Rainfall Anomaly Index over Mouhoun Basin in Southwestern Burkina Faso

Rainfall as one of the main components of the hydrological cycle (Shiklomanov et al. 2000; Schneider et al.; 2010; Li et al.,2022b) is undergoing changes worldwide. Indeed, global warming is substantially leading to potential change in rainfall occurrence allover the world. As reported by IPCC (2022), an increase of temperature up to degree, may results in huge rainfall variability. Consequently, climate change, is greatly affecting the patterns of rainfall. The Sahel region, highly exposed to climate change, is gradually becoming hotter, with some areas experiencing increased and erratic rainfall. Historical records and climate projections show that the magnitude and frequency of precipitation extremes are increasing in Sahelian areas considering global, regional, and local scales. According to Nicholson et al, (2018) significant rainfall fluctuations has been registered in the Sahel. Indeed, over the last few decades, rainfall in this region has experienced profound changes such as drought and flood. Moreover, Martinet al (2016) revealed that high variabilities sound to occur in the next coming decades leading to an inappropriate water resources management. Consequently, analyzing rainfall records stand to be an important step in water resources management (Kim et al 2012; Dastorani, 2016; Borah et al; 2021). Several studies have investigated temporal and spatial changes in rainfall within Burkina Faso. Various rainfall-based indices were used to investigate the rainfall variability conditions in Burkina Faso. Using Standardized Anomaly Index (SAI), Kima et al (2015) found an upward trend, with high inter-annual variability in the Boulgou’ province of Burkina Faso. While, Sagnan et al (2021) applied a standardized precipitation index to analyze the water balance in Burkina Faso. They found a slight increase in rainfall over the country in the 1990s. Over the recent years, Rainfall anomaly index analysis has become more popular in recent years (Costat 2017, Raziei 2021). Hänsel et al. (2016) applied the Modified Rainfall Anomaly Index (mRAI) in evaluating its suitability as an alternative to the Standardized Precipitation Index (SPI) in assessing future precipitation conditions. Applying Standardized precipitation evapotranspiration index (SPEI) and standardized precipitation index (SPI) to classify the precipitation and water balance anomalies in four deleted Sub-Saharan Countries (Senegal, Burkina Faso, Tanzania, and Malawi.), Mbaye et al. 2022 found high rainfall variability of extreme events in all the four countries. In a recent study, Aryal et al. (2022) applied the RAI to characterize the droughts in Népal and to evaluate their impacts on crop yields. Although rainfall analyses in Mouhoun watershed have been done by many methods such as SPI and, rainfall anomaly index analysis within the basin is still lacking. However, a number of studies have shown a comparable performance of RAI to the SPI (Keyantash and Dracup 2002; Loukas et al. 2003). According to Keyantash and Dracup (2002), the RAI offers a higher degree of transparency and tractability and demands a lower degree of sophistication than the SPI with regard to the evaluation criteria for drought indices. Analyzing the comparative performance of three meteorological drought indices, Oladipo (1985) found that differences between the RAI and the more complicated indices were negligible. Moreover, the results of the analysis suggest that precipitation is the most important climatic element as an input into meteorological drought. Therefore, the present study estimates the value of RAI within the Mouhoun watershed in order to provide robust evidence on historical rainfall characteristics for decision making.