Rainfall erosivity attributes on central and western Mauritius

Abstract

Rainfall can be the most erosive agent with respect to rainfall induced erosion, particularly within the context of a tropical maritime environment. Mauritius provides an example of such an environment, which, due to its location and elevated topography, is subject to frequent erosive rainfall events as well as occasional cyclones which potentially threaten loss of soil and may accelerate land degradation. Such intense rainfall forms a key part of the “R-factor” in the USLE and RUSLE soil loss equations, which are commonly used worldwide in deriving the soil loss of an area. This project focuses on various attributes of rainfall erosivity on the central and western parts of Mauritius over a six year assessment period. A steep rainfall gradient exists; 600mm in the western plains and 4000mm per year in the higher central region. Rainfall and erosivity attributes are investigated in these two regions on the island to assess the role that topographic elevation has on rainfall erosivity. Using the EI30 method to find the “R-factor”, erosivity is calculated for the period of 2003 – 2008. Varying time intervals were used in calculating EI30 to determine the value that high resolution data has in erosivity calculations and is compared to the use of the Modified Fournier Index. This project also speculates on the potential impacts of changing rainfall intensity and erosivity associated with climate change in the future. A difference was found in the erosivity experienced in the elevated central interior and the rain-shadowed western lava plains. Stations on the western plains recorded 25% of the erosivity experienced by stations in the interior and large differences were found in the number of erosive events, rainfall, erosive rainfall totals, seasonality, and annual erosivity totals of erosivity. The central interior showed greater variability in R-factor values; however these remained similar in extent despite the large difference in total annual rainfall and the number of events that each station recorded. High resolution data did account for erosivity that lower resolution does not, but the extent of erosivity for all stations within the respective regions were markedly similar. Use of the Modified Fournier Index caused erosivity to be overestimated on the island when compared to the EI30 method. Changes in erosivity are speculated to occur with changes in rainfall intensities but the central interior of the island will notice fluctuations in climate (with respect to rainfall erosivity) more than the western plains.