Bull. Séanc. Acad. R. Sci. Outre-Mer Meded. Zitt. K. Acad. Overzeese Wet. 55 (2009-2) : 187-198 How Extreme is the Precipitation at Lubumbashi, Congo, or the Elaboration of Intensity-Duration-Frequency (IDF) Curves for Precipitation, under the Hypothesis of Inadequate Data * by Gaston DEMARÉE ** & Hans VAN DE VIJVER *** KEYWORDS . — Congo; IDF Curves; Precipitation; Extremes and Inadequate Data. SUMMARY . — The Intensity-Duration-Frequency (IDF) curves for precipitation are a probabilistic tool which has proven its usefulness in water resources management. In par - ticular, the IDF curves for precipitation enable to answer questions on the extreme char - acter of precipitation. The establishment of IDF curves for precipitation is made difficult or impossible in tropical areas by the lack of long-term extreme precipitation data. The authors propose a technique to overcome this shortcoming by using limited high-fre - quency information on rainfall extremes together with long-term daily rainfall informa - tion. Using this technique, IDF curves for precipitation are elaborated for Lubumbashi in Congo and a table with rainfall depths for different durations and return periods is given. TREFWOORDEN . — Congo; IDF-krommen; Neerslag; Extremen en onvoldoende gege - vens. SAMENVATTING . — Hoe extreem is de neerslag in Lubumbashi, Congo, of het opstellen van intensiteit-duur-frequentiekrommen voor de neerslag met een beperkte dataset . — De Intensiteit-Duur-Frequentie (IDF) krommen voor de neerslag is een probabilistisch in - strument dat zijn nut heeft bewezen in het waterbeheer en -beleid. Specifiek laten de IDF- krommen voor de neerslag toe van vragen te beantwoorden over het extreme karakter van de neerslag. Het opstellen van IDF-krommen voor de neerslag in tropische gebieden wordt bemoeilijkt of zelfs onmogelijk gemaakt door het ontbreken van lange termijn gegevens van extreme neerslagwaarden. The auteurs stellen een methodologie voor die deze tekortkoming overbrugd door gebruik te maken van een beperkte hoeveelheid hoog - frequente neerslaggegevens gekoppeld aan lange termijn dagelijkse neerslagwaarden. Die techniek wordt geïllustreerd door het aanmaken van IDF-krommen voor Lubumbashi, Kongo, en een tabel met neerslaghoeveelheden voor verschillende duren en terugkeerpe - rioden wordt gegeven. ————— * Paper presented at the meeting of the Section of Technical Sciences held on 29 January 2009. Text received on 31 March 2009. ** Member of the Academy; Royal Meteorological Institute of Belgium, Ringlaan 3, B-1180 Brussels (Belgium). *** Royal Meteorological Institute of Belgium, Ringlaan 3, B-1180 Brussels (Belgium). — 188 — MOTS -CLES . — Congo; Courbes IDF; Précipitations; Valeurs extrêmes et données insuf - fisantes. RESUME . — Précipitations extrêmes à Lubumbashi, Congo, ou la création de courbes d’intensité-durée-fréquence des précipitations à l’aide d’un ensemble de données limité . — Les courbes Intensité-Durée-Fréquence (IDF) des précipitations sont un outil probabi - liste qui a démontré son utilité dans la gestion des ressources en eau. En particulier, les courbes IDF permettent de donner une réponse au caractère extrême des précipitations. En région tropicale, l’établissement des courbes IDF est souvent rendu difficile, voire impossible, par le manque de données à long terme des valeurs extrêmes. Les auteurs pré - sentent une technique qui surmonte cette difficulté en utilisant un jeu de données à haute fréquence de portée réduite, combiné à un jeu de données à long terme des valeurs extrêmes des cotes pluviométriques journalières. Cette technique est illustrée par l’éta - blissement de courbes IDF pour Lubumbashi au Congo et un tableau des quantités de pluies pour des durées et des périodes de retour est proposé. Introduction In June 1973, a WMO-UNESCO-IAHS Symposium was held in Madrid deal - ing with the design of water resources projects with inadequate data as a contrib - ution to the International Hydrological Decade (WMO-UNESCO-IAHS 1974). The design of water resources projects with inadequate data remains a continu - ing engineering challenge in spite of the modern and widespread observational techniques. In particular, Intensity-Duration-Frequency (IDF) curves for precip - itation are urgently needed for water resources projects in tropical Africa but rainfall data with aggregation times less than one day are rarely available over long reference periods. These data are required for water resources projects, sewer system design, or water quality management projects of large urban areas such as Kinshasa, Lubumbashi, Kisangani, Mbuji-Mayi and others in Congo. However, data sets of extreme rainfall depths for aggregation times less than one day are, in general, lacking or do exist only for a few locations and even then, for only a few years ( MOHYMONT et al. 2004, MOHYMONT & DEMAREE 2006). These data are also needed for the rehabilitation of the inadequate or outdated existing sewer system network due to the demographic expansion, the extension of the urbanized periphery, the increase in impervious areas, and the increased needs for water followed by the increased volume of the waste water flow (for the case of the town of Lubumbashi, Congo, see KABANGE -N UMBI 2007). Occurrence of Extreme Rainfall and Floods in Congo In this section some examples of severe rainfall followed by floods in the urbanized areas of large Congolese towns are given to illustrate the importance of the subject and to underline the necessity of determining IDF curves for rain - fall to cope with this occurrence. — 189 — Following Marcel Crabbé in his yearly descriptions of the main features of the climate in Kinshasa: The rainfall of January 12th, 1969, at Kinshasa-N’djili shows an hourly rainfall intensity record of 81 mm depth, and a total depth from 15 h 25 to 18 h 50 of 88.4 mm (Crabbé, Kinshasa, 1969) The maximum of the 24-hour rainfall depth is 112.5 mm at Kinshasa-N’djili and 89.4 mm at Kinshasa-Binza for a rain occurring in the night of November 24th, 1970. However, these amounts are still less than the absolute record of 150.2 mm registered in one day in March 1965. (Crabbé, Kinshasa, 1970) More recent descriptions of rainfall events from the rainy season 2008-2009 are given below: Kinshasa: The torrential rain that beat down last night on the capital has caused important material damage and even loss of human lives. (Radio Okapi, 03 November 2008) At least two deaths in the town of Lubumbashi and as many at Likasi. This is the toll of the rain that beat down last Sunday in several towns of Katanga and that has caused important material damage. ( Le Potentiel , 20 November 2008) 178 houses collapsed throwing on the street hundreds of families in the town of Nyunzu, Katanga, following the torrential rains in the night of 30 to 31 December 2008. (Reliefweb, Caritas, 2 January 2009) More than 100 houses collapsed in the quarter Kilobelobe, in the commune Annexe, at Lubumbashi, following torrential rains from 31 January to 1 February 2009. (Digitalcongo.net, 05/02/2009) Formulation of the Problem This paper shows a technique to overcome data limitation. In many cases, long-term daily precipitation time-series are available for climatological stations. From these series Annual Maxima (AM) or Peak-Over-Threshold (POT) sets may be derived and an extreme value analysis may be carried out providing an excellent basis for the analysis. The paper shows that such an analysis is a trust - ful basis for the establishment of the IDF curves of precipitation provided that limited material for shorter aggregation times than one day is available. The IDF curves for precipitation ( VEN TE CHOW 1964, REMENIERAS 1972, MAIDMENT 1992) determine the relationship between the mean intensity i (in mm h-1 ), the duration, or better said the aggregation time d (in min), and the frequen - cy of the event expressed as a return period T (in year). The return period is defined according to the selected approach, AM or POT. Little is known on IDF curves for precipitation in the tropical area of the Congo River basin. There are the outdated publications of BULTOT (1956) and — 190 — PIRE et al. (1960). Recently MOHYMONT et al . (2004) published a comparison between the IDF curves for in precipitation Yangambi, Kinshasa-N’djili and Kinshasa-Binza in Congo and the station of Uccle in Belgium. MOHYMONT & DEMAREE (2006) modelled the IDF curves for precipitation in Yangambi by dif - ferent forms of the Montana formula. Data and Climate of Lubumbashi The town of Lubumbashi is located at 1,200 m elevation. Its climate belongs to the type Cw6 in the Köppen classification, a tropical dry climate with two alternating seasons, a dry one and a wet one of approximately six months (from October to March/April). At Keyberg (Kisanga), a station located at 8 km south- west of Lubumbashi, the duration of rainfall was studied by BULTOT (1956). The afternoon rains are less frequent than the night rains; they are of short duration and do not reach high depths. The night rains last longer and are located between 21 h and 7 h. The afternoon rains are short but intense while the intensity of night rains is more uniform. 50 % of showers in the day take longer than two hours and more than four hours in the night; 20 % last longer than four hours in the day and longer than five hours in the night. HARJOABA & MALAISSE (1978) studied the rainfall at Lubumbashi in great detail. Early sporadic climatological observations in Katanga were carried out dur - ing the exploratory expeditions in the early 1890s ( AMERLINCK 1893, BRIARD 1893). Daily rainfall observations for individual months in three different loca - tions in Katanga were carried out by the scientific expedition of Commandant Lemaire between October 1898 and December 1899. The largest daily amount of 57.3 mm was observed at Lukafu in December 1899. Daily precipitation depths were published in the Bulletin Officiel de l’Etat Indépendant du Congo and in the Bulletin Officiel du Congo Belge at Kambove (near Likasi) for the period August 1907 – December 1909 (Anonymous 1908, 1908-1909, 1910) and the mine Etoile du Congo , near Elisabethville, for the period January – August 1910 (Anonymous 1910).