Contrasting stream water temperature responses to global change in the Mid-Atlantic Region of the United States: A process-based modeling study

The accurate estimation of stream water temperature is essential for understanding environmental controls on the structure and functioning of aquatic ecosystems. Few studies have coupled soil and stream water temperatures to capture the synergy of thermal balances between terrestrial and riverine systems. As a result, little is known about how multiple environmental stresses have affected water temperature, particularly for different orders of streams. Here we incorporated a new water transport scheme into the Dynamic Land Ecosystem Model (DLEM) to predict water temperature in 1st order and higher-order streams (>1st order). Driven by a 4-km geo-referenced dataset of multiple environmental factors, our new water temperature model was utilized to predict the spatiotemporal variations of water temperature in the U.S. Mid-Atlantic Region during 1900–2015. Results revealed that water temperature during 1970–2015 increased significantly (p < 0.05), and the rate of increase of the 1st order streams 0.32 °C∙decade⁻¹ is higher than that of higher-order streams 0.28 °C ∙ decade ⁻¹. The buffering effect of groundwater on water temperature in 1st order streams diminished under the context of climate warming. Factorial analysis showed that climate change and variability explain most of the changes (~80%) in stream water temperature since 1900. Land-use conversions (mostly from cropland to forest), CO₂ fertilization, and land nitrogen management collectively explained a greater percent of change in water temperature in 1st order streams (24%) than higher-order streams (9%), implying that 1st order streams are particularly vulnerable to human activities.