Global radiation, energy budget and heat island mitigation in Santiago de Chile Eugenio Collados and Rodrigo Sánchez School of Architecture, Universidad de Santiago de Chile, E-Mail:
[email protected] ABSTRACT: Climate in Santiago de Chile (-33° lat) is Mediterranean with a long dry season. Monthly average Global Radiation exceeds 200 W/m2 during at least 6 months due to low cloudiness, resulting in heat flows dominated by clear sky radiation exchange. Some factors affecting energy budget at surface level were investigated. A profile of global reflectance (0.3 to 2.8 µm) was measured along an 80 km path at 4,500 feet altitude during Summer solstice. The measured values (in the range 0.13 to 0.20) were compared to ground surface type classified in 4 categories and visually assessed from samples of visible and IR Landsat images along the path. Correlation between reflectance, land cover percentage and IR bands were found for each category. Vegetation was found positively correlated to reflectance and low rise built areas were found negatively correlated. High rise built areas were found weakly correlated to reflectance. Differences between IR profile and global reflectance profile can be explained by the fractions of absorbed heat that are either transferred to air or absorbed by evaporation through irrigated soil and vegetation. Heat stored in ground warm-up is not a significant factor of the energy budget. Also energy dissipated from human activities is not the main energy source of the heat island. The feasibility of modifying the heat gain from radiation absorption is evaluated, concluding that increasing surface reflectance, surface irrigation and vegetation cover would be effective ways of mitigating or reversing the current heat island effect. SAMPLING OF AERIAL PHOTOGRAPHS
CLIMATE Climate in Santiago de Chile (-33° lat) is Mediterranean with a long dry season. Typical Summer conditions on clear days are: • DBT max above 30°C • DBT daily mean 21°C • DBT daily swing 17°C • WBT below 21 °C • vapour pressure 1.5 kPa • wind speed below 3 m/s • global radiation above 8 kWh/day/m2 • sunshine time 14 hours
Flight path and spatial distribution of samples over Santiago
High income area. Streets shaded by trees and gardens covering a significant fraction of private plots. Public areas are irrigated during summer. Planning regulations demand for minimum distance around buildings.
Sampling was focused on a series of 2 by 2 km samples, each divided into 100 cells of 200 x 200 meters. Samples covered most types of land use, including agriculture, barren areas, suburban and all sort of urban types. Land cover was classified into four types and their percentage was visually estimated in all cells. 1. Bare land 2. Vegetation 3. Low rise neighbourhoods
Low income area. single story buildings, maximum land occupation and little vegetation. Most roofs are fiber-cement corrugated. Public spaces are mostly bare, with no irrigation and streets are fully exposed. Same regulations as above, but plots are smaller and they end almost fully occupied by buildings, leaving little room for intermediate spaces.
4. High rise neighbourhoods
CORRELATION ANALYSIS Correlation analysis between measured reflectance and land cover fraction was calculated for each cover type. Samples with vegetation show the expected positive correlation with global reflectance. However there is a contrast between low rise and high rise trends. Low rise areas are negatively correlated while high rise areas exhibit a weak positive correlation, showing that a deep built geometry may decrease the energy contribution to heat island, as predicted by Pearlmutter. That is the positive effects of current building spacing and landscaping balances the negative effects of increased surface area, reduced sky factor and lower wind speed caused by high rise buildings.
High rise neighbourhood. Building regulations for higher population densities demand wider spacing and limit land occupation, thus avoiding a close canyon geometry. Spaces between buildings are usually landscaped, as do street corridors.
REFLECTANCE ALONG FLIGHT PATH
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Reflectance was measured with a global radiation pyranometer (0.3 to 2.8 µm) carried by a plane flying at 4,500 feet altitude. Flight was done at noon (solar time) on 2002 Summer solstice (December) over an 80 km path.
SAMPLING OF LANDSAT IMAGES False colour Landsat images of Santiago were generated from bands TM1, TM4 and TM7. Band TM1 luminance (0,45 to 0,52 m) was taken as a parameter related to built areas. Concrete, roofs and asphalt surfaces have a almost flat spectrum in the range 0,35 to 0,80 m [2] On the contrary band TM4 luminance (0,76 to 0,90 m) was taken as a parameter related to vegetation since reflectance values much higher above 0,7 m. [2] Following the same 2x2 sq. km. sampling grid the images were analysed by computing the ratios band luminance to total luminance.
100% PYRANOMETE R RESPONSE
80% 60%
SOLAR IRRADIANCE AT GROUND SURFACE
40% VISIBLE
20% 0%
LANDSAT TM BANDS TM 1 BLUE
TM 4 NIR
TM 7 MIR
HEAT ISLAND MITIGATION
CONCLUSIONS
Only 16,5% of urban area in Santiago is covered by vegetation, mostly irrigated with drinking water, using 14% of city supply. This is equivalent to around 1 liter/day m2 or 2.4 m3/s for the whole city. Each square meter of additional vegetation cover would mean a reduction of 0.5 kWh/day/m2 from increased reflectance and 0,4 kWh/day/m2 from latent heat absorbed by additional evapotranspiration, at a cost of 0.08 US$/m2, assuming only 50% irrigation efficiency and use of drinking water. Global radiation available for ground and air warming would be reduced by 11%. As a comparison, anthropogenic energy amounts for only 0.7% of global radiation. These facts confirm the feasibility of mitigating the current heat island and its environmental impacts. Further mitigation steps could be taken in order to achieve better results making more efficient use of resources, such as white roofs, green roofs and other.
It was found evidence that in the climatic and urban conditions of Santiago de Chile the urbanisation effects on the heat island are twofold: − Built areas increase the absorption of solar radiation as compared to bare land. The increased absorption is greater when compared to cultivated land − Vegetation is particularly effective in reducing the absorption of solar radiation, by both increasing IR reflectance and absorbing latent heat through evapotranspiration. On the other hand, effects of canyon geometry and anthropogenic sources are less significant.
REFERENCES CONCRETE
Luminance for all three bands is positively correlated to global reflectance as expected. Band 4 (Near IR) exhibit positive correlation as well. Band 1 (Visible) exhibit a strong negative correlation, indicating that built areas without vegetation have lower global reflectance values despite high visible luminance.
VEGETATION
LANDSAT TM 7
LANDSAT TM 4
LANDSAT TM 1
• Romero A., Hugo and Ordenes S., Fernando (2001). El Gran Santiago y su medio ambiente. Encuentro por Santiago (Ordenamiento Territorial Ambientalmente Sustentable), Proyectos Fondecyt 1970470 y 1000828. • Hammer, Philip D., Johnson; Lee F., Strawa, Anthony W.; Dunagan, Stephen E.; Higgins, Robert; Brass, James; Slye, Robert E.; Sullivan, Donald V.; Lobitz, Brad M.; Smith, William H. and Peterson, David L. (1998). Surface Reflectance Mapping using Interferometric Spectral Imagery from a Remotely Piloted Aircraft, Ames Research Center, Moffett Field, CA.
• Pearlmutter, David (1998) Street canyon geometry and microclimate: Designing for urban comfort under arid conditions, ENVIRONMENTALLY FRIENDLY CITIES, Proceedings of PLEA '98, Lisbon, Portugal, James & James Publishers Ltd. • De la Maza, Carmen Luz, (2003). Desarrollo de un Sistema de Gestión de la Vegetación Urbana con Fines de Descontaminación Atmosférica y de Apoyo a la Toma de Decisiones a Nivel Municipal, Proyecto FONDEF D00I 1078. • Alvarez, Servando, Sánchez, Francisco J., Velázquez, David and Perez-Lombard, Luis (2001). Use of the vegetation and water to promote passive cooling. Information Paper as part of the ‘Renewables in the City Environment’ project, BRE- ALTENER.
