Evaluating model-based relationship of cone index, soil water content and bulk density using dual-sensor penetrometer data

The relationship among cone index (CI), soil water content (θ) and bulk density (Db) plays a critical role in assessing soil physical conditions. To predict Db as functions of the measurements of CI and θ, a variety of semi-empirical CI-models have been established historically, however a study for validating these models has not been found. In this study four CI-models, one considered the penetration depth as variable but others did not, were evaluated under laboratory condition. The methodology was to use our own developed dual-sensor vertical penetrometer (DSVP) to simultaneously measure CI and volumetric soil water content (θv), and then to compare the bulk density (Db) core-measured to that model-predicted by the DSVP data. Two types of soil samples (silt-loam and clay) were tested. Because a previous study speculated that penetration depth could confound the CI measured, two depth-dependent factors were incorporated into each CI-model for validating this speculation. Our study found that two of the four models tested fit the experimental data with acceptable R2 (>0.70) and RMSE (<0.093 g cm−3). In contrast, the experimental results confirmed that CI in Model-1 had a peak value adapting a wide range of θ. More ever, the results indicated that the DSVP combined with Model-1 or Model-2 can be used as a tool to predict Db when CI and θ are simultaneously measured.