Abstract

This study proposes a new drought index based on several precipitation-based parameters to quantify drought hazard in semi arid region. In addition to the practice of using only rainfall volume for indexing drought, the proposed index verifies the potentials of nine other precipitation effectiveness variables (PEVs) onset of rain, cessation of rain, length of rainy and dry season, wet days and dry days within a wet season, dry days within the year, maximum dry spell length within a wet season and mean seasonal rainfall depth (MAR) in quantifying the drought conditions over a place. The conjunctive Precipitation Effectiveness Index (CPEI), as proposed in this study, utilizes a mathematical model, which algebraically combines “standardized seasonal PEV difference or deficit in each prevailing PEV” and terms of their sequent higher powers to define a single numerical value for this “at-site” drought index approach. Some statistical comparison tests were employed to determine the most appropriate set of PEV that can be employed in the CPEI model to quantify the drought conditions at each study location. The daily rainfall data obtained from seven synoptic stations in the semi-arid region of Nigeria were obtained, tested and then used to verify the effectiveness of this new method. Results obtained showed that the optimum no of PEVs that can be effectively combined to get the optimum CPEI values for indexing the drought in the study area is three PEVs for Gusau and Kano, five PEVs for Sokoto and Maiduguri and four for the rest stations under study. The trends observed in drought values obtained using the CPEI models employing these optimum PEVs also clearly earmarks the 1970 -73 and 1983 -1987 historical drought years within the study area. This approach seems to be significant for the specific area in the Sudano-Sahelian Region of Nigeria but would need to be verified in a wider regional context in similar future study.

Key words: Drought indexing, precipitation effectiveness variables, semi arid regions.