Intra-storm attributes and climatology of extreme erosive events on Mauritius 2004 - 2008

Abstract

Topographic complexity and island-scale weather systems on Mauritius result in highly variable spatial and temporal rainfall distribution. This, in combination with the intense agricultural activity, predisposes the island to a high risk of rainfall induced soil erosion. Thus it is important to investigate intra-storm attributes of erosive events, as these events are most likely to cause significant degradation and reduced productivity of the soils on the island. Intra-storm analysis allows for the identification of critical intensity peaks in rainfall events that potentially impact the severity of erosion. Six Mauritius Meteorological Services automated weather stations (measuring rainfall at 6 minute intervals) located on the west coast and in the interior providing rainfall data over a 5 year period (2004 to 2008), enabled the first detailed intra-storm analysis on the island to occur. For the purpose of this study, erosive events were defined as a total rainfall exceeding 12.5 mm and a maximum 6-minute intensity exceeding 30 mm/h. The analysis found that there were 444 erosive events during the study period which are responsible for generating the bulk of the rainfall erosivity. A total of 120 erosive events (the top twenty erosive events for each weather station with the highest total kinetic energy generated ) were analysed to investigate the intra-storm distribution of rainfall depth, extreme rainfall intensity and cumulative kinetic energy. General climatological characteristics and weather circulation patterns were also determined. Erosive events were found to vary both in rainfall depth and duration, but all the stations indicate a clear exponential distribution of cumulative kinetic energy generated over the duration of the rainfall events. Extreme rainfall intensities display noticeable temporal differences between the stations in different climatic regions on the island. All the stations received more than 80% of the potential kinetic energy content generated by the storms within the first 2500 minutes of the storm, as well as 80% of the cumulative rainfall available. Investigating the distribution of the extreme rainfall intensity (above 30 mm/h) as a function of storm duration, reveals that 57% of the erosive events generate peak intensities within the first half of the storm duration. Erosive events were not restricted to tropical cyclones, but include other weather systems such as cold fronts. The elevated centre of Mauritius, which is responsible for a high rainfall gradient across the island, influenced the spatial and temporal variability in erosive events. Results indicate that the intra-storm attributes of rainfall events are strongly dependent on the geographic features within the immediate surroundings of the weather stations, and distance between weather stations did not always lead to predictable differences in intra-storm attributes. Although the erosive events on Mauritius share common characteristics, the within-storm distribution shows that no two events are similar and no two stations show comparable event pattern distributions. However, the intra-storm analysis of the erosive events suggests that, despite the spatial differentiation in the structure and nature of the erosive rainfall generalisations can be made regarding the erosion experienced in the coastal and interior regions of the island. The inclusion of more automated weather stations is warranted as this will provide a better representation of rainfall characteristics across the remaining regions of the island. Further research is necessary to determine the relationship between event structure and synoptic conditions experienced on the island.