Determining the influence of transpiration on soil moisture pathways using electrical resistivity imaging

Abstract

Many hydrologic models assume the vegetation and streamflow are fed by the same source of water, but isotopic studies have shown this is not always the case, especially in semi-arid climates. We investigate how soil moisture pathways are affected by daily transpiration using time-lapse electrical resistivity imaging (ERI) on a ponderosa pine and the surrounding soil during the growing season. By comparing sapflow measurements to the ERI data, we find that high periods of sapflow within the diel cycle are aligned with decreases in ground conductivity and soil moisture due to drying out of the soil during uptake. As the sapflow slows during the night, the ground conductivity increases as the soil is replenished. The mean and variance of the ground conductivity decreases into the summer dry season, describing drier soil and smaller diel fluctuations as the summer progresses. Sapflow does not significantly decrease through the summer, however, suggesting use of a deeper water source to maintain sapflow in a time of soil moisture depletion. ERI successfully captured spatiotemporal variability of soil moisture on daily and seasonal timescales, and with further research could be used to monitor temporal changes in tree water content.