Measuring and modeling water content in stony soils

Addressing the impact of soil heterogeneities on soil water patterns at the field scale involves measuring and/or modeling water content evolution with fine spatial and temporal resolution. The presence of stones introduces difficulties for both the measurement of the water content by TDR probes and the soil hydraulic properties. In this study, the role of stones was explicitly considered for interpreting TDR-based measurements of water content and its variability in the field, as well as for adjusting the hydraulic properties of the fine fraction to that of the bulk soil including stones. The in situ TDR measurements of the bulk dielectric constant were converted to the bulk water content by adopting an approach explicitly accounting for the contribution of the volumetric fraction of stones and their dielectric properties. The water content evolution was also simulated by using laboratory-based hydraulic properties as input in a numerical model. The soil hydraulic properties of the bulk soil (stones plus fine soil) were deduced from the soil core-based hydraulic properties according to the volumetric proportion of stones in the bulk soil. For the hydraulic conductivity, we assumed that the effect of stones is simply to reduce the cross-sectional area of the bulk sample available for flow, by assuming that, on the average, the areal fraction of stones is equal to the volume fraction of stones. In modeling soil water content, we also analyzed the effect of reducing hydraulic conductivity and water retention on the evaporation process and thus on the areal distribution of soil water content. Finally, the effect of stoniness was considered as a possible explanation of the differences frequently observed between the measured hydraulic behavior and that estimated by using PTFs. Overall, both the measured and PTF-based hydraulic properties, when scaled for stoniness, proved to be adequate for describing the average evolution of the water content. However, measured hydraulic properties describe the areal distribution of water contents in the field better than PTF.