Determining sediment yield in olur sub-watershed within Çoruh River basin using erosion pin method, suspended solid measurement and GeoWEPP prediction model

The objectives of the study were to determine and compare soil erosion losses from Olur sub-watershed (37517 ha) within Çoruh River Basin using three different measurement approaches including GeoWEPP erosion prediction model, sediment station for the assessment of suspended load, and erosion pin method for assessment of gully and stream bank (1st, and 2nd order) erosion within two year study period. The gully and streambank soil loss measured during the first (2017) and second year (2018) of the study period were 24472 and 159746 tons/yr, respectively. Significant soil loss was measured during the second year of the study period due to increase in the rainfall amount and its event number. Gully erosion accounted almost 90% of the total soil loss compared to 1st and 2nd order streams. The streambank and gully erosion loss (2017: 58 ton/yr; 2018: 19989 ton/yr) predicted by the GeoWEPP model were much lower than the measured ones. The total sediment yield predicted by the model was 87.6 ton/yr (0.002 ton/ha/yr) and 89124 ton/yr (2.80 ton/ha/yr) for the year 2017 and 2018, respectively. The suspended loads measured at the sediment station were 36312 ton/yr (0.97 ton/ha/yr), 26108 ton/yr (0.70 ton/ha/yr) for the year 2017 and 2018, respectively. According to results; the bank erosion values were underestimated by the model and this was pronounced even more during the dry period. Additionally, the measured suspended load for the second year of the study was lower than the first year, probably due to rehabilitation efforts undertaken across the watershed. Beside all these results, the shortcoming in the measurement of the bed load was the biggest obstacle to estimate the sediment budget for the whole watershed.