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

Authors: MUMBI, ROSE - Purdue University; Williams, Mark; King, Kevin; Penn, Chad; CAMBERATO, JAMES - Purdue University

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2025
Publication Date: 6/14/2025
Citation: Mumbi, R.C., Williams, M.R., King, K.W., Penn, C.J., Camberato, J.J. 2025. Drought to inundation: Precipitation extremes exacerbate phosphorus loss in artificially drained watersheds. Agricultural Water Management. https://doi.org/10.1016/j.agwat.2025.109606.
DOI: https://doi.org/10.1016/j.agwat.2025.109606

Interpretive Summary: Water flow and nutrient management cause phosphorus (P) loss from agricultural fields. The study looks at a 12-year dataset of rainfall, runoff, P concentrations, and management practices. The goal was to find the conditions that affect P loss. In 2021, concentrations of dissolved reactive P and total P were two times greater than in other years. Fertilizer application during drought conditions resulted in P accumulation in the topsoil. A wet period then transported the accumulated P along with newly applied fertilizer. This 9-month period had 43% of the total dissolved P loading over 12 years. A single rainfall in September 2021 following manure application caused 8% of the cumulative dissolved P load. This shows the importance of extreme rainfall events on P loss. Findings showed that drought and ponding combined with the timing of nutrient application can worsen P loss. Coordinated timing of conservation and nutrient management plans can help reduce the risk of P loss.

Technical Abstract: 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, were used to assess how hydrometeorological and management factors influenced P loading during 2020–2021 period 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 in discharge, exported within a 9-mo period. A single precipitation event (71 mm) following manure application in Sep. 2021 accounted for 8% of cumulative 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.