Changing rainfall patterns over the Western Lake Erie Basin (1975-2017): Effects on tributary discharge and phosphorus load

Authors: Williams, Mark; King, Kevin

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2020
Publication Date: 2/14/2020
Citation: Williams, M.R., King, K.W. 2020. Changing rainfall patterns over the Western Lake Erie Basin (1975-2017): Effects on tributary discharge and phosphorus load. Water Resources Research. 56(3). Article E2019WR025985. https://doi.org/10.1029/2019WR025985.
DOI: https://doi.org/10.1029/2019WR025985

Interpretive Summary: Assessment of rainfall patterns over time is often conducted at regional (e.g., Midwestern U.S.) or national spatial scales. These assessments are important, but lack detail on specific changes in rainfall amount and intensity that have relevance for local agriculture and water resource managers. In this study, our objective was to evaluate long-term trends in rainfall amount, frequency, and intensity across the Western Lake Erie Basin (WLEB) from 1975-2017 and determine if changes in rainfall have impacted stream flow and phosphorus loads. Results showed that annual rainfall has increased significantly over the past 40 years in the WLEB due to increases in heavy (daily rainfall between 1 and 3 inches) and very heavy (daily rainfall greater than 3 inches) rainfall. Stream flow in ten subwatersheds of the Maumee River increased in response to changing rainfall patterns. Increasing rainfall amount and intensity may not only affect discharge, but also dissolved phosphorus concentration due to an increased number of days with high flows and potentially greater probability of surface runoff. The effects of increasing rainfall amount and intensity observed in this study combined with increasing connectivity (i.e., increased drainage intensity) and accumulated nutrient stores (i.e., legacy phosphorus) in agricultural watersheds create substantial challenges for traditional conservation approaches. To ensure sustainable agricultural adaptation to changing rainfall patterns, innovative conservation practices that address these challenges are needed.

Technical Abstract: In agricultural watersheds, shifting climate and hydrologic patterns present an immediate and localized risk to both farm productivity and downstream aquatic ecosystems. Here, our objective was to evaluate long-term spatiotemporal trends in rainfall amount and intensity within the Maumee River-Lake Erie system and quantify the effects of rainfall patterns on streamflow and phosphorus loading. Across the watershed, annual rainfall increased by 102±115 mm from 1975-2017. Heavy and very heavy rainfall accounted for most of the increase in total rainfall, with observed increases of 45±32% that occurred primarily during spring and summer. Trends in streamflow for twelve monitored subwatersheds generally followed patterns in rainfall, as increases were observed in ten subwatersheds. Discharge to rainfall ratio also increased in nine of twelve subwatersheds despite no change or increases in watershed storage capacity suggesting that increasing rainfall intensity that exceeds short-term watershed storage capacity (i.e., during and immediately following the rainfall event) may control patterns of discharge more than long-term gains in watershed storage (i.e., changes in soil water holding capacity). Monthly long-term trends in total phosphorus load were weakly correlated (r2=0.25) to monthly trends in discharge; however, dissolved phosphorus was strongly correlated (r2=0.98) to discharge. These findings indicate that increasing rainfall amount and intensity combined with increasing hydrologic connectivity and accumulating nutrient stores in the Maumee watershed over the past four decades create significant challenges for contemporary approaches for dissolved phosphorus load mitigation. Innovative drainage design and management practices are needed to ensure that agricultural adaptation to climate change is sustainable.