Fourier Index Research Articles

Spatio-temporal variations of rainfall erosivity in Rwanda

This study aims at investigating the spatio-temporal distribution of rainfall erosivity in Rwanda between the 1940s and the 1990s. The study gives new insights into rising trends in rainfall erosivity and, therefore, prepares agriculturalists to take precautionary measures for better land conservation. Modified Fournier Index (MFI) for consecutive decades from 1941 to 1990 and mean differences in Fourier Index (FI) and (MFI) values between the decades 1981-1990 and 1941-1950, were calculated. A geographical information system (GIS) was used to interpolate the values and to present spatially the variations of rainfall erosivity over Rwanda. MFI showed a clear upward pattern in rainfall erosivity from the 1960s, becoming more pronounced in the 1980s. The highest MFI values were in the northern highlands, the Congo-Nile crest and southwest region around Kivu Lake. On the other hand, the eastern lowlands accounted for the lowest MFI values. A comparative analysis of the above-mentioned decades reveals a massive FI decrease in May and January, and a significant increase in April and November, which are the wettest months of the year. The high values of MFI and FI observed in highlands with steep slopes are most likely to lead to more soil loss; the lowlands with the highest values will likewise be more exposed to flooding mainly during the wet seasons. Key words: Rainfall erosivity, fournier index, modified fournier index, Rwanda.

ASSESSING LAND DEGRADATION RISK THROUGH THE LONG‐TERM ANALYSIS OF EROSIVITY: A CASE STUDY IN SOUTHERN SPAIN

ABSTRACTDespite the high variability of the precipitation regime characterizing the Mediterranean area, the records of rainfall depth are usually not appropriate for long‐term calculations of erosivity and soil losses, because they do not reveal details of short lengths or long durations (daily, monthly). In this work, we present a simple approach to calculate annual erosivity through monthly precipitation records. The study area (olive groves on steep slopes) has a high erosion risk associated to the main soil land use, combined with an irregular and erosive rainfall regime. The relationships between rainfall data at intervals of 10 min for a period of 3 years, daily rainfall records over 10 years and a long‐term monthly dataset of 60 years were checked to calculate the annual erosivity values through daily data, Fourier's index and modified Fourier's index values. A good, adjusted linear relationship between modified Fourier's index and the erosivity was found, which allowed us to optimize the use of the 60‐year monthly data series and to carry out a long‐term analysis of the erosivity quantiles in the study area. The estimated mean erosivity showed a return period of between 2 and 5 years and a variation coefficient of over 50 per cent, which illustrate its high variability and frequency. This approach to calculate erosivity and the use of quantiles could be applied in other areas with month‐long data series in order to study and model the erosion risk using suitable temporal periods. Copyright © 2011 John Wiley & Sons, Ltd.