Quantifying the Linkage Between Agricultural Mulch Film Watershed Abundance and Riverbank Accumulation

Abstract

This study quantified the linkage between the watershed abundance of agricultural mulch film and its accumulation along riverbanks in the Hamada River catchment, Japan, by integrating riverbank monitoring, catchment-wide surveys, hydrological connectivity assessment, and delivery ratio estimation. Mulch film covered 41,748 m² of the watershed during the 2025 growing season, with abundance varying by crop type. Black film was predominantly associated with red perilla, while silver film was almost exclusively used for tobacco. Riverbank monitoring from 2023 to 2025 showed that mulch film accounted for 22.5% of cumulative surface area coverage but only 5.6% of cumulative weight density, reflecting its thin, sheet-form geometry. Its accumulation was associated with episodic hydrological connectivity, with integrated hydrological exposure significantly correlating with both surface area coverage (ρ = 0.72, p < 0.05) and weight density (ρ = 0.66, p < 0.05). In contrast, the accumulation of other macroplastics showed few significant associations with the examined hydrological variables. This study provides the first quantitative estimate of the delivery ratio of agricultural mulch films from agricultural land to river systems. The annual delivery ratio of silver film ranged from 0.17% to 0.31%, with the highest value in the year for the lowest watershed abundance, indicating that delivery was not determined solely by abundance. In 2025, the delivery ratios were 0.18% for silver and 0.06% for black film, exhibiting the same order of magnitude despite a threefold difference in watershed abundance. These findings highlight the need for an abundance-based plastic mulch management strategy at the catchment scale.

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