The Influence of Rainfall Erosivity Temporal Variation on Suspended Sediment Load Seasonality (Case Study: Kasiliyan Basin)

Erosion rate has temporal variability and shows a cyclic fluctuation in each year as well as the factors influence it due to the seasonality of climate. This study was done to determine the seasonal variability of rainfall erosivity and suspended sediment and also to found out critical times in term of erosion risk in Kasiliyan watershed. The appropriate indices of monthly rainfall erosivity were determined for the aforementioned watershed based on regression analysis between EI30 and several easily accessible indices for 27 rain gauges inside and around the watershed. Finally, monthly maps of erosivity were produced by Kriging method. For estimating the suspended sediment, we used a combination of sediment rating curve of mean loads within discharge classes and average daily discharge data. Then, correlation between monthly sediment loads and corresponding erosivities was investigated. The results showed a strong seasonality for both rainfall erosivity and suspended sediment. Two peaks were observed in annual erosivity/precipitation in Kasilian basin, one in the late fall in Aban and Azar (Iranian month equal to Oct. 23 to Nov. 21) which was 41 MJmmha-1h-1 and another in spring in Farvardin (Iranian month equal to Mar. 21 to Apr. 20 which was 24 MJmm ha-1h-1(. A change in precipitation type (rain to snow) due to the change in temperature is a possible reason for lower rainfall erosivity during winter. In contrast, the suspended sediment showed just one peak during spring in Farvardin (equal to Mar. 21 to Apr. 20 which was 13175 ton). Absorption of early rains by dry soils in the fall is most probable reason for little runoff and consequently low rate of sediment (3300 ton during Aban) in the outlet of watershed.