Soil moisture measurement techniques for remote sensing ground truth: evaluation and performance test of soil moisture sensors

Remote sensing technology requires fast and sufficiently accurate devices to take repetitive and less destructive soil moisture measurement techniques for validation of remotely sensed data. This study was conducted at Winfred Thomas Agricultural Research Station (WTARS) in Hazel Green, Alabama. The objectives of this study were to compare volumetric water content values measured with the time domain reflectometry (TDR) and water content reflectometry (WCR) instruments to the values obtained by the standard gravimetric technique for the upper soil depth and to examine the performance of the different types of soil moisture sensors and the effect of the probe length on the accuracy of soil moisture determination. From Huntsville '96 field research, we found that the emitting depth is 5 cm or less, possibly as low as 1 cm. This suggests that, in order to validate remotely sensed data, it is necessary to have fast and sufficiently accurate instruments to take repetitive and non-destructive soil moisture measurement to measure soil moisture. Our results indicated no significance difference between the Delta-T 6 cm probe output with GSM, MESA 10 cm probe output with GSM, and WCR30 and 20 cm probe output with GSM measurements. Even though the standard gravimetric technique is very reliable to measure soil moisture content, it is relatively time consuming and very destructive. Therefore, it may not be used for repetitive measurement at exactly the same location. The different types of TDR and WCR probes we tested can be used for measuring the moisture content. Except the WCR 5 and 10 cm probes, all probes tested in this experiment provided similar results. Therefore, this probe can replace the traditional gravimetric technique as long as the proper calibration is performed for a range of soil moisture and soil types.