Soil Moisture Sampling

For logistical reasons, the AACES transect has been divided into 10 patches, with each patch being the focus of a single day of ground monitoring and coincident aircraft/SMOS coverage. Thus, a total of 20 focus farms have been selected over the entire catchment to provide ground truth for several 1km resolution PLMR pixels along the flight track.

Schematic of HDAS soil moisture sampling strategy

Within each of the flight patches two 2km × 5km areas (oriented along the direction of flight lines) have been established and subdivided by six ground sampling transect lines, each 5km in length and 330m apart. The ground teams made three surface soil moisture measurements (within a radius of 1m) every 50m along these transects using the Hydraprobe Data Acquisition System (HDAS). This allows the effect of random errors in local scale soil moisture measurements to be minimised. To facilitate comparisons between SMOS, airborne observations, and the detailed ground measurements, ground soil moisture sampling took place from 5-9am. The soil moisture calibration equation for the HDAS system has been confirmed prior to the AACES campaign.

Hydraprobe Data Acquisition System (HDAS)	HDAS structure

In addition to the collected soil moisture data, a number of further ancillary information has been entered and recorded in the HDAS system for each sampling site. This information includes:

land cover type
vegetation canopy height
a visual observation of dew presence and characteristics
a visual estimate of surface rock cover fraction

HDAS soil moisture measurements

The soil moisture data that has been collected at each focus farm is shown as overview below. Users can download the data of the corresponding patch via the Download Centre