2.2 Micrometeorology and soil moisture
Meteorological data have been monitored in this watershed since 2007 as part of a Best Management Practice (BMP) project (Boggs et al., 2015; 2016). Precipitation was measured in an open area with a Hobo Data-Logging Rain Gauge - RG3 (Onset Corporation, Bourne, MA, USA) approximately 450 m from the farthest sap flux station. Relative humidity (RH) and air temperature measurements were recorded with a Hobo Micro Station (Onset Corporation, Bourne, MA) next to the rain gauge every 10 minutes and averaged every hour. The hourly vapor pressure deficit (VPD) was calculated from RH and air temperature data and reported in kilopascals (kPa).
Soil moisture and sap flux density were measured in the riparian buffer, mid-hillslope, and upland-hillslope zones of the watershed from May 2015 to December 2016. The definition of the riparian buffer was based on field observations of sandy loam soils in the flat (i.e., <12% slope) stream valley bottom (Dreps et al., 2014). Mid-hillslope units correspond with the Tatum soil series and are on relatively steep (i.e., 12-50%) slopes with eroded soils that have a shallow depth to bedrock. Shallow-to-deep water tables and upland-hillslope units are relatively flat slopes on deep soils that correspond to Cecil, Appling, and Georgeville soil series (Dreps et al., 2014). Volumetric soil moisture (m3m-3 or %) readings were taken using a water content reflectometer (CS 615; Campbell Scientific, Logan, UT, USA). Two reflectometers were installed at each zone near the sap flux monitored trees. The reflectometers were inserted parallel to the ground surface at 10 and 30 cm depths in the riparian buffer, and at 10 and 60 cm depths in the mid-hillslope and upland-hillslope. Soil moisture data were logged every 10 minutes using a CR1000 datalogger (Campbell Scientific, Logan, UT, USA).