EFFECTS OF URBANIZATION ON AIR TEMPERATURE/WATER TEMPERATURE RELATIONSHIP IN TWO SMALL, MOUNTAIN STREAMS

Boone Creek is a headwater stream of low to moderate gradient located in Boone, North Carolina, USA. The 5.2 km­² catchment includes a 1.9 km study reach. Hardin Creek is a moderate gradient, low discharge tributary to Boone Creek with a study reach of 0.7 km. Impervious surface coverage in the catchment increases downstream from 13.4% to 24.3%. Other markers of urbanization, including culverting, lack of riparian vegetation, and bank armoring, increase downstream along both streams as they flow from the forested hillsides through town and the campus of Appalachian State University.

We quantified the effect of urbanization on the thermal inertia of the streams by comparing the thermal exchange coefficient, K, among 11 monitored sites. K is a measure of the strength of the relationship between stream temperature and air temperature, or an inverse measure of the stream’s thermal inertia. Hourly air temperature data and 15-minute water temperature data from the monitoring sites were collected from January 1, 2013 to December 31, 2015. An analytical model of the relationship between air and water temperature was developed in MATLAB using an optimized sine curve and a second order Markov process. The observed air and water temperatures were used to calibrate the model and determine the optimal K for each site and year.

In a uniform catchment environment, K values are expected to decrease downstream as the stream gains discharge volume and, therefore, thermal inertia. Our analysis of data from 2015 reveals that along Boone Creek, K values decrease downstream through the upper half of the study reach from 0.284 to 0.216. However, K values for the lower half of the reach, after the stream had crossed into the more urbanized environment, increased from 0.216 to 0.308. Both Hardin Creek sites are located on campus and also exhibited a downstream increase in K from 0.190 to 0.211. Our study suggests that urbanization may reduce the thermal inertia of these small streams.