The Science

Soil moisture plays a key role in the hydrological, biogeochemical, and energy budgets of terrestrial ecosystems. However, accurate soil moisture measurements in the Amazon are difficult because of logistical constraints. To improve the understanding of ecohydrological processes within tropical forests, realistic soil moisture data in the Amazon are required. Such data also would improve models of these systems in the face of changing environmental conditions.

The Impact

Time domain reflectometry (TDR) sensors are widely used to monitor soil moisture but require calibration. In this study, a team of researchers from Lawrence Berkeley National Laboratory developed the first field‐based calibration of TDR sensors in an old‐growth upland forest in the central Amazon, evaluated the performance of the calibration, and then applied the calibration to determine the dynamics of soil moisture content within a 14.2‐m‐deep soil profile. They found that the widely used Topp model underestimated volumetric water content by 22 to 42%, suggesting that site‐specific calibration of TDR sensors for tropical soils is necessary. This new calibration will enable more accurate measurements of soil moisture in tropical soils, improving model representation of system hydrology and providing researchers with a better understanding of drought effects, forest vulnerability to water stress and mortality, vegetation succession under changing environmental conditions, and water cycling across the soil-plant-atmosphere system.

Summary

Depth-specific TDRs were calibrated using local soils in a controlled laboratory experiment, producing a novel calibration. The sensors were later installed to their specific calibration depth in a 14.2 m pit. The widely used Topp model underestimated the site-specific volumetric water content (θ_v) by 22-42%, indicating significant error in the model when applied to well-structured, clay-rich tropical forest soils. The calibrated wet- and dry-season θ_v data showed a variety of depth and temporal variations, highlighting the importance of soil textural differentiation, root uptake depths, and event- to seasonal-precipitation effects.

Principal Investigator

Robinson Negrón-Juárez
Lawrence Berkeley National Laboratory
robinson.inj@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was supported as part of the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, under contract number DE-AC02-05CH11231. Laura Borma would like to acknowledge Go-Amazon (2013/50531-2) for retrofitting the pit structure.

References