Soil moisture and vegetation effects on the ASCAT backscatter-incidence angle dependence
Authors: Pfeil Isabella , Wagner Wolfgang , Hahn Sebastian , Quast Raphael , Steele-Dunne Susan , Vreugdenhil Mariette 
Published: March 2021

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&lt;p&gt;Soil moisture (SM) datasets retrieved from the advanced&amp;#160;scatterometer&amp;#160;(ASCAT) sensor are well established and widely used for various hydro-meteorological, agricultural,&amp;#160;and&amp;#160;climate monitoring applications. Besides SM, ASCAT is sensitive to vegetation structure and vegetation water content, enabling the retrieval of vegetation optical depth (VOD;&amp;#160;1). The challenge in the retrieval of SM and vegetation products&amp;#160;from ASCAT observations&amp;#160;is to separate the two effects. As described by Wagner et al. (2), SM and vegetation affect the&amp;#160;relation between&amp;#160;backscatter&amp;#160;and incidence angle differently.&amp;#160;&amp;#160;At high incidence angles, the response from bare soil and thus the sensitivity to SM conditions is significantly weaker than at low incidence angles,&amp;#160;leading to decreasing backscatter with increasing incidence angle.&amp;#160;The presence of vegetation on the other hand&amp;#160;decreases the backscatter dependence on&amp;#160;the incidence angle. The dependence of backscatter on the incidence angle&amp;#160;can be&amp;#160;described by a second-order Taylor polynomial based on a slope and a curvature coefficient.&amp;#160;It was found empirically that SM conditions have no significant effect on the steepness of the slope, and that therefore,&amp;#160;SM and&amp;#160;vegetation effects can be&amp;#160;separated&amp;#160;using&amp;#160;the&amp;#160;slope&amp;#160;(2).&amp;#160;&amp;#160;This is a major assumption in the&amp;#160;TU&amp;#160;Wien&amp;#160;soil moisture retrieval algorithm used in several operational soil moisture products. However,&amp;#160;recent&amp;#160;findings by&amp;#160;Quast et al. (3)&amp;#160;using&amp;#160;a first-order radiative transfer model for the inversion of soil and vegetation parameters from&amp;#160;scatterometer&amp;#160;observations&amp;#160;indicate that SM may influence the slope, as the SM-induced backscatter increase is more pronounced at low incidence angles.&amp;#160;&lt;/p&gt;
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&lt;p&gt;The aim of this analysis is to&amp;#160;revisit&amp;#160;the assumption that&amp;#160;SM&amp;#160;does not affect the&amp;#160;slope of the backscatter incidence angle relations by&amp;#160;investigating&amp;#160;if&amp;#160;short-term variability,&amp;#160;observed&amp;#160;in ASCAT slope&amp;#160;timeseries&amp;#160;on top of the seasonal vegetation cycle,&amp;#160;is caused by SM.&amp;#160;We therefore compare timeseries and anomalies of&amp;#160;the&amp;#160;ASCAT slope&amp;#160;to&amp;#160;air temperature,&amp;#160;rainfall&amp;#160;and SM&amp;#160;from the ERA5-Land dataset. We carry out the analysis in a&amp;#160;humid continental&amp;#160;climate (Austria) and a&amp;#160;Mediterranean&amp;#160;climate&amp;#160;study region (Portugal).&amp;#160;First results show&amp;#160;significant negative&amp;#160;correlations between slope and SM anomalies. However,&amp;#160;correlations between temperature and slope anomalies are&amp;#160;of a similar magnitude,&amp;#160;albeit&amp;#160;positive,&amp;#160;which may reflect temperature-induced vegetation dynamics. The fact that temperature and SM are&amp;#160;strongly correlated&amp;#160;with each&amp;#160;other&amp;#160;complicates the&amp;#160;interpretation of the results.&amp;#160;Thus, our second approach is to&amp;#160;investigate&amp;#160;daily slope values and their change between dry and wet days.&amp;#160;The results of this study shall&amp;#160;help to quantify&amp;#160;the&amp;#160;uncertainties&amp;#160;in ASCAT SM products caused by&amp;#160;the potentially&amp;#160;inadequate&amp;#160;assumption&amp;#160;of a SM-independent&amp;#160;slope.&amp;#160;&lt;/p&gt;
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&lt;p&gt;(1) Vreugdenhil, Mariette, et al. &quot;Analyzing the vegetation parameterization in the TU-Wien ASCAT soil moisture retrieval.&quot; IEEE Transactions on Geoscience and Remote Sensing 54.6 (2016): 3513-3531.&lt;/p&gt;
&lt;p&gt;&lt;span&gt;(2) Wagner, Wolfgang, et al. &quot;Monitoring soil moisture over the Canadian Prairies with the ERS scatterometer.&quot; IEEE Transactions on Geoscience and Remote Sensing 37.1 (1999): 206-216.&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;(3) Quast, Raphael, et al. &quot;A Generic First-Order Radiative Transfer Modelling Approach for the Inversion of Soil and Vegetation Parameters from Scatterometer Observations.&quot; Remote Sensing 11.3 (2019): 285.&lt;/p&gt;
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