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Topsoil Structure Influencing Soil Water Retrieval by Microwave Radiometry

^a Institute of Terrestrial Ecology, ETH Zurich, Grabenstrasse 11a, 8952 Schlieren, Switzerland
^b Institute of Terrestrial Ecology, ETH Zurich, Grabenstrasse 11a, 8952 Schlieren, Switzerland
^c EAWAG, Ueberlandstrasse 13, 8600 Duebendorf, Switzerland
^d Centre d'Etudes Spatiales et de la Biosphere (CESBIO) 18 av. Edouard Belin, bpi 2801, 31401 Toulouse Cedex 4, France
^e Institute of Applied Physics, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland

View larger version (92K): [in a new window]   Fig. 1. Photograph of the soil surface with the reference frame (left) and derived digital surface model (DSM) (right). Bright gray values indicate high elevations, and dark values low elevations.

View larger version (28K): [in a new window]   Fig. 2. Schematic diagram of water content estimation from a passive microwave signal. State variables are shown within rectangle frames. Several models (elliptic frames) used to evaluate the radiometric signal. Model references are given in the bibliography.

View larger version (22K): [in a new window]   Fig. 3. Flowchart used for calculating the reflectivities Rt from the in situ measured profiles i,t and Ti,t. (Subscript i refers to layer i and t to time; h,tapp is defined by Eq. [10]; 2,t is the water content at 2 cm; h is the roughness parameter.)

View larger version (26K): [in a new window]   Fig. 4. Illustration of the principles implemented in the air-to-soil transition model. Sh(z*) is the surface height distribution and vhsoil(z*) the cumulated value corresponding to the volume fraction of the soil material; z = 0 is the average surface height of a soil area A of the order by ; z* = 0 and z* = h refer to the highest peak and the lowest depression within A. The TDR and temperature probes are located at depths of 2 and 7 cm. tsoil(z*) is the dielectric constant of the bulk soil material (including soil water), and h,tapp(z*) is the apparent dielectric constant of the two-phase (air, soil material) transition layer of thickness h.

View larger version (15K): [in a new window]   Fig. 5. Influence of the thickness h of the air-to-soil transition zone on the soil reflectivity for different assumptions about the shape of the water content profiles in 0 z 20 mm (dotted and dashed lines of the insets). The calculations were performed for typical water content profiles (a) dry and (b) wet as observed under dry and wet conditions (Fig. 10). For comparison, the radiometrically observed soil reflectivities Rradio, dry and Rradio, wet are also indicated. The relationship between the height variance and the transition zone thickness h is given by Eq. [15].

View larger version (27K): [in a new window]   Fig. 6. Rainfall (gray line) and in situ measured water content (TDR at 2 cm, black line) during the experiment. The measurements were segmented into three cycles following the precipitation periods. The times of the digital surface model (DSM) measurements are also shown.

View larger version (43K): [in a new window]   Fig. 7. Comparison between volumetric water contents measured in situ with TDR at the 2-cm depth and those calculated based on the Fresnel equations (Eq. [6] and [7]) using the ELBARA (1.4 GHz) radiometer data. For the calculations the soil profile and the surface were assumed to be smooth.

View larger version (44K): [in a new window]   Fig. 8. Comparison of reflectivities Rradio measured with the L-band radiometer ELBARA (thick black line) at horizontal (upper graph) and vertical (lower graph) polarization with reflectivities calculated using the Fresnel equations (Eq. [6] and [7]) based on water contents measured in situ by TDR at the 2-cm depth (thin black line), the coherent radiative transfer model (Rt, thick dark-gray line), and the same model but introducing a roughness correction to the coherent reflectivities according to Shi et al. (2002) (thick light-gray line).

View larger version (22K): [in a new window]   Fig. 9. Reflectivities measured with ELBARA at horizontal vs. vertical polarization. The solid line represents the values expected for a perfectly (Lambertian) rough surface, whereas the dash-dotted line holds for a perfectly smooth surface. Symbols refer to the three cycles as specified in Fig. 6.

View larger version (30K): [in a new window]   Fig. 10. (a) Reflectivities measured with the L-band radiometer at horizontal polarization (black thick line) compared with reflectivities Rt calculated by using the air-to-soil transition model considering a water content dependent transition zone of thickness h(2,t) (thick gray line). The gray dotted line indicates reflectivities calculated during period 1 if the transition zone dependence h(2,t) from period 2 is used, and reflectivities during period 2 calculated using h(2,t) from period 1. (b) Water content 2,t measured with the TDR 2 cm below the soil surface. The data section labeled as wet condition and dry condition refers to the discussion in the section Effect of the Extrapolation Approach within the Topsoil.

View larger version (26K): [in a new window]   Fig. 11. Dependence of the roughness parameter h on the volumetric water content 2,t measured at the 2-cm depth for the two periods 1 and 2 (solid lines). The dashed lines show h values for 1 and 2 averaged over the measured water content range. The calculated corresponding to h during 1 or 2 are indicated on the right axis. These values are of the same order as the optically measured values opt shown in Table 1.