Total Water Storage Change in Cameroon: Calculation, Variability and Link with Onset and Retreat Dates of the Rainy Season

Abstract

Abstract:Total water storage change (TWSC) was calculated using CRU (Climatic Research Unit) monthly gridded data for the period 1962-1993 over Cameroon. Investigations were conducted to link its annual cycle with both the beginning and the end of the rainy season. A method was derived as an alternative to determine onset and retreat dates of the rainy season. Two methods were used for the calculation of TWSC. The first method used potential evapotranspiration (PET) from the Thornthwaite formula (PET TH ) and the second, CRU gridded PET data estimated from the Penman-Monteith formula (PET PM ). A comparative study of the corresponding TWSC, namely TWSC TH and TWSC PM , respectively, was done. According to the preliminary results, the study area is classified as humid below latitude 8 • N and semiarid above. The results of the spatial and temporal variations showed a close correlation between the two methods, but with a slight gap between their different values, those of TWSC PM being larger and fluctuating less. The annual cycles of TWSC and PR generally showed similar patterns, and their intensities decreased from the southern part of the area (Equatorial forest zone) to the northern part (Sahelian zone). For mean TWSC = 0, two different points were identified: the first and the second corresponding dates matching the onset and retreat months of the rainy season, respectively, except in the arid area (Sahelian zone), where only the retreat month of the rainy season was perfectly determined. The delay observed in the determination of rainfall onset date in that area is assigned to PET formulas that are defined only for humid areas and to the influence of high temperature just before the beginning of the rainy season, promoting the rapid evaporation of soil water immediately after the first rains. Application of the same method (TWSC = 0) for the individual year showed similar performances. Although TWSC is always negative in Zone 3 and positive in Zones 1 and 2, the study of the interannual variabilities revealed an overall declining trend due to a stronger decrease in precipitation compared with PET. Moreover, the decrease during dry months is more remarkable than during wet months.