## 3 Results and Discussion
### Second Experiment  
The main objective of this experiment is to analyze if the RADAR could be used for soil moisture monitoring.  
To achieve the goals of this experiment, we recorded the data from the sensors for three and a half hours, without changing the soil and the sensor's conditions, to acquire  
Figure 3: Experiment 1 results in a small container in two rounds. The blue line corresponds to the RADAR’s signal, and the red line corresponds to the control signal. The room temperature is the purple line and the brown line corresponds to the room humidity. The vertical yellow dotted line represents the moment when the water was added. (**a**) First round. (**b**) Second round.  
the values for dry soil. After that, we added to the soil 500 mL of water, directly below the RADAR, without saturating the soil.  
We mapped the measurements from the RADAR into colors based on the signal's intensity. Then, we arranged the readings to make a graphic of depth by the time to see the evolution of the RADAR's signal over time. Figure 4 presents the results from this experiment.  
To compare the soil moisture sensor's and the RADAR's readings, we extracted the values from the RADAR at the same depth as the other sensors, producing Figures 5 and 6. These figures show that the more water the soil has, the lower the RADAR's signal intensity. This relation is due to the water having a much bigger dielectric constant than the soil.  
We calculated Spearman's correlation between the signals of both sensors, giving 0.75 and 0.64 at a depth of 2 cm and 21 cm, correspondingly. Near the surface, the correlation is lower due to the water, making the soil and the sediments more compact, even after it dries, increasing both RADAR readings. This can be seen in Figure 6, where the soil moisture sensor near-surface ends with a higher value than moments before adding water.  
We also calculated the cross-correlation function (CCF) between the signals of both sensors. At a depth of 2 cm, the maximum value of correlation was 0.73 with a delay of 21 min and at depth of 21 cm, the maximum value of correlation was 0.82 without any delay.  
The RADAR sensor could not detect the three different sediment layers used in our experiments. Nevertheless, the effect on water absorption and evaporation is relevant. The layers have different compositions, densities, and water absorption capabilities. That discontinuity in soil characteristics is likely to create additional small water pockets between soil layers when water is added. Small condensation zones between soil layers appear when the water evaporates. These phenomena are the most likely explanation why the RADAR's readings at 21 cm depth show some peaks and valleys throughout the experiment 2. The soil moisture sensor does not show this change because the capacitive sensing principle makes them not sensitive to condensation.  
Figure 4: Spatiotemporal pattern of the RADAR’s readings. Warmer colors represent a higher intensity of the signal that relates to drier soil. The vertical red line represents the moment when the water was added. The blue and black dotted lines represent the depths where the two ground-truth sensors were positioned.