Quantifying the Evapotranspiration Rate and Its Cooling Effects of Urban Hedges Based on Three-Temperature Model and Infrared Remote Sensing

The evapotranspiration (ET) of urban hedges has been assumed to be an important component of the urban water budget and energy balance for years. However, because it is difficult to quantify the ET rate of urban hedges through conventional evapotranspiration methods, the ET rate, characteristics, and the cooling effects of urban hedges remain unclear. This study aims to measure the ET rate and quantify the cooling effects of urban hedges using the ‘three-temperature model + infrared remote sensing (3T + IR)’, a fetch-free and high-spatiotemporal-resolution method. An herb hedge and a shrub hedge were used as field experimental sites in Shenzhen, a subtropical megacity. After verification, the ‘3T + IR’ technique was proven to be a reasonable method for measuring the ET of urban hedges. The results are as follows. (1) The ET rate of urban hedges was very high. The daily average rates of the herb and shrub hedges were 0.38 mm·h−1 and 0.33 mm·h−1, respectively, on the hot summer day. (2) Urban hedges had a strong ability to reduce the air temperature. The two hedges could consume 68.44% and 60.81% of the net radiation through latent heat of ET on the summer day, while their cooling rates on air temperature were 1.29 °C min−1 m−2 and 1.13 °C min−1 m−2, respectively. (3) Hedges could also significantly cool the urban underlying surface. On the summer day, the surface temperatures of the two hedges were 19 °C lower than that of the asphalt pavement. (4) Urban hedges had markedly higher ET rates (0.19 mm·h−1 in the summer day) and cooling abilities (0.66 °C min−1 m−2 for air and 9.14 °C for underlying surface, respectively) than the lawn used for comparison. To the best of our knowledge, this is the first research to quantitatively measure the ET rate of urban hedges, and our findings provide new insight in understanding the process of ET in urban hedges. This work may also aid in understanding the ET of urban vegetation.

SUSTAINABLE HOUSE-SCALE PASSIVE RAINWATER CAPTURE LANDSCAPE IN THE DESERT SOUTHWEST

A passive rainwater harvesting technique was used to design a sustainable landscape for a residential lot located in the desert. The design was adapted to the Desert Southwest region of the United States based on thirty years of daily historical climate data including precipitation and reference evapotranspiration (ET0). Four cities including El Paso, TX, Albuquerque, NM, Phoenix, AZ, and Pahrump, NV, were selected to represent the area. The residential lot was broken up into micro-watersheds reflecting the runoff of water from each separate portion of the house roof, driveway, and lawn area. The paper explains in detail the design steps for one of the micro-watersheds where water retention and infiltration structures were distributed throughout the soil area to capture stormwater runoff close to its source. A passive rainwater capture landscape was obtained by using the stormwater captured in the infiltration structures and stored in the surrounding soil. Native vegetation (shrubs and trees) will use this water exclusively for growth. These plants will not require watering once their root establishment period has passed, except in extreme droughts. Meanwhile, stormwater discharge from the lot will decrease and the groundwater recharge will increase. Results indicate that the current urban water budget can be made sustainable by replacing watering of landscape by municipal water with harvested stormwater. This results in a relatively lush and shady environment even in desert climates. The success is an artifact of the tendency of urban watersheds to increase the volume of stormwater relative to pre-development conditions.