Intra-event characteristics of extreme erosive rainfall on Mauritius

Mauritius is a typical tropical volcanic island with a raised interior where extreme rainfall events generate the bulk of the erosivity. Intra-event characteristics of the 120 highest erosive events at six selected locations between 2004 and 2008 were analysed to provide the first detailed intra-storm data for a tropical island environment. On Mauritius, spatial variation is evident in the characteristics of the extreme erosive rainfall recorded at the stations, with a noticeable increase in rainfall depth, duration, kinetic energy and erosivity of extreme events with altitude. Extreme events in the raised interior (central plateau) show a high variability in peak intensity over time as well as a higher percentage of events with the greatest intensities in the latter part of the event. Intra-event distribution of rainfall in the interior of the island shows that rainfall has a higher potential to exceed infiltration rates as well as the ability to generate high peak runoff rates and substantial soil loss. The study suggests that even though the within-event rainfall characteristics is complex it has implications for soil erosion risk, and that in tropical island environments the within-storm distribution of rainfall must be incorporated in soil loss modelling.National Research Foundation (NRF)http://www.tandfonline.com/loi/tphy202017-05-31hb2016Geography, Geoinformatics and Meteorolog

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

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.Dissertation (MSc)--University of Pretoria, 2016.tm2016Geography, Geoinformatics and MeteorologyMScUnrestricte