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ŠUMARSKI LIST 5-6/2021 str. 56     <-- 56 -->        PDF

amount of TSS transported from the watersheds was high because of sudden snow melts rather than from rainfall intensity.
In the two years, a total of 6511.7 ton of TSS were transported from the FW and 5503.8 ton from the AW. However there is no significant difference between watersheds for two years in terms of monthly TSS yields (P>0.05). Excluding the exceptionally high TSS yield in June 2014 when extreme rainfall occurred, the 2-year TSS yield was 1161.5 ton and 2534.5 ton for the FW and AW. The amounts in this month constituted 82% and 54% of the sediment transported from the FW and AW in the two years. Similarly, Nearing et al. (2007) determined that one of seven watersheds produced 60% of the 11-year total sediment during two storm events and that this was due to the difference in watershed geomorphology and vegetation cover. Although the unit surface water yield of the FW was higher than the AW, the presence of high amounts of sediment from the FW in only a small number of months shows that forests play an important role in reducing sediment. Therefore, it can be concluded in this study that, aside from extreme precipitation events, there was a positive relationship between the ratio of agricultural land in the watershed and the TSS yield and a negative relationship with the ratio of forested land. This finding was consistent with those of Ahearn et al. (2005), Yan et al. (2013) and Kibena et al. (2014).
Rainfall intensity was also effective in producing high TSS for several months in the FW. Indeed, Nu-Fang et al. (2011) found a significant correlation between sediment transport and total rainfall, peak flow, total water yield, and maximum 30-min precipitation intensity and stated that peak discharge and duration of rainfall were the most important factors controlling suspended sediment transport. Similarly, Zhang et al. (2010) found a positive correlation between precipitation and surface runoff and sediment yield in four watersheds. In a study conducted in watersheds with different land uses, it was found that the difference between suspended sediment concentrations of agricultural and forestred watersheds decreased at high streamflow and increased at low streamflow (Lenat and Crawford, 1994). This finding showed that the amount of TSS of FWs may be higher than in AWs due to the increase in the streamflow.
It is thought that there were two main reasons that more TSS was transported from the FW after an heavy rainfall events. The first reason was that the infiltration was reduced due to the high average slope and short main channel of the FW compared to AW. Thus, in heavy rainfall events, the speed and amount of surface runoff increased in a short time and caused stream bank and bottom erosion. Ebabu et al. (2018), in a similar study carried out in the paired watersheds, found that the unit surface sediment yield of the steep sloped watershed was greater. The second reason was the collapse of the suspended sediment in the stream bed due to be wide of the main channel in the outlet section of the AW and its very low slope. The third reason was that the speed and amount of the surface flow was decreased due to its storage in limestone fissures and fractures in the AW.
CONCLUSION
ZAKLJUČAK
This study evaluated the effects of some watershed characteristics on water and total suspended sediment yields in two adjacent subwatersheds of the Big Melen watershed having the similar climate and soil characteristics. As a result, the unit surface water yield in the steep sloped forested watershed was found to be higher than in the hazelnut plantation dominated watershed, where limestone bedrock occupied a large area. This result showed that the effect on reducing surface runoff in steep sloped forested watersheds was low and therefore, water yields may be higher than in hazelnut plantation dominated watersheds. After heavy rainfall events, a greater amount of suspended sediment was transported from the forested watershed. The forests have a more pronounced effect on reducing sediment transport during low rainfall and in periods of no rainfall. This finding indicated that the positive or negative effect on sediment transport in the forested watershed depended on topography and rainfall intensity. The results of the study showed that the effects of various factors such as slope, rainfall intensity, geological structure, drainage density and frequency of streams may be more dominant on the water and sediment yield of the watersheds than the effect of land use.
This study revealed that when investigating the effect of different land uses on water and sediment yields in paired watersheds, for a better understanding of the effects, the watershed characteristics should be similar. Due to the difficulty of finding paired watersheds and the expense of monitoring streamflows, long-term studies that provide more reliable results at lower cost can be carried out on single watersheds. Moreover, the factors affecting hydrological processes in single watersheds can remain constant over a long time period.
ACKNOWLEDGEMENTS
ZAHVALA
This study was funded by Duzce University Scientific Research Projects (No: 2013.02.02.186) and was based on the PhD dissertation of the first author under the supervision of the second author.