Examining Diel Patterns of Soil and Xylem Moisture From the Single-Tree to Hillslope Scale

Wednesday, 26 July 2017: 11:15 AM
Paul Brest West (Munger Conference Center)
Kamini Singha1, Ryan Harmon1, Rachel Mares1 and Holly R Barnard2, (1)Colorado School of Mines, Golden, CO, United States, (2)University of Colorado at Boulder, Boulder, CO, United States
The feedbacks among forest transpiration, soil moisture, and subsurface flowpaths are poorly understood. We investigate how soil moisture is affected by daily transpiration using time-lapse electrical resistivity imaging (ERI) on (1) a highly instrumented ponderosa pine and the surrounding soil throughout the growing season at the Boulder Creek Critical Zone Observatory, and (2) the hillslope scale over multiple seasons from the H.J. Andrews in Oregon. By comparing sap flow measurements to the ERI data at the single-tree scale, we find that periods of high sap flow within the diel cycle are aligned with decreases in ground electrical conductivity and soil moisture due to drying of the soil during moisture uptake. As sap flow decreases during the night, the ground conductivity increases as the soil moisture is replenished. The mean and variance of the ground conductivity decreases into the summer dry season, indicating drier soil and smaller diel fluctuations in soil moisture as the summer progresses. Sap flow did not significantly decrease through the summer suggesting use of a water source deeper than 60 cm to maintain transpiration during times of shallow soil moisture depletion. ERI captured spatiotemporal variability of soil moisture on daily and seasonal timescales. ERI data on the tree showed a diel cycle of conductivity, interpreted as changes in water content due to transpiration, as well as a night-time signature of tree refilling. Seasonal variations indicate that the tree might be showing osmotic adjustment. At the hillslope scale, lags between transpiration, soil moisture as interpreted by ERI, groundwater and stream water help to distinguish between processes controlling evapotranspiration and groundwater fluxes.