Читать книгу Congo Basin Hydrology, Climate, and Biogeochemistry - Группа авторов - Страница 92

The average interannual rainfall on the Ubangi basin at Mobaye is 1,506 mm over the period 1938–2015. On the one hand, the search for breaks with Hubert’s segmentation reveals two breaks (1968 and 2006) and thus three homogeneous periods (1938–1968, 1969–2006, and 2007–2013). On the other hand, a single break appears in 1969 with the Pettitt and Lee‐Heghinian tests (Table 6.3). This time series is thus characterized by a definite rainfall break in 1968/1969 (Figure 6.3), marking the passage from a previous wet period to a dry period, and a probable second break in 2006 to be confirmed in the future, and which would characterize the return to a wetter period. The average rainfall amounts for the periods 1938–1968, 1969–2006, and 2007–2015 are 1,568 mm, 1,436 mm, and 1,583 mm respectively. The dry period of 1969–2006 corresponds to a reduction of 8% in rainfall compared to the previous period. And the current wet period (2007–2015) describes a 10% increase in rainfall compared to the previous period.

Table 6.3 Statistical segmentation of annual rainfall in the Ubangi basin at Mobaye (1938–2015), and in the upstream basins from 1951–1995 (in the Mbomu basin at Bangassou and Zemio, and in the Kotto basin at Kembe and Bria).

Hydrological basins	Interannual average (mm)	Pettitt	Lee and Heghinian	Segmentation of Hubert (mm)	Standard Deviation	Hydro‐climatic trends	P divided by interannual average (%)
Ubangi at Mobaye (403,800 km2)	1,506	1969	1969	1938–1968: 1,568 1969–2006: 1,436 2007–2013: 1,583	109 88 81	Wet Dry Wet	+4 –5 +5
Mbomu at Bangassou (115,000 km2)	1,491	1970		1951–1994: 1,851
Mbomu at Zemio (29,300 km2)	1,421	1970		1951–1994: 1,748
Kotto at Kembe (77,500 km2)	1,348	1969	1969	1948–1980: 1,458 1981–1991: 1,306 1992–1993: 1,577	94 89 32	Wet Dry Wet	+2 –8 +11
Kotto at Bria (61,500 km2)	1,348	1967	1967	1948–1966: 1,436 1967–1991: 1,267 1992–1993: 1,515	94 105 83	Wet Dry Wet	+7 –7 +12

Figure 6.3 Rainfall index and flow index of the Ubangi basin at Mobaye, 1938–2015.

The 1938–2015 time series of annual flows from the Ubangi at Mobaye is marked by a sharp decline in flows in 1970 (Figure 6.2). Statistical tests indicate only one break with the Pettitt and Lee‐Heghinian tests: it is noted in 1981 on the 1938–2015 series, and in 1970 on the 1948–1995 series. And Hubert’s segmentation describes five homogeneous periods for the two series tested (1938–2015 and 1948–1995) but only the years 1968 and 1980 appear for the two hydrological series tested, attesting to the importance of the statistical break at these two dates. Three hydroclimatic periods are thus retained for the Ubangi at Mobaye (Table 6.4): 1954–1968, 1969–1980, 1981–2013, with interannual mean flows of 3,919 m³/s, 2,986 m³/s (–24%) and 2,372 m³/s (–21%), respectively.