Determination of surface, channel, and gully erosion amounts in the arhavi kapisre stream basin and measurement of some water quality parameters

The Arhavi Kapisre Stream Basin (AKDH), located within the borders of Artvin Province, constitutes the primary research area of this study. The basin receives over 2000 mm of annual precipitation, and the various types of erosion occurring in the basin were investigated over three years (2021-2023) using three different measurement methods. The first method, referred to as the "erosion pin method," involved measuring erosion on channel (1st-3rd order streams) and gully banks using iron rods. The second method utilized erosion plots in different land use types (tea, hazelnut, forest, and pasture) to measure surface runoff volume, sediment, and associated water quality parameters (TN, TP, pH, EC). The third method involved measuring discharge and suspended sediment in the main Kapisre stream and its tributaries to estimate the total sediment, TN, and TP amounts reaching the basin outlet. Additionally, the study aimed to determine the impact of the 2021 flood disaster on channel and gully erosion. According to the erosion pin method, the total soil loss from channel and gully banks was calculated as 144770 tons over three years, with 106341 tons (408,27 tons/km/year) in 2021, 22898 tons (87,89 tons/km/year) in 2022, and 15531 tons (59,46 tons/km/year) in 2023. When averaged over the three years, soil loss was calculated as 5,46 cm/year and 298,6 tons/km/year in 3rd-order streams, 7,5 cm/year and 258,3 tons/km/year in 2nd-order streams, 6,94 cm/year and 194 tons/km/year in 1st-order streams, and 6,81 cm/year and 158,9 tons/km/year in gully streams. Of the total erosion, 61,6% occurred on gully stream banks, 14,4% in 1st-order streams, 15,8% in 2nd-order streams, and 8,2% in 3rd-order streams. Had the flood disaster not occurred in 2021, the calculated soil loss would have been 12545 tons/year (49,12 tons/km/year). However, the flood increased soil loss in 2021 by 93796 tons (359,15 tons/km/year, 13,96 cm of erosion), raising erosion by approximately 7,5 times. Over the three years, the average soil loss was calculated as 48256 tons/year (179,7 tons/km/year). In terms of surface runoff measured in erosion plots, the highest value was recorded in hazelnut fields at 11,9 L/day (60,9%). Pasture areas ranked second with 4,7 L/day (23,8%), followed by upper forest areas with 1,4 L/day (7,1%), tea fields with 1,1 L/day (5,8%), and lower forest areas with 0,5 L/day (2,4%). Tea fields were found to have low surface runoff, similar to forest areas. The highest hygroscopic moisture in soil was measured in tea fields at 12,1%. The total soil loss from surface runoff over two years was 288 tons/2 years, with an average soil loss of 144 tons/year and 0,0049 tons/ha/year. Hazelnut fields had the highest soil loss at 89,8 tons/year (0,0637 tons/ha/year). Lower forest areas ranked second with 33,2 tons/year (0,0022 tons/ha/year), while pasture areas ranked third with 17,8 tons/year (0,0019 tons/ha/year). Tea fields and upper forest areas had lower soil loss values at 1,8 tons/year (0,0008 tons/ha/year) and 1,3 tons/year (0,0006 tons/ha/year), respectively. In terms of total phosphorus (TP) concentration, the highest value was recorded in lower forest areas at 0,99 mg/L. This value was measured as 0,46 mg/L in upper forest areas and 0,29 mg/L in hazelnut fields, falling into the 3rd-class water category. Tea fields and pasture areas had lower TP concentrations at 0,09 mg/L and 0,06 mg/L, respectively, falling into the 2nd-class water category. For total nitrogen (TN) concentration, lower forest areas had the highest value at 8,39 mg/L, followed by upper forest areas (4,68 mg/L), both falling into the 2nd-class water category. Hazelnut fields (2,10 mg/L), pasture areas (1,27 mg/L), and tea fields (1,14 mg/L) fell into the 1st-class water category. At the Arhavi Kapisre stream outlet, the total suspended sediment measured over three years was 124193 tons. When compared with other measurement methods, 144770 tons of soil loss occurred from channels and gullies over three years, while 288 tons of soil loss occurred from surface runoff over two years. Thus, approximately 145058 tons of soil loss occurred from channels, gullies, and surface runoff over three years. Considering that the measured sediment at the basin outlet was 124193 tons, it is estimated that approximately 20865 tons of sediment were deposited in the stream bed and did not reach the outlet, indicating a sediment delivery ratio of approximately 85,6%. In stream water samples, Yolgeçen (forest) and Güvercinli (agriculture) had average sediment concentrations of 0,003 g/L and 0,043 g/L, respectively, indicating that forested basins produce lower sediment. Mençuna (pasture) also produced moderate sediment at 0,024 g/L. In terms of discharge, Yolgeçen (12315 m³/ha/year), Mençuna (11874 m³/ha/year), and Güvercinli (11021 m³/ha/year) were ranked in descending order. The highest water yield was found in the Yolgeçen sub-basin, which is densely forested. According to water quality data based on the (YSKY), total phosphorus (TP) values in all stream samples fell into the 1st-class (excellent) category. Total nitrogen (TN) values were also in the 1st-class category for Mençuna (0,99 mg/L) and Yolgeçen (1,70 mg/L) streams, while Güvercinli (2,29 mg/L) and Arhavi Inlet (1,60 mg/L) streams were at the 1st-class threshold. The lowest water quality values were found in the Güvercinli sub-basin, where agricultural activities are intensive.