Integrating soil water measurements from plot to catchment scale in a snow-dominated, mixed-conifer forest of the southern Sierra Nevada

Soil moisture was measured in the southern Sierra Nevada using cosmic-ray, time domain reflectometry (TDR), dielectric, neutron probe, and gravimetric or volumetric sampling techniques. These techniques are compared to develop better a understanding of shallow (0-50 cm) soil moisture and to determine the feasibility of decoupling vegetation moisture storage from soil-moisture storage within the cosmic-ray signal. Multiple embedded sensors (TDR and dielectric) were deployed across varying soil depths, aspects, and canopy covers to capture spatial and temporal variations of soil volumetric water content within the spatial range of a COsmic-ray Soil Moisture Observing Systems (COSMOS). Soil samples were collected within the COSMOS footprint for calibration and comparison during the COSMOS installation, June 2011. Through a one-year period, June 2011-June 2012, area-average volumetric water contents observed by COSMOS were compared to real-time, in situ observations of soil moisture using TDR and dielectric sensors, and with measurements of soil moisture taken periodically during surveys within the COSMOS footprint. Surveys of soil moisture in the upper 40 cm of soil were made along transects around the COSMOS with handheld TDR and gravimetric sampling techniques. A neutron probe was also used to measure soil moisture at 14 locations within the COSMSO footprint. Results show that the COSMOS and the embedded sensor networks effectively observed trends of snow disappearance and soil drainage throughout the summer and fall, and track diurnal and seasonal trends in the near-surface soil profile. The addition of snow during the winter of WY2012 complicates the COSMOS signal. Timber harvest during spring and summer 2012 appear to have no immediate effect on shallow soil moisture throughout the area. However, we had anticipated that the loss of water stored in vegetation during timber harvest would be apparent in the COSMOS signal, which should include water stored in vegetation. Three possible causes for the lack of moisture change are: 1) too few trees were removed; 2) canopy vegetation left on the forest floor had approximately equal water content as tree-stem wood removed; 3) tree-root water storage remained relatively unchanged. This research is part of the NSF-supported Southern Sierra Critical Zone Observatory, which is co-located within the U.S. Forest Service, Kings River Experimental Watershed.