Changing Rainfall Patterns Over the Western Lake Erie Basin (1975–2017): Effects on Tributary Discharge and Phosphorus Load

Abstract

AbstractIn agricultural watersheds, shifting climate and hydrologic patterns present an immediate and future 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 (11 ± 13%) from 1975–2017. Heavy (25.4–76.2 mm) and very heavy (>76.2 mm) 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 12 monitored subwatersheds generally followed patterns in rainfall, as discharge increased by 19 ± 32%. Discharge‐to‐rainfall ratio increased in 9 of 12 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/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). Long‐term trends in phosphorus (total phosphorus and soluble reactive phosphorus) load indicate that changes in rainfall amount and intensity have substantially contributed to increased loading through increased discharge volumes. Differences in total phosphorus and soluble reactive phosphorus concentration trends also highlight how concomitant changes in rainfall and land management practices have influenced loading. Findings therefore suggest that changing patterns in rainfall amount and intensity have and will likely continue to pose a significant challenge to conservation efforts in agricultural landscapes aiming to decrease phosphorus delivery to surface waters.