Drought to inundation: Precipitation extremes exacerbate phosphorus loss in artificially drained watersheds

Changing precipitation patterns coupled with phosphorus (P) abundance and availability in agricultural landscapes pose significant risks to water quality. This study examines the impact of drought followed by extreme precipitation on P export dynamics in an artificially drained watershed in the U.S. Midwest. A 12-year dataset (2012–2023) of precipitation, discharge, P concentrations, and management practices was used to assess how hydrometeorological and management factors influenced P loading during 2020 and 2021 which experienced drought followed by extreme precipitation relative to long-term trends. Results show that the combined effect of seasonal precipitation contrasts, extreme events, and P management practices led to a 2-fold increase in annual flow-weighted mean concentrations of dissolved reactive P (DRP) and total P (TP) in 2021 compared to previous years. Drought from 2020 to early 2021 accompanied by fertilizer additions caused P accumulation in surface soils. A period of excess wetness followed, reactivating hydrological pathways and mobilizing accumulated soil P and newly applied fertilizer, with 43 % (DRP) and 29 % (TP) of cumulative 12-yr loading exported within a 9-mo period. A single precipitation event (71 mm) following manure application in Sep. 2021 accounted for 8 % of cumulative 12-yr DRP load, emphasizing the disproportionate role of extreme events on P export. Findings highlight how seasonal- and event-scale precipitation extremes combined with nutrient management practices exacerbate P loss. Targeted conservation efforts, including optimized application timing and placement, and legacy P mitigation are essential for reducing short- and long-term eutrophication risks associated with precipitation variability.