Water Retention and Preferential States of Soil Moisture in a Cultivated Vertisol

Vertisols are well suited for rainfed agriculture in water-limited environments as a result of their unique water transfer and retention characteristics. Despite their importance, the agro-hydrological behavior of these soils under seasonally dry climates is not yet fully understood. We collected water retention data for a Vertisol, measured from saturation to oven dryness on 27 undisturbed topsoil (0-0.05 m) samples from an experimental field in southcentral Spain and related this information to the occurrence of field-observed preferential soil moisture states. A continuous function was fitted to the mean gravimetric water retention data, circle minus, consisting of the sum of a double exponential model and the Groenevelt and Grant model. An inflection point at pressure head |h| = 1.1 X 10(5) cm, circle minus = 0.12 kg kg(- 1), and an equivalent pore radius, delta = 14 nm, was interpreted as the boundary between the clay inter-and intra-aggregate pore spaces, corresponding with the transition from the intermediate to the dry field soil moisture state. The mode of the textural pore space (|h| = 7.3 X 10(3) cm, circle minus = 0.21 kg kg(- 1), and delta = 200 nm) matched the transition from the wet to the intermediate field soil moisture state. We related these characteristics of the soil water retention curve with the spatiotemporal soil moisture dynamics and patterns observed in the field, characterized by fast transitions between preferential soil moisture states. The proposed framework is suitable for comparing the effects of different soil management strategies on the agro-hydrological performance of Vertisols.