Predicting surface porosity using a fine-scale index of roughness in a cultivated field

Both soil porosity and surface elevation can be altered by tillage operation. Even though the surface porosity is an important parameter of a tilled field, however, no practical technique for rapid and non-contact measurement of surface porosity has been developed yet. On the contrary, the surface elevation of tilled soil can be quickly determined with a laser profiler. Working under the assumption that the surface elevation of a tilled field is a complicated superposition of the soil terrain profile at a larger-scale and the roughness at a fine-scale, this study included three aspects: (i) to present a differential index (RI) at a fine-scale to associate the surface roughness with porosity; (ii) to examine the correlation between surface porosity and the proposed RI by four types of tillage treatment in the field; and (iii) to check the influence of different grid sizes of calculating RI on predicting porosity. Consequently, the statistical results from each tilled plot demonstrated a strong correlation (0.889 ≤ R2 ≤ 0.982) between the surface porosity and the defined RI in an early stage (ca. 20 days) after tillage. Moreover, a combined data analysis showed a linear approximation between both parameters with an R2 value of 0.707 when the grid size was adequately small (2 cm × 2 cm). The pronounced influence of the grid size on the predicting accuracy (R2) of the statistics model further confirmed the assumption that the surface elevation had scale-dependency characteristics. Therefore, according to the results presented in this study, the laser profiler is a promising rapid and non-destructive tool for assessing the surface porosity.