Impact of urban growth on surface climate: A case study in Oran, Algeria
Lahouari
Bounoua, NASA-GSFC, lahouari.bounoua@nasa.gov
(Presenting)
Jeff
L.
Masek, NASA-GSFC, jeffrey.g.masek@nasa.gov
Christa
P
Lidars, NASA-GSFC, christa d. lidars@nasa.gov
We use Enhanced Thematic Mapper (ETM+) data to develop a land use map discriminating urban surfaces from other land cover types over a semiarid region in North Africa and use this map in a land surface model to assess the impact of urbanized land on near surface energy, water and carbon balances at different time scales.
Unlike in temperate climates where urbanization creates a marked heat island effect, in semi-arid regions its impact does not result in a marked contrast compared to surrounding areas composed mainly of bare soils and sparse vegetation. During summer, the urban class resulted in an additional warming of 1.45 oC during daytime and 0.81 oC at night compared to that simulated for needleleaf trees under similar prevailing climate conditions. The asymmetry in the diurnal temperature cycle is a direct consequence of the lack of evapotranspiration in the urban class during the day. Our analysis indicates that in this semi arid region, the temperature and precipitation phases do not concur for the development of dense vegetation and so result in low annual rates of carbon uptake. During the winter, vegetation activity is reduced by low temperatures and during the summer by high temperatures and lack of rainfall. Consequently, the simulated seasonal temperatures show the urban area warmer than it’s surrounding during summer and slightly cooler in winter. The hydrological cycle is practically “shut down” during summer and characterized by relatively large amounts of runoff during winter.
We estimate the annual carbon uptake from all land cover types to 1.94 million metric tons. However, we project that if urbanization expands to reach 50% of the total area using equally the available land, excluding forests, the annual total carbon uptake will decline by 0.68 million metric tons (35%) and the July mean temperature would increase only by a small fraction, compared to current configuration. In contrast if urbanization expands to 50% of the total available land excluding forests and cropland, but replaced all short vegetation by a native broadleaf deciduous tree, the carbon uptake would increase by 0.76 million metric tons per year (39%) and the July mean temperature would decrease by 0.9oC, compared to current configuration.
While it is not possible to reengineer cities, our results provide important guidelines for city planners and land use managers in semi arid regions.
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