Soil water dynamics in a toposequence under savanna woodland (Cerradao) in Assis, SP, Brazil

The aim of this study was to characterize the morphological, physical and hydraulic soil properties in a toposequence under native plant cover in a permanent observation plot of 320 x 320 m installed in the Assis Ecological Station, Sao Paulo State, Brazil. The plot is covered by a remnant Savanna Woodland (Cerradao) vegetation of the southern border zone of the great Cerrado domain. The bi-dimensional geometry (horizontal and vertical) of the soil horizons in a toposequence provided details on the soil morphology. The soil water dynamics in the native plants environment was studied through determination of the soil-water retention curves as well as soil particle-size distribution, and saturated soil hydraulic conductivity using the Guelph permeameter. Soil moisture was measured in situ using a WCR sensor. Soil moisture and rainfall distribution were monitored from November 2003 to November 2004. The soils were classified, from the summit down to the footslope, as Rhodic Haplustox, Typic Haplustox and Epiaquic Haplustult. The high clay content in the subsurface horizon of the Epiaquic Haplustult, when compared to the Oxisols with greater sand content, determines less drainage, greater water retention and lower hydraulic conductivity. The soil moisture in this layer never attained the saturation during the study period. Soil water dynamics were influenced by relief conditions, as evidenced by the continuous lateral color transition, soil texture and structure, and by rainfall. At the soil surface, the soil moisture oscillation is closely related to the rain events. In the subsurface, this effect is less intense, and lagged in time. The water availability along the toposequence is limited in the dry season and in part of the wet season. The floristic characteristics of the forest formation and its mosaic distribution in the plot can be explained by the seasonal water availability.