Kaleidoscope of Urban Evapotranspiration: Exploring the Science and Modeling Approaches

Urban evapotranspiration is a complex physical process. It depends on various critical drivers, including the land surface temperature (LST), surface albedo, landscape types, and building orientations. All of these factors create difficulties in the estimation of evapotranspiration (ET) by changing the microclimate conditions. The literature has oversimplified microclimate conditions by considering temperature difference as the only variable defining climate. The physical process depends on land-use changes, building proximities, and landscape types. This study devised three objectives to understand the microclimate effects on ET. In the first objective, land-use change effects on LST, surface albedo, and ET were analyzed over a period of twenty-seven years in the Las Vegas Valley. The analysis employed trends and shifts using Mann Kendal\u27s test and Pettit\u27s test, respectively. Land use encompassed four prominent urban surfaces, including residential, commercial, asphalt, and turf grass surfaces. The commercial and asphalt surfaces proved to be the main contributors to increased LST and decreased surface albedo. However, the increase in LST was lower than the rural surface increase, illustrating overall cooling in the summertime due to development. The removal of turf grass over the study period showed a significant increase in LST, while turf grass development showed an overall increase in ET. This study can help water managers and urban planners to understand the role of land-use change in irrigation water demand and urban thermal comfort. This study has been submitted to the Urban Climate Journal. The second objective was devised to understand the surface energy budget due to the presence and proximity of buildings. The study analyzed net radiation and soil heat flux, as well as the surface temperatures of canyons, rooftops, and turf grass, to understand day-time and nighttime warming. A 68 sq. km parcel in Phoenix, AZ was studied for the analysis. The findings suggest that canyons\u27 land surface temperatures (LST) were lower than rooftop surfaces, while turf grass surfaces were cooler than canyon surfaces. Moreover, north and south (N-S) oriented canyons were cooler than east and west (E-W) oriented canyons. No significant changes were observed in the net radiation for rooftop, turf grass, and canyon surfaces. However, the soil heat flux, warranting nighttime warming, showed higher absorption on rooftop surfaces than in canyons. The turf grass reported nighttime cooling, as the heat absorption was lower than the rooftop surfaces and the canyons. Additionally, a significant difference in heat absorption was observed between N-S oriented canyons and E-W oriented canyons. The study concluded that canyons and their orientations are major causes of daytime cooling and nighttime warming. For Phoenix, the N-S oriented streets are cooler than the E-W oriented streets. This study recommends studying canyons\u27 local municipalities, and developing a master plan for cities\u27 construction accordingly. This study has been submitted to the International Journal of Remote Sensing. The third objective investigated the microclimate effects and irrigation water requirements of three landscape types in an arid region of Phoenix, AZ. The microclimate effect encompassed surface temperature, air temperature, and wind speed. The three landscapes include mesic, oasis, and xeric. The simulation was conducted using ENVI-met software for the hottest day of the year (23rd June 2011). The simulated model was validated using ground data. The results showed that the mesic landscape induced cooling effects, both in the day-time and nighttime, by reducing the surface temperature, air temperature, and wind speed. However, the mesic landscape showed high-water consumption because of high leaf area density. The oasis landscape showed more day-time cooling than the mesic landscape, but the nighttime warming was like a xeric landscape. However, the potential irrigation water requirement was lower than the mesic landscape. Moreover, the surfaces between buildings showed varying microclimate conditions. In the case of mesic landscape, the surfaces showed high wind speeds and higher temperatures. The xeric landscape showed lower wind speeds and air temperatures between the buildings. Overall, the oasis landscape proved to be the most efficient of the three landscapes for water consumption and day-time cooling. This study will be submitted to the Journal of Advances in Modeling Earth Systems (JAMES), AGU. To sum up, both surface properties (land use) and orientation (canyons) affect the surface energy budget. Landscape type also contributes to air temperature and surface temperature changes, while air temperature changes related to wind speed. Changes in the surface energy budget affect ET rates in arid regions (Las Vegas Valley and Phoenix)