Spatio-temporal variability of droughts and terrestrial water storage over Lake Chad Basin using independent component analysis

Abstract

9Lake Chad has recently been perceived to be completely desiccated and almost extinct due 10 to insufficient published ground observations. Given the high spatial variability of rainfall in 11 the region, and the fact that extreme climatic conditions (for example, droughts) could be 12 intensifying in the Lake Chad basin (LCB) due to human activities, a spatio-temporal ap-13 proach to drought analysis becomes essential. This study employed independent component 14 analysis (ICA), a fourth-order cumulant statistics, to decompose standardised precipitation 15 index (SPI), standardised soil moisture index (SSI), and terrestrial water storage (TWS) de-16 rived from Gravity Recovery and Climate Experiment (GRACE) into spatial and temporal 17 patterns over the LCB. In addition, this study uses satellite altimetry data to estimate vari-18 ations in the Lake Chad water levels, and further employs relevant climate teleconnection 19 indices (El-Niño Southern Oscillation-ENSO, Atlantic Multi-decadal Oscillation-AMO, and 20 Atlantic Meridional Mode-AMM) to examine their links to the observed drought temporal 21 patterns over the basin. From the spatio-temporal drought analysis, temporal evolutions of 22 SPI at 12 month aggregation show relatively wet conditions in the last two decades (although 23 with marked alterations) with the 2012 − 2014 period being the wettest. In addition to the 24 improved rainfall conditions during this period, there was a statistically significant increase of 25 0.04 m/yr in altimetry water levels observed over Lake Chad between 2008 and 2014, which 26 confirms a shift in the hydrological conditions of the basin. Observed trend in TWS changes 27 during the 2002 − 2014 period shows a statistically insignificant increase of 3.0 mm/yr at the 28 center of the basin, coinciding with soil moisture deficit indicated by the temporal evolutions 29 of SSI at all monthly accumulations during the 2002 − 2003 and 2009 − 2012 periods. Further, 30 SPI at 3 and 6 month scales indicated fluctuating drought conditions at the extreme south of 31 the basin, coinciding with a statistically insignificant decline in TWS of about 4.5 mm/yr at 32 the southern catchment of the basin. Finally, correlation analyses indicate that ENSO, AMO, 33 and AMM are associated with extreme rainfall conditions in the basin, with AMO showing 34 the strongest association (statistically significant correlation of 0.55) with SPI 12 month ag-35gregation. Therefore, this study provides a framework that will support drought monitoring 36 in the LCB. 37

Analysis of rainfall variability in the Logone catchment, Lake Chad basin

“…Maidment et al () in their analysis observed a positive trend in rainfall in the LCB covering the semi‐arid zone of the Logone catchment while Okonkwo et al () also reported a similar trend in the southern part of LCB covering the Sudano zone of the catchment. In a recent study, Ndehedehe et al () also reported an increasing trend in rainfall over the LCB since 1990. In addition, the positive trends in rainfall observed in the catchment are reported to be a global trend (Westra et al , ).…”

Recent Surface Water Extent of Lake Chad from Multispectral Sensors and GRACE

“…The slight increase in lake area is in line with a study which showed improving wet conditions in the last two decades with 2002–2014 described as the wettest period. The authors attributed this to increasing rainfall in the area and the lack of any major long-term drought besides the usual seasonal fluctuations [51]. Lake Chad’s average area from 2003–2016 was estimated to be 1694 km 2 with a standard deviation of about 233.…”

“…Increase in estimated lake area could also be partially explained by the slight increase in altimetric lake levels (Figure 2a). Between 2008 and 2014, there was a yearly increase of about 0.4 m/year in altimetry lake levels [51]. Lake area estimates also match with an increasing agricultural productivity.…”

“…As previous studies indicated two seasons over the Lake Chad basin [42,51,58], we focused our studies during the rainy and dry seasons for analyses. A total of 416 images were downloaded between 2003 and 2016.…”

“…Recent surface area estimates are critical to understanding the desiccation story of the basin in recent times [51]. Cadre Harmonisé, a food security analysis tool unique to West Africa, demonstrated that this crisis has contributed to soaring food insecurity, with more than 6 million affected in the region of the four countries surrounding the lake [52].…”