Laboratory, Field and Remote Sensing

Combined use of field and laboratory measurements to test remote sensing applications to soil physics.

Clay soils have the specific property to swell upon wetting and shrink when drying out. This causes very small, but measureable, soil surface elevation, which can be linked to the soil water content. Driven by a need to measure soil water storage on large scales and with a high resolution, Bram te Brake of the Soil Physics and Land Management group of Wageningen University & Research, currently studies the possibilities to measure clay swell and shrinkage on large scales to estimate soil water storage change. The nested scales of the soil physical processes involved ask for an innovative combination of small scale laboratory measurements, field data collection and remote sensing techniques.

Fig. 1: Testing satellite based Radar Interferometry to measure swell and shrinkage
Fig. 1: Testing satellite based Radar Interferometry to measure swell and shrinkage

Large scale measurements in hydrology and soil physics nowadays highly rely on remote sensing techniques. Satellite based Radar Interferometry (InSAR) is a technique capable of measuring mm to cm scale surface elevation changes. These can result from clay swell and shrinkage. The specific property of clay to swell and shrink, combined with its mineralogical and hydraulic properties, make that relations between soil moisture content, soil volume and soil moisture sensor output are not straightforward.

Fig. 2: Soil moisture sensor calibration.
Fig. 2: Soil moisture sensor calibration.
Fig. 3: Clay aggregates to determine soil shrinkage characteristic curve.
Fig. 3: Clay aggregates to determine soil shrinkage characteristic curve.

The soil physics laboratory facilitated techniques to establish the relation between soil water content, shrinkage cracks and sensor output. Field measurements and measurements in the soil physics laboratory were carried out to quantify the relation between clay aggregate volume and water content. Both measurements show a highly linear relation between volume and water content of the soil. This relation can be used to convert surface elevation changes to soil water content changes.

The possibilities and limitations of this approach using Radar Interferometry are investigated. The study shows great potential for this new application, although limitations arise in vegetated areas. Careful selection of measurement sites is therefore required. Upcoming data analysis techniques and satellite missions can boost the use of Radar Interferometry in soil physical research. Soil physical laboratory and field measurements play a crucial role to revealing the underlying processes and calibrate and validate remote sensing observations.

Further reading

Bakker, G. , M.J. van der Ploeg, H. F. Gertsen, 2011. Quantifying the influence of soil cracks on the permittivity measured with Time Domain Reflectometry. Submitted.

Te Brake, B., M.J. Van der Ploeg, and G.H. De Rooij. Applicability of soil water storage change estimation from in-situ shrinkage measurements of clay soils. In preparation.

Te Brake, B., R.F. Hanssen, M.J. Van der Ploeg, and G.H. De Rooij, 2012. Satellite based radar interferometry to estimate large-scale soil water depletion from clay shrinkage: possibilities and limitations. Submitted.