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- Author or Editor: O. Taconet x
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Abstract
The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters.
As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.
Abstract
The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters.
As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.
Abstract
This paper compares surface sensible heat flux and soil moisture values derived by inverting two boundary layers models with a surface/vegetation formulation, using surface temperature measurements made from NOAA-7 satellite (the AVHRR) with measured values for a wheat-growing area of the Beauce in France. The vegetation parameterization enables the models to reproduce the dramatic increase in surface sensible heat flux and decrease in soil moisture which occurred over a 5-day period during the field experiment. A bare soil model proved incapable of capturing the increase of the sensible heat flux during the 5-day period even though it yielded similar values of root-zone moisture.
The vegetation model responds sensitively to small changes in canopy temperature by producing large changes in surface sensible heat flux due to the parameterization of the foliage resistance and the fact that the foliage is considered a layer of zero thermal inertia. Both the vegetation and bare soil models showed a continuous moisture decrease to values near or below the wilting point in the upper part of the root zone.
The sensitivity of the results to errors in the initial sounding values or measured surface temperature were tested by varying the initial sounding temperature, dewpoint and windspeed, and the measured surface temperature by amounts corresponding to typical measurement error. Accordingly, we found that an unlucky combination of such errors can totally mask even large variations in surface heat flux from day to day, such as was measured during the field experiment. The vegetation component, therefore, is apparently more sensitive to error than the bare soil model.
Abstract
This paper compares surface sensible heat flux and soil moisture values derived by inverting two boundary layers models with a surface/vegetation formulation, using surface temperature measurements made from NOAA-7 satellite (the AVHRR) with measured values for a wheat-growing area of the Beauce in France. The vegetation parameterization enables the models to reproduce the dramatic increase in surface sensible heat flux and decrease in soil moisture which occurred over a 5-day period during the field experiment. A bare soil model proved incapable of capturing the increase of the sensible heat flux during the 5-day period even though it yielded similar values of root-zone moisture.
The vegetation model responds sensitively to small changes in canopy temperature by producing large changes in surface sensible heat flux due to the parameterization of the foliage resistance and the fact that the foliage is considered a layer of zero thermal inertia. Both the vegetation and bare soil models showed a continuous moisture decrease to values near or below the wilting point in the upper part of the root zone.
The sensitivity of the results to errors in the initial sounding values or measured surface temperature were tested by varying the initial sounding temperature, dewpoint and windspeed, and the measured surface temperature by amounts corresponding to typical measurement error. Accordingly, we found that an unlucky combination of such errors can totally mask even large variations in surface heat flux from day to day, such as was measured during the field experiment. The vegetation component, therefore, is apparently more sensitive to error than the bare soil model.
