Comparing effects of climate and land use on surface water quality using future watershed scenarios

Authors: Motew, Melissa; CHEN, XI - University Of Cincinnati; CARPENTER, STEPHEN - University Of Wisconsin; BOOTH, ERIC - University Of Wisconsin; SEIFERT, JENNY - University Of California; QIU, JIANGXIAO - University Of Florida; LOHEIDE, STEVEN - University Of Wisconsin; TURNER, MONICA - University Of Wisconsin; ZIPPER, SAMUEL - University Of Victoria; KUCHARIK, CHRISTOPHER - University Of Wisconsin

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2019
Publication Date: N/A
Citation: N/A

Interpretive Summary: In this study we compared the independent, long-term effects of land use and climate on phosphorus (P) pollution in a well-studied watershed-lake system, the Yahara Watershed of southern Wisconsin. We used biophysical models to simulate future scenarios of climate and land use, and and found that climate had the stronger influence overall, overwhelming the effects of land use at the annual timescale and during very wet periods. However, land use had a significant influence on long-term outcomes of water quality due to the legacy effects of excess manure application and high soil P. The previous decades manure application rate was the most influential aspect of management affecting stream and lake water quality, outweighing erosion risk and land cover type. Reducing over-application of P was an effective and consistent management strategy under all future climates investigated. Ultimately, our findings suggest that surface water quality will face considerable challenges from a changing climate, yet local land use and management decisions will still play a vital role in determining future outcomes.

Technical Abstract: Eutrophication of freshwaters occurs in watersheds with excessive pollution of phosphorus (P). Factors that affect P cycling and transport, including climate and land use, are changing rapidly and can have legacy effects, making future freshwater quality uncertain. Focusing on the agricultural Yahara Watershed (YW) of southern Wisconsin (USA), we explored the relative influence of land use and climate on annual indicators of water quality over a span of six decades and at three spatial scales: field, stream, and lake. We also investigated which aspects of land use were the most important for water quality indicators at each scale. Using biophysical model simulations of future watershed scenarios, we found that climate exerted a stronger influence than land use on annual water quality indicators, yet land use had an important role in influencing decadal scale outcomes at field, stream, and lake scales. The most influential aspect of land use was the previous decade’s manure application rate, which superseded erosion risk and land cover type. The effect of land use was strongest at the field scale and attenuated at stream and lake scales, where the influence of weather variability was greater. These findings underscore the dominant role of climate in driving inter-annual nutrient fluxes within the hydrologic network and suggest a challenge for land use to influence water quality within streams and lakes over timescales shorter than at least a decade. Over longer timescales, reducing over-application of P throughout the watershed was an effective management strategy under all four climates investigated, even during decades with wetter conditions and more frequent extreme precipitation events.