The Influence of Soil Moisture and Tree Evapotranspiration on an Urban Microclimate

Authors

Melanie A. Mayes1* (mayesma@ornl.gov), Jeffrey Warren1, Yaoping Wang1, Chris DeRolph1, Jiafu Mao1, Jennifer First2, Jon Hathaway2

Institutions

1Oak Ridge National Laboratory, Oak Ridge, TN; 2University of Tennessee, Knoxville, TN

Abstract

The heat island effect in urban environments is primarily associated with impervious surfaces and buildings that retain heat. However, the heat island effect can be moderated by natural ecosystem components such as soil moisture and tree evapotranspiration. Impervious surfaces and green space tend to be unequally distributed based on neighborhood economic value and social constructs such as redlining. The goal of this project is to understand how microclimates (e.g., temperature and relative humidity) in urban environments are affected by the activities of plant evapotranspiration and soil moisture. Soil moisture, the extent of impervious surfaces, and tree canopy are hypothesized to influence local microclimate. Specifically, low soil moisture, high percentages of impervious surfaces, and low extent of tree canopy will cause more intense heating in the summer and more intense cooling in the winter. Public parks in Knoxville, TN, were examined and three paired sites were chosen, with each pair having contrasting temperature, percent impervious surfaces, greenness, and social vulnerability. The paired sites have overall similar underlying geology, elevation, and soil water capacity to minimize confounding factors. The sites were instrumented with small weather stations, photosynthetically active radiation sensors, incoming and outgoing radiation sensors, and soil tension and soil moisture sensors, while trees were instrumented with sapflow sensors to serve as an indicator of evapotranspiration. Soil samples were collected for basic soil properties. The data will be used to determine relationships between urban microclimate and natural components (e.g., soil moisture and tree evapotranspiration) as a function of diurnal differences throughout the growing season. This research will be used to assess the utility of models in simulating the urban heat island effect with explicit consideration of natural ecosystem components.