DIURNAL STREAM TEMPERATURE REGIMES IN THE COTERMINOUS UNITED STATES BASED ON ECO-REGIONS

Stream temperature which varies over daily to seasonal timescales is a primary control on myriad ecological, biogeochemical and physical processes. Yet geographic patterns of its diurnal variations have not been fully characterized. Using daily temperature records over 15 years (2000-2014), monthly-averaged mean daily temperature and diurnal temperature range were calculated for 310 streams distributed across six eco-regions within the coterminous United States. To consider the effect of stream size on diurnal variation, streams were grouped into three average annual discharge categories: small with <100 cfs, medium with 100-1000 cfs, and large with >1,000 cfs. Daily discharge data was used to compare seasonal discharge patterns for each of the ecoregions to the seasonal changes in diurnal temperature conditions. Across all six eco-regions, diurnal temperature fluctuations were lowest during the winter, around 1-2 °C. During the summer there was a much wider distribution in diurnal temperatures (2 °C – 12 °C), with the Plains and Mountain West having systematically higher diurnal variation than the other regions. The medium and large flow streams typically displayed peak diurnal variation in the summer. The peak fluctuation in diurnal temperature for small flow streams occurred during the spring in the Central and Eastern US and in the summer in the Western US. Across all eco-regions the annual progression of diurnal temperature ranges displayed a hysteretic pattern with larger diurnal temperature ranges in the spring than in the fall – despite there being lower flows in the fall than in the spring across all of the ecoregions. As stream size increases, diurnal variation decreases and the annual cycles of diurnal temperature range display less hysteresis.