Temporal Scales of Water Retention Dynamics Observed in Arable Soils of Weighing Lysimeters.

Soil water retention is frequently described by main drying curves measured in the lab on intact soil cores. In the field, however, soil pore structure is known to change dynamically due to swelling and shrinkage, wetting and drying or tillage operations. Due to soil management changes, the water retention dynamics could be even more complex. The objective was to separate shorter-term hysteretic from longer-term dynamics in field-measured water retention data. Tensiometers and TDR sensors were installed in 10, 30, and 50 cm depths of six lysimeter soil monoliths from two field sites. The water content and suction data of three years (2012-2014) allowed identifying drying and wetting periods for which separate parameters of the van Genuchten (VG) retention function were fitted. The water retention curves of the initial or main drying in spring were generally steeper than those obtained in the lab. During intra-seasonal wet/dry cycles, steepness increased and the saturated VG parameter successively decreased; these data indicated that re-wetting rates decreased with each dry/wet cycle. In each spring, water retention curves returned to a very similar level except for soil monoliths that had been transferred from a differently managed neighboring field. The inter-annual comparison of maximum drying curves showed an increase in water retention that corresponded with an increase in pH-values. Modeling should consider management-induced gradual changes in hydraulic properties in addition to hysteresis and seasonal dynamics. The disentangling of dry/wet cycles from highly-resolved time series’ could help identifying processes responsible for retention dynamics, which can be useful for improving soil water flow and solute transport models.