In-situ measurement of soil moisture at the KUREX-91 Streletskaya steppe sites

Measurements were obtained at Kursk Biospheric Reserve Station (BRS) throughout the period from 6 July through 27 July, 1991. Both gravimetric and dielectric measurements were obtained daily for all sites on the steppe. Soil sampling points were selected near each corner of the three sites (i.e. southeast, southwest, northwest, and northeast field corners). This scheme was slightly modified for the primary site, field 12, to achieve better coincidence with Soviet measurements. The moisture content of the top 10 cm of soil was examined by extracting a cylindrical core approximately 2cm in diameter and 10cm long at four sampling locations in each of the three test sites/fields. Two cores were extracted from each sampling location and combined in a single sampling container for subsequent gravimetric analysis. These samples were weighed in the laboratory several hours after being obtained and placed in drying ovens at a temperature of 110°C for approximately 48 h. Dry weights were then obtained and the original soil moisture content calculated. The complex electrical impedance at 10 MHz was measured to determine the dielectric properties of a cylindrical volume of soil contained within a specially designed probe. Probes were placed in the southeast corners of each site at depths of 5, 10, and 20cm. These probes were collocated with Soviet neutron activation tubes at the southeast sampling location for each of the three test sites. Additional sets of probes were placed at the other three sampling locations in the primary test area (site 12). These probes were placed at 5, 10, and 20 cm for the northwest and northeast sampling locations. An even more elaborate scheme was employed at the site 12 southwest location in the vicinity of the Fritchen flux measurement station. Six probes were placed as follows: two at 5 cm, one at 10 cm, one at 15 cm, and two at 20 cm. The dielectric properties of soil are strongly dependent on the moisture content since the dielectric constant of pure water is almost two magnitudes greater than that of soil containing no free water. Therefore the probe measurements were combined with the gravimetric measurements to produce estimated daily soil moisture profiles from the surface to 20 cm depth for each of the three steppe sites. The within field variability was also estimated for each site on a daily basis. These estimates are shown to be consistent with vegetation cover and precipitation history.