Spatial and temporal variations of rainfall erosivity on Pearl River basin during 1960-2012

Abstract: Soil erosion is recognized as one of the most serious, global ecological environmental crises in progress today. A rainfall-runoff erosivity factor, combined with the effects of duration, magnitude and intensity of rainfall event, can be used to measure the rain's potential ability to cause erosion. In this paper, the rainfall erosivity model proposed by the Chinese scholar Zhang Wenbo was used to calculate the rainfall erosivity. Taking the Pearl River basin as the study case, daily rainfall data from 1960 to 2012 in 43 meteorological stations were applied in the model. Methods of linear regression, Mann-Kendall, wavelet analysis and Kriging interpolation were applied to analyze the spatial and temporal variations of rainfall erosivity. The results showed that the range of annual rainfall erosivity in the Pearl River basin was 1858.0-14656.6 MJ·mm/(hm2·h) with an average value of 7177.1 MJ·mm/(hm2·h). The average annual rainfall erosivity decreased from east to west in general. Larger values mainly appeared in most areas of Pearl River Delta region, Dongjiang River basin and Beijiang River basin, but the values in Nanpanjiang and Beipanjiang River basin which are the upstream regions of the Pearl River basin were smaller. The distribution of average annual rainfall erosivity was similar with the average annual rainfall and there was a strong correlation (R=0.95, P<0.01) between them. Moreover, the average annual rainfall erosivity generally increased with the increasing of longitude (R=0.712, P<0.01), but decreased with the increasing of latitude (R=0.449, P<0.01). Trends of rainfall erosivity were not significant among years, four seasons, flood and non-flood seasons and no significant mutations occurred in these periods. Among them, the rainfall erosivity showed a slight downward trend in spring and autumn, but a slight upward trend in other periods. Among the periods of upward trend, the rainfall erosivity rising in summer was the fastest with a climbing speed of 11.251 MJ·mm/(hm2·h·a) and the average summer rainfall erosivity reached up to 5 414.530 MJ?mm/(hm2?h). The rainfall erosivity rising in year was the second fastest with a climbing speed of 8.469 MJ?mm/(hm2?h) and the average annual rainfall erosivity reached up to 10235.962 MJ·mm/(hm2·h). In winter, the rainfall erosivity of 39 meteorological stations accounting for about 90.7% of the total stations showed an upward trend, suggesting that the erosivity in winter rose overall. The mutation analysis of rainfall erosivity in the Pearl River basin indicated that the average annual rainfall erosivity sequences of the 7 sub-regions and the whole basin did not have significant mutations. The annual rainfall erosivity of most areas in the Pearl River basin showed significant upward trends (P<0.05), especially at Shaoguan station. In winter, the annual rainfall erosivity at Zhanyi, Fengshan, Hechi, Baise, Liuzhou, Rong'an and Guilin stations also showed significant upward trends. Evidently, the regions represented by these stations faced great pressure in the water and soil conservation. The rainfall erosivity sequence from 1960 to 2012 had the periods of 2.3, 3.8, 6.9, 12.7 and 23.4 a. Among them, only 2.3 and 3.8 a passed the red noise test at confidence level of 95%. The peak value of wavelet variance in 3.8 a was larger than 2.3 a, which suggested that 3.8 a was the main period. The red noise test also indicated that there was an oscillation period of 2.0-7.0 a in the basin. Generally speaking, the rainfall erosivity in the Pearl River basin showed an exacerbated trend, and therefore water and soil conservation should be well prepared. This study has the potential to provide an important reference for soil and water conservation planning, agricultural protection, ecological protection and disaster control in the Pearl River basin.