Abstract:The effect of pot plant density on plant transpiration rate was examined in a series of field experiments. Three spatial densities were created using 203 nearly homogeneous pot plants; the ratios of plant separation to plant height were 0Ð25, 0Ð5, and 3 for the 'high,' 'medium,' and 'low' groups respectively. The daily transpiration rate of 55 pot plants was measured for 28 days. During that period, the plants were randomly rotated each day to statistically eliminate individual characteristics and to successfully ascertain the effect of plant spatial density on the transpiration rate. As a best-case scenario, the soil for each plant was saturated at the start of each experiment. The results showed that the transpiration rate of potted plants in the 'low' group was about 1Ð5 times greater than that of the 'high' group. On the basis of the transpiration rate per unit of vegetation area projected on a horizontal plain, which is a general index used in meteorological modeling, the plants in the 'low' group evaporated 2Ð7 times as much water as those in the centre of the 'high' group. These results indicate the need for modified models that can consider the relative increase in evapotranspiration from vegetation in small-size plants or low spatial density of vegetation to estimate latent heat flux in urban areas.

The convective heat transfer coefficient (CHTC) of an urban canopy is a crucial parameter for estimating the turbulent heat flux in an urban area. We compared recent experimental research on the CHTC and the mass transfer coefficient (MTC) of urban surfaces in the field and in wind tunnels. Our findings are summarised as follows.(1) In full-scale measurements on horizontal building roofs, the CHTC is sensitive to the height of the reference wind speed for heights below 1. 5 m but is relatively independent of roof size.(2) In full-scale measurements of vertical building walls, the dependence of the CHTC on wind speed is significantly influenced by the choice of the measurement position and wall size. The CHTC of the edge of the building wall is much higher than that near the centre. (3) In spite of differences of the measurement methods, wind-tunnel experiments of the MTC give similar relations between the ratio of street width to canopy height in the urban canopy. Moreover, this relationship is consistent with known properties of the flow regime of an urban canopy. (4) Full-scale measurements on roofs result in a non-dimensional CHTC several tens of times greater than that in scale-model experiments with the same Reynolds number. Although there is some agreement in the measured values, our overall understanding of the CHTC remains too low for accurate modelling of urban climate.