Model-based study of the role of rainfall and land use–land cover in the changes in the occurrence and intensity of Niger red floods in Niamey between 1953 and 2012

Abstract

Abstract. Since 1950, the Niger River basin has gone through three main climatic periods: a wet period (1950)(1951)(1952)(1953)(1954)(1955)(1956)(1957)(1958)(1959)(1960), an extended drought (1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980) and since 1990 a recent partial recovery of annual rainfall. Hydrological changes co-occur with these rainfall fluctuations. In most of the basin, the rainfall deficit caused an enhanced discharge deficit, but in the Sahelian region the runoff increased despite the rainfall deficit. Since 2000 the Sahelian part of the Niger has been hit by an increase of flood hazards during the so-called red flood period. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate versus land use-land cover (LULC) changes are still debated and are investigated in this paper. The evolution of the Niger red flood in Niamey from 1950 to 2012 is analysed based on long-term records of rainfall (three data sets based on in situ and/or satellite data) and discharge, and a hydrological model. The model is first run with the present LULC conditions in order to analyse solely the effect of rainfall variability. The impact of LULC and drainage area modification is investigated in a second step. The simulations based on the current surface conditions are able to reproduce the observed trend in the red flood occurrence and intensity since the 1980s. This has been verified with three independent rainfall data sets and implies that rainfall variability is the main driver for the red flood intensification observed over the last 30 years. The simulation results since 1953 have revealed that LULC and drainage area changes need to be invoked to explain the changes over a 60-year period.

The paradoxical evolution of runoff in the pastoral Sahel: analysis of the hydrological changes over the Agoufou watershed (Mali) using the KINEROS-2 model

“…If this variable is only taken into account (simulation "P"), simulated surface runoff decreases rather than increasing over time. This is in line with Descroix et al (2013), Casse et al (2016) and Aich et al (2015), who found that the modest increase in large rainfall amount (events > 40 mm) observed during the 2000s cannot alone explain the Sahelian paradox. However, this should be taken with caution because changes in precipitation not statistically detectable here may have occurred elsewhere, due to the high natural variability, and further studies are required to address this question in more detail.…”

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…In the same MNRB, Cassé et al [54] observed that the Upper Niger contribution has followed the climatic trend, and inversely, that "the local runoff contribution has been continuously increasing over the 1953-2012 period, which is paradoxical given the rainfall signal" ( [54], p. 2853). The opposite evolution between upstream and downstream signals results in the progressive increase of the "red" flood since the 1980s.…”

“…Aich et al [93] and Cassé et al [54] showed that during the dry period (1st Sahelian paradox), LULCC was the main explanatory factor of the increasing stream flows, while, since the annual rainfall recovery period (since the end of the 20th century), rainfall intensification has also contributed to this trend, the respective roles of both being difficult to determine. In addition, two series of facts could complete this statement:…”

“…In these three basins, runoff coefficients are at least 4 times larger (see Figure 4b), and discharges at least 3 times larger, than the ones observed during the 1950-1970 wet period. This is why this area is the most studied (focused on) within the Sahel [2,3,6,7,9,19,46,51,54,[67][68][69][70]76,78]. Contrary to the sole Niamey area, MNRB is therefore representative of the whole Sahel in terms of land cover evolution [10,[12][13][14]33,34].…”

Process refinements improve a hydrological model concept applied to the Niger River basin

“…In response, a regional committee was set up to provide seasonal drought outlooks, informed by various monitoring and forecasting systems (e.g., Sheffield et al, 2014). Since then, land cover changes and wetter conditions has increased runoff coefficients and river discharge, particularly in the Sahelian zone (Aich et al, 2015;Amogu et al, 2010;Casse, Gosset, Vischel, Quantin, & Tanimoun, 2016). This has resulted in higher flood peaks, arriving earlier in the season (Mahé & Paturel, 2009).…”