Spatial and Trend Analyses of Rainfall Seasonality and Erosivity in the West of Andalusia (Period 1945–2005)

Abstract

Rainfall analyses are essential to evaluate the risk of soil erosion by water. Seasonality and concentration of precipitation need to be considered in the assessment of erosive potential of the rainfall. Spatial variations and trends in annual rainfall, seasonality and rainfall erosivity, have been studied in this work for the Western region of Andalusia in order to improve the analysis of soil degradation associated with erosivity and to determine potential alterations related to climate change. The present study is based on data from monthly rainfall recorded in the period 1945 to 2005 in a total of 225 rain-gauge stations spread all over region. Rainfall seasonality has been obtained by means of the use of two seasonality indices: (i) the Walsh and Lawler index (SI) and (ii) the precipitation concentration index (PCI). Soil erosion risk has been determined estimating the erosivity by the Modified Fournier Index (MFI). For each year, the SI, PCI, MFI, and precipitation values for different seasons (fall, winter, spring, summer, fall-winter, and spring-summer) were calculated. The time-series trend of the annual and seasonal precipitation and the annual values of the SI, PCI, and MFI indices have been examined by using the nonparametric Mann-Kendall test. The results show a marked seasonality in the rainfall. A small, although relevant, fraction of the area has a high erosivity index. Finally, a decreasing trend in the spring-summer rainfall has been detected in 72% of the rain gauges considered.