ORIGINAL PAPER Comparison of the spatiotemporal variability of rainfall from four different interpolation methods and impact on the result of GR2M hydrological modeling—case of Bani River in Mali, West Africa S. Louvet & J. E. Paturel & G. Mahé & N. Rouché & M. Koité Received: 17 October 2013 /Accepted: 22 December 2014 /Published online: 8 January 2015 # Springer-Verlag Wien 2015 Abstract The climatic evolution of the Bani river watershed, the main tributary to the upper Niger River, is approached through the spatiotemporal variability of rainfall grids over the 1950–2006 period. The analyses are conducted, and their results compared, using four different methods of spatial interpolation of rainfall fields: the spline, kriging, weighted inverse distance, and nearest neighbor methods. The largest changes are observed for all of these grids, but differences— and in some cases divergent results—appear in the details. The analysis shows a substantial decline in rainfall, particu- larly marked in the center of the basin, during the 1970–2000 period with respect to the 1950–1969 period, and a slight upturn in the northern part, mainly since the beginning of the 1990s. The rainfall deficit can be attributed to a combina- tion of factors: an earlier and drier end of the rainy season, less precipitation in the middle of the rainy season, more dry days and lower amounts of precipitation on rainy days. Two drought indices —the Effective Drought Index and Standardized Precipitation Index—revealed that the maxi- mum duration of drought events increased most in the central part of the basin. Lastly, to supplement this comparison of methods of spatial interpolation of rainfall fields, the sensitiv- ity of a hydrological model (GR2M) to rainfall data was tested. Given the distribution and density of rain gauge sta- tions available in the Bani watershed, the kriging method is found to yield the best hydrological modeling performance. 1 Introduction In sub-Saharan West Africa, rainfall is the main factor distinguishing the seasons, in the absence of a marked thermal winter. Since economic development, agriculture, and the health of the population are dependent on the spatiotemporal distribution of rainfall, changes in the dynamics of the mon- soon system and rainfall regimes can have disastrous conse- quences, and the fact is that sub-Saharan West Africa is one of the regions of the world that experienced the largest relative decrease in precipitation over the second half of the twenteith century (Redelsperger et al. 2006). The Sahelian and Sudano– Sahelian regions, which have low total annual rainfall and a strong rainfall gradient, were particularly affected by this decline (Nicholson and Palao 1993; Mahé et al. 2001). The Guinean band, further south, saw a similar drop in annual precipitation (Le Barbé et al. 2002). This period of drought has had disastrous consequences for the food resources, econ- omies, and health situations of the countries affected (Folland et al. 1986). Most of the rivers in the Sudano–Sahelian region show a discharge deficit greater than the rainfall deficit (Mahé and Olivry 1999), but in the Bani basin, where annual rainfall has decreased by 15 to 25 % since the beginning of the drought in 1970 (depending on the location within the water- shed), this disproportion reaches an extreme value: annual discharge fell by 69 % over the period, as measured at the Douna station near the outlet of the basin (13.2° N, 5.9° W; Mahé 2009). This exceptional situation calls for detailed research on the precipitation patterns and hydrology of the Bani basin in order to examine the sensibility of this area which is subject to the hazards of surface water resources, mainly in the form of droughts. Very little hydro-climatological documentation exists concerning the Bani basin. The earliest studies considered a single gauging station and an annual time step, without much consideration for spatial distribution over the basin. Bamba S. Louvet (*) Ecoclimasol, Paris, France e-mail: samuel.louvet@gmail.com J. E. Paturel : G. Mahé : N. Rouché UMR HydroSciences Montpellier, Montpellier, France M. Koité DNM Bamako, Mali, Africa Theor Appl Climatol (2016) 123:303–319 DOI 10.1007/s00704-014-1357-y