Multi-model evaluation of longitudinal temperature fluctuations and the dominant influencing factors among Michigan streams

Stream temperature is an important determinant of fish growth, migration, and survival, and can thus impact the structure and function of stream ecosystems. Fluctuations in water temperature can occur spatially and temporally, occurring naturally or because of anthropogenic pressures. Many streams in Michigan and elsewhere in North America receive groundwater inputs that help regulate instream conditions by stabilizing discharge as well as stream temperature. However, groundwater withdrawal through high-capacity wells is important to the agricultural industry and water users for irrigation or municipal water supplies. Withdrawal can cause reductions in streamflow which typically results in increased stream temperature. Other atmospheric and hydrologic variables (i.e. overland discharge) also impact the rate at which stream temperature changes as it flows downstream. In this study we deployed paired up- and downstream water pressure and temperature loggers within 21 stream reaches throughout the state of Michigan to quantify and model relationships between stream discharge, air temperature, and longitudinal change in stream temperature (i.e., temperature flux). Using multi-model selection criteria, we evaluated the performance of a hierarchical suite of models that predict temperature flux rates as a function of potential driving variables. The multi-model selection criteria identified a best-fitting model that was able to model the diurnal, seasonal, and annual variations in rates of longitudinal temperature fluctuations across a majority of sample streams. Partial regression analysis indicated that proxy variables representing solar radiation at the stream surface were generally the most influential predictors of longitudinal changes in stream temperature, but air temperature and components of streamflow including groundwater input were significant predictors and important in many streams.