In hydrogeological assessment, groundwater balances are usually carried out at catchment scales. Tt and Tg can be upscalled to the catchment scale using high resolution remote sensing. The upscalling of Tt at the plot scale uses biometric relation between stem area and xylem area and at the catchment scale between canopy projection area and xylem area. The canopy areas can be identified using very high resolution multiband images such as QuickBird and Ikonos. Applying biometric relations, they can be converted to xylem areas. The xylem areas are then multiplied by species dependent sap flux densities which are categorized with regard to species type and eventually size of a tree. Obtaining Tg map in the catchment scale is pretty similar to obtaining Tt although it requires additional species dependent partitioning transfer functions (PTF) which represent relations between Tt and T .
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