Phosphorus dynamics in agricultural surface runoff at the edge of the field and in ditches during overbank flooding conditions in the Red River Valley

Abstract

Agricultural fields in the Red River Valley of the Northern Great Plains are located on flat clay soils, often drained by shallow, roadside ditches that are not graded and lacking relief. These conditions can result in flow reversals and subsequent flooding of adjacent fields during large runoff events, which can mobilize phosphorus (P). Surface runoff from two agricultural fields and their adjacent ditches was monitored from 2015 to 2017 in southern Manitoba, Canada. Overbank flooding of fields adjacent to ditches was observed in 5 of 21 hydrologic events, and such events dominated annual runoff and P budgets (>83% of losses over the 3-year study period). Flooding events were often dominated by soluble P fractions (57–67%) relative to events where flooding was not observed (39–63%). Concentrations of soluble reactive P in water standing on fields increased with time during flooding events, suggesting that P was mobilized during such events; however, the source of the soluble reactive P is not clear. This study has highlighted temporal differences in hydrologic and biogeochemical interactions between fields and ditches and demonstrated the need for an improved understanding of mechanisms of P mobilization in the landscape, which has direct implications for predicting P mobility in agricultural watersheds.

RÉSUMÉ

Les terres agricoles de la vallée de la rivière Rouge, dans les Grandes Plaines du Nord, reposent sur des sols argileux en terrain plat et sont souvent drainées par des fossés peu profonds. Ces conditions, combinées à de grands événements de ruissellement, peuvent entrainer des inondations et une mobilisation du phosphore d’origine agricole. Cette étude présente des données de ruissellement de surface pour deux champs agricoles du sud du Manitoba et leurs fossés de drainage. Parmi les 21 événements hydrologiques identifiés entre 2015 et 2017, cinq correspondaient à des conditions d’inondation et représentaient la majorité du ruissellement et des exports de phosphore (>83% des pertes sur trois ans). Les événements d‘inondation étaient caractérisés par de grandes proportions de phosphore soluble (57 à 67%), comparativement aux autres évènements hydrologiques (39 à 63%). Quant aux concentrations de phosphore soluble dans les eaux stagnantes, elles augmentaient au fil du temps pendant les événements d‘inondation, ce qui suggère une mobilisation accrue pendant ces événements. Cette étude met en évidence la nécessité de mieux comprendre les interactions hydrologiques et biogéochimiques entre les champs et les fossés en terrain plat, pour mieux prédire la mobilité du phosphore dans les bassins versants agricoles.

Keywords:

• Snowmelt
• overbank flooding
• soluble reactive phosphorus
• roadside ditch
• runoff
• riparian zone

Acknowledgements

Northern Plains Drainage Systems, Brendan Brooks, Anthony Buckley, Eva Slavicek, Vito Lam, Bo Pan and Reza Habibiandehkordi are thanked for the field and laboratory assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

Data available on request from the authors.

Additional information

Funding

This project was funded by the Manitoba Agriculture, Food and Rural Development (MAFRD), Manitoba Conservative Districts Association (MCDA), Environment and Climate Change Canada’s (ECCC) Lake Winnipeg Basin Stewardship Fund, the Natural Sciences and Engineering Research Council of Canada (Macrae–DG and Lobb–DG), the Canada First Excellence Research Fund (Agricultural Water Futures), and the Canadian Agri-Food Policy Institute (CAPI).