Vegetation indices and the calibration of transpiration models in the American Southwest

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Ochoa, Francisco, M.A.

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Land management decisions should incorporate reliable estimates of energy and water available within a managed ecosystem or watershed. Studies in Central Texas savannas and in Southern New Mexico’s open-shrublands have demonstrated that evapotranspiration accounts for 60% to 90% of incoming precipitation within the same watershed. Models of evapotranspiration from various governmental agencies have been incorporated into different water conservation plans and policies in multiple southwestern states which in turn supported land and wildlife management decisions. Using the International Geosphere-Biosphere Programme vegetation classification system, carbon uptake was analyzed at a diurnal scale to assess the impact of timing (of space as well as ground derived measurements) on the calibration of transpiration models based on vegetation indices that incorporate photosynthetic active radiation (0.38 – 0.70 μm). Beer-Lambert’s Law was used to model the non-linear and linear relationships that exists between crop coefficients derived from eddy covariance systems and MODIS based measurements of various vegetation indices at different temporal scales in woody savannas, open-shrublands, and grasslands of the American southwest. Using high temporal resolution ground-based measurements of the normalized vegetation index along a southern California climate gradient, we determined the range and peak values and compared them to values derived from MODIS’ Terra and Aqua satellites at their respective times of overpass in the study area. Results indicate that temporal differences [on a 24 hour basis] need to be considered during the calibration of estimates involving vegetation indices and transpiration modeling.


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