Climate or Land Use?—Attribution of Changes in River Flooding in the Sahel Zone

Abstract

This study intends to contribute to the ongoing discussion on whether land use and land cover changes (LULC) or climate trends have the major influence on the observed increase of flood magnitudes in the Sahel. A simulation-based approach is used for attributing the observed trends to the postulated drivers. For this purpose, the ecohydrological model SWIM (Soil and Water Integrated Model) with a new, dynamic LULC module was set up for the Sahelian part of the Niger River until Niamey, including the main tributaries Sirba and Goroul. The model was driven with observed, reanalyzed climate and LULC data for the years 1950-2009. In order to quantify the shares of influence, one simulation was carried out with constant land cover as of 1950, and one including LULC. As quantitative measure, the gradients of the simulated trends were compared to the observed trend. The modeling studies showed that for the Sirba River only the simulation which included LULC was able to reproduce the observed trend. The simulation without LULC showed a positive trend for flood magnitudes, but underestimated the trend significantly. For the Goroul River and the local flood of the Niger River at Niamey, the simulations were only partly able to reproduce the observed trend. In conclusion, the new LULC module OPEN ACCESSWater 2015, 7 2797 enabled some first quantitative insights into the relative influence of LULC and climatic changes. For the Sirba catchment, the results imply that LULC and climatic changes contribute in roughly equal shares to the observed increase in flooding. For the other parts of the subcatchment, the results are less clear but show, that climatic changes and LULC are drivers for the flood increase; however their shares cannot be quantified. Based on these modeling results, we argue for a two-pillar adaptation strategy to reduce current and future flood risk: Flood mitigation for reducing LULC-induced flood increase, and flood adaptation for a general reduction of flood vulnerability.

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

“…Finally, in research on the relative impact of human and natural factors, Aich et al [93], observed both a general return to wet conditions since the end of the 1980s and an increase in annual maximum discharge in the NRB (Niger River Basin) for the same time period.…”

“…In cultivated areas of the Sahel, the extension of soil "erosion-crusting" (see description below) [72,76] has led to directly exposing topsoil to successive intense rainfall events and severe drying episodes. This "erosion-crusting" mainly concerns newly cropped typical Sahelian sandy soils and is most often assumed to be a consequence of land clearing and extension of cropping [6,93,94] (land use change). In pastoral areas, during the drought, the removal of the thin lithosol by wind or water erosion, occurring because of the vegetation dying, led rocks to outcrop [16,49,72,95] (land cover change).…”

“…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:…”

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

“…Many authors have used hydrological modelling (based on different scale, basin, data set and model) to infer the role of climate and LULC on hydrological changes in West Africa since 1950(1950-1998: Seguis et al, 2004, 1951: D'Orgeval and Polcher, 2008, 1950-2009: Aich et al, 2015. The conclusions differ among these studies: D'Orgeval and Polcher (2008) found that LULC was less important than rainfall changes, in contrast to Seguis et al (2004), while Aich et al (2015) concluded on the role of both LULC and climate. presented a preliminary work between 1983 and 2012, and showed that rainfall variability alone could explain the observed changes in the Niger River hydrograph in Niamey over the last 30 years.…”

Tracing Temporal Changes of Model Parameters in Rainfall-Runoff Modeling via a Real-Time Data Assimilation

“…The first strategy is to couple dynamic drivers (e.g., climate, LULC, soils, topography and human infrastructure) to hydrological models [22]. For example, a dynamic LULC module can quantify the contribution of land use change to river flooding [24]. The other approach is to adapt physical or conceptual hydrological models to changing conditions by estimating parameter travel in real time or near real time [19].…”