Thermal imaging of a hydrophobic soil’s response to surfactant application at the Avon River Catchment Critical Zone Observatory

Abstract The hydrology of water repellent soil is poorly understood, in part because measuring hydrophobicity often interferes with subsequent water flow. Since soil surface temperatures provide information on near surface water content, thermography offers the potential to indirectly monitor this hydrology non-invasively. Using a near-infrared thermal camera, 15 plots, each 1 m2, of a hydrophobic sandy topsoil were recorded continuously for three weeks in the field. Treatments included plots with surfactant applied in bands from 16 cm to 100 cm width and a control (no surfactant). Soil sampling and continuous monitoring of soil moisture, soil temperature and weather conditions supplemented surface temperature measurements. Surface temperature and surface soil moisture were shown to be negatively correlated during the day and positively at night. Soil moisture probes indicated that water from three 10 mm irrigation events was largely restricted to depths less than 10 cm throughout the experiment. Dynamics of the semi-variance for treated plots were different to that of the control. Empirical orthogonal functionals (EOF) initially showed the dominant variability in spatial pattern reflected the surfactant application. During the course of the experiment, changes in the EOFs revealed the gradual re-emergence of background soil heterogeneity related to prior seeding rows. The monitoring did not reveal any lateral expansion of wetted areas from the edges of surfactant treated strips. Thermography was able to inform the dynamics of soil moisture related to spatial and temporal variability in hydrophobicity under field conditions. The approach offers the potential to non-invasively image dynamic soil hydraulic properties and to evaluate water repellence amelioration methods.