Spatial patterns and frequency distribution of Late Quaternary water budget tendencies in Africa

CATENA

vol. 16, p. 163-188

Cremlingen 1989 ]

SPATIAL PATTERNS A N D F R E Q U E N C Y DISTRIBUTION OF LATE QUATERNARY WATER B U D G E T TENDENCIES IN AFRICA T.

Littmann, B o c h u m

Summary

to humid innertropical Africa wheras the humidification gradient shows a reverse Regional water budget tendencies re- structure. Humid tropical Africa is less ported by 14C-dated field data are corre- susceptible to climatic change than semilated with Late Quaternary cooling and arid regions and the arid zone is least warming phases. Areal frequency dis- affected. In those regions being susceptitribution analysis of these correlations ble to both cooling and warming trends shows cooling phase negative and warm- non-climatic factors may play an imporing phase positive water budget tendency tant role in ecosystem fluctuations. Wato be negatively correlated, with a sig- ter budget tendency analysis does not innificantly larger area reacting in cool- dicate large-scale glacial and interglacial ing phases. Six main phases of climatic dislocations of continental atmospheric and environmental change are deduced circulation patterns. from functional time and elevation plots. The only clear phase dislocations of posi1 Context of the problem tive and negative water budget tendency in all climatic zones are identified for The number of field data of Late Quathe period before 23,000 yr B.P. and for ternary environmental change in Africa the Full Glacial 20,000 to 17,000 yr B.P. which is rapidly increasing over the last There are clear differences in chronologi- few years brought about the realisation cal sequences between the subtropics and that there is a relation of variations in retropics, but during Holocene very com- gional water budgets to global and hemiplex time and elevation patterns are ap- spheric temperature trends (VAN ZINparent. This implies a strong cooling DEREN BAKKER 1976, NICHOLphase intensification of subtropical anti- SON & FLOHN 1980). This seems cyclonic belts. The aridification gradient to be true at least for all long-term rises exponentially from subtropical arid ecosystem fluctuations, i.e. on a 103 ISSN0341-8162 (~)1989 by CATENAVERLAG, D-3302 Cremlingen-Destedt,W. Germany 0341-8162/89/5011851/US$ 2.00+0.25

to 104-year-scale. As FAURE already stated in 1969 that the "humidity curve in the Sahara is not simply parallel to temperature curves in Europe", short-

CATENA~An Interdisciplinary Journal of SOIL SCIENCE--HYDROLOGY GEOMORPHOLOGY

164

Littmann

or intermediate-term variations may run counter to the overall thermal trend (NICHOLSON 1978). In this context, any paleoclimatic reconstruction has to be based on a continental-scale inventory of field data. Distribution pattern analysis of Late Quaternary regional water budget variations correlated with hemispheric warming and cooling trends should provide information about largescale fluctuations of African continental ecosystems and atmospheric parameters. However, the SASQUA workshop stressed in 1983 that there need to be a synchroneity of northern hemisphere glacial and southern African terrestrial sequences (DEACON et al. 1984). On the other hand, paleoclimatic modeling has been rather controversial during the last decade with West Wind Drift penetration into the continent (ROGNON & WILLIAMS 1977, DEACON 1983) and fluctuations of the ITZC (VAN ZINDEREN BAKKER 1976, NICHOLSON & FLOHN 1980) being discussed most frequently.

2

Correlation

It is only when the duration of biodynamic, geomorphodynamic and pedodynamic ecosystem fluctuations in Late Quaternary is known by means of dated local and regional chronostratigraphies that the deduced water budget tendency (more positive or negative than today) may be correlated with global or hemispheric trends in temperature. These trends since at least 40.000 B.P. are fairly well documented in the Camp Century ice core (DANSGAARD et al. 1971), in East Atlantic deep sea sediments (SARNTHEIN et al. 1982) and in northern hemisphere terrestrial stratigraphy (MORNER 1973).

Analysis of field data provides several possible response modes of regional water budgets (tab.l). There can be a single phase water budget tendency in long-term cooling phases (e.g. Full Glacial 25,000 - 14,500 yr B.P., Dryas 14,500 12,500, 11,800 - 11,500 and 11,000 - 10,000 yr B.E, Boreal 8400 7400 yr B.E, Subboreal 4500 - 2500 yr B.P.) or in global warming phases (e.g. Bolling/Allerod 12,500 - 11,800 and 11,500 - 11,000 yr B.P., Preboreal 10,000 - 8400 yr B.P., Atlantic 7500 4500 yr B.E). An example of such single phase water budget tendency is the formation of the Shati lakes in the Fezzan region of Central Sahara. Lake formation occurred between 40,000 and 26,000 yr B.E, i.e. during the Denekamp-Hengelo warming phase in Europe and again from 5500 to 4500 yr B.P. during the Atlantic stage (PETIT-MAIRE et al. 1980). A single phase development provides two options of water dubget tendency: more positive than today (water budget tendency mode 1H in cooling phases, 2H in warming phases) or more negative than today (water budget tendency mode 1X). On the other hand, double phase water budget tendency cases show several possible mono- and bimodal combinations. In Africa some areas became wetter in both cooling and warming phases (water budget tendency mode K), drier in warming and wetter in cooling phases (water budget tendency mode P) or drier in cooling and wetter in warming phases (water budget tendency mode S). Finally, there are regions without any apparent Late Quaternary environmental change (water budget tendency mode Z). Still we do not know whether African temperature variations correspond with the Blytt-Sernander scheme but a correlation of the European Late Quaternary tern-

CATENA An Interdisciplinary Journal of SOIL SCIENCE

HYDROLOGY GEOMORPHOLOGY

Quaternary Water-Budget, Africa

T e n d e n c y in: cooling phases warming phases

165

WBT-mode

1. single phase wetter drier --

--wetter

1H 1X 2H

2. double phase wetter wetter drier

wetter drier wetter

K P S

--

Z

3. static

perature sequence with African hydrology seems to be valid. There has been extensive discussion about those biological, geomorphological and pedological processes indicating Late Quaternary drier or wetter ecosystem conditions (see B U T Z E R 1966, M A B B U T T 1977 and S T R E E T & GROVE 1979 for further references). Although past water budget variations can be deduced from field data on a regional basis only, wetter conditions than today generally are proved by a denser vegetation cover, immigration of animal populations, the formation of lakes, fluvial terraces and in some cases alluvial fans, paleosoils and groundwater infiltration whereas dune activity, more arid taxa and increased slopewash features in semiarid ecosystems point to more negative water budgets than today ( L I T T M A N N 1988).

3

Methods

The available field data of a given locality or region were assigned to their respective climatic zone using the T R O L L P A F F E N classification. The following categories were found to be most useful: 100: subtropical humid zone

CATENA

An interdisciplinary

Journal of SOIL SCIENCE

HYDROLOGY

1: Late Quaternary response modes of African water budgets. Tab.

200: 300: 400: 500: 600:

subtropical semiarid zone subtropical arid zone tropical margin, arid tropical margin, semiarid tropical humid zone.

Other parameters for data grouping were the respective elevation a.s.1, and the 14-C datings which were generalised to 3 digit entries for reasons of simplification. The (tentative) data record used here is summarised in tab.2. All data implying positive or negative water budget tendency were then plotted as a function of time and elevation in order to obtain possible graphic patterns of spatial and temporal distribution. Frequency distribution analysis was conducted on the basis of areal comparison. All identified or implied areas with known water budget tendency mode were graphically transformed into polygonal structures by means of 166 × 166 km grid coordinates. These polygons were structured into regular triangular and trapezoid forms and their area was calculated. Each area was then expressed as percentage of the total continental area, with Late Quaternary glacial and periglacial areas (0.5 • 1 0 6 km 2) being excluded. In this context, "frequency distribution" means the percentage of any

GEOMORPHOLOGY

Littmann

166 IbentlftcatIon 8o. 101 102

Mode

Area

gate soorce

SouthernCape Province Moroocan Coast

biotic biotic, terrace

Tell-Atlas, Auras KroumJrie(Tunisla)

biotic, terrace, glacis biotic

201

Gab~o (Tunisia)

biotic, terrace,

802

biotic, dunes, glacis terrace, denudadation biotic, alluvia! fan biotic, lake, groundwater terrace, soll, mass movement biotic, lake, terrace lake

1 X g

105 104

1

501

I

Eastern Algerian Hauts Plateaux, Ziban Oj. a] Akhdar

5O2

1

MelrhJri Rhlr

505

2

504

2

305

2

ShatL Wadi 8arjuj, (Fezzan) Messak-OjadoScarp, (Fezzan) 6 i l f Kebir (Egypt)

Altltude (m) 1500 50

Tendency d w

Reference

52,51, 20,18, 16.7, 16, 14.2 28,7, 27.7, 26.4, 15.9, 6.5, 6.5, 5.0, 5-0 I0.0, 8.2

1200

>53, 20.0 (±)

600

d

Rognon 1987

w

Deaconet al. 1984 Weisrock A Rognon 1977, Dognon 1987 Lubbell & Gautier 1979

50, 27, 24, 21, 18, 8.5, 5,5, 5.2, 5.1, 7.5 (w): >14 (d): 14.0, 8.5, 7.5, 6.5 52, >12

20

w

800

w/d

800

w

Stelnmann A Bartels 1982, Rognon 1987 8allais et al. 1979, Dognon 1987 Hey 1968

16, >12, 8.6,

-30

d

Williams 1970

500

w

900

w

Gaven et el. 1981, Pachur & Braun 1982 Grunert & Hagedorn 1976

1800

w

Pachur & ROper 1984

50

w

Gautier 1981

50 600

w w

5

26, 14,> 8, 6.8, 5.5, 4.5 >14, 7.0,

11.0,

4.0

7.0

11.0, 9.7, 9.1, 8.6, 8.2 7.7, 7.2, 5,8 g7.0, 7.0, ).0 26, 14.0, 7.5, 7.0, 6.0, >5.0, 2.0 52.0, 28,9, 27,2, 25.0, 25.0, 82.0, 7.5, 5.0 55, 18 11, 9.5, 9.2, 7.4, 7.0, 6.9, 6.5, 5.5, 4.9, 4.5

)07 )O8

ZH 2 H

509

2 8

WesternDesert 8epresslons (Egypt) West Saharan coast Serlr Tibesti, Serir Calansclo Chott Djerid (Tunisia)

510 311

2 H 2 H

Tidlkelt (Algeria) Iaoudennl Basin (Mall)

lake, terrace biotic, lake

512

2 H

Kulseb River (Namibia)

terrace

Oattara depression (Egypt) M'zab, Tademalt, (Algeria) Libyan Desert plain (Egypt) lanezrouft (Algeria) Pjouf (Mali/Mauret.) Matmata(southern Tunisia) O. al kkarit (southern Tunisia) lgderltz Bey, fsauchab River, Tsondab (Namibla) Grand Er9 Occidental (Algeria) Erg er gaoui (Algeria) Er9 Chech (Algeria) Grand £rg Oriental (Algeria) SaouraSystem (Algeria) Ouarzazate basin, AntiAtlas, fafJlalet (soothern Morocco) Djeffara Plain (Libya) LaghouatHamada, Ouargla depression El Haouita (Algeria) NW-Kalahari

sediment

55.0, 50.0, ZT.O, 25.0, >19.0, 11.Z, 10.0 none

sediment

none

600

sediment

none

600

s

Haynes 1982

dune, sediment

none

550

s

biotic, terrace

150

w

biotic, terrace

100

w

Meckeleln 1988, 8esler 1982 Michel 1980 Brosche& Molle 1976, Coud@-Gaussen et aS. 1985 Rognon 1987

506

Z

Dating (±) in kyr

515 514 515 516 517

2

518

2

519

2

5gO 581 522 525 524

K

525

K

526

2 H

527

P

528

2 8

401

B

4O2

2 H

Bajuda Plain (Sudan)

905

28

404

2 H

4O5

2 8

Lake gesaka, panakil depression Paieolake Chad (northern part) Central f~n~r~

4O6

2 H

HeDger, Tfbesti, klr

4O?

2 H

4O8

2 H

Erg gee Sakane, Adrar des Iferls (Mall) DjebaI Marra

biotic biotic, terrace, lake, soil, terrace, glacis

> 52.0,29.0, 28.0, 27.0, 20.0, 19.8, 12.0, 10.0, 8.6,7.8,6.1 28,28.6,19.9,9.2,8.7,8.6,8.5, 8.2,8.1,6.7,5.9, 4.5,5.7, 3.6 lake, groundwater 25, Z5, 9.6, 9.5, 8.6

20

w

60 150

w w

700

w

-133

s

Petit Moire 1981 Pachur 1980, Pachur & Braun 1982 Coque 1969) Rognon 1987 Conrad 1969 Petit-Maire A Riser 1981, Riser et el. 1985, Celles& Schulz 1985, Guerin A Faure 1985 Hesler 1980, Vogel 1982, Deacon et al. 1984, Ward 198~ Said 1981

s

Littmann, in press

w

Deacon et el. 1984

biotic~ lake, groundwater lake, biotic

50.~, 29, 24, 19.8, 7, 5, 4

500

w

Conrad 1969

7, ~, 4

500

w

Alimen 1981

lake, groundwater groundwater

26, 22, 20 18, 17, 7, 5, 4 40, 58, 25, 21

500 500

w w

Conrad 1969, Street & Grove 1979 Sonnta0 et el. 1980

biotic, lake,

50.~, 29, 22, 18, 17, 16.), 9, 0, 7, 5, 4 50, 19, 10.4, 8.5, 6.5, 5

550

w

Allmen 1981, 8eucher 1975

1000

w

Schmidt 1986, tittmann 1987 a,b Rognon 1987 Hugenroth & Meyer 1979

terrace, lake, glacis fluvial accumulation, pedoganic lake, terrace, dune pedogenlc, groundwater lake, pedogenic

biotic, fixed dune, glacis, lake, soil lake

biotic, lake, groundwater lake, 9roundwater, biotic biotic, lake, terrace, glacis, groundwater biotic, lake biotic

ZOO

12, 10

100

w

(w): > 12 (d): 9.5, 8.5, 8.3, 7.9, %5 7.5

150

w/d

700

w

(w): >51 (d): 22, 16, 12 27.0, 10.0, 9.2, 8.6, 7.8

1000

w/d

500

w

Heine 1982, Deacon et at. 1984 Pflaumbaum1987

12,11

-100

w

Williams at el. 1981

500

w

Servant 1985

550

w

16, 15, 14, 12~ 7.5, 61 5,

2000

w

Street & Grove 1979, Servant 1985 gognon 1980, 8agedorn 1980, Morel 1984

9.6,9.9,9.5,8.9,8.5,?.9,7.5, 7.5,6.5,5.8,5.5,4.8,4.4,4.Z 12,> 8

500

w

gO00

W

29, 25, 22, 12, 11, 10, 7.5, 7, A, 5, 2.5 10, g~ 8, 6, 5, 2.5

Rognon1987 Aumassip1986

Petit-Moire & Riser 1981 flillalre-Marcel 1985 Wickens1975

Tab. 2: A preliminary record of paleonvironmental data in Africa since 30.000 yr B.P.

CATENA -An Interdisciplinary Journal of SOIL SCIENCE -HYDROLOGY X3EOMORPHOLOGY

Quaternary Water-Budget, Africa Identification Mode NO.

Area

Data source

167 gating (±) In KIt

kltlrude

Tendency

Reference

(m)

4O9

2 H

410

2 B

411

S

412

S

413

S

414 415 416 417

Z Z 2 8

418

2

501

1

502

1

503

I

504

2

Wadi Newer, Show (Nordofan) Nile givers, ktbara (Egypt/Sedan) Oarfur (Sudan) Lake kbh~, central Afar Makgadikgadl depression (Botswana) Namlb (northern part) Somali coast Seltma Sand Sheet Chemchane (Mauritania) Azawaghvalley (NW Niger) Sahelosudanlan dune complexes Dune system, southern KaIahar[ Swazt]aod Southern Balahari pans Gezira Plain (Sudan) kuob, NOSSObrivers (Botswana) Ziway-Shala system (Ethiopia) Alexandersfonteln, Orange free State

505 506 507 508

biotic, lake, terrace biotic, lake, dune lake, dune biotic, lake, dune sediment deep sea sediment lake lake, dune biotic, lake biotic, dune dune denudation, (terrace) lake lake, fluvial accumulations terrace lake biotic, lake

509

p

Veal river

terrace

510

S

511

S

White Nlle, Sudd (Sudan) Senegal, Niger rivers

biotic, terrace, lake, dune terrace, dune

512

S

513

S

514

S

Paleolake Chad (southern part) Niger interior delta (Mail) transvaal sites

biotic, lake~ dune, groundwater terrace, lake, dune biotic

515

S

Lakes Victoria, Baring% Natron, Magadi Lakes Iurkana, Nakuru, Ktvu Lake Chew Bahir, Ahlyata (Ethiopia) Coastal Kenya, Tanzania Malawl Plateau Mr. Kenya

biotic, lake

516

S

51?

2 H

60T 602 603

1X 1X S

11, 9.4, 8, 7, 6.5, 6 > 27, 11, 10, g, 7, 6, 5, 4

lake lake biotic, denubiotic biotic

604

S

Ruwenzori

biotic

605

1X

Central African rainforest area

biotic, denudatlon

606

1 X

Zaire Basin

sediment, biotic

(we: 12, 10, 9, 7 (d): 20, 18, 16, 14, 12, 7, 6 (we: >17, 11, 10,8.4~ 7, 5, 4 (d): 17, 15, 13,11.5,8.3, 7.2 (w): 25.6~ 21.9, 12.5, 10, 9, 6.2, 1.7, 1.5 (d): 19.6, 19.4, 19.1 none none 9,7~ %4,8.7,84, 7.9, 7.5, 5 (we: >ZO, 9.2, 8.5, ?.3 (de: 20, 18, 16, 10 9.3,9.0,8.5,7.7,7.3,6.4,6.5, 6.0,5.8,5.1,4.8,4.4,4.1, 3.7 20,>12 58, 28 (stable), 18, 16, 13 30, 20, 12 18, 8 17.2, 16.2, 15.6, 7.3, 7.0, 5-9, 5.0, 4.4, 3.6, 2.9 (we: 12, 6, 4 (de: 20, 18, 16, 15 19, 16.5, 15.5

100 2~ )00 400 500 600

: : : : : :

Gabriel & KrOpelin 1983, Pachur &RBper 1984, Kempf 1986 Butzer 1980

600

w/d

Parry N Wickens 1981

250

w/d

Gasse & Street 1978

250

w/d

500 150 500 250

s s w w/d

Cooke A Verstapben 198G, Cooke 1984, Deacon et el. 1984 Besler 1976, Heine 1982 Vernier & Froget 1984 Haynes 1982 Chamard 1972

500

w

400

d

500

d

800

d

450

w

500

w/d

450

w

Heine 1982

w

Gasse 1980

Durand A Paris 1986 Mensching 1979, gurand 8 tang 1986 Heine 1982, Lancaster 1981 Watson et at. 1984 Lancaster 1981, Butzer 1984, geacon eL el. 1984 Williams & Adamson 1980

w/d

Dutzer 1984

w/d

8utzer 1984

w/d

Adamson 1982, Williams & Adamson 1980 Blanck 1968, Michel 1980, Pastouret et el. 1978 Servant 1983, gurand el el. 198G Riser G Petit-Maire 1981

w/d w/d w/d w/d w/d w/d w

Scott & Vogel 1985, Butzer 1984 Hamilton 1982, HillalreMarcel N Casanova 1987 Begens G Hecky 1974, Gasse 1980, Butzer 1980 Grove et ai. 1979

I00 1200 2200

d d w/d

Hamilton 1982 Meadows 1985 Coetzee 1967

2600

w/d

Hamilton 1972

400

d

24.8, 23.7, 19.9, 17.7, 16.6

900

d

subtropical humid area subtropical semiarid area subtropical arid area tropical margin, arid area tropical margin, semiarid area tropical humid area

5. Area : in SOle cases, lodtvlduel date have been sumerised for a larger area. 4. Data source : origin of datable w t e r I e l and type of environmental change. 5- Bating : for simplification, only two or three digit ages have been used. 6. Altitude : In meters a.s.1. ?. Tendency : water-budget tendency Implied. w = wetter, d = drier, s = stable. 8. Refere,ce : best-fit publication r e f e r e n c e s .

Tab. 2: Continuation.

Interdisciplinary Journal of SOIL SCIENCE

w

21, 13 18, 14, 12 (w): 27, I0.5, 7, 4 (a): 26, 22, 18, 14 (we: 7.5, 6, 4.5 (de: 15.5, 12.6 21, 13.5, 18, 15, 4.5, 3.7

2. Mode : see Table 1.

CATEN~An

w

200

30,28,25,25,17,18,11,10.4, 1500 9.8, 9, 8.5, 7.2, 6,6,1.8 (uS: 16,15.6,15.2,12.9,11.2, 8, 7.7, 6, 4.5, 3.6 1000 (de: 15.5,13.2,12.8,11.5,1.8 (we: 13.7, 10, 7, 3, 2 800 (de: 18.5, 6.9, 3.2 (uS: 12.5,10.5,10.2,8.7,3.9,2 250 (de: 20, 18, 12.5 (we: 11, B, 6, 4, 2.9 250 (d): 14, 12 (we: 2.4,8,7.1,12,10.8,3-9,1-8 GOD (de: 21, 17, 14 (we: 8.8,8.7,8.5,5.9,5,3.9,3.5 300 (de: 20, 18 (we: T2~ 10, 8, 7, 4, 3 1200 (de: 29, 13, 12, 8, 7 (we: 12.5, 10.5, 8, 6, 4 800 (de: 22, 21, 16, 12.5, D, 6 (we: 12,10,8,6,5,3, 6.5, 3-2 400 (de: 16, 12, 8, 7 12, 11, 9, 7, 5-5 BOO

Explanation 1. Identification Number:

500

HYDROLOGY~EOMORPHOLOGY

Hervieu 1970, Hurault 1972, Flenley 1979, Hamilton 1982, Talbot 1981 Preuss 1986 a,b

168

Littmann

C

Q

Tendency in global cooling phases

warndngphases

wetter ~

~

wefter

drier wetter

~

wafter

wetter

~

drier

drier

~ ~]

wefter supposedstatic cell forest refuge

t 10

LII~MANN

Fig. 1: Spatial macrostructures o f long-term thermal trend - water budget tendency correlations in Africa.

area with defined tendency mode (tab.l, fig. 1) in relation to the African continental area. However representative any assessment of this kind may be, there are several restrictions given by the data base and data selection. As the data base is continuously expanding, it seems impossible to include all data in this analysis. CATENA

The results and implications presented here are only as good as the data base itself. About one third of the African continental area (which totals 30. 106 km 2) is still lacking paleoenvironmental field data (figs.3, 4).

An Interdisciplinary Journal of SOIL SCIENCE

HYDROLOGY- GEOMORPHOLOGY

Quaternary Water-Budget, Africa

169

Field data as evidence for regional water-budget tendencies

active soil water percolation (HUGENROTH & MEYER 1979) and in the Jbel al Akhdar region of northern Cyrenaica where periglacial screes and fluvial terraces were formed during the same period (HEY 1962). The northwesternSaharan Saoura Valley experienced a positive budget tendency from 30,300 _+ 1500 to 16,300 _ 350 yr B.P. (ALIMEN 1981) when the Saourien terrace was accumulated and Mediterranean marsh plants occurred in isolated stillwater patches thoughout the year (BEUCHER 1975). In the adjacent dune areas of Grand Erg Occidental and Erg Chech, groundwater-fed shallow interdune lakes appeared 24,000 - 17,000 yr B.P. (STREET & GROVE 1979) and the dunes were fixed by sparse steppe vegetation (CONRAD 1969). In Middle Holocene these wetter conditions returned to the Atlas region and northwestern Sahara (water budget tendency mode K, fig.l). The rest of the Sahara shows a relatively simple water budget tendency pattern. Most of its area remained stable in full glacial times, other parts became wetter in warming phases (water budget tendency mode 2H). Drainage from the Hoggar Mountains caused shallow lake and fluvial terrace formation in the Algerian Tidikelt region between 38,000 and 18,000 yr B.P. (CONRAD 1969). In the central Saharan massifs a significantly more positive water budget tendency is indicated by the formation of "middle terraces" between 12,000 and 8000 yr B.P. in the Atakor Mountains of the Hoggar (ROGNON 1980), from ca. 15,000 or 12,000 to 8000 yr B.P. in Tibesti (HAGEDORN 1980) and from ca. 16,000 to 8000 yr B.P. in Air (MOREL 1984). Slope processes seem to have been subdued during this phase

Full glacial (cooling phase) wetter conditions reached the Atlas region between 30,000 and 14,000 yr B.P. and caused widespread pedimentation (glacis) around the High and Tell Atlas slopes (SCHMIDT 1986) as well as fluvial fine material terraces in the coastal plains and major valleys (WEISROCK & ROGNON 1977). Starting around 22,000 yr B.P., fine material accumulation decreased in favour of coarse material terraces. Fine material fluvial accumulation was resumed around 8,000 yr B.P. (ROGNON 1987). However, all variations of regional water budgets were not strong enough to interrupt the generally arid conditions in southern Morocco or adjacent parts of the Sahara in Late Quaternary (LITTMANN 1987a,b, ALIMEN, M.-H., personal communication 1986). ROGNON (1987) implies an arid phase in southern Morocco because of erosion of older semidesert soils after 12,000 yr B.P. There are, however, some exceptions to the prevailing K-modal water budget tendency of northern North Africa. The eastern parts of the Algerian Hauts Plateaux showed dune formation ca. 12,000 to 6300 yr B.P. (BALLAIS et al. 1979) while extensive alluvial fan formation 19,000 - 12,000 yr B.P. on the northern slopes of the Algerian chott depression (WILLIAMS 1970) indicates drier cooling phase conditions with widespread decrease of the vegetation cover in mountainous catchment areas. The only case of single cooling phase positive water budget tendency (1H) in Africa can be found in northern Hamada al Hamra where calcrete formation in full glacial loess deposits indicates

CATENA- AnlnterdisciplinaryJoumalofSOILSCIENCE HYDROLOGY GEOMORPHOLOGY

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170

of global warming and the main valleys were characterised by stillwater lakes, marsh plant associations and a rich molluscan and mammalian fauna. However, mediterranean (Olea laperrini, Pinus halepensis in Atakor) and sahelosudanian (Acacia in Tibesti) arboreal flora was concentrated on wetter river valleys only (SCHULZ 1980). To the northeast of Tibesti two alluvial covers were deposited 14,000 - 7500 yr B.P. and 5000 - 2000 yr B.P. As there are some interspersed fluviolimnic features 14C-dated 12,000 - 7500 and 5000 - 2000 yr B.P. (PACHUR & BRAUN 1982) and local Late glacial and Early Holocene groundwater infiltration in the Sirte Basin is connected with these superficial deposits (EDMUNDS & WRIGHT 1979) there is good evidence for an extended drainage system from northern Tibesti into the Libyan Desert. The Fezzan had limnic periods in the Shati area from 40,000 to 26,000 yr B.P. and again from 5500 to 4500 yr B.P. A euryhaline molluscan fauna with Cardium glaucum and Melania tuberculata indicates the extreme salinity of these desert lakes (PETITMAIRE et al. 1980). Holocene shallow lakes with seasonal floodplains in dune areas and in cuesta scarp landscapes of the Malian Sahara existed 9500 - 6500 and 5500 - 4500 yr B.P. in the Taoudenni Basin and around the Adrar des Iforas and allowed isolated spots of Sahelian flora (Acacia maerua, SCHULZ, E., personal communication 1986; Phragmites, CELLES & SCHULZ 1983) and fauna (tropical freshwater species with fish, molluscs, turtles, crocodiles; floodplains with terrestrial pulmonae and mammalian savanna species; PETIT-MAIRE & RISER 1981, GUERIN & FAURE 1983). Similar features appeared in the

Bilma and Fachi region of Niger (SERVANT 1983). In the Lake Chad area full glacial Saharan dunes overrode the northern lake basin (SERVANT 1983) but inflow into the northwestern basin and into the southern part (DURAND 1982) was continuous during this generally dry period. It was only after the onset of Late Glacial warming that Lake Chad showed its first two transgressions around 12,000 yr B.P. and from 10,000 to 9000 yr B.P. Its maximum expansion was reached during the postglacial climatic optimum 7000 to 4000 yr B.P. (SERVANT 1983). In the eastern parts of the Libyan Desert and in the Western Desert of Egypt shallow and continuously brackish lakes were formed in several depressions during Holocene warming phases (HAYNES 1980, KEMPF 1986) allowing isolated stands of mesic vegetation and Neolithic occupation (PACHUR & ROPER 1984). As the neighbouring Nile Valley is fed by drainage from the Ethiopian Mountains and the western Rift lakes system, cooling phase incision and warming phase aggradation follows the same water budget tendency mode 2H (BUTZER 1980). During the Full Glacial the southern Saharan margin became significantly drier than today. However, the sahelosudanian paleodune belt seems not to indicate a complete full glacial southward shift of the Sahara by 400-600 km as postulated by GROVE & WARREN (1968) because the dunes show several Early Glacial, Full glacial and Holocene phases of reactivation and were not entirely active at the same time (DURAND & LANG 1986). Consequently, the extremely arid character of the Full glacial on the south side of the Sahara has to be challenged (DURAND, A., personal communication 1986). Some

CATENA An Interdisciplinary Journal of SOIL S C I E N C E - - H Y D R O L O G Y ~ E O M O R P H O L O G Y

Quaternary Water-Budget, Africa Mauretanian dune areas were the only parts of the Sahara proper becoming drier in cooling phases (dune activation) and wetter in warming phases (interdune lakes; MICHEL 1980). The same water budget tendency mode (S) can be detected in the valleys of the Senegal and Niger rivers. The Senegal was dammed off by Ogolien II dunes until 14,000 yr B.P.; wetter conditions than today returned between 11,000 and 8000 yr B.P. when fine material terraces were accumulated and interdune lakes appeared (MICHEL 1980). After 9500 yr B.P. hydrographic connections were established between the Niger River and the Taoudenni basin in Mali as well as between the Senegal and Niger systems as indicated by the occurrence of coastal fish species (PETIT-MAIRE 1983). There are, however, several large parts of the Sahara where field data implies a relative water budget stability throughout Late Quaternary. These "static cells" (fig.l) are centered in the Mauretanian and Malian Sahara, in central Algeria and in the eastern Libyan Desert, others are located in Somalia, in the northern and southern Namib Desert and in the central Kalahari of Southern Africa (LITTMANN 1988). In the Ethiopian parts of the East African Rift system several lakes expanded or were formed in Late Glacial and Holocene warming phases, others fell dry during Full Glacial (GASSE & STREET 1978). This S-modal water budget tendency is the general pattern of the Kenyan, Ugandan and Tanzanian Rift lakes (HAMILTON 1982). The effect of full glacial cooling in the East African highlands was a strong compression of ecologic zones in high altitude areas, with afroalpine elements being especially favoured. However, there was

171 no expansion of montane forest elements on the surrounding plains (HAMILTON 1982). More likely is a full glacial increase in dry savanna plant associations (VAN ZINDEREN BAKKER, E.M., personal communication 1986). Unfortunately, only few geomorphological investigations support these hypotheses. As far as paleoclimatic data is available, Southern Africa shows a rather complex pattern of long-term water budget tendency. There are three areas that became wetter in warming phases (2H): the Makgadikgadi depression had limnic phases ca. 31,000 to 19,000 yr B.P. and again 14,000 - 9000 yr B.P. (COOKE & VERSTAPPEN 1984); the Auob, Nossob and Molopo rivers had discharge 30,000 to 10,000 B.P. (HEINE 1982), freshwater lakes appeared near the Kwihabe Hills prior to 31,000 yr B.P. (BUTZER 1984) and the Kuiseb River valley of Central Namib was active (reed beds) 33,000 to 27,000 yr B.P. and 12,000 to 10,000 yr B.P. (VOGEL 1982). Speleothem formation and higher groundwater tables in caves throughout the Great Escarpment, Kalahari and Transvaal (Border Cave, Bushman Rock Shelter; BUTZER 1984) clearly indicate lower temperatures 30,000 to 20,000 yr B.P. (-8 to -10°C; DEACON et al. 1984), but not necessarily higher regional precipitation. However, most of tropical and subtropical Southern Africa was subject to large-scale full glacial aridification. In many parts of the Kalahari paleodune complexes were active between 19,000 and 13,000 yr B.P. (LANCASTER 1981) and the Kalaharian thornveld expanded in Transvaal before 12,000 yr B.P. and 8000 - 7000 yr B.P., combined with low species diversity (DEACON et al. 1984). A reverse water budget tendency in Late glacial and Early Holocene

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warming phases 12,000 to 8000 yr B.P. (SCOTT & V O G E L 1983) indicates a Smodal development in Transvaal. Even the Cape Province shows cooling phase drier conditions. A full glacial expansion of afroalpine grassland and grazing mammalian populations is proved for the southern littoral (VAN ZIND E R E N B A K K E R 1982) whereas in the southwestern Cape full glacial paleosoils dating 25,000 to 15,000 yr B.P. might be out of phase with the other parts of the Cape (DEACON et al. 1984). A reverse P-modal water budget tendency is shown for the Orange Free State where the maximum transgression of paleolake Alexandersfontein coincides with full to late glacial cooling 16,000 to 13,000 yr B.P. whereas late glacial warming caused local wind erosion features (BUTZER et al. 1973). The Holocene showed possibly drier conditions in the southern Cape (pedogenic horizons 8000 to 7000 yr B.P. but Middle Holocene moister conditions in the winter rainfall area (DEACON et al. 1984)). Fig.1 shows for most of tropical Central and Western Africa a tendency towards negative water budgets during cooling phases. Between 20,000 and at least 14,000 yr B.P. the rain forest was replaced by more xeric plant associations but not in terms of a lowland expansion of montane forest biomes (FLENLEY 1979). Stable areas were (speculative) rainforest refuges where monsoonal influence originating in the Gulf of Guinea prevailed ( H A M I L T O N 1982, VAN Z I N D E R E N B A K K E R 1982). However, direct paleobotanical evidence is rare. Most desiccation indicators originate from geomorphological data about large-scale slopewash, alluvial fan formation and coarse material terrace accumulation processes that indicate a massive

surface destabilisation caused by a decrease in biomass ( L I T T M A N N 1988). In the Zaire Basin, the Ruki River catchment area was clearly drier between 24,800 and 16,600 yr B.P. with erosion of older sandy sediments and pollenanalytical data about gallery forest vegetation (PREUSS 1986a). Another phase of Early Holocene erosion in the Ruki catchment from 10,000 to 8000 yr B.P. (PREUSS 1986b) is not certain but possible. This means that wetter conditions returned to the Zaire Basin some 4000 to 2000 years earlier than to East Africa (cf. P E Y R O T & L A N F R A N C H I 1983).

5

Chronological sequences and qualitative patterns

Figs. 2.1 and 2.2 plot the recorded field data from tab.2 as a function of time and elevation. As the recorded subtropical localities reach an elevation maximum of only 1300 m, their altitudinal pattern is dependent on topographic parameters. Isolated groupings on different elevation levels are called groups in fig.2.3, condensed groupings without elevational gaps are called complexes. An analysis of the data plots in fig.2.1 and 2.2 reveals six main phases of large-scale water budget tendency (fig.2.3). . 30,000 to 22,000 yr B.P. There is a clear phase dislocation between positive and negative water budget tendency. Both the subtropics and tropics were wetter than today and no individual reaction of arid, semiarid or humid regions can be found. Wetter conditions ceased earlier in the tropics (around 23,000 yr B.P.) whereas the northern and southern African subtropical latitudes show

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173

m 8,8.1, 2800

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2.2: Negative water-budget tendency in Africa since 30 kyr: data plot.

C a r b o n 1 4 - a g e s x 1 0 0 0 yr,, g e n e r a l i s e d 100

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i

Interdisciplinary Journal of SOIL SCIENCE-

poelthm

~

itegatl~

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29

31

Fig. 2.3: Positive and negative water-budget tendency: chronological groups and complexes.

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Littmann

only a short full glacial break in wetter conditions (20-17 gap).

B.P. but the equatorial mountains experienced another wet phase 8000 to 4000 yr B.P. On the tropical margins, elevations around 1200 m were drier from 11,000 to 7000 yr B.P. but there is a rather complex superposition of positive and negative water budget tendency at lower elevations.

2. 22,000 to 20,000 yr B.P. Subtropical arid regions (mainly the northern Saharan plains) were still wetter (due to high relief influence?) but in the tropics, especially on the tropical margins, the Full to Late Glacial arid complex set in.

. 12,000 to 10,000 yr B.P. The only clear pattern to be isolated within the 13,000 to 2000 yr B.P. sequence is the subtropical 12--10 gap in fig.2.3. There is no positive entry for this period, negative entries come from semiarid North Africa on the 800 m level and from the North Saharan lowland (below 300 m). However, in Morocco only the Saharan margin became drier.

3. 20,000 to 17,000 yr B.P. The Full Glacial temperature minimum brought negative water budget tendencies for all climatic zones and all elevation levels. However, the 20-17 subtropical negative group appears only in Southern Africa, for the 2017 gap in the Maghreb is not confirmed by negative entries in fig.2.2. 4. 17,000 to 13,000 yr B.P. The subtropics became rapidly wetter after 17,000 yr B.P. whereas the tropics show a very complicated pattern. On the tropical margins only the Saharan mountains have a clearly positive tendency, the lowland complexes were both wetter and drier with negative tendency dominating in elevations between 300 and 800 m and positive tendency concentration on the 500, 800 and 1000 m levels (figs. 2.1, 2.2). While the tropical lowlands remained dry, after 17,000 yr B.P. the tropical mountains became even drier. This implies a highland - lowland aridification time lag of about 5000 years.

This implies that the only clear phase dislocation between positive and negative water budget tendency is the interstadial warming phase before at least 23,000 yr B.P. and the Full Glacial between 20,000 and 17,000 yr B.P. During Holocene most climatic regions show parallel or heterogeneous developments without distinctive altitudinal differentiation (subtropics) or below the 1200 m level (tropics). Fig.5 gives a summary of spatial distribution patterns during 5 main water-budget tendency phases 30,000 to 10,000 yr B.P.

6

Frequency distribution of long-term water budget tendencies

5. 13,000 to 2000 yr B.P. There is no clear differentiation of Late Glacial Frequency distribution analysis of all and Holocene water budget varia- water budget tendency modes gives some tions. The humid tropics generally details about the overall patterns of returned to modern ecological con- Late Quaternary environmental change figurations after 13,000 or 12,000 yr in Africa. The basis of the analysis CATENA--An Interdisciplinary Journal of SOIL SCIENCE

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175

Quaternary Water-Budget, Africa tendency mode 1H lx

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CATENA--An Interdisciplinary Journal of SOIL SCIENCE

Fig. 3.3:Single and double phase waterbudget tendency•

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tendency mode 1H

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Fig. 4.3: Water-budget tendency on different altitudinal levels.

Quaternary Water-Budget, Africa is the total area of any water budget tendency mode (fig.3) and the respective area of several spatial and climatic parameters (fig.4). Of course, as field data are not always reliable, an assignment of percentages to different water budget tendency-modes should be seen as a first assessment. Fig. 3.1 shows the percentage of water budget tendencies in proportion to the total continental area. Most frequent is a negative water budget tendency in cooling phases (1X: 24.0%, S = 7.0%) followed by a positive water budget tendency in warming phases (2H: 12.6%, S = 7.0%). 17.2% of all continental ecosystems remained stable (West, Central and East African rainforest refuges after HAMILTON, 1982 included). This main pattern of African water budget tendency is reflected in fig. 3.2. In cooling phases 31.0% of the continent became drier and only 5.7% wetter; in warming phases 23.6% became wetter but 1.4% drier. Thus, the relation of wetter/drier in both thermal trends is strongly negatively correlated. In fact, cooling phases include more regions into the reactive, i.e. affected area (36.7%) wheras only 25% are reactive in warming phases. This general pattern is reflected in the frequency distribution of single and double phase water budget tendency cases (fig. 3.3): 36.9% of the continental area show a single phase water budget tendency whereas only 12.4% show double phase water budget tendency (more frequently wetter conditions in warming phases: K: 4.0%, S: 7.0%). Fig.4 plots the respective water budget tendency area against the total area of several climatic and spatial parameters. The first three parameters are large-scale climatic zones (arid, semiarid, humid in fig. 4.1). The percentage of paleoelimatically unknown area is highest in humid

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177

Central Africa (55.0%). However, the data basis reveals some clear differences in long-term water budget tendency of the main climatic zones. Arid regions (less than 250 mm p.a.) mainly become wetter in warming phases (2H: 30.0%) or remain unchanged (Z: 33.6%). Semiarid regions (250-500 mm p.a.) show a very strong tendency towards cooling phase desiccation (IX: 36.8%, S: 17.5%) but became only moderately wetter in warming phases (2H: 10.5%, S: 17.5%). The humid parts of Africa (over 500 mm p.a.) show the same large-scale desiccation in cooling phases (IX: 36.5%) but no significant change towards wetter conditions in warming phases (2H: 0.0%, S: 1.1%). This implies that the reactive area affected by climatic fluctuations is largest in semiarid Africa (75.3% of total) and smallest in arid regions where about one third remained stable. On the other hand, the aridification gradient (IX in fig. 4.1) rises exponentially from arid to semiarid and humid regions whereas warming phase wetter conditions (2H) are a phenomenon centered on arid and semiarid regions. Similar interrelations are shown by the second set of parameters (latitudinalclimatic parameters in fig. 4.2). The dry and humid subtropical latitudes of Africa tend to become wetter in warming phases (2H: 22.7%, S: 6.1%) or are considerably stable (30.8%). Cooling phase wetter conditions can be found in the northern and southern parts of the continent (K: 11.5%, P: 3.9%). The dry tropical margins become dearly drier in cooling phases (IX: 21.5%, S: 9.9%) but to a far less extent wetter in warming phases (2H: 10.4%) than the subtropical latitudes. Again, humid innertropical Africa shows the strongest cooling aridification (IX: 66.8%) or tends to be stable

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(forest refuges: 15.8% of total innertropical area). In this category the reverse structure of desiccation and humidification gradients is most apparent (fig. 4.2). As Central and Southern African highland areas have a very uncertain data basis (66.9% unknown) there is no significant concentration on any water budget tendency but stable areas are negligible (Z: 7.4% in fig. 4.3). As static cells are centered in Saharan and Kalaharian lowlands, the lowland parameter shows a higher percentage of stable area (Z: 24.1%). On the other hand, highlands and lowlands seem to follow the same water budget tendency pattern. The peak of cooling phase negative water budget tendency is assigned to the sahelosudanian and Central African lowland. However, there is a remarkable time lag in highland negative tendency compared to lowland areas (fig. 2.3), possibly due to prolonged Full Glacial monsoonal influence in high altitude East Africa.

7

Conclusions and paleoclimatic problems

Recent paleoclimatic discussion and modeling (ROGNON & WILLIAMS 1977, GATES 1976, SALTZMAN & VERNEKAR 1975, WILLIAMS 1978, LEROUX 1986) centers on the question whether glacial and interglacial phases brought about large-scale shifts of African climatic and vegetational zones and on physical causes for supposed dislocations. In this context, critical parameters are glacial and interglacial hemispheric temperature gradients, sea surface temperatures, the position of the polar and tropical fronts and the stability and intensity of subtropical anticyclonic belts with trade wind

circulations. First of all, water budget tendency mode analysis reveals cooling phase environmental change (36.7%) with negative water budget tendency (31.0%) especially in semiarid and humid tropical Africa to be most frequent. This should be due to the full glacial global decrease in surface temperature resulting in a stronger thermal contrast between pole and equator that led to a significant intensification of subtropical anticyclonic belts and trade wind circulation in both hemispheres (indicated by increased eolian activity in the Sahara and Kalahari) with the northern ITCZ limited to equatorial latitudes (NICHOLSON & FLOHN 1980) or (as shown by widespread rainforest withdrawal) being dissolved. However, no significant latitudinal shifts of the trade wind circulation have been verified (HARE 1983). Full glacial Sahelian dust storm outbreaks showed the same latitudinal position as today and oceanic circulation patterns off West Africa remained relatively stable (SARNTHEIN et al. 1982). Deep sea sedimentation off southern Africa at 18,000 yr B.P. also seems to indicate rather stable but intensified anticyclones over the Kalahari (DEACON 1983). On the other hand, all "static cells" of the Sahara may be correlated with presentday centers of northern hemisphere anticyclonic influence. The extreme gradient of cooling phase negative water budget tendency rising exponentially from the arid subtropics to semiarid and humid tropical Africa (fig.4) should be due to a reduction of humid air mass transfer into the tropics up to 30% (STREET & GROVE 1979) caused by reduced oceanic evaporation and a cutting off of monsoonal precipitation. Another problem is the extent of West Wind Drift penetration into the north-

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'1

100 200

weffer

drior

• •

z~ D

300

•

V

400 500 6OO

• • •

0 D 0

I 0

km

4 600

Fig. 5 shows generalised maps o f main water-budget tendencies in Africa between 30,000 and 10,000 yr B.P. Fig. 5a: From 30,000 to 22,000 yr B.P a widespread interstadial wet phase comes to its end and the tropics become significantly drier. The northern extratropics and both tropical margins, however, are still more humid than today. ern and southern parts of the continent in cooling phases. An increase in full glacial winter rainfall was modeled by SALTZMAN & VERNEKAR (1975) and ROGNON & WILLIAMS (1977) for the Atlas region and northern Sahara, by VAN ZINDEREN BAKKER (1982) for the Cape Province and southern Kalahari. There were indeed cooling phase wetter conditions in the Maghreb and northwestern Sahara (fig.l) and winter rains may have gained intensity in the Atlas region but field data in

southern Morocco (LITTMANN 1987b) and in the Saoura system (ALIMEN 1981) does not imply an all-year budget more positive than today. A penetration of regular winter rainfall as far as 26° n.1. (ROGNON & WILLIAMS 1977) has to be rejected the more so as deep sea sedimentation off Northwest Africa indicates continuously stable Late Quaternary oceanic circulation patterns (SARNTHEIN et al. 1982). Recently, ROGNON (1987) modified his earlier hypothesis and dated more evenly dis-

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f

•

• V

•

•

/

Fig. 5b: Between 22,000 and 20,000 yr B.P. only the Atlas region and northwestern Sahara are still wetter than today. The onset o f Full glacial addification is charactensed by fluctuating regional patterns on the tropical margins and expanding dryness in the tropics. - - Explanation see 5a. tributed rainfall patterns in the Maghreb back to the period 40,000 to 20,000 yr B.P. whereas after 20,000 yr B.P. an increase of anticyclonic pressure systems led to the well-known phase of full glacial aridity in the subtropics (18-20 gap in fig. 2.3). Full glacial seasonal incursions of polar frontal cyclones into the northern Sahara could have been not more frequent than today (LEROUX 1986). However, after the onset of Late Glacial climatic instability (17,000 to 13,000 yr B.P.) wetter conditions expanded all over North-Central Sahara (fig.5d). In this CATENA

context, Southern Africa presents an interesting paradox. Increased winter rains in the Cape Province and in the southern Kalahari cannot be verified (fig.l) but data in the Orange Free State implies cooling phase wetter conditions. This might be due to a stronger temperature decrease than supposed in GATES' paleotemperature model (1976). In this region winter and summer rainfall are out of phase today and might have been as well in Late Quaternary (DEACON et al. 1984). The impact of warming phases was far

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Quaternary Water-Budget, Africa

"~)

181

/

D

[]

Fig. 5c: During the Full Glacial 20,000 to 17,000 yr B.P, only the Atlas region remains wetter than today. Intertropical aridit~cation has reached its maximum but also extratropical southern Africa is in its driest phase. - - Explanation see 5a. less pronounced (23.0% in fig. 3.1). As the gradient of warming phase positive water budget tendency rises from equatorial regions to the arid and semiarid subtropics (fig. 4.2), innertropical Africa seems not to have been much susceptible to Late Glacial and Holocene temperature and air humidity increases, despite of an inevitable Middle Holocene increase in tropical convection (LEROUX 1986). Wetter conditions in Northern and Southern Africa should be correlated with poleward expansions of the ITCZ (NICHOLSON & FLOHN 1980,

PETIT-MAIRE 1983) but a precipitation concentration and water table rise on the tropical margins could have topographic reasons (Sahelian rivers, East African lakes, depressions and valleys of the Kalahari, western Sahara and Namib). However, large-scale shifts of subtropical atmospheric circulation patterns are not very likely since the Atlas region and northwestern Sahara still had winter rain influence and the Cape Province remained relatively unchanged (rigA). In this context, long-term African wa-

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Fig. 5d: Late Glacial climatic instability 17,000 to 13,000 y r B.P led to a clear expansion o f wetter conditions from the Atlas region over Central Sahara. However, water-budget patterns did not change for the rest o f the continent. - - Explanation see 5a.

ter budget tendency patterns do not verify significant latitudinal shifts of continental atmospheric circulation patterns. Moreover, water budget tendency patterns as presented in figs. 1 and 3 rather point to a more cellular than latitudinal structure of climatic and environmental change as previewed in FLOHN's (1953) meridional paleoclimate model. Static cells like Western Sahara, the Namib and Somalia obviously are (except Saharan and Kalaharian inland areas) correlated with stable off-shore upwelling bodies expanding in cooling phases (SARN-

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THEIN et al. 1982, VAN ZINDEREN BAKKER 1975). Although Africa is to at least 62% of its total continental area susceptible to long-term climatic change, it is still problematic why single phase water budget tendency (in either cooling or warming phases) is far more frequent (36.9%) than double phase water budget tendency (in both cooling and warming phases; 12.4%). Double phase, i.e. extremely susceptible water budget tendency areas seem to be dependent on edaphicaUy and topographically

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•

?

[]

Fig. 5e: Late Glacial warming 12,000 to 10,000 yr B.P. had some reverse effects on regionM water budgets. The Atlas region, wetter for most o f the preceeding 20,000 years, became drier. At4dity ceased in southern Africa and in the tropics, which returned to modern constellations. Only the northern tropical margin became signiticantly wetter than today. - - Explanation see 5a. favoured mountainous areas, large valleys, lake basins and dune areas (fig.l, fig.5). Non-climatic parameters may play an important role in long-term variations of regional water budgets.

stimulating discussion of the paper.

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Acknowledgements

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Address of author: Thomas Littmann Geographisches Institut Angewandte Physische Geographic Ruhr-Universit~it Bochum 4630 Bochum 1 West Germany

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