Congo Basin Hydrology, Climate, and Biogeochemistry. Группа авторов

Table 6.1 Hydrological data gaps.

Hydrological stations	Period	Months missing	Years missing	Missing periods
Ubangi / Bangui	1938–2015 (77 years)	None	None	None
Ubangi / Mobaye	1938–2015 (77 years, 924 months)	475 months (51%)	21 years + 20 non complete years (53%)	1938, 1950, 1958–1969 1971, 1975–2014
Kotto / Kembe	1948–1995 (47 years, 564 months)	256 months (45%)	14 years + 16 non complete years (64%)	1949–1952, 1956, 1965–1969, 1976–1994
Kotto / Bria	1948–1995 (47 years, 564 months)	242 months (43%)	17 years + 5 non complete years (47%)	1948–1953, 1964, 1967, 1968, 1977–1989
Mbomu / Bangassou	1951–1995 (44 years, 528 months)	274 months (52%)	14 years + 20 non complete years (77%)	1951–1954, 1956–1962 1964–1973, 1976–1986, 1992, 1994
Mbomu / Zemio	1951–1995 (44 years, 528 months)	100 months (19%)	4 years + 13 non complete years (39%)	1951–1953, 1957–1961, 1969, 1976, 1979–1982 1984, 1985, 1993
Uele + Bili	Estimated	No interest	No interest

Rainfall data are from the Agence pour la Sécurité de la Navigation Aérienne en Afrique et Madagascar (ASECNA) in Bangui for CAR and from the IRD database for Congo. Annual rainfall data were collected from eight stations in CAR and three stations in Congo, spread over the entire Ubangi basin at Mobaye (Figure 6.1), based on the work of Orange et al (1994). From west to east (Figure 6.2), there are: Ippy, Mobaye, Bria, Ouadda Djalle, Yalinga, Bangassou, Rafaï, and Obo in CAR; Bondo, Niangara, and Watsa in Congo. The two rainfall stations at Yalinga and Obo are complete from 1951 to 1995; most of the other stations have common data from 1972 to 1992 (Table 6.2). Reconstruction of the missing data was done by analysis of annual correlations between stations, according to the work of Orange et al. (1994).

      The rainfall (P) and hydrology (Q) data have been arranged according to the hydrological year beginning April 1. The annual rainfall for each sub‐basin was calculated using the Thiessen method (Orange et al., 1994). The annual rainfall and hydrological indices (respectively, rainfall index and flow index) are standardized indices. They are calculated at the outlet of each watershed studied according to the following formula:

      (6.2)

where Xi is the value of the variable X for year i; Xm and Si are, respectively, the mean and the standard deviation of the variable X observed for the duration of the series studied. The variables considered are the annual precipitated water depth P (expressed in mm) and the annual flow Q (expressed in m³/s). This index makes it possible to define the severity of the drought. Negative annual values indicate drought in relation to the chosen reference period and positive values indicate a wet situation (Ardoin et al., 2003).

Table 6.2 Rainfall data gaps.

Rainfall stations	Period	Missing years	Missing periods
Yppi(1)	1951–1995 (44 years)	11 years (25%)	1951–1959, 1993, 1994
Mobaye	1951–1995 (44 years)	23 years (48%)	1951–1971, 1993, 1994
Bria	1951–1995 (44 years)	11 years (25%)	1951–1959, 1993, 1994
Ouadda Djalle(1)	1951–1995 (44 years)	11 years (25%)	1951–1959, 1993, 1994
Yalinga	1951–1995 (44 years)	None (0%)
Bangassou	1951–1995 (44 years)	11 years (25%)	1951–1959, 1993, 1994
Rafai	1951–1995 (44 years)	27 years (61%)	1951–1971, 1989–1994
Obo	1951–1995 (44 years)	None (0%)
Bondo	1951–1995 (44 years)	27 years (61%)	1951–1971, 1989–1994
Niangara	1951–1995 (44 years)	27 years (61%)	1951–1971, 1989–1994
Watsa	1951–1995 (44 years)	33 years (75%)	1951–1971, 1983–1994

      ⁽¹⁾ Yppi et Bambari are assimilated; Ouadda Djalle et Birao are assimilated.

      6.3.2. Break Tests for Hydropluviometric Data

      The break tests on the rainfall P and hydrometric Q series were performed with the Khronostat 1.01 software (IRD, 1998) to characterize the stability periods of these two long time sequences, establishing if possible homogeneous flow subsequences, or hydroclimatic periods. Pettitt, Lee and Heghinian rank tests, and Hubert’s segmentation were applied (Hubert & Carbonnel, 1987). The combination of these various methods is justified by the fact that the first two