Evaluation of the effect of soil texture and compaction on the precision of water content measurement by Theta Probe (model ML2)

Theta Probe is one of the new methods for measuring soil water content. Rapid measurement,high precision and direct measurement in field condition are important advantages of thi method. In this study, the effects of soil texture and compaction on water content measurement precision by Theta Probe (model ML2) were evaluated. For this purpose, three types of soil (sandy, loam and clay) were selected. Subsequently, 17 samples, from each type of soils, with various compaction and moisture content (between dry soils to saturated soil) were taken. Then, the Theta Probe was calibrated for each soil by linear and nonlinear techniques. Results of Theta Probe method compared with usual gravimetric method. The results indicated that ML2 Theta Probe have the highest precision (RMSE=% 2.7) in the sandy soil and precision of the Theta Probe have inverse relationship with clay content. Also, results showed that the effect of soil compaction on Theta Probe precision is very low and this effect increases by increasing in clay content. The correlation curves between results of Theta Probe method and gravimetric method showed the highest correlation (R2 =0.95) in sandy soil and the lowest correlation (R2 =0.91) in clay soil, Loamy soil had intermediate condition (R2 =0.93).