Effect of stones on soil hydraulic properties: measurement and modeling

Stony soils are soils that contain a high amount of stones and are widespread all over the world.  The effective soil hydraulic properties (SHP), i.e. the water retention curve (WRC) and the hydraulic conductivity curve (HCC) are influenced by the presence of stones in the soil. This influence is normally neglected in vadose zone modeling due to the considerable measurement challenges in stony soils. The available data on the effect of stones on SHP is scarce and there is not a systematic modeling approach to obtain the effective SHP in stony soils. Most of the past studies are limited to the effect of stones on the WRC and saturated hydraulic conductivity and low and medium stone contents (up to 40 % v/v). We investigated the effect of stone content on the effective SHP of stony soils through a series of evaporation experiments. Two soil materials a) sandy loam and b) silt loam as background soils were packed with different volumetric contents (0, 10, 30 and 60 %) of medium stones were in containers with a volume of 5060 cm³. Volumetric stone contents were chosen in a way to present stone-free, moderately stony and highly stony soils. All of the experiments were carried out in two replicate packings with an almost identical bulk density. Packed samples were saturated with water from the bottom and subjected to evaporation in a climate-controlled room. During the evaporation experiments, the pressure head and soil temperature were continuously monitored and the water loss from the soil columns was measured with a balance. The dewpoint method provided additional data on the WRC in the dry soil. The resulting data were evaluated by inverse modeling with the Richards equation to identify effective SHP and to analyze the effect of stone content on the evaporation rate, soil temperature, the effective WRC and the effective HCC. The applied methodology was successful in identifying effective SHP with high precision over the full moisture range. The results reveal a quicker transition from stage I to stage II of evaporation in highly stony soils. Evaporation rate reduces with the increase of the volumetric stone content. The existence of a high amount of stone content shorten stage II of evaporation driven by the vapor diffusion through the restricted soil evaporative surface.