Evapotranspiration Impacts on Summer Surface Urban Heat Island Distributions and Trends in Santa Clara, California: Moderating Effects of the Human Environment

Increased urbanization impacts surface albedos and alters surface energy balances, reducing urban evaporative cooling and thus forming surface urban heat islands. Neighborhoods with higher vegetative densities, therefore, tend to have lower temperatures than those with sparse vegetation. In the United States, racial and ethnic segregated neighborhoods have limited green infrastructure, exposing populations to higher ambient temperatures. This study determined seasonal daytime and nighttime land surface temperature (LST) trends from MODIS data and distributions from ECOSTRESS data in Santa Clara County, California. It related these results to vegetation (NDVI) and evapotranspiration (ET) values, as well as to median household income, percentage of Hispanic/Latino populations, and the built environment per the Local Climate Zone (LCZ) classification framework. The combination of these assessments comprised a comprehensive human-environment approach for heat exposure evaluation. Results show upward trends in daytime summer and winter mean LST and weak nighttime trends. Winter NDVI and LST values exhibit positive correlations. Negative correlations are found for summer NDVI and LST values, which are stronger during daytime hours, indicating that the cooling effects of vegetation occur primarily during the daytime. Maximum LSTs occur in low-income neighborhoods characterized by high densities and scarce vegetation with a high percentage of Hispanic/Latino populations.