Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel (Gourma region, Mali)

Abstract

Abstract. Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought. The seasonal and interannual variations of the areas covered by surface water are assessed using multidate and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, -TM, and -ETM, CORONA, and MODIS) and aerial photographs (IGN). Water body classification is adapted to each type of spectral resolution, with or without a middle-infrared band, and each spatial resolution, using linear unmixing for mixed pixels of MODIS data. The high-frequency MODIS data document the seasonal cycle of flooded areas, with an abrupt rise early in wet season and a progressive decrease in the dry season. They also provide a base to study the inter-annual variability of the flooded areas, with sharp contrasts between dry years such as 2004 (low and early maximal area) and wetter years such as 2001 and 2002 (respectively high and late maximal area).The highest flooded area reached annually greatly depends on the volume, intensity and timing of rain events. However, the overall reduction by 20% of annual rains during the last 40 years is concomitant with an apparently paradoxical large increase in the area of surface water, starting from the 1970's and accelerating in the mid 1980's. Spectacular for the two study cases of Agoufou and Ebang Mallam, for which time series covering the 1954 to present period exist, this increase is also diagnosed at the regional scale from LANDSAT data spanning 1972-2007. It reaches 108% between September 1975 and 2002 for 91 ponds identified in central Gourma. Ponds with turbid waters and no aquatic vegetation are mostly reCorrespondence to: P. Hiernaux (pierre.hiernaux@cesbio.cnes.fr) sponsible for this increase, more pronounced in the centre and north of the study zone. Possible causes of the differential changes in flooded areas are discussed in relation with the specifics in topography, soil texture and vegetation cover over the watersheds that feed each of the ponds. Changes in rain pattern and in ponds sedimentation are ruled out, and the impact of changes in land use, limited in the area, is found secondary, as opposed to what has often been advocated for in southern Sahel. Instead, major responsibility is attributed to increased runoff triggered by the lasting impact of the 1970-1980's droughts on the vegetation and on the runoff system over the shallow soils prevailing over a third of the landscape.

Water storage changes as a marker for base flow generation processes in a tropical humid basement catchment (Benin): Insights from hybrid gravimetry

“…Hydrological responses differed greatly between these two regions. In Sahel, the drought coincided with an observed increase in flow [Leblanc et al, 2008;Mah e and Paturel, 2009;Gardelle et al, 2010]. This paradox has been explained by simultaneous land clearance during extensive agriculture development.…”

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

“…During the same dry years the phenomenon know as "Sahelian paradox" (Descroix et al, 2009) was observed in many part of the Sahel: an increase in runoff despite the deficit in rainfall (Albergel, 1987;Amani and Nguetora, 2002;Mahé et al, 2003Mahé et al, , 2005Mahé and Paturel, 2009). This phenomenon resulted from a discharge increase in exorheic 1 basins (Amani and Nguetora, 2002;Mahé et al, 2003Mahé et al, , 2005Mahé and Paturel, 2009;Descroix et al, 2009;Amogu et al, 2010) and in larger pond surfaces, infiltration and water table levels in the endorheic 2 areas (Desconnet et al, 1997;Leduc et al, 2001;Gardelle et al, 2010).…”

Spatial characterization of long-term hydrological change in the Arkavathy watershed adjacent to Bangalore, India

“…Although the results of our statistical model imply a non-trivial amount of unexplained variation, Gardelle et al (2010) reported similar performance (R 2 = 0.78) for a model relating precipitation and water extent in a single lake, and noted that the correlation was valid only for a 9-year subset of the 5-decade study period. The sources of uncertainty include the complex hydrological processes that relate precipitation, streamflow, and tank water storage, as well as the nonlinear and heterogeneous relationship between water extent and water storage, the neglect of pre-monsoon tank water extent in the model, and the non-stationary behavior of dry-season losses in the two northernmost watersheds.…”

“…For instance, in situ measurements of tank water storage have been successfully used to calibrate and validate hydrological models in Andhra Pradesh (Perrin et al, 2012) and Tamil Nadu (Van Meter et al, 2016). Other studies in southern India (Mialhe et al, 2008), the USA (Halabisky et al, 2016), Africa (Meigh, 1995;Liebe et al, 2005;Sawunyama et al, 2006;Liebe et al, 2009;Gardelle et al, 2010), and South America (Rodrigues et al, 2012) also use surface water bodies as aggregators of streamflow.…”