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sur le développement des zones arides et semi-arides

Titre : Spatial and temporal variability of transpiration in the Gwydir and Namoi catchments from 2000-2004

Auteur : Weidemann, Stephanie Lyn

Université de soutenance : Australian National University

Grade : Master of Environmental Science 2006

Résumé
Vegetation links the atmosphere and the hydrologic cycle. Rapid advances in satellite technology have allowed scientists to monitor changes in vegetation productivity through time. Data from the moderate resolution imaging spectrometer (MODIS) was used to estimate transpiration fluxes in the Gwydir and Namoi catchment areas in northern New South Wales Australia from 6 April 2000 to 31December2004. The MODIS NDVI imagery was used to compare differences in gross primary productivity (GPP) and to estimate catchment scale transpiration fluxes between croplands and non croplands. Two solar radiation parameters were estimated ; daily solar irradiance (Rs) grids were created in ESOCLIM and the global solar irradiance (R0 ) using Roderick (1999). These outputs were then used to estimate the diffuse fraction of radiation (Rd/Rs) reaching the vegetation canopy. The fraction of photosynthetic radiation (jP AR) was estimated from the MO DIS NDVI data. Then the diffuse fraction of radiation was used to yield canopy efficiency (e) (mol C0 2 mor1 PAR). Canopy efficiency and the Rd/Rs were used to estimate GPP using a radiation use efficiency approach. Monthly GPP grids were used to estimate monthly transpiration fluxes from May 2000 to December 2004. Then the monthly transpiration grids were summed to yield annual catchment scale transpiration fluxes from 2001-2004. Temporal estimates of transpiration fluxes display characteristics of wet and dry years. Transpiration fluxes increased with elevation. Vegetation on southerly aspects had higher transpiration rates due to increased radiation received at the surface. Higher annual transpiration fluxes were observed in non cropland pixels and lower annual transpiration fluxes occurred in cropland pixels. Intra-annual and inter-annual patterns of transpiration fluxes were detected from individual pixel analysis. Catchment scale estimates of transpiration fluxes are useful in monitoring the state and productivity of agricultural crops, observing the spatial arrangement of transpiration fluxes across catchments, observing differences in transpiration fluxes for contrasting vegetation types, and monitoring changes in water use efficiency of different vegetation types (e.g. forest versus cropland). Estimates of catchment scale transpiration fluxes have important implications for catchment management in that it provides an explicit functional link between land use/land cover change and catchment-level water budgets.

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