SEASONAL VARIATION IN SOIL WATER CONTENT FOR FOREST AND PASTURE IN SOUTHERN BRAZIL

Abstract. The objective of this study was to monitor the behavior and spatial variability of soil water content under native pasture and forest vegetation while considering the influence of different seasons and different depths in an area of Atlantic forest in southern Brazil. To obtain the soil water content, electronic tensiometers with pressure transducers were installed 0.10, 0.30, and 0.70 m below the soil surface from October 2010 to May 2011. The values of tension, measured by the tensiometers, were transformed into soil water content values based on a soil water retention curve for each depth. The obtained results showed that the greatest variations of tension, soil water content, and soil water storage in the pasture and forest happened at depths of 0.10 and 0.30 m. At a depth of 0.70 m, these variables presented less variation, mainly for the pasture. The soil water content was greater for the forest than for the pasture. Different seasons throughout the year directly influenced the analyzed variables. In summer, the tension values observed for the forest were higher than those observed for the pasture. In winter, there was a change in that behavior at depths of 0.30 and 0.70 m, and the tension values became smaller for the forest. Great variation in the tension was observed from October to April (summer) and became steady after May (the beginning of winter). Generally in the pasture, the greatest soil water content was observed at a depth of 0.10 m, followed by 0.70 m and 0.30 m, respectively. However, during dry periods, the greatest soil water content was observed at 0.70 m, followed by 0.10 m and 0.30 m, respectively. In the forest, except for rainy periods, the greatest soil water content occurred at a depth of 0.70 m. For the conditions of this study, at 0.10 m depth, a difference of 5.73% between the soil water storage of the forest and the pasture was observed. At depths of 0.30 and 0.70 m, these differences reached 41.12% and 35.8%, respectively. The total soil water storage, considering an effective depth of the profile equal to 0.90 m, was 31.05% higher in the native forest than in the pasture.