Estimating Seasonal Changes in Volumetric Soil Water Content at Landscape Scales in a Savanna Ecosystem Using Two-Dimensional Resistivity Profiling

Abstract

Water distributed in deep soil reservoirs is an important factor determining the ecosystem structure of water-limited environments, such as the seasonal tropical savannas of South America. In this study a two-dimensional (2D) geoelectrical profiling technique was employed to estimate seasonal dynamics of soil water content to 10-m depth along transects of 275 m in savanna vegetation during the period between 2002 and 2006. Methods were developed to convert resistivity values along these 2D resistivity profiles into volumetric water content (VWC) by soil depth. The 2D resistivity profiles revealed the following soil and aquifer structure characterizing the underground environment: 0?4 m of permanently unsaturated and seasonally droughty soil, less severely dry unsaturated soil at about 4?7 m, nearly permanently saturated soil between 7 and 10 m, mostly impermeable saprolite interspaced with fresh bedrock of parent material at about 10?30 m, and a region of highly conductive water-saturated material at 30 m and below. Considerable spatial variation of these relative depths is clearly demonstrated along the transects. Temporal dynamics in VWC indicate that the active zone of water uptake is predominantly at 0?7 m, and follows the seasonal cycles of precipitation and evapotranspiration. Uptake from below 7 m may have been critical for a short period near the beginning of the rainy season, although the seasonal variations in VWC in the 7?10-m layer are relatively small and lag the surface water recharge for about 6 months. Calculations using a simple 1-box water balance model indicate that average total runoff was 15?25 mm month?1 in the wet season and about 6?9 mm month?1 in the dry season. Modeled ET was about 75?85 mm month?1 in the wet season and 20?25 mm month?1 in the dry season. Variation in basal area and tree density along one transect was positively correlated with VWC of the 0?3-m and 0?7-m soil depths, respectively, during the wettest months. These multitemporal measurements demonstrate that the along-transect spatial differences in soil moisture are quasi-permanent and influence vegetation structure at the scale of tens to hundreds of meters.