Evaluating Drought-Induced Reductions in the Cooling Capacity of Urban Vegetation during the 2012–16 Megadrought in Southern California

Vegetation is an important driver of urban climate. Plants in cities can shade the surface, promote latent heat exchange, and provide a source of surface moisture. These effects modify the surface energy balance and cool surface and air temperatures in and around vegetated patches. The cooling capacity of vegetation is sensitive to plant health, and fractional cover, and irrigation and management practices. These factors may change during drought. Reduced precipitation and elevated vapor pressure deficit may decrease plant transpiration rates and lead to plant mortality. Water conservation may cause a reduction in irrigation. In addition, drought effects in cities may be particularly intensive as urban areas generally have lower ambient humidity and higher ambient air temperatures than their surroundings (Oke et al. 2017). Thus, we hypothesize that the cooling capacity of urban vegetation decreases over drought. To investigate this, we track urban land surface temperature (LST) in greater Los Angeles over the 2012 – 2016 mega-drought in Southern California and analyze changes in LST relative to sub-pixel vegetation fractional cover.

In this project, we use a combination of high-spatial and high-temporal resolution aerial and satellite thermal imagery to track drought-induced changes in the vegetation-LST relationship. High spatial resolution thermal imagery are from the 2013 – 2018 HyspIRI airborne campaign (HAC), which flew the MODIS/ASTER (MASTER) airborne simulator over the Los Angeles basin sub-seasonally between 2013 and 2015 and once per year between 2016 and 2018 with 36-meter spatial-resolution. High-temporal resolution thermal imagery are from the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua daily LST product with a 1-kilometer spatial resolution. LST from both sources is analyzed against vegetation fractional cover maps derived from a ~0.22-meter land cover classification generated by the Los Angeles Region Imagery Acquisition Consortium (LARIAC). These fractions represent grass, shrub, and tree cover near the midpoint of the drought (2014 – 2015).

Results indicate that the cooling effect of urban vegetation decreased by more than 20% over the drought. In addition, urban LST increased and became more spatially homogenous over the drought – mean LST increased by ~3 K and standard deviation of LST decreased by ~0.3 K. Tree dominated areas, on average, had double the cooling capacity compared to grass (~0.54 K per 10% increase in tree cover versus ~0.24 K for grass), but lost a larger fraction of their cooling effect over the drought than grass dominated areas. These results suggest that drought effects may be strongest in areas that are normally relatively cool (i.e. highly vegetated neighborhoods) and that urban water conservation during drought may exacerbate extremes of urban temperatures.