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United States Department of Agriculture

Agricultural Research Service

Title: The Impact on Area-Averaged Heat Fluxes Using Remotely Sensed Data at Different Resolutions: a Case Using Washita '92 Data

Authors
item Kustas, William
item Jackson, Thomas

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 21, 1999
Publication Date: N/A

Interpretive Summary: An energy balance model has been developed to use land cover and soil moisture estimates derived from remote sensing data for estimating evapotranspiration (ET) over land surfaces. Since energy fluxes are non-linearly related to model input variables, defining model variables at resolutions significantly larger than the field-scale from satellite data may cause significant errors in flux calculations. Potential errors in flux calculations were investigated by evaluating differences in area-averaged flux estimates for the 18 x 45 km study area using the model with the remotely sensed data at coarser pixel resolutions. Differences in the area-averaged ET computed by the model at the lower resolutions were within 10%. This result suggests that heterogeneity in key model inputs, namely soil moisture and vegetation type and cover, was not significant enough for this surface to cause marked differences in the area-averaged flux estimates. Thus with satellite-based remote sensing systems the model has potential for mapping ET over complex landscapes and will have the capability of monitoring water use at watershed and regional scales. This information is important in assessing the impact of agricultural management practices on local and regional climate and to address environmental impact issues. In addition, the capability of monitoring water use over large regions will assist resource managers in assessing vegetation health of agricultural and natural ecosystems.

Technical Abstract: Landscape-scale fluxes are derived using an energy balance model with remotely sensed input data at different pixel resolutions. The model explicitly evaluates energy flux contributions from the soil and vegetation using remotely sensed near-surface soil moisture and leaf area index to define key model variables. The remotely sensed data used in the model were collected as part of the Washita 92 Experiment conducted in the Little Washita watershed, a sub-humid catchment in southwest Oklahoma. Since fluxes are non-linearly related to model input variables, defining model variables at resolutions significantly larger than the patch-scale may cause significant errors in flux calculations. Potential errors in flux calculations were investigated by evaluating differences in area-averaged flux estimates for the 18 x 45 km study area using the model with the remotely sensed data at the following pixel resolutions: 1) 0.2 km m2)9 km and 3) the whole image, which has an effective pixel resolution on the order of 28 km. Differences in the area-averaged sensible and latent fluxes computed by the model at the three resolutions were within 10%. This result suggests that heterogeneity in key model inputs, namely soil moisture and vegetation type and cover, was not significant enough for this surface to cause marked differences in the area-averaged flux estimates. There was a distinct increase in the area-averaged midday Bowen ratio with decreasing pixel resolution, although this trend was found to depend on the method used in averaging leaf area index for the coarser resolutions.

Last Modified: 9/22/2014
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