Fingerprinting the Contribution of Soil Erodible Units to Sediment Yield and Its Relationship with Organic Carbon Stock in Kouhdasht Watershed, Lorestan Province

Accelerated soil erosion is a serious problem in Iran, leading to degradation of soil and water resources, reduction of soil fertility, destruction of range and agricultural lands, desertification, recurring floods, sedimentation of reservoirs, and pollution of fishery habitats. Hence, understanding of the potential soil erosion process and opposition to this erosion are necessary environmental. To this end, uptake and refinement of sediment source tracing or fingerprinting techniques has expanded dramatically as an alternative approach to traditional methods of identifying key sediment sources. Sediment source fingerprinting involves discriminating potential sediment sources on the basis of differences in source material properties or tracers and determining the relative contributions of these sources to sampled target sediment. different kinds of sediment sources have been used so far in sediment fingerprinting techniques (e. g., land use, geology, sub-basins, surface and subsurface erosion) but, there is a little attention paid to the selecting the soil erodibility groups as sediment sources. Therefore, the main objective of this study are the Kouhdasht watershed dividing into different erodible units based on soil erodibility index and determination of the contribution of each unit in sediment yield using an un-mixing Bayesian uncertainty model and to find its relationship with soil organic carbon stock.

Kouhdasht basin with 1138 km2 area located in  33°  17´ to 33° 41´ north latitude and 47°  20´ to 47°  50´ eastern longitude in western of Lorestan province. samples were collected in two stages; first, 81 samples in order to estimate erodibility, second, in order to determine the contribution of each source to sediment yield, 70 soil samples were collected form sources and 12 sediment samples collected at  the basin outlet.  The soil erodibility was calculated based on the soil texture and based on the geometric mean of the soil particle diameter. Based on the amount of soil erodibility, the area was divided into three different erosion units as sediment sources. To determine the contribution of sediment sources to sediment yield used fingerprinting technique is based on estimation of uncertainty.

The erodibility of the study area varied from 0.0386 to 0.0663. Erodible units were identified as sediment sources based on the values obtained from the erosion parameter and according to the results of selecting the optimal combination of tracers. The results showed that the first erosion unit 2%, the second erodible unit 5%, and the third erosion unit 93% contributed in the region's sediment yield. The relative importance of erodible units in sediment yield was obtained by dividing the share of each resource in the production of sediment into the percentage covered by each source. The relative importance of the first, second and third erosion units is 0.08, 0.28, and 1.57, respectively. Regarding the role of organic carbon in erosion, the amount of organic carbon storage in different erosion units of the area was also measured. The amount of organic carbon storage in each erosion unit is first, second and third ones were 70.5, 64.3 and 54.6 mg / ha respectively.

The third unit with 93% has the largest contribution in sediment yield and with 54.6 mg / ha, it has the lowest amount of organic carbon storage in the area. Considering that this unit is most used in agriculture and geologically under quaternary sediments, showed that the parts that are under cultivation and quaternary sediments have both high erodibility and the highest contribution to sediment yield. Measurements of organic carbon storage also showed, there is the least amount of organic carbon storage in this unit and this suggests that in the third unit, the damage caused by the loss of fine sediments such as clay is higher. Given that the third unit is under agricultural use this can be attributed to the type of land use and exploitation. Therefore recommended more attention to the type of use of land and soil management and conservation programs implemented in the region.