Quantifying Water and Energy Fluxes Over Different Urban Land Covers in Phoenix, Arizona

Abstract

The impact of urbanization on water and energy fluxes varies according to the characteristics of the urban patch type. Nevertheless, urban flux observations are limited, particularly in arid climates, given the wide variety of land cover present in cities. To help address this need, a mobile eddy covariance tower was deployed at three locations in Phoenix, Arizona, to sample the surface energy balance at a parking lot, a xeric landscaping (irrigated trees with gravel) and a mesic landscaping (irrigated turf grass). These deployments were compared to a stationary eddy covariance tower in a suburban neighborhood. A comparison of the observations revealed key differences between the mobile and reference sites tied to the urban land cover within the measurement footprints. For instance, the net radiation varied substantially among the sites in manners consistent with albedo and shallow soil temperature differences. The partitioning of available energy between sensible and latent heat fluxes was modulated strongly by the presence of outdoor water use, with the irrigated turf grass exhibiting the highest evaporative fraction. At this site, we identified a lack of sensitivity of turbulent flux partitioning to precipitation events, which suggests that frequent outdoor water use removes water limitations in an arid climate, thus leading to mesic conditions. Other urban land covers with less irrigation, however, exhibited sensitivity to the occurrence of precipitation, as expected for an arid climate. As a result, quantifying the frequency and magnitude of outdoor water use is critical for understanding evapotranspiration losses in arid urban areas.

Regional Impacts of Urban Irrigation on Surface Heat Fluxes and Rainfall in Central Arizona

“…For the nonirrigated areas, the changes of surface heat fluxes are related to the hotter and drier environment caused by decreased rainfall and soil moisture. These results are in line with previous observational analyses showing that irrigation plays an important role in surface energy partitioning in arid/semiarid urban environments (e.g., Templeton et al, ). Both the CTRL and IRR simulations produce comparable rainfall accumulations over the entire study region and have similar temporal distributions of rainfall during the entire simulation period. The IRR simulation, however, captures the spatial distribution of rainfall in a way that is more consistent with observations.…”

“…For the nonirrigated areas, the changes of surface heat fluxes are related to the hotter and drier environment caused by decreased rainfall and soil moisture. These results are in line with previous observational analyses showing that irrigation plays an important role in surface energy partitioning in arid/semiarid urban environments (e.g., Templeton et al, 2018). 3.…”

“…The eddy covariance tower is used to characterize surface heat fluxes over the Phoenix metropolitan region. The eddy covariance tower is located in a suburban residential area with its footprint mainly composed of heterogeneous urban land cover (including low‐rise buildings, pavement, undeveloped surface cover; see, e.g., Chow et al, ; Templeton et al, for details). Observations of surface heat fluxes (i.e., sensible and latent heat flux) are also integrated to hourly scale for further analyses.…”

“…Enhanced evapotranspiration from increased soil moisture contributes substantially to latent heat flux in the surface energy budget. The outdoor water use modulates the partitioning of available energy between sensible and latent heat fluxes (Taylor et al, ; Templeton et al, ). Due to decreased sensible heat flux, urban‐canyon air temperature can be more than 3 °C cooler, which helps to reduce thermal stress in the cities during summer (Yang & Wang, ).…”

Temporal Evolution of Green Stormwater Infrastructure Strategies in Three US Cities

“…There is likely to be a strong difference in evapotranspiration (ET) between the basin cover types. While this also has not been directly measured in these SCMs, research on ET focusing on different land covers in the Phoenix region demonstrates much higher ET and associated cooling in turfgrass vs. xeric landscapes (Templeton et al, 2017). Some of the turfgrass infiltration facilities also provide recreation benefits, as they are constructed within parks to be shallow and flat to allow for baseball or other activities.…”