The hydrological effects of two extreme rainfall events over East Africa: 1961 and 1997

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1 FRIEND 2002 Regional Hydrology: Bridging die Gap between Research and Practice (Proceedings of (he Fourth International FR1I-ND Conference held at Cape Town. South Africa. March 2002). IAHS Publ. no The hydrological effects of two extreme rainfall events over East Africa: 1961 and 1997 DECLAN CONWAY School of Development Studies, University of East Anglia, Norwich NR4 7TJ, UK d.conwav(s).uea,ac,uk Abstract An assessment of the hydrological effects of the two most extreme October-November rainfall events in East Africa during the twentieth century is provided. Flow records for major rivers draining large parts of East (White Nile and Tana), central (Congo and Oubangui), the Horn (Blue Nile and Atbara) and southern Africa (Zambezi) are used for the analysis. During and after the 1961 event flooding was widespread throughout the region. Between 1961 and 1964 the cumulative excess volume of flow above the mean of all the rivers analysed was 357 km 3 year" 1. The availability of hydrological data for 1997 is restricted to the Blue Nile (unexceptional flows), Congo (very high flows) and Lake Victoria (significant rise in lake level). The historical records are surveyed for possible precedents to these events and the year most similar is 1878, which is generally noted as an extremely wet year across the region. Key words rainfall; extremes; flooding; East Africa; Nile INTRODUCTION During the last few months of 1997 widespread heavy rainfall caused flooding across East Africa and the Horn of Africa. For example, the October-February dry season in was the wettest on record over much of Ethiopia (Conway, 2000) and the levels of many lakes in East Africa rose significantly in response to heavy rainfall (Birkett et al, 1999). This event produced wide-ranging agricultural, hydrological, ecological and economic impacts across the region. The event in 1997 prompted studies into its nature and causes and renewed interest in an earlier event with similar characteristics that occurred in The 1961 event over East Africa was widely documented at the time and in subsequent analyses. Odingo (1962) described the social and economic disruption caused by the event and estimated the total flood damage costs at the time for Kenya to have been around five million GB pounds. Research into the 1997 event (and also the 1961 event) has shown that it is associated with a reversal in atmospheric circulation and sea surface temperatures in the Indian Ocean (Murtugudde & Busalacchi, 1999). The 1961 and 1997 Indian Ocean events were associated with extremely heavy rainfall over East Africa mainly during October and November (ON), primarily affecting the area between 10 N and 10 S and from at least 25 E to the Indian Ocean (Conway, in press). The spatio-temporal characteristics of the 1961 and 1997 events are quite exceptional in terms of magnitude and extent during the twentieth century. This study provides a comprehensive assessment of the hydrological effects of the 1961 and 1997 rainfall events over East Africa, and surveys the historical records for possible precedents. The implications of these events for water resource management are then discussed.

2 476 Declan Conway THE HYDROLOGICAL EFFECTS OF THE 1961 EVENT Flow records for major rivers draining large areas of East, central and the Horn of Africa were analysed. The impacts of the 1961 event are well documented for the East African lakes (Grove, 1996) and only Lake Victoria is presented here. Flow and lake level data for the Nile are primarily from Hurst & Phillips (1933). The Congo, Oubangui and Tana series are from UNESCO (1995, Congo updated from Laraque et al, 1998) and the Zambezi series from Grove (1996). Table 1 lists the river and lake gauging locations and basin areas. Annual flow series are shown in Fig. 1 and monthly mean ( ) and monthly flows ( ) are shown in Fig. 2. Flows for the Equatorial Lakes are calculated as the difference between river flow at the outlet to Lake Victoria and downstream at Mongalla, just upstream of the Sudd swamps. The flows of the Main Nile, Atbara and Blue Nile were unexceptional during Neither the Blue Nile nor Atbara show unusual flows during ON 1961 (Fig. 2, Main Nile not shown) produced the Main Nile's highest flow since the 1890s due to a combination of high Blue Nile and Equatorial Lake flows and maximum Lake Victoria levels. Sobat flows were above average during 1962 due to very high flows between December 1961 and March 1962 (Fig. 2). Monthly flows were high throughout 1962 and October 1964-February The monthly Equatorial Lakes time series show that a massive increase in flows occurred from August 1961, three months earlier than Lake Victoria (see below). It produced high annual runoff in 1961, whereas the increase in Lake Victoria levels only becomes notable by 1962 (Fig. 1). The return in flows to magnitudes typical of the pre-1961 event was faster than in Lake Table 1 Characteristics of lake level and river flow series. River or Lake Gauge location Latitude Longitude Period of record Basin area (km 2 ) MAR c MAR Main Nile Dongola" N E Atbara 3 kilo 7.41 N E Blue Nile EI Deim 1 ' N E Per cent change" Sobat Hillet 9.20 N E Doleib Equatorial Lakes Owen Falls Mongalla 0.43 N 5.12 N E E Lake Owen Falls 0.43 N E Victoria ' Tana Garissa 0.45 S E Congo Kinshasa 4.30 S E Oubangui Bangui 4.37 N E Zambezi Livingstone /Victoria Falls S E 1907/8-1993/ a Main Nile, Aswan (dashed line), Wadi Haifa, Kajnarty, Dongola. 11 Blue Nile, Khartoum (dashed line), Roseires, El Deim. c or earlier, depending upon record length. " L ' % difference between and MAR = mean annual river flow (nr s"').

3 The hydrological effects of two extreme rainfall events over East Africa: 1961 and 1997 All Fig. 1 Annual flows (km 3 ) for major East African rivers and Lake Victoria levels (note different v scales). Dotted vertical lines highlight six years, i.e. 1878, 1916, 1917, 1961, 1964 and 1997.

4 478 Declan Conway Victoria. Downstream impacts of high flows along the White Nile were significant. A significant and sustained increase in Lake Victoria levels and outflows occurred in late Lake Victoria levels peaked in 1964 and decreased steadily, except for increases in and , but even by 1997 they remained well above their pre levels. The Tana River has the smallest gauged basin area of the rivers examined here (Table 1) and provides the clearest and most short-lived example of flow response to the 1961 event. The anomaly lasts from October to December, peaking in November 1961, and generating the second highest annual flow on record. October, November and December 1961 produced the highest monthly flows on record in the Congo basin making 1961 the sixth highest annual flow on record recorded maximum monthly flows from January to October and produced an even higher annual flow (over 30% higher than the long-term mean). Flows remained exceptionally high until the end of 1964, probably maintained by a combination of high rainfall in , storage within this vast complex river system with areas of seasonal and permanent wetland, and possibly additional inputs from Lake Tanganyika via the River Lukuga outlet as occurred in 1878 (Nicholson, 1999). Annual flow in the Oubangui, a major tributary of the Congo River, is very similar (annual flows not shown) except that the anomalies begin one month earlier (September 1961, Fig. 2). Further to the south, 1961 produced the third highest Zambezi flood on record but was followed by three years with fairly low flows. THE HYDROLOGICAL EFFECTS OF THE 1997 EVENT The availability of in situ hydrological data for 1997 is restricted in this study to the Blue Nile, Lake Victoria and Congo along with satellite observations of some East African Lake levels. Blue Nile flows were unexceptional in 1997, much lower than in 1961, but basin rainfall during the following dry season (October-February) was the highest on record (Conway, 2000). A significant rise in Lake Victoria level occurred in 1998, a rise of approximately 1.0 m, slightly less than the 1.2 m rise from 1961 to 1962 (mean annual levels) which was the largest annual increase on record. Whether levels remain high as after 1961, or will fall more rapidly as after the rises in and will largely depend upon subsequent rainfall over the lake basin. Conway (in press) found that 1961 was followed by a sequence of wet years over the main lake basin whereas rainfall after 1997 has not been particularly high. Satellite altimetry data also show similar Lake Victoria level increases to the in situ observations during 1997 and 1998 (a rise of about 1.7 m by June 1998, Birkett et al., 1999), however, by December 2000 levels had returned to conditions similar to those before 1997 (Birkett, personal communication). Birkett et al. (1999) also identified large increases in other lake levels across East Africa as a result of heavy rainfall in 1997, for instance Lake Tanganyika rose by about 2.1 m and Lake Malawi by about 1.8 m and a rise occurred in the levels of the Sudd swamps. IRD (1999) showed very high flows for the Congo at Kinshasa during December 1997-March 1998, in fact during January-March 1998 flows were almost as high as during the same months in

5 The hydrological effects of two extreme rainfall events over East Africa: 1961 and Fig. 2 Mean ( ; dashed line) and monthly ( ; solid line) river flows (km 3 ) (note different y scales). Dotted vertical line highlights November 1961.

6 480 Declan Conway HISTORICAL PRECEDENTS The period of extremely high Main Nile flows prior to 1899 has been the subject of a number of studies especially concerning the accuracy of the early gauge data. Of note during this high flow period are the floods of 1878 and Sutcliffe & Parks (1999) identified periods of high flows at Aswan (January-May, indicative of high Lake Victoria levels) during 1879, , and similar flow increases upstream at Dongola after The Atbara and Blue Nile had high flows during and The Sobat displays similar flood years to the Blue Nile, in and and 1962 (one year later than Blue Nile floods). Flows from the Equatorial Lakes basin, along the Nile region north of Lake Victoria and south of the Sudd swamps, were also extremely high in Lake Victoria levels had two short-lived high periods during and but the annual series is dominated by the sudden change from 1962 onwards (Fig. 1). Small floods occurred in the Congo and Oubangui in 1909 and in 1917, as in the White and Blue Niles. The Zambezi flows show quite different temporal variability to the other series. The period is unexceptional and the prolonged increase in flows from 1940 to 1956 does not feature in any of the other series. Apart from the extreme floods in there are no other hydrological events during the twentieth century similar to the wide-ranging and extreme characteristics of the period. There is not enough evidence here to compare in detail the extent of the 1997 event with 1961 in terms of spatial extent and prolonged impacts on lake levels. The episode was smaller in magnitude and extent, mainly limited to the area of Lake Victoria, southern Sudan and southwestern Ethiopia. The most similar year to the early 1960s peak flows is 1878 which was a year of record floods on the Blue Nile, very high Lake Victoria levels and is generally noted as an extremely wet year across the region. Nicholson (1999), for example, cites the explorer Thompson as reporting Lake Tanganyika's maximum level was in Lyons (1906) reports travellers' observations of high rainfall and lake levels in both the March-May and ON seasons in The lake rose three feet (0.91 m) above its usual June maximum in August and September 1878, in consequence of the heavy rains. Lyons also mentions that rainfall in 1878 was particularly a very late rainfall, extending far into the autumn. Some stage readings do exist for the Blue Nile at Khartoum prior to 1900, for the years between 1869 and 1883 (Walsh et al., 1994). These indicate floods were more frequent and severe in the period than at any time during the twentieth century, with 1878 producing the highest flood on record. Tate et al. (2001) use the long Aswan flow series to derive new estimates of Lake Victoria levels for This series, supported by other historical evidence, suggests the 1878 levels were the highest on record. DISCUSSION The results of this study go some way towards understanding the regional hydrological response to the extreme rainfall events of 1961 and The events of 1961 and 1997 and the possibility of similar events occurring in the future raises the question of their predictability and has implications for longer-term resource management strategies in

7 The hydrological effects of two extreme rainfall events over East Africa: 1961 and the region. There is also the possibility that climate change may alter the nature and frequency of Indian Ocean events and their relationship with rainfall over East Africa. The immediate hydrological impacts of such events include disruption and damage due to temporary inundation of lakeside and wetland areas and river flooding. Longer term management implications revolve around the dynamic nature of water resources over time and the need for flexible management systems that consider the inherent uncertainty in the resource base. This undermines traditional assumptions of reliable yields for planning water supply projects. Fluctuating lake levels also present management challenges and opportunities for fishing and other lakeside activities. Further study of the impacts and response strategies associated with this type of extreme event is necessary to provide an indication of the region's vulnerability and adaptive capacity in relation to present day climate variability and also to future climate change. REFERENCES Birkett, C. M., Murtuguckle, R. & Allan,.1. A. (1999) Indian Ocean climate event brings floods to East Africa's lakes and Sudd Marsh. Geophys. Res. Lett. 26, Conway, D. (2000) The climate and hydrology of the Upper Blue Nile, Ethiopia. Geogr. J. 166, Conway, D. (in press) Extreme rainfall events and lake level changes in East Africa: recent events and historical precedents, in: The Second International Symposium on the Limnology. Climatology and Palaeoclimatology ofthe East African Lakes (ed. by E. O. Odada) (Proc. Club Makakola, Malawi Symp., 2000). Kluwer, Dordrecht. Grove, A. T. (1996) African river discharges and lake levels in the twentieth century, in: The Limnology, Climatology and Paleoclimatology ofthe East African Lakes (ed. by T. C. Johnson & E. O. Odada), Gordon & Breach, The Netherlands. Hurst, H. E. & Phillips, P. (1933) The Nile Basin, vol. IV. Ten-day Mean and Monthly Mean Discharges of the Nile and its Tributaries. Ministry of Public Works, Physical Department, Cairo. IRD (1999) Observatoire hydrologique regional de l'afrique de l'ouest et Centrale. Accessed online 19 May 1999 at Laraque, A., Maziezoula, B., Orange, D. & Olivry,.1. C. (1998) Origine des variations de débits du Congo à Brazzaville durant le XXième siècle. In: Water Resources Variability in Africa during the XXth Century (ed. by E. Serval, D. Hughes, J.-M. Frilsch & M. Hulme) (Proc. Abidjan'98 Conf., November 1998), IAHS Publ. no Lyons, H. G. (1906) The Physiography ofthe River Nile and its Basin. Royal Meteorology Society, Cairo. Murtugudde, R. & Busalacchi A..1. (1999) Interannual variability ofthe dynamics and thermodynamics ofthe tropical Indian Ocean. J. Climate 12, Nicholson, S. E. (1999) Historical and modern fluctuations of lakes Tanganyika and Rukwa and their relationship to rainfall variability. Climatic Change 41, Odingo, R. S. (1962) The abnormal and unseasonal rains in East Africa. Geogr. Rev. 52, Sutcliffe, J. V. & Parks, Y. P. ( 1999) The Hydrology ofthe Nile. IAHS Special Publ. no. 5. Tate, E. L., Sene, K. J. & Sutcliffe, J. V. (2001) A water balance study ofthe upper White Nile basin Hows in the late nineteenth century. Hydrol. Sci. J. 46(2), UNESCO (1995) Discharge of Selected Rivers of Africa. UNESCO Studies in Hydrology no. 52, Paris. Walsh, R. P. D., Davies, H. R. J. & Musa, S. B. (1994) Flood frequency and impacts at Khartoum since the early nineteenth century. Geogr. J. 160,