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Rainfall in Central Sudan: An asset or a liability?
Geoforum, Vol. 18, No. 3, pp. 321-331,
1987 0
Printed in Great Britain
OOlh-7185/87$3.00+0 00 19X7 Pergamon Journals Ltd
Rainfall in Central Sudan: an Asset or a Liability?
M. HULME,*
Salford,
U.K.
Abstract: Sudan has been at the centre of the debate of drought and famine issues over the last 3 or 4 years. While the ideas of a simplistic casual link between these two phenomena has been partially slain, there still remains the desire in some circles to find a scapegoat for famine. Drought remains one of the readiest to hand. Rather than being seen as a basic characteristic of the rainfall resource which requires management, drought is seen as such an abnormality that it provides too convenient an excuse for the failure of agriculture, inadequacy of water supplies, exhaustion of soils and other environmental phenomena that have afflicted Sudan within the last 3-4 years (and longer). Such an attitude is in danger of converting rainfall in Sudan from an asset to a liability. This paper examines rainfall in Sudan as a resource and hence a potential asset. Drought is recognised as an inherent characteristic of that resource and its management discussed. It is suggested that the performance of indigenous traditional adaptive strategies has contrasted favourably with those of Governmental institutions. Reasons for this are given. Some broad options for future perspectives on the effective management of rainfall are outlined.
Introduction
rainfall, i.e. the interface between the supply of rainfall (climate) and the utilisation of rainfall (socioeconomic processes). The notion of climate as a consumable, however, is not new, with LANDSBERG (1946) discussing climate both as a ‘natural resource’ and as ‘national income’ and, for example, AUSUBEL (1980) presenting climate as ‘matter and energy organised in a certain way’ for supply purposes. More recently GLANTZ and KATZ (1985) have discussed drought as a constraint on development in Africa and PALUTIKOF (1986) has reviewed the role of the climatologist in devising drought strategies in East Africa.
The most recent bout of famine since 1983 in several countries of Sub-Saharan Africa has accelerated research into the causes of drought in the Sahel. Such research was first tackled in the mid-1970s following the 1972-1974 food crisis in Ethiopia and the West African Sahel. A recent report from the Climatic Research Unit (FARMER and WIGLEY, 1985) has summarised much of the work on Sahelian rainfall completed over the last lo-15 years. This work has aimed at monitoring, explaining, modelling and forecasting regional rainfall with varying degrees of success for the first three aims and minimal success for the last.
This paper therefore builds on such emerging work and re-examines the way in which semi-arid rainfall might be regarded in managerial terms. Rather than discussing such an issue in broad, regional terms, comments here are closely wedded to examples drawn from work by the author and others on the Sudanese climate. The supply and demand charac-
There has not been a commensurate focus of work upon the resource implications of variable Sahelian *Department of Geography, M5 4WT, U.K.
University of Salford, Salford 321
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teristics of rainfall in central Sudan are outlined, followed by a discussion of the anatomy of one of the supply characteristics, drought itself. Traditional and institutional adaptations to drought are contrasted, before the question is addressed, ‘Can drought in central Sudan be managed?’ Although acknowledging the existence of a wide variety of drought definitions (meteorological, agricultural and social), the word ‘drought’ will here be taken simply to mean a deficit of rainfall compared to some long-term reference period which in turn leads to soil moisture deficits. Specific examples are taken primarily from central Sudan, where semi-arid marginality is greatest. Central Sudan is defined as between 12 and 16”N.
Supply
and Demand
Central
Sudan
Characteristics
of Rainfall
in
Rainfall is a resource. Sudanese rainfall is a scarce resource. It is a resource that is currently subject only partially to management strategies. For progress to be made toward its more effective management, the basic supply and demand characteristics need to be
(a)
18 Number 3/1987
understood. This is the case with any natural resource, but particularly so with a marginal, unstable resource such as semi-arid rainfall. Supply characteristics The spatial and temporal distribution of rainfall in central Sudan creates several distinctive characteristics of rainfall supply, several of which are shared with other semi-arid regions. These are investigated here as magnitude, reliability, spatial organisation, storm intensity and frequency and diurnal distribution. By definition annual magnitudes of rainfall in central Sudan are low, typically between 100 and 600 mm. The marginality of these low magnitudes is exaggerated by high rates of potential evapotranspiration, ranging from 1500 to over 2000 mm per annum. Using this simple accounting system no locality in central Sudan has an annual water surplus and few have monthly surpluses even in the height of the wet season in July and August. Reliability is poor, not only year-to-year decade-to-decade, as demonstrated by familiar patterns of twentieth-century
400.
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Annual
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Full permd reference medmn
350,
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1910
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1920
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1930
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Figure l(a).
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1960
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1980
1990
Geoforum/Volume l-7
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323
15-23
24-26
15-20
Number
of
10
-2-o
-2.5 1’
0
I
I
I
1910
1920
1930
Figure 1. Annual
-
Raw
-
Filtered
,
1940
series series
I
1950
I
1960
I
1970
rainfall series in central Sudan, 1900-1986, for: (a) Khartoum,
(b) standardised
I
I
1980
1990
and
regional series integrating 26 stations.
behaviour (Figure 1). Prolonged periods of aboveand below-average annual rainfalls (runs) are a fundamental feature of all Sahelian rainfall series. A statistical experiment by GLANTZ and KATZ (1985) demonstrated that even under an hypothesis of a stable climate (i.e. sampling from a single population), runs of 12 years length can occur legitimately. The spatial organisation of Sudanese rainfall is also distinctive. The localised nature of semi-arid convective storms leads to very large daily, monthly and annual rainfall gradients over only, say, 5-10 km. Figure 2 illustrates the non-uniform surface of standardised annual rainfalls for 2 years, 1978 and 1985, in central Sudan. Note especially the sequence of values in the north-south transect down the Blue Nile in 1978. Whether such localisation should be regarded as a consequence of basically random smallscale convection operating is not so certain. Following on from work by SHARON (1979,198l) in Tanzania and Namibia, a preliminary investigation of central
Sudan stations suggests a high degree of spatial regularity in storm spacing, at least in some years (LIGHTWOOD, 1987) (see Figure 3). Storm frequencies also suggest a degree of rainfall organisation. Quasi-regular pulses in easterly wave activity result in temporal spacing of storms at given locations of the order of 3-6 days during the middle of the wet season (HAMMER, 1973). In principle this leads to beneficial temporal spacing of storms for soil moisture storage and crop water use. Rainfall intensities are high. The majority of rain falls in heavy, intense storms with approximately75% falling at between 5-10 mm/hr in central Sudan, compared to <5 mm/hr for temperature regimes (OLIVER, 1969). Daily falls of >25 mm are frequent. The diurnal distribution of rainfall varies between eastern and western central Sudan. Maximum incidence of rainfall during later afternoon and evening (often considered characteristic of a tropical area) is dominant only at locations far to the west of
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1978
1985
Figure 2. Spatiai patterns of standardised
the Ethiopian Highlands. Elsewhere, rainfall is more evenly distributed throughout the day, although an early morning maximum is common in the Nile valleys (PEDGELEY, 1969).
annual rainfalls in central Sudan, 1978 and 1985. evaporation loss (20-30 cm/month) capacity for year-to-year management.
and
have
no
It is also necessary to sketch in briefly the major demand characteristics of rainfall in central Sudan. The rainfall resource is utilised primarily for domestic and livestock water supply, crop growth and natural vegetation (HUL~E, 1986).
Shallow groundwater aquifers, too, are highly dependent upon yearly recharge from rainfall events. Generally these aquifers (at less than 10 m depth) are completely decoupled from one wet season to the next, thus eliminating any year-to-year storage capacity to offset poor wet season performance. Such shallow wells are still the major supplier of domestic and livestock water in rural central Sudan {EL SA~~ANI, 1978).
The generation of surface runoff requires highintensity storms and is fundamental for the supply of both surface water storage reservoirs and recharge of shallow groundwater aquifers. The former comprise both natural and artificial features and also contribute (minimally) to permanent discharge from the region through the Nile rivers (the active Nile catchment in central Sudan is negligible in area and contribution). These surface water supplies are subject to large
Crop growth in rural Sudan is primarily rainfed. Irrigated cultivation in any large scale is almost totally restricted to the Nile systems. The large populations of western Sudan (Darfur and Kordofan Provinces combined number about 6 million) are subsistence, rainfed cultivators, a practise which is directly dependent upon rainfall events. These cultivators are acutely aware of rainfall performance and the adaptation of crop practise to rainfall resource
Geoforum/Volume characteristics (1984)].
325
18 Number 3/1987 is often
intricate
[see
for surface reservoirs and shallow aquifers, by reducing transpired water loss. The loss of thus released rainwater through increased surface evaporation, combined with absolute declines in rainfall magnitudes, however, argues against such a net increase for water supply.
IBRAHI~
Natural vegetation provides a building, fuel and grazing resource for rural communities. Annual grasses and stands of acacia are thus essential for the functioning of such communities and again are directly sensitive to rainfall events as mediated through soil moisture. Here there may be a trade-off in rainfall utilisation between natural vegetation cover and surface runoff. Vegetation depletion in recent decades should have increased input supplies
Rainfall as Liability
Having sketched the characteristics of rainfall
N
key supply and demand in central Sudan what of the
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Significant
0
Not
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Probable signtficonfe ... at 0.07 ievei _I”“““” fincomplete data) “F_._..
150
IKm
150 Km
Figure 3. Composite
correlation field of pentad rainfalls for 15 stations in central Sudan for the years, 1948,1949,1962 and 1963 (lower half is reverse mirror image of upper half). Organisation of storms at around 35 km spacing is suggested.
326
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issue of rainfall as asset or liability? There is of course an immediate sense in which rainfall must be seen as an asset. None of the above water demands could be met should rainfall cease completely. There is another sense, however, in which rainfall appears to be regarded as a liability. The recent consciousness and perception of drought in the Sahel has equated it alongside war and civil unrest as a factor of ill-health, and on its own as a constraint upon development and as the source of both famine and desertification. Thus, in relation to disease: . . . warfare, drought and civil unrest are the three factors which have enhanced disease liability among children (RECELLIS. 1986). to famine: The famine that has hit Eritrea and much of SubSaharan Africa has been caused by drought and erosion (SUAN, 19X5). and to desertification:
. [desertification’s] major cause is thought to bc a change in the climate [drought] on the edge of deserts (BLUNDY and VALLELY, 1985). As we have seen, however, drought is one of the supply characteristics of rainfall in central Sudan (and the Sahel). One cannot therefore experience rainfall without experiencing drought. It is as fundamental a characteristic of rainfall in central Sudan as a wet season-dry season rainfall regime. Drought cannot be separated from rainfall. As GLANTZ (1986) has “Drought is a part of climate not apart commented, from it.”
Viewing drought therefore in such a negative way is an admission of failure to adapt to, or to manage adequately, the totality of the rainfall resource. Some of the perceptions of drought in the Sahel still regard drought as in some way an abnormal part of the climate in the region as if the removal of drought from the climate would guarantee effective rainfall management. This false perception of one characteristic of climate has therefore transferred the rainfall resource from being an asset to becoming a liability.
Supply Characteristics
of Drought
As drought is an integral part of the Sudanese climate, an awareness of the supply characteristics of drought years is necessary for its proper management,
18 Number
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i.e. what are the characteristics of rainfall in dry years, compared to wet years? What is the anatomy of drought from the standpoint of the demand characteristics for waler in this semi-arid region? Recent research in both central Sudan and the West African Sahel has established certain common features. Annual reductions of rainfall in drought years do not lead to uniform reductions in monthly totals. The drought of the 1970s and 1980s has witnessed larger reductions in August and September totals (i.e. late wet season) than June and July totals (DENNETT et al., 1985; HULME, 1987). This drought characteristic does not seem to hold, however, for the earlier twentieth century droughts of the 1910s and 194%. Such skewed hehaviour of the monthly rainfall regime will affect rainfed crop production, since seeding schedules are determined by early-season, not late-season, rainfall. Storm intensities are reduced to a greater extent than storm frequencies. The frequency of heaviest storms (e.g. those with a yield of >40 mm) has shown a disproportionately large decrease in recent drought years over frequencies of light storms, i.e. storm events have not been reduced substantially, but the rainfall contribution of each event has. There are clear implications here for the generation of runoff, and hence for rural water supplies. The reliability of the wet season is reduced in drought years. Using a simple water balance model with wetseason definitions based on prolonged soil moisture surpluses, it has been shown that not only have periods of inadequate soil moisture levels increased within the wet season but that the frequencies of years when the wet season failed to commence (null starts) have increased (HULME, 1987). The probability of null starts between 1968 and 1985 increased severalfold over the 1900-1967 period (Table 1). Furthermore, in the 1970s null starts occurred for the first time at locations with between 200 and 300 mm median annual rainfall, the zone where rainfed cultivation is conventionally regarded as viable. Such phenomena have an inevitable detrimental impact upon crop yields. The spatial coherency of rainfall in drought. years is different from wet years. Analysis in central Sudan for the years 1901-1985 has demonstrated that dry years exhibit greater spatial coherency than wet years. The association between the standardised median regional rainfall value [cf. Figure l(b)] and a standardised measure of spatial variation (inter-
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Table 1. Frequencies
of null starts for 12 stations in central Sudan, 1968-1985
compared with 1900-1967
Probability of null start No. of years
No. of null starts
1900-1967
Khartoum
85
17
0.17
0.29
142
El Geteina Abu Deleiq Kamlin El Fasher Bara Ed Dueim Manaqil El Obeid Kosti En Nahud Sennar
80 78 80 68 78 84 76 82 52 71 55
13 7 4 1 2 1 1 -
0.14 0.05 0.02 -
0.24 0.26 0.18
185 197 222 254 276 287 300 353 385 388 439
Station
quartile range), is highly significant (Figure 4). A wet year in central Sudan is more likely to have localities of low rainfall than a drought year is likely to have localities of high rainfall. This particular characteristic has implications for national coordination and planning strategies for the alleviation of drought impact, through food and population redistribution.
Adaptation to Drought Why has adaptation to drought fallen so far behind the adaptation to many of the other supply characteristics of rainfall in Sahelian societies? Traditional rural practises of the past have responded to many of the supply characteristics of Sudanese rainfall, including drought.
Artificial water harvesting techniques using surface topography have enabled rainfed crops to be cultivated in areas of only 100 mm, thereby exploiting low annual rainfall magnitudes. Through multiple cultivation plots and the use of numerous water supply sources operated by both villagers and nomads, subsistence farmers have adapted to the spatial inhomogeneity of the rainfall field. IBRAHIM (1984) reported an interesting relationship between polygamy (which is practised by about 10% of Darfur farmers) and multiple plots. Those farmers which have more than one wife are often better able to obtain a fair crop from one of their wives’ plots. Since wives are often drawn from neighbouring but
Median 1968-1985 annual rainfall (mm>
0.06 0.12 0.08 0.06 -
separate villages, the probability of adequate rainfall being received at one of the areas cultivated is increased. Variable seeding schedules, with locally established critical dates, allows adaptation to the unreliability of the commencement of the rains. Similarly, an indigenous calendar of eight periods of 13 days each is used by farmers in western Sudan. The periods lo-23 July and 23 July-5 August are critical in the assessment of crop performance. If both periods have poor rains then preparation for a poor harvest begins. If both experience good rainfalls, it is assumed that the subsequent harvest will be fair. It is worth noting that the failure of late-season rains in August as noted above would not be detected by this procedure. Migration, of course, remains the oldest and probably most suitable form of adaptation to the drought characteristic of rainfall in central Sudan practised by nomadic groups of the region. Why then has drought, in particular the most recent drought, caused such disruption and devastation in countries such as Sudan? Why has not drought been so adapted to? The failures, it is argued, have tended to arise in the scale of Governmental planning initiatives and interventions rather than indigenous rural technologies and adaptations. Four reasons why Sahelian G~v~r~~en~s, such as the Sudanese, have not responded adequately to drought may be suggested [these exclude issues of more fundamental managerial incompetence as discussed by REILLY (1986)].
328
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Figure 4. Scatter of regional standardised annual rainfall value (median, x-axis) against standardised measure (interquartile range, y-axis) of within-year variation. These data form the basis of the regional series displayed in Figure l(b), i.e. 19011985, central Sudan.
Rainfall data are not adequately measured and/or disseminated. Many Sahelian countries have a large area and a small rainguage network. Within the last lo-15 years these networks have been contracting rather than expanding. Figure 5 demonstrates this for Sudan, which reached a peak of recording gauges and published data in the 1960s. Often there has been inappropriate analysis of the existing rainfall data. Emphasis has been placed upon the central tendency of rainfalls, rather than examining the whole rainfall distribution. Skewed rainfall distributions (where the central tendency is a poor indicator of rainfall likelihood) and outliers (occasional very wet or very dry years) are common in these regions [see KATZ and GLANTZ (1986)]. A confusing of drought and climatic change has occurred. The two concepts are not the same. As has been argued above, drought is a result of an inherent variability which occurs under any particular climatic regime. It is a fundamental manifestation of the workings of the Sahelian atmosphere. A failure to perceive this has led to confusion and diversion of effort particularly in some planning and Government circles. There should be less urgency with the consequences of hypothetical long-term climatic change [as espoused by, for example, WINSTANLEY
(1985)] and more with the variability, which is drought.
reality
of short-term
The continuity of Government perceptions and policies in relation to drought has been poor (GLANTZ and KATZ, 1985). Frequent changes of Government in some Sahelian countries and high turnovers of professional staff in civil services (e.g. high professional labour migration rates from Sudan to the Gulf) has created short-term horizons and a failure to maintain and implement the drought strategies of predecessors. Each fresh new round of appointees has started as if from scratch. Also there is the more basic problem that, as with all political issues (and drought is a major political issue in these and goes’ in response to regions), it ‘comes movements and trends in national and international societies. There is a need for a continuity of approach and it is arguable whether indigenous Governmental organisations or external non-Governmental organisations (NGOs) are the better equipped to meet this need.
Can Rainfall
in Central
Sudan
be Managed?’
In the terms which have been used in this paper there are two options which can be pursued:
Geoforum/Volume
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I
1950
1
I
Figure 5. Number
of meteorological
1
1980
1967
1960
and rainfall stations
published
1
I
1
I
1985
in
Sudan Meteorological Service Reports, 1950-1985.
(a) (b)
to manage
the demand of the rainfall the light of supply, to manage the supply of rainfall
resource
in
in view
of
demand. These options can be re-expressed. First, can we reach a situation where one of the fundamental supply characteristics of rainfall (drought) is appropriated into Governmental and non-Governmental planning strategies, thereby encouraging demand to adapt to supply (e.g. through agricultural policy, resettlement and attitudes to nomadic communities)? Second, what are the possibilities of manipulating rainfall supply so that the demands of an expanding population in a marginal, semi-arid zone can be met? The second option is not feasible yet. Even simple forecasting of specific rainfall supply characteristics presently eludes the scientific community: Put bluntly, and despite occasional contrary, our forecasting capability rainfall] is close to zero (FARMER 1985, p. 101).
claims to the [for Sahelian and WIGLEY,
Consequently, we are well short of a full explanation of rainfall supply which is of course the necessary precursor to the manipulation of supply. What is becoming increasingly clear, however, is that ‘management by default’ (improvident management or inadvertant climatic change) is actually occurring
with respect to Sahelian rainfall. Three potential mechanisms for Sahelian drought are intimately linked with human activity (Figure 6). The reduction of surface biomass, either in the subhumid zone to the south of the Sahel or in the Sahel itself, is a socially driven process. This possibility remains a powerful hypothesis for Sahelian drought through moisture supply perturbation and albedo modification, respectively. Second, massive dust loading of the Sahelian troposphere has been identified in recent years (MIDDLETON, 1985; PROSPER0 and NEES, 1986), such an increase being primarily an anthropogenic process. Radiative modification leading to convection suppression remains a viable hypothesis. Third, the regional climatic signals of increasing atmospheric carbon dioxide (and other trace gases) are still poorly defined, particularly with regard to precipitation, yet it remains a possibility (albeit currently speculative) that regional circulation forcing could be traced back to rising carbon dioxide levels. There is therefore the looming spectre of the human depletion of the climatic resource, not just in the Beneficial Sahel, but worldwide. climatic management remains distant and, although not agreeing with the dogma of the following statement, the danger is undoubtedly real: Human
population
growth
is driving climatic
change
in
GeoforumiVolume
330 directions
that
(BROWN,
1986, p. 14).
will not
benefit
the
people
18 Number
marginal localities is making traditional adaptations to drought obsolete and, second, a degree of social integration and economic politicisation, dependency has already been thrust upon these societies which would prove impossible to revoke. A decoupling move would therefore undoubtedly qqeav as a degenerative step to organisations intent on extending their domain.
affected
Since we have dearly shown that the second option above is not viable, the first option (integrating drought characteristics into the planning process) becomes very pressing. Here again, one may subdivide policy into two options. First, Governmental organisations (and NGOs who are playing an increasingly important role in ‘governing’ activities in Sudan) must take on board the responsibility to respond at a national and institutional level to the recurring phenomena of drought, one of the basic supply characteristics of rainfall in central Sudan.
What we may, however, witness is the large-scale resettlement of semi-nomadic communities presently in central Sudan to some of the more subhumid regions of the country, e.g. North Darfur and Kordofan to South Darfur, South Kordofan and the Blue Nile provinces. One or two of the NGOs in Khartoum are already developing strategies along these lines. Certainly, in the face of climatic change it is one possible management strategy to match demand with supply (KATES, 1985) and has been practised to some extent in neighbouring Ethiopia. How much it will be a Governmental decision with a degree of grass-roots approval, rather than an imposed policy from outside the community, remains to be seen.
Second, there remains the option of returning these marginal, semi-arid rural societies to the control of of local local inhabitants, i.e. the decoupling communities from the demands and constraints national and international institutions place upon their economy and practise. Adaptation has been far more successfully carried out at a local and instinctive level in past decades and centuries than now. This would appear somewhat suggestion, however, idealistic and will prove impossibie for two reasons. The increasing density of population in these
The rainfall resource of central Sudan, which by incorporates drought, is an asset. de~nition
DROUGHT (Moisture
def kit)
I LOCAL
sea
Surface
ENERGY
Local
/
iemDerol”ies
GLOBAL
TELECONNECTI
Global
I
_. I,,
Figure 6. Framework
of possible
3/1987
explanations
for drought
in the Sahel.
Geoforum/Volume
18 Number
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However, by inappropriate exploitation and through poor or improvident (unforseeing and heedless) management it is increasingly becoming a liability.
Acknowledgements We thank Dr A. Trilsbach for comments on a earlier draft of this paper and Gustav Dobrzynski for the construction of the diagrams. Earlier visits to Sudan were supported by an N.E.R.C. studentship and British Council lecturing tour.
Note 1.
In this secion, the wider Sahel region is referred to since mechanisms of climatic change occur on scales larger than central Sudan.
References AUSUBEL, J. (1980) Economics in the air, In: Climatic Constraints and Human Activities, pp. 13-60, J. Ausubel and A. K. Biswas (Eds). Pergamon Press, Oxford. BLUNDY, D. and VALLELY, P. (1985) With Geldof in Africa. Times Books, London. BROWN, L. R. (1986) State of the World, 1985. W. W. Norton, New York. DENNETI, M. D.., ELSTON, J. and RODGERS, J. A. (1985) A reappraisal of rainfall trends in the Sahel, J. Clint., $353-361. EL SAMMANI, M. 0. (1978) Gaps in the water provision map of the Sudan. Sudan Notes Rec., 59,97-106. FARMER, G. and WIGLEY, T. M. L. (1985) Climatic Trends for Tropical Africa. Unpublished report for the Overseas Development Administration, Climatic Research Unit, Norwich. GLANTZ, M. H. and KATZ, R. W. (1985) Drought as a constraint to development in Sub-Saharan Africa, Ambio, 6,334-349. GLANTZ, M. H. (1986) Drought and economic development in Sub-Saharan Africa, In: Drought and Hunger in Africa: Denying Famine a Future, Chap. 3, M. H. Glantz (Ed.). Cambridge University Press, Cambridge. HAMMER, R. M. (1973) Applications and consequences of precipitation observations in the Republic of Sudan in
331 view of the nomadic life and economy, Geoforum, 14, 11-18. HULME, M. (1986) The adaptability of a rural water supply system to extreme rainfall anomalies in central Sudan, Appl. Geogr., 6,89-105. HULME, M. (1987) Secular changes in wet season structure in central Sudan, J. Arid Envir. (in press). IBRAHIM, F. (1984) Ecological Imbalance in the Republic of the Sudan. Druckhaus Bayreuth, Bayreuth. KATES, R. W. (1985) The interaction of climate and society, In: Climate Impact Assessment, pp. 3-37, R. W. Kates, J. H. Ausubel and M. Berberian (Eds). SCOPE 27, John Wiley, Chichester. KATZ, R. W. and GLANTZ, M. H. (1986) The anatomy of a rainfall index, Mon. Weath, Rev., 114,764-771. LANDSBERG, H. (1946) Climate as a natural resource, Scient Mon., 63,293-298. LIGHTWOOD, D. S. (1987) The spatial variability of convective rainfall: a tri-partite analysis in Africa, BSc dissertation, Department of Unpublished Geography, University of Salford. MIDDLETON, N. J. (1985) Effect of drought on dust production in the Sahel, Nature, 316,431-434. OLIVER, J. (1969) Evapotranspiration in the semi-arid tropics, J. trop. Geogr., 28,64-74. PALUTIKOF, J. P. (1986) Drought strategies in East Africa: the climatologists role, Clim. Change, 8,67-78. PEDGELEY, D. E. (1969) Diurnal variation of monsoon rainfall, Met. Mug., 98, 97-107 (part a), 129-134 (part b). PROSPERO, J. M. and NEES, R. T. (1986) Impact of the North African drought and El Nino on mineral dust in the Barbados trade winds, Nature, 320,735-738. RECELLIS, C. (1986) Interview with the UNICEF Director for eastern and southern Africa, Zimbabwe, T.V., July. REILLY, W. (1986) Administrative and managerial constraints to rural development, Paper presented at the workshop on Arid Lands: Problems and Prospects in the Sahel, I.T.E., Grange-over-Sands, May. SHARON, D. (1979) Correlation analysis of the Jordan Valley rainfall field, Mon. Weath. Rev., 107, 1042-1049. SHARON, D. (1981) The distribution in space of local rainfall in the Namib desert, J. Clim., 1,69-76. SUAN, A. (1985) Nat. geogr. Mug., June. WINSTANLEY, D. (1985) Africa in drought: a change in climate?, Weatherwise, April, 65-71.
1987 0
Printed in Great Britain
OOlh-7185/87$3.00+0 00 19X7 Pergamon Journals Ltd
Rainfall in Central Sudan: an Asset or a Liability?
M. HULME,*
Salford,
U.K.
Abstract: Sudan has been at the centre of the debate of drought and famine issues over the last 3 or 4 years. While the ideas of a simplistic casual link between these two phenomena has been partially slain, there still remains the desire in some circles to find a scapegoat for famine. Drought remains one of the readiest to hand. Rather than being seen as a basic characteristic of the rainfall resource which requires management, drought is seen as such an abnormality that it provides too convenient an excuse for the failure of agriculture, inadequacy of water supplies, exhaustion of soils and other environmental phenomena that have afflicted Sudan within the last 3-4 years (and longer). Such an attitude is in danger of converting rainfall in Sudan from an asset to a liability. This paper examines rainfall in Sudan as a resource and hence a potential asset. Drought is recognised as an inherent characteristic of that resource and its management discussed. It is suggested that the performance of indigenous traditional adaptive strategies has contrasted favourably with those of Governmental institutions. Reasons for this are given. Some broad options for future perspectives on the effective management of rainfall are outlined.
Introduction
rainfall, i.e. the interface between the supply of rainfall (climate) and the utilisation of rainfall (socioeconomic processes). The notion of climate as a consumable, however, is not new, with LANDSBERG (1946) discussing climate both as a ‘natural resource’ and as ‘national income’ and, for example, AUSUBEL (1980) presenting climate as ‘matter and energy organised in a certain way’ for supply purposes. More recently GLANTZ and KATZ (1985) have discussed drought as a constraint on development in Africa and PALUTIKOF (1986) has reviewed the role of the climatologist in devising drought strategies in East Africa.
The most recent bout of famine since 1983 in several countries of Sub-Saharan Africa has accelerated research into the causes of drought in the Sahel. Such research was first tackled in the mid-1970s following the 1972-1974 food crisis in Ethiopia and the West African Sahel. A recent report from the Climatic Research Unit (FARMER and WIGLEY, 1985) has summarised much of the work on Sahelian rainfall completed over the last lo-15 years. This work has aimed at monitoring, explaining, modelling and forecasting regional rainfall with varying degrees of success for the first three aims and minimal success for the last.
This paper therefore builds on such emerging work and re-examines the way in which semi-arid rainfall might be regarded in managerial terms. Rather than discussing such an issue in broad, regional terms, comments here are closely wedded to examples drawn from work by the author and others on the Sudanese climate. The supply and demand charac-
There has not been a commensurate focus of work upon the resource implications of variable Sahelian *Department of Geography, M5 4WT, U.K.
University of Salford, Salford 321
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teristics of rainfall in central Sudan are outlined, followed by a discussion of the anatomy of one of the supply characteristics, drought itself. Traditional and institutional adaptations to drought are contrasted, before the question is addressed, ‘Can drought in central Sudan be managed?’ Although acknowledging the existence of a wide variety of drought definitions (meteorological, agricultural and social), the word ‘drought’ will here be taken simply to mean a deficit of rainfall compared to some long-term reference period which in turn leads to soil moisture deficits. Specific examples are taken primarily from central Sudan, where semi-arid marginality is greatest. Central Sudan is defined as between 12 and 16”N.
Supply
and Demand
Central
Sudan
Characteristics
of Rainfall
in
Rainfall is a resource. Sudanese rainfall is a scarce resource. It is a resource that is currently subject only partially to management strategies. For progress to be made toward its more effective management, the basic supply and demand characteristics need to be
(a)
18 Number 3/1987
understood. This is the case with any natural resource, but particularly so with a marginal, unstable resource such as semi-arid rainfall. Supply characteristics The spatial and temporal distribution of rainfall in central Sudan creates several distinctive characteristics of rainfall supply, several of which are shared with other semi-arid regions. These are investigated here as magnitude, reliability, spatial organisation, storm intensity and frequency and diurnal distribution. By definition annual magnitudes of rainfall in central Sudan are low, typically between 100 and 600 mm. The marginality of these low magnitudes is exaggerated by high rates of potential evapotranspiration, ranging from 1500 to over 2000 mm per annum. Using this simple accounting system no locality in central Sudan has an annual water surplus and few have monthly surpluses even in the height of the wet season in July and August. Reliability is poor, not only year-to-year decade-to-decade, as demonstrated by familiar patterns of twentieth-century
400.
-
Annual
ro/nfoll
-
F//teed
ser,es
.---......-
Full permd reference medmn
350,
but also the now rainfall
300
-c E - 250 2 Q is 4: 200 (L 2 ZJ 150 z 2 100
50
0
c
19ot 3
I
1910
I
1920
I
1930
1
1940
I
1950
Figure l(a).
I
1960
I
I
I
1970
1980
1990
Geoforum/Volume l-7
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323
15-23
24-26
15-20
Number
of
10
-2-o
-2.5 1’
0
I
I
I
1910
1920
1930
Figure 1. Annual
-
Raw
-
Filtered
,
1940
series series
I
1950
I
1960
I
1970
rainfall series in central Sudan, 1900-1986, for: (a) Khartoum,
(b) standardised
I
I
1980
1990
and
regional series integrating 26 stations.
behaviour (Figure 1). Prolonged periods of aboveand below-average annual rainfalls (runs) are a fundamental feature of all Sahelian rainfall series. A statistical experiment by GLANTZ and KATZ (1985) demonstrated that even under an hypothesis of a stable climate (i.e. sampling from a single population), runs of 12 years length can occur legitimately. The spatial organisation of Sudanese rainfall is also distinctive. The localised nature of semi-arid convective storms leads to very large daily, monthly and annual rainfall gradients over only, say, 5-10 km. Figure 2 illustrates the non-uniform surface of standardised annual rainfalls for 2 years, 1978 and 1985, in central Sudan. Note especially the sequence of values in the north-south transect down the Blue Nile in 1978. Whether such localisation should be regarded as a consequence of basically random smallscale convection operating is not so certain. Following on from work by SHARON (1979,198l) in Tanzania and Namibia, a preliminary investigation of central
Sudan stations suggests a high degree of spatial regularity in storm spacing, at least in some years (LIGHTWOOD, 1987) (see Figure 3). Storm frequencies also suggest a degree of rainfall organisation. Quasi-regular pulses in easterly wave activity result in temporal spacing of storms at given locations of the order of 3-6 days during the middle of the wet season (HAMMER, 1973). In principle this leads to beneficial temporal spacing of storms for soil moisture storage and crop water use. Rainfall intensities are high. The majority of rain falls in heavy, intense storms with approximately75% falling at between 5-10 mm/hr in central Sudan, compared to <5 mm/hr for temperature regimes (OLIVER, 1969). Daily falls of >25 mm are frequent. The diurnal distribution of rainfall varies between eastern and western central Sudan. Maximum incidence of rainfall during later afternoon and evening (often considered characteristic of a tropical area) is dominant only at locations far to the west of
GeoforumWolume
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18 Number 311987
1978
1985
Figure 2. Spatiai patterns of standardised
the Ethiopian Highlands. Elsewhere, rainfall is more evenly distributed throughout the day, although an early morning maximum is common in the Nile valleys (PEDGELEY, 1969).
annual rainfalls in central Sudan, 1978 and 1985. evaporation loss (20-30 cm/month) capacity for year-to-year management.
and
have
no
It is also necessary to sketch in briefly the major demand characteristics of rainfall in central Sudan. The rainfall resource is utilised primarily for domestic and livestock water supply, crop growth and natural vegetation (HUL~E, 1986).
Shallow groundwater aquifers, too, are highly dependent upon yearly recharge from rainfall events. Generally these aquifers (at less than 10 m depth) are completely decoupled from one wet season to the next, thus eliminating any year-to-year storage capacity to offset poor wet season performance. Such shallow wells are still the major supplier of domestic and livestock water in rural central Sudan {EL SA~~ANI, 1978).
The generation of surface runoff requires highintensity storms and is fundamental for the supply of both surface water storage reservoirs and recharge of shallow groundwater aquifers. The former comprise both natural and artificial features and also contribute (minimally) to permanent discharge from the region through the Nile rivers (the active Nile catchment in central Sudan is negligible in area and contribution). These surface water supplies are subject to large
Crop growth in rural Sudan is primarily rainfed. Irrigated cultivation in any large scale is almost totally restricted to the Nile systems. The large populations of western Sudan (Darfur and Kordofan Provinces combined number about 6 million) are subsistence, rainfed cultivators, a practise which is directly dependent upon rainfall events. These cultivators are acutely aware of rainfall performance and the adaptation of crop practise to rainfall resource
Geoforum/Volume characteristics (1984)].
325
18 Number 3/1987 is often
intricate
[see
for surface reservoirs and shallow aquifers, by reducing transpired water loss. The loss of thus released rainwater through increased surface evaporation, combined with absolute declines in rainfall magnitudes, however, argues against such a net increase for water supply.
IBRAHI~
Natural vegetation provides a building, fuel and grazing resource for rural communities. Annual grasses and stands of acacia are thus essential for the functioning of such communities and again are directly sensitive to rainfall events as mediated through soil moisture. Here there may be a trade-off in rainfall utilisation between natural vegetation cover and surface runoff. Vegetation depletion in recent decades should have increased input supplies
Rainfall as Liability
Having sketched the characteristics of rainfall
N
key supply and demand in central Sudan what of the
250Km 0
Significant
0
Not
sign/ficont
Probable signtficonfe ... at 0.07 ievei _I”“““” fincomplete data) “F_._..
150
IKm
150 Km
Figure 3. Composite
correlation field of pentad rainfalls for 15 stations in central Sudan for the years, 1948,1949,1962 and 1963 (lower half is reverse mirror image of upper half). Organisation of storms at around 35 km spacing is suggested.
326
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issue of rainfall as asset or liability? There is of course an immediate sense in which rainfall must be seen as an asset. None of the above water demands could be met should rainfall cease completely. There is another sense, however, in which rainfall appears to be regarded as a liability. The recent consciousness and perception of drought in the Sahel has equated it alongside war and civil unrest as a factor of ill-health, and on its own as a constraint upon development and as the source of both famine and desertification. Thus, in relation to disease: . . . warfare, drought and civil unrest are the three factors which have enhanced disease liability among children (RECELLIS. 1986). to famine: The famine that has hit Eritrea and much of SubSaharan Africa has been caused by drought and erosion (SUAN, 19X5). and to desertification:
. [desertification’s] major cause is thought to bc a change in the climate [drought] on the edge of deserts (BLUNDY and VALLELY, 1985). As we have seen, however, drought is one of the supply characteristics of rainfall in central Sudan (and the Sahel). One cannot therefore experience rainfall without experiencing drought. It is as fundamental a characteristic of rainfall in central Sudan as a wet season-dry season rainfall regime. Drought cannot be separated from rainfall. As GLANTZ (1986) has “Drought is a part of climate not apart commented, from it.”
Viewing drought therefore in such a negative way is an admission of failure to adapt to, or to manage adequately, the totality of the rainfall resource. Some of the perceptions of drought in the Sahel still regard drought as in some way an abnormal part of the climate in the region as if the removal of drought from the climate would guarantee effective rainfall management. This false perception of one characteristic of climate has therefore transferred the rainfall resource from being an asset to becoming a liability.
Supply Characteristics
of Drought
As drought is an integral part of the Sudanese climate, an awareness of the supply characteristics of drought years is necessary for its proper management,
18 Number
3/19X7
i.e. what are the characteristics of rainfall in dry years, compared to wet years? What is the anatomy of drought from the standpoint of the demand characteristics for waler in this semi-arid region? Recent research in both central Sudan and the West African Sahel has established certain common features. Annual reductions of rainfall in drought years do not lead to uniform reductions in monthly totals. The drought of the 1970s and 1980s has witnessed larger reductions in August and September totals (i.e. late wet season) than June and July totals (DENNETT et al., 1985; HULME, 1987). This drought characteristic does not seem to hold, however, for the earlier twentieth century droughts of the 1910s and 194%. Such skewed hehaviour of the monthly rainfall regime will affect rainfed crop production, since seeding schedules are determined by early-season, not late-season, rainfall. Storm intensities are reduced to a greater extent than storm frequencies. The frequency of heaviest storms (e.g. those with a yield of >40 mm) has shown a disproportionately large decrease in recent drought years over frequencies of light storms, i.e. storm events have not been reduced substantially, but the rainfall contribution of each event has. There are clear implications here for the generation of runoff, and hence for rural water supplies. The reliability of the wet season is reduced in drought years. Using a simple water balance model with wetseason definitions based on prolonged soil moisture surpluses, it has been shown that not only have periods of inadequate soil moisture levels increased within the wet season but that the frequencies of years when the wet season failed to commence (null starts) have increased (HULME, 1987). The probability of null starts between 1968 and 1985 increased severalfold over the 1900-1967 period (Table 1). Furthermore, in the 1970s null starts occurred for the first time at locations with between 200 and 300 mm median annual rainfall, the zone where rainfed cultivation is conventionally regarded as viable. Such phenomena have an inevitable detrimental impact upon crop yields. The spatial coherency of rainfall in drought. years is different from wet years. Analysis in central Sudan for the years 1901-1985 has demonstrated that dry years exhibit greater spatial coherency than wet years. The association between the standardised median regional rainfall value [cf. Figure l(b)] and a standardised measure of spatial variation (inter-
GeoforumlVolume
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Table 1. Frequencies
of null starts for 12 stations in central Sudan, 1968-1985
compared with 1900-1967
Probability of null start No. of years
No. of null starts
1900-1967
Khartoum
85
17
0.17
0.29
142
El Geteina Abu Deleiq Kamlin El Fasher Bara Ed Dueim Manaqil El Obeid Kosti En Nahud Sennar
80 78 80 68 78 84 76 82 52 71 55
13 7 4 1 2 1 1 -
0.14 0.05 0.02 -
0.24 0.26 0.18
185 197 222 254 276 287 300 353 385 388 439
Station
quartile range), is highly significant (Figure 4). A wet year in central Sudan is more likely to have localities of low rainfall than a drought year is likely to have localities of high rainfall. This particular characteristic has implications for national coordination and planning strategies for the alleviation of drought impact, through food and population redistribution.
Adaptation to Drought Why has adaptation to drought fallen so far behind the adaptation to many of the other supply characteristics of rainfall in Sahelian societies? Traditional rural practises of the past have responded to many of the supply characteristics of Sudanese rainfall, including drought.
Artificial water harvesting techniques using surface topography have enabled rainfed crops to be cultivated in areas of only 100 mm, thereby exploiting low annual rainfall magnitudes. Through multiple cultivation plots and the use of numerous water supply sources operated by both villagers and nomads, subsistence farmers have adapted to the spatial inhomogeneity of the rainfall field. IBRAHIM (1984) reported an interesting relationship between polygamy (which is practised by about 10% of Darfur farmers) and multiple plots. Those farmers which have more than one wife are often better able to obtain a fair crop from one of their wives’ plots. Since wives are often drawn from neighbouring but
Median 1968-1985 annual rainfall (mm>
0.06 0.12 0.08 0.06 -
separate villages, the probability of adequate rainfall being received at one of the areas cultivated is increased. Variable seeding schedules, with locally established critical dates, allows adaptation to the unreliability of the commencement of the rains. Similarly, an indigenous calendar of eight periods of 13 days each is used by farmers in western Sudan. The periods lo-23 July and 23 July-5 August are critical in the assessment of crop performance. If both periods have poor rains then preparation for a poor harvest begins. If both experience good rainfalls, it is assumed that the subsequent harvest will be fair. It is worth noting that the failure of late-season rains in August as noted above would not be detected by this procedure. Migration, of course, remains the oldest and probably most suitable form of adaptation to the drought characteristic of rainfall in central Sudan practised by nomadic groups of the region. Why then has drought, in particular the most recent drought, caused such disruption and devastation in countries such as Sudan? Why has not drought been so adapted to? The failures, it is argued, have tended to arise in the scale of Governmental planning initiatives and interventions rather than indigenous rural technologies and adaptations. Four reasons why Sahelian G~v~r~~en~s, such as the Sudanese, have not responded adequately to drought may be suggested [these exclude issues of more fundamental managerial incompetence as discussed by REILLY (1986)].
328
Geoforum/Volume
18 Number
311987
INTERQUARTILE RANGE (dq) 2-o .
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Figure 4. Scatter of regional standardised annual rainfall value (median, x-axis) against standardised measure (interquartile range, y-axis) of within-year variation. These data form the basis of the regional series displayed in Figure l(b), i.e. 19011985, central Sudan.
Rainfall data are not adequately measured and/or disseminated. Many Sahelian countries have a large area and a small rainguage network. Within the last lo-15 years these networks have been contracting rather than expanding. Figure 5 demonstrates this for Sudan, which reached a peak of recording gauges and published data in the 1960s. Often there has been inappropriate analysis of the existing rainfall data. Emphasis has been placed upon the central tendency of rainfalls, rather than examining the whole rainfall distribution. Skewed rainfall distributions (where the central tendency is a poor indicator of rainfall likelihood) and outliers (occasional very wet or very dry years) are common in these regions [see KATZ and GLANTZ (1986)]. A confusing of drought and climatic change has occurred. The two concepts are not the same. As has been argued above, drought is a result of an inherent variability which occurs under any particular climatic regime. It is a fundamental manifestation of the workings of the Sahelian atmosphere. A failure to perceive this has led to confusion and diversion of effort particularly in some planning and Government circles. There should be less urgency with the consequences of hypothetical long-term climatic change [as espoused by, for example, WINSTANLEY
(1985)] and more with the variability, which is drought.
reality
of short-term
The continuity of Government perceptions and policies in relation to drought has been poor (GLANTZ and KATZ, 1985). Frequent changes of Government in some Sahelian countries and high turnovers of professional staff in civil services (e.g. high professional labour migration rates from Sudan to the Gulf) has created short-term horizons and a failure to maintain and implement the drought strategies of predecessors. Each fresh new round of appointees has started as if from scratch. Also there is the more basic problem that, as with all political issues (and drought is a major political issue in these and goes’ in response to regions), it ‘comes movements and trends in national and international societies. There is a need for a continuity of approach and it is arguable whether indigenous Governmental organisations or external non-Governmental organisations (NGOs) are the better equipped to meet this need.
Can Rainfall
in Central
Sudan
be Managed?’
In the terms which have been used in this paper there are two options which can be pursued:
Geoforum/Volume
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I
1950
1
I
Figure 5. Number
of meteorological
1
1980
1967
1960
and rainfall stations
published
1
I
1
I
1985
in
Sudan Meteorological Service Reports, 1950-1985.
(a) (b)
to manage
the demand of the rainfall the light of supply, to manage the supply of rainfall
resource
in
in view
of
demand. These options can be re-expressed. First, can we reach a situation where one of the fundamental supply characteristics of rainfall (drought) is appropriated into Governmental and non-Governmental planning strategies, thereby encouraging demand to adapt to supply (e.g. through agricultural policy, resettlement and attitudes to nomadic communities)? Second, what are the possibilities of manipulating rainfall supply so that the demands of an expanding population in a marginal, semi-arid zone can be met? The second option is not feasible yet. Even simple forecasting of specific rainfall supply characteristics presently eludes the scientific community: Put bluntly, and despite occasional contrary, our forecasting capability rainfall] is close to zero (FARMER 1985, p. 101).
claims to the [for Sahelian and WIGLEY,
Consequently, we are well short of a full explanation of rainfall supply which is of course the necessary precursor to the manipulation of supply. What is becoming increasingly clear, however, is that ‘management by default’ (improvident management or inadvertant climatic change) is actually occurring
with respect to Sahelian rainfall. Three potential mechanisms for Sahelian drought are intimately linked with human activity (Figure 6). The reduction of surface biomass, either in the subhumid zone to the south of the Sahel or in the Sahel itself, is a socially driven process. This possibility remains a powerful hypothesis for Sahelian drought through moisture supply perturbation and albedo modification, respectively. Second, massive dust loading of the Sahelian troposphere has been identified in recent years (MIDDLETON, 1985; PROSPER0 and NEES, 1986), such an increase being primarily an anthropogenic process. Radiative modification leading to convection suppression remains a viable hypothesis. Third, the regional climatic signals of increasing atmospheric carbon dioxide (and other trace gases) are still poorly defined, particularly with regard to precipitation, yet it remains a possibility (albeit currently speculative) that regional circulation forcing could be traced back to rising carbon dioxide levels. There is therefore the looming spectre of the human depletion of the climatic resource, not just in the Beneficial Sahel, but worldwide. climatic management remains distant and, although not agreeing with the dogma of the following statement, the danger is undoubtedly real: Human
population
growth
is driving climatic
change
in
GeoforumiVolume
330 directions
that
(BROWN,
1986, p. 14).
will not
benefit
the
people
18 Number
marginal localities is making traditional adaptations to drought obsolete and, second, a degree of social integration and economic politicisation, dependency has already been thrust upon these societies which would prove impossible to revoke. A decoupling move would therefore undoubtedly qqeav as a degenerative step to organisations intent on extending their domain.
affected
Since we have dearly shown that the second option above is not viable, the first option (integrating drought characteristics into the planning process) becomes very pressing. Here again, one may subdivide policy into two options. First, Governmental organisations (and NGOs who are playing an increasingly important role in ‘governing’ activities in Sudan) must take on board the responsibility to respond at a national and institutional level to the recurring phenomena of drought, one of the basic supply characteristics of rainfall in central Sudan.
What we may, however, witness is the large-scale resettlement of semi-nomadic communities presently in central Sudan to some of the more subhumid regions of the country, e.g. North Darfur and Kordofan to South Darfur, South Kordofan and the Blue Nile provinces. One or two of the NGOs in Khartoum are already developing strategies along these lines. Certainly, in the face of climatic change it is one possible management strategy to match demand with supply (KATES, 1985) and has been practised to some extent in neighbouring Ethiopia. How much it will be a Governmental decision with a degree of grass-roots approval, rather than an imposed policy from outside the community, remains to be seen.
Second, there remains the option of returning these marginal, semi-arid rural societies to the control of of local local inhabitants, i.e. the decoupling communities from the demands and constraints national and international institutions place upon their economy and practise. Adaptation has been far more successfully carried out at a local and instinctive level in past decades and centuries than now. This would appear somewhat suggestion, however, idealistic and will prove impossibie for two reasons. The increasing density of population in these
The rainfall resource of central Sudan, which by incorporates drought, is an asset. de~nition
DROUGHT (Moisture
def kit)
I LOCAL
sea
Surface
ENERGY
Local
/
iemDerol”ies
GLOBAL
TELECONNECTI
Global
I
_. I,,
Figure 6. Framework
of possible
3/1987
explanations
for drought
in the Sahel.
Geoforum/Volume
18 Number
3/1987
However, by inappropriate exploitation and through poor or improvident (unforseeing and heedless) management it is increasingly becoming a liability.
Acknowledgements We thank Dr A. Trilsbach for comments on a earlier draft of this paper and Gustav Dobrzynski for the construction of the diagrams. Earlier visits to Sudan were supported by an N.E.R.C. studentship and British Council lecturing tour.
Note 1.
In this secion, the wider Sahel region is referred to since mechanisms of climatic change occur on scales larger than central Sudan.
References AUSUBEL, J. (1980) Economics in the air, In: Climatic Constraints and Human Activities, pp. 13-60, J. Ausubel and A. K. Biswas (Eds). Pergamon Press, Oxford. BLUNDY, D. and VALLELY, P. (1985) With Geldof in Africa. Times Books, London. BROWN, L. R. (1986) State of the World, 1985. W. W. Norton, New York. DENNETI, M. D.., ELSTON, J. and RODGERS, J. A. (1985) A reappraisal of rainfall trends in the Sahel, J. Clint., $353-361. EL SAMMANI, M. 0. (1978) Gaps in the water provision map of the Sudan. Sudan Notes Rec., 59,97-106. FARMER, G. and WIGLEY, T. M. L. (1985) Climatic Trends for Tropical Africa. Unpublished report for the Overseas Development Administration, Climatic Research Unit, Norwich. GLANTZ, M. H. and KATZ, R. W. (1985) Drought as a constraint to development in Sub-Saharan Africa, Ambio, 6,334-349. GLANTZ, M. H. (1986) Drought and economic development in Sub-Saharan Africa, In: Drought and Hunger in Africa: Denying Famine a Future, Chap. 3, M. H. Glantz (Ed.). Cambridge University Press, Cambridge. HAMMER, R. M. (1973) Applications and consequences of precipitation observations in the Republic of Sudan in
331 view of the nomadic life and economy, Geoforum, 14, 11-18. HULME, M. (1986) The adaptability of a rural water supply system to extreme rainfall anomalies in central Sudan, Appl. Geogr., 6,89-105. HULME, M. (1987) Secular changes in wet season structure in central Sudan, J. Arid Envir. (in press). IBRAHIM, F. (1984) Ecological Imbalance in the Republic of the Sudan. Druckhaus Bayreuth, Bayreuth. KATES, R. W. (1985) The interaction of climate and society, In: Climate Impact Assessment, pp. 3-37, R. W. Kates, J. H. Ausubel and M. Berberian (Eds). SCOPE 27, John Wiley, Chichester. KATZ, R. W. and GLANTZ, M. H. (1986) The anatomy of a rainfall index, Mon. Weath, Rev., 114,764-771. LANDSBERG, H. (1946) Climate as a natural resource, Scient Mon., 63,293-298. LIGHTWOOD, D. S. (1987) The spatial variability of convective rainfall: a tri-partite analysis in Africa, BSc dissertation, Department of Unpublished Geography, University of Salford. MIDDLETON, N. J. (1985) Effect of drought on dust production in the Sahel, Nature, 316,431-434. OLIVER, J. (1969) Evapotranspiration in the semi-arid tropics, J. trop. Geogr., 28,64-74. PALUTIKOF, J. P. (1986) Drought strategies in East Africa: the climatologists role, Clim. Change, 8,67-78. PEDGELEY, D. E. (1969) Diurnal variation of monsoon rainfall, Met. Mug., 98, 97-107 (part a), 129-134 (part b). PROSPERO, J. M. and NEES, R. T. (1986) Impact of the North African drought and El Nino on mineral dust in the Barbados trade winds, Nature, 320,735-738. RECELLIS, C. (1986) Interview with the UNICEF Director for eastern and southern Africa, Zimbabwe, T.V., July. REILLY, W. (1986) Administrative and managerial constraints to rural development, Paper presented at the workshop on Arid Lands: Problems and Prospects in the Sahel, I.T.E., Grange-over-Sands, May. SHARON, D. (1979) Correlation analysis of the Jordan Valley rainfall field, Mon. Weath. Rev., 107, 1042-1049. SHARON, D. (1981) The distribution in space of local rainfall in the Namib desert, J. Clim., 1,69-76. SUAN, A. (1985) Nat. geogr. Mug., June. WINSTANLEY, D. (1985) Africa in drought: a change in climate?, Weatherwise, April, 65-71.