Urban parks are called "park cool islands" since air temperature is lower than their surroundings. This effect is produced by the capacity of plants to take in a great amount of energy (2.454 MJ kg-1) to transpire making with this mechanism not only a cooler area but more humid. Typical rates of heat loss by evaporation range from 24.5 to 29.5 MJm-2 per day in arid environments with good irrigation whereas, in temperate climates range <0.7 (winter) to 7.4 MJm-2 (summer) per day. The release of water vapor corresponding to heat loss values ranges from 12 to 0.28 lm-2 per day. Therefore, the determination of sensible and latent heat flux in urban parks could be important because of their direct effect on air temperature and humidity.
Energy Balance equation/Bowen ratio or Eddy Correlation methods are used to determine heat fluxes in different urban areas. Nevertheless, for small urban vegetated areas these micrometeorological methods are not adequated because they require areas with an appropriate fetch distance that often is not fulfilled. For example in Mexico City there are only three urban parks that could fulfill this requirement. In this research the method of sap flow measurments was used to determine the latent heat flux from the amount of transpiring water in individual trees in a small urban park.
A series of measurements of transpiring water (TRP), net radiation above the canopy and energy soil storage was carried out in an irrigated park in the south of Mexico City. Also wind intensity and direction and leaf area index (LAI) were measured. Four trees were selected for sap flow measurements in a completely randomized design. Latent and sensible heat fluxes were calculated from TRP and Energy Balance equation, respectively. Measuremenmts were made fro 08:00 to 17:00 LST for safety reasons.
Transpiration was higher in September (x=606 g m-2 s-1) than in January (x=432 g m-2 s-1). Maximum transpiration rates registered 0.032 and 0.025 g m-2 s-1 in September and January, respectively. Net radiation was mainly dissipated by sensible and latent heat fluxes by a 50 and 35% in the dry season and 72 and 20% in the wet season, respectively. LAI was 4.2 and 2.8 in the wet and dry season, respectively. This change in LAI was due to the deciduous character of some tree species in the park.
It is evident from results reported here that small urban vegetated areas can play a key role in the urban microclimate. It is also evident that sap flow measurements could be very helpful when it is necessary to measure the components of energy balance in small vegetated areas. However, latent heat flux may be underestimated mainly in the dry season because evaporation from the soil surface was not evaluated and soil evaporation is important when LAI is less than three. More studies to investigate this mechanism are in progress.