Seasonal and interannual variability of precipitation and rainfall erosivity in Northeastern Japan: insights from 14 years of observations after Fukushima Dai-ichi nuclear accident (2011–2024)

Abstract. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011, led to the deposition of about 2.0 PBq of ¹³⁷Cs across Fukushima Prefecture, resulting in widespread long-term environmental contamination. The accumulation of ¹³⁷Cs in the uppermost soil layer, led rainfall-driven soil erosion to become a major mechanism for ¹³⁷Cs redistribution across the landscape. Previous research (Laceby, et al., 2016b), which analysed data over 1995–2015, identified the June–October period as critical for rainfall erosivity. However, this analysis covered only the immediate five years period after the accident, leaving spatial and temporal patterns, event-scale dynamics, and long-term trends unquantified in the longer post-accidental context. To address this gap, the current research analysed 10-minute precipitation records from 58 weather stations located within a 110-km radius of the FDNPP, to include most of ¹³⁷Cs deposition onland, during the 14 years that followed the accident (2011–2024), calculating event-level precipitation amount and rainfall erosivity (EI₃₀), and aggregating metrics across annual, monthly, seasonal, and extreme-event timescales. Between 2011 and 2024, median annual precipitation was 1318 mm, with erosive events contributing 917 mm and generating 3,290 MJ mm ha^-1 h^-1 of erosivity. The June–October period accounted for 61 % of erosive events, representing 46 % of annual precipitation (602 mm) but 86 % of annual erosivity (2,698 MJ mm ha^-1 h^-1), with a clear precipitation-erosivity relation (r² = 0.62). Erosivity was highly concentrated during a few extreme events: the three most erosive events of each year contributed 55 % of total annual erosivity (1,753 MJ mm ha^-1 h^-1; 224 mm), whilst the single most erosive event alone accounted for 28 % (912 MJ mm ha^-1 h^-1; 94 mm). In 2011, 2015, and 2019, single events dominated annual erosivity, generated 38–44 % of annual erosivity. These findings quantify the erosive drivers of ¹³⁷Cs remobilisation since the accident with at large spatial and a high temporal resolution, supporting the retrospective reconstruction and prediction of long-term contamination dynamics. The predominance of extreme rainfall events emphasises their role in controlling sediment and contaminant transport in the post-accidental Fukushima context. All datasets, reproducible analytical workflows, and scripts (R package RainErosivity) are openly available via Zenodo and GitHub, supporting research on rainfall-driven ¹³⁷Cs redistribution, sediment transfer, erosion risk assessment and long-term geomorphological evolution of catchments.