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An Overview of the “ Triangle Method ” for Estimating Surface Evapotranspiration and Soil Moisture from Satellite Imagery

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ABSTRACT

An overview of the ‘triangle’ method for estimating soil surface wetness and evapotranspiration fraction from satellite imagery is presented here. The method is insensitive to initial atmospheric and surface conditions, net radiation and atmospheric correction, yet can yield accuracies comparable to other methods. We describe the method first from the standpoint of the how the triangle is observed as obtained from aircraft and satellite image data and then show how the triangle can be created from a land surface model. By superimposing the model triangle over the observed one, pixel values from the image are determined for all points within the triangle. We further show how the stretched (or ‘universal’) triangle can be used to interpret pixel configurations within the triangle, showing how the temporal trajectories of points uniquely describe patterns of land use change. Finally, we conclude the paper with a brief assessment of the method's limitations.

No MeSH data available.


Scatter plot of satellite pixel values of NDVI versus radiant surface temperature from an AVHRR image approximately 100 km on a side located near Philadelphia, Pennsylvania, August 17, 1991. The warm edge, denoted with an arrow, is evident from the sharply defined right side of the pixel envelope. Pixels likely representing clouds and water are labeled with arrows.
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f1-sensors-07-01612: Scatter plot of satellite pixel values of NDVI versus radiant surface temperature from an AVHRR image approximately 100 km on a side located near Philadelphia, Pennsylvania, August 17, 1991. The warm edge, denoted with an arrow, is evident from the sharply defined right side of the pixel envelope. Pixels likely representing clouds and water are labeled with arrows.

Mentions: Consider a ‘raw’ scatter plot of surface radiant temperature versus NDVI for an AVHRR image over eastern Pennsylvania in summertime (Figure 1). (Unlike most published articles pertaining to the triangle, our figures are plotted with the ordinate as NDVI (or Fr) and the abscissa as some transform of surface radiant temperature.) One aspect immediately strikes the observer: a sharp edge to the data on the warm side of the envelope (along with plausible borders for the top and bottom of the scatter plot). The cold side of the envelope is poorly demarcated, exhibiting a tail toward low values of temperature and NDVI. One can make the case that the well-defined borders represent physical limits, such as zero available soil water content, zero vegetation cover and full vegetation. Figure 2, a scatter plot made from aircraft measurements using the NASA NS001 radiometer (5 m surface resolution), shows a better defined cold edge.


An Overview of the “ Triangle Method ” for Estimating Surface Evapotranspiration and Soil Moisture from Satellite Imagery
Scatter plot of satellite pixel values of NDVI versus radiant surface temperature from an AVHRR image approximately 100 km on a side located near Philadelphia, Pennsylvania, August 17, 1991. The warm edge, denoted with an arrow, is evident from the sharply defined right side of the pixel envelope. Pixels likely representing clouds and water are labeled with arrows.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3814871&req=5

f1-sensors-07-01612: Scatter plot of satellite pixel values of NDVI versus radiant surface temperature from an AVHRR image approximately 100 km on a side located near Philadelphia, Pennsylvania, August 17, 1991. The warm edge, denoted with an arrow, is evident from the sharply defined right side of the pixel envelope. Pixels likely representing clouds and water are labeled with arrows.
Mentions: Consider a ‘raw’ scatter plot of surface radiant temperature versus NDVI for an AVHRR image over eastern Pennsylvania in summertime (Figure 1). (Unlike most published articles pertaining to the triangle, our figures are plotted with the ordinate as NDVI (or Fr) and the abscissa as some transform of surface radiant temperature.) One aspect immediately strikes the observer: a sharp edge to the data on the warm side of the envelope (along with plausible borders for the top and bottom of the scatter plot). The cold side of the envelope is poorly demarcated, exhibiting a tail toward low values of temperature and NDVI. One can make the case that the well-defined borders represent physical limits, such as zero available soil water content, zero vegetation cover and full vegetation. Figure 2, a scatter plot made from aircraft measurements using the NASA NS001 radiometer (5 m surface resolution), shows a better defined cold edge.

View Article: PubMed Central

ABSTRACT

An overview of the ‘triangle’ method for estimating soil surface wetness and evapotranspiration fraction from satellite imagery is presented here. The method is insensitive to initial atmospheric and surface conditions, net radiation and atmospheric correction, yet can yield accuracies comparable to other methods. We describe the method first from the standpoint of the how the triangle is observed as obtained from aircraft and satellite image data and then show how the triangle can be created from a land surface model. By superimposing the model triangle over the observed one, pixel values from the image are determined for all points within the triangle. We further show how the stretched (or ‘universal’) triangle can be used to interpret pixel configurations within the triangle, showing how the temporal trajectories of points uniquely describe patterns of land use change. Finally, we conclude the paper with a brief assessment of the method's limitations.

No MeSH data available.