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)

Role of Urban Landscapes in Changing the Irrigation Water Requirements in Arid Climate

The estimation of urban irrigation water requirements has often been approached from an agricultural perspective. This approach is flawed, as the intention of estimating agricultural water is to optimize yield. Recent studies have reported that urban irrigation systems waste about 34% of water, an alarming number for arid cities. The intention for urban irrigation is complex and dependent on the microclimates created by the development of the landscape. In this paper, we investigate the role of the urban landscape on the irrigation water requirements in arid cities. The role of the landscape in determining the irrigation water requirements is examined through the changes in surface-heat energy exchanges. The effects of landscapes are examined through land use change, buildings’ geometry and orientation, and vegetation types. The irrigation water requirement is assessed as the function of urban evapotranspiration and irrigation efficiency. The development of land use characteristics includes the transition from undeveloped (natural) surfaces to residential, commercial, road surfaces, or vegetated surfaces. The orientation and geometry of the streets are assessed by changes in sky view factor values due to building geometry. Three landscapes varying in vegetation type and water use are investigated. The study focuses on understanding the heat exchanges and their effects on irrigation water requirements in arid climates. Two major cities were studied: Las Vegas Valley and Phoenix metropolitan. The study concludes that the development of hardscapes, including commercial and road infrastructures, increases the overall surface temperature by 2 °C per unit change in albedo, thereby increasing evapotranspiration and urban irrigation water requirement. In addition, landscape diversity also plays a crucial role in changing the irrigation water requirement. This study highlights the importance of making development decisions in urban settings and their effects on water resources. It also contributes by providing the major factors changing the urban irrigation requirement. The study can help urban water managers and climatologists to develop improved urban irrigation models

Role of Urban Landscapes in Changing the Irrigation Water Requirements in Arid Climate

The estimation of urban irrigation water requirements has often been approached from an agricultural perspective. This approach is flawed, as the intention of estimating agricultural water is to optimize yield. Recent studies have reported that urban irrigation systems waste about 34% of water, an alarming number for arid cities. The intention for urban irrigation is complex and dependent on the microclimates created by the development of the landscape. In this paper, we investigate the role of the urban landscape on the irrigation water requirements in arid cities. The role of the landscape in determining the irrigation water requirements is examined through the changes in surface-heat energy exchanges. The effects of landscapes are examined through land use change, buildings’ geometry and orientation, and vegetation types. The irrigation water requirement is assessed as the function of urban evapotranspiration and irrigation efficiency. The development of land use characteristics includes the transition from undeveloped (natural) surfaces to residential, commercial, road surfaces, or vegetated surfaces. The orientation and geometry of the streets are assessed by changes in sky view factor values due to building geometry. Three landscapes varying in vegetation type and water use are investigated. The study focuses on understanding the heat exchanges and their effects on irrigation water requirements in arid climates. Two major cities were studied: Las Vegas Valley and Phoenix metropolitan. The study concludes that the development of hardscapes, including commercial and road infrastructures, increases the overall surface temperature by 2 °C per unit change in albedo, thereby increasing evapotranspiration and urban irrigation water requirement. In addition, landscape diversity also plays a crucial role in changing the irrigation water requirement. This study highlights the importance of making development decisions in urban settings and their effects on water resources. It also contributes by providing the major factors changing the urban irrigation requirement. The study can help urban water managers and climatologists to develop improved urban irrigation models

Assessing the Microclimate Effects and Irrigation Water Requirements of Mesic, Oasis, and Xeric Landscapes

Urban irrigation is an essential process in land–atmosphere interactions. It is one of the uncertain parameters of urban hydrology due to various microclimates. This study 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 simulations of the three landscapes were conducted using ENVI-met software for the hottest day of the year (23 June 2011). The simulated model was validated using ground data. Results show that the mesic landscape induced cooling effects, both in the daytime and nighttime, by reducing surface and air temperatures. However, the mesic landscape showed high-water consumption because of a high leaf area density. The oasis landscape showed 2 °C more daytime cooling than the mesic landscape, but the nighttime warming (surface temperature) was comparable to the xeric landscape. The potential irrigation water requirement was 1 mm/day lower than the mesic landscape. Moreover, microclimate conditions varied spatially in each neighborhood. The xeric landscape showed lower wind speeds and air temperatures between the buildings. The wind speed variations in the three landscapes were inconclusive due to differences in building orientations and discrepancies in trees’ heights. The findings can have implications for restricting the municipal irrigation budget. In addition, they can help water managers in choosing a landscape in urban areas. Urban scientists can adapt the methodology to quantify urban ET in arid regions