The Campbell Soil-Water Retention Function: Predictions Using Visible Near-Infrared Spectroscopy or Soil Fines.

The unsaturated hydraulic conductivity is one of the most uncertain soil properties while, at the same time, it is essential for modelling water and solute movement in the vadose zone. The Campbell soil-water retention function and its b parameter (pore-size distribution index) is a simple method to assess the soil unsaturated hydraulic conductivity when water retention data are available. However, measuring water retention is time consuming. A method to accurately predict the Campbell relation from either textural parameters such as clay and organic matter (soil fines) or from rapid, visible near-infrared spectroscopy (vis-NIR) measurements will be highly useful. To enable this, we suggest to anchor the Campbell retention model not at water saturation but rather with a reference point at the volumetric water content at -1000 cm H₂O of soil-water matric potential (pF 3). The soil-water content at the reference-point and the Campbell b was predicted using either vis-NIR (with spectral range from 400-2500 nm) or the soil fines content. Water retention, texture, and vis-NIR measurements for more than 200 soils were used for the model development. Vis-NIR measurements were used in order to correlate absorptions in specific wavelengths to Campbell b and volumetric water content at pF 3. The volumetric water content at pF 3 and Campbell b could both be well predicted from soil fines content and vis-NIR measurements. The hereby predicted Campbell function anchored at pF 3 using both methods, compared closely with measured water retention data for a majority of the soils.