## 4 Results and Discussion
### SM-LST Relationship at the Daily Scale
Segments show the linear regression fitting lines. Figure 6 (left) shows the correlation of SMOS SM at the morning passes and MODIS LST at different platforms and acquisition times during the study period. Correlations are less variable for satellite than for in situ observations, likely due to their different spatial scales; satellite observations represent the four 25-km pixels covering the REMEDHUS area whereas ground-based measurements are made at point scale. The correlation values obtained (in median) are: Terra day (R \(\approx-0.68\)), Terra night (R \(\approx-0.54\)), Aqua day (R \(\approx-0.70\)), and Aqua night (R \(\approx-0.44\)). The strongest anticorrrelations are obtained for MODIS LST day times, both for Terra and Aqua satellites. We hypothesize that MODIS LST from day overpasses captures better the spatial pattern induced by topography that may not be captured at night time. Note that the anticorrelation for Aqua day is slightly stronger than for Terra day. This can be explained by Aqua day passes being closer to \(T_{max}\) over the REMEDHUS region than Terra day passes. These results agree with those obtained in Figure 5 (left). Further research using a geostationary satellite as MSG SEVIRI, which has a temporal resolution of 15 min, would be needed to analyze the relationship of SM with LST diurnal range. When analyzing the anomaly time-series, Figure 6 (right) displays correlations that are weaker than when using the direct variables in absolute value, as expected, but significant in all cases: Terra day (R \(\approx-0.47\) in median), Terra night (R \(\approx-0.13\)), Aqua day (R \(\approx-0.46\)), and Aqua night (R \(\approx+0.17\)). Note that Terra/Aqua at night overpasses produce very weak correlations, suggesting that only Terra/Aqua at day times maintain their relationship with the SM under anomalous conditions. Similar behavior is obtained at the afternoon passes (not shown). According to these results, only daily maximum LST (for the in situ analysis) and MODIS LST Aqua day (for the satellite analysis) will be shown hereafter. Figure 5: Correlation (R) of in situ SM acquired at the SMOS morning/afternoon passes and LST derived parameters (**left**) and their anomalies (**right**) for: (i) instantaneous \(T_{i}\); (ii) daily mean \(\overline{T}\); (iii) daily median \(T_{med}\); (iv) daily maximum \(T_{max}\); (v) daily minimum \(T_{min}\); and (vi) diurnal range \(\Delta T=T_{max}-T_{min}\). Outliers are depicted with red crosses. Figure 6: Correlation (R) of SMOS SM at the morning passes and MODIS LST (**left**) and their anomalies (**right**) for different MODIS platforms and passes: (i) Terra day; (ii) Terra night; (ii) Aqua day; and (iv) Aqua night. Similar behavior is obtained at the afternoon passes (not shown). Two-dimensional frequency density diagrams of in situ SM acquired at the SMOS morning passes and \(T_{max}\), and SMOS SM at the morning passes and MODIS LST Aqua day during the study period are displayed in Figure 7. They inform of the frequency of occurrence of all SM-LST combinations. The normalized SM-LST space visually shows the relationship between the two variables through its distributions. A triangular shape emerges from the SM-LST relationships, with sharper edges when using in situ data (left) than when using satellite data (right). This is likely to the fact that MODIS LST Aqua day is not acquired at the maximum daily LST time, when the strongest SM-LST correlation is obtained. It is important to note that MODIS LST measurements are obtained at the skin level whereas the ground-based LST observations are acquired at 0-5 cm below the land surface. Also, daily LST fluctuations are less pronounced with depth [59]. These differences could explain the higher MODIS LST Aqua day than \(T_{max}\) during very dry conditions (0-0.1 m\({}^{3}\)/m\({}^{3}\)). Similar triangles are obtained at the afternoon passes and/or using Terra day (not shown).