & Hutchinson, J. (1934). The flora of the Libyan desert. Bulletin of Miscellaneous Information, Kew 281-289. Smith, J. (1949). Distribution of Tree Species in the Sudan in Relation to Rainfall and Soil Texture. Bull. No.4. Khartoum: Ministry of Agriculture. 68 pp. Thornthwiate, C. W. (1948). An approach towards a rational classification of climate. Geographical Review, 38: 55-94. Tilho, J. (1920). The exploration of Tibesti, Erdi, Borkou and Ennedi in 1912-1917. GeographicalJournal, 56: 81-99, 241-267. Vesey-FitzGerald, D. F. (1957) The vegetation of eastern and central Arabia. Journal of Ecology, 45: 779-798. Watson, R. M., Tippett, C. I., Razk, F., Jolly, F., Birkett, J. J., Scholes, V. & Casbon, F. (1977). Sudan National Livestock Census and Resource Inventory. Range Management and Research, Ltd, Nairobi (mimeo). Worrall, G. (1961). A brief account of the soils of the Sudan. African Soils 6: 53-81.