Partitioning and analyzing temporal variability of wash and bed material loads in a forest watershed in Iran

The amount of transported material from a hillslope or channel, mirrors the watershed health, which needs to be quantified. However, the contribution of different sediment sources to sediment load has not been adequately studied. In this study, 24 samples of suspended load, bed load and channel material were taken bi-weekly for a period of one year from the Kojour River of the Educational and Research Forest Watershed of Tarbiat Modares University in Iran. The suspended sediment concentration and particle-size distribution were determined. The total sediment load was then partitioned into bed load, wash load and suspended bed material load based on three criteria: (i) the upper limit of 63 micron, (ii) the Einstein method, and (iii) the lower limit of sediment particles in the channel material. The results suggested a significant contribution of wash load compared to suspended bed material and bed loads ratio of 96.8, 2.65 and 0.55%, respectively. The ratio of suspended bed material load to wash load varied from 13.12% (the maximum) in winter to 11.02, 2.70 and 0.91% in autumn, spring and summer, respectively. The results also showed the effects of flow discharge on different components of sediment loads. While, the flow discharge significantly affected bed load and wash load with respective correlation coefficients ( r ) of 0.520 and 0.464 < r < 0.535 for three methods with significant level ( p ) between 0.007 and 0.022. It had little influence on suspended bed material load (–0.243 < r < 0.277 with 0.190 < p < 0.253).