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Soil Material, Temperature, and Salinity Effects on Calibration of Multisensor Capacitance Probes

^a USDA-ARS, Conservation and Production Research Lab., P.O. Drawer 10, Bushland, TX 79012-0010 USA
^b Texas Agricultural Experiment Station, Route 3, Box 219, Lubbock, TX 79401-9757 USA

View larger version (67K): [in a new window]   Fig. 1 Diagram (not-to-scale) showing the relative position of soil temperature and water content sensors in each soil column. Thermocouples measured soil temperature 0.05, 0.1, and 0.3 m below the surface. Soil water content was measured in 0.1-m intervals beginning at the 0.05-m depth for time domain reflectometry (TDR) wave guides and 0.1-m depth for multisensor capacitance probe (MCAP) sensors with a lateral separation of 0.05 m to minimize potential interference

View larger version (41K): [in a new window]   Fig. 2 Measured v plotted as a function of the multisensor capacitance probe (MCAP)-scaled frequency, SF for the calcic (open diamond) and surface (filled diamond) soil materials and calculated using the regression relationships listed in Table 2. (A) The least squares regression using Eq. [3] and ln-transformation of the pooled Texas-data appears to fit poorly. Nonlinear fits of Eq. [4] to the untransformed calcic and surface Texas-data were similar and, therefore, pooled. (B) Measured v and water contents calculated using calibrations reported by Paltineanu and Starr (1997), supplied by the factory, or developed in this paper are shown

View larger version (40K): [in a new window]   Fig. 3 Diurnal (A) air and soil temperature and (B) the multisensor capacitance probe (MCAP)-measured water content for sealed air dry soil columns. Note that the v scale is expanded to 0.001 m3 m-3 for detail (0.1 of error). Greenhouse heating began on DOY 103, resulting in 15° minimum air temperature. The gradual warming of the soil is also reflected by similarly increasing MCAP water contents

View larger version (34K): [in a new window]   Fig. 4 Diurnal (A) air and soil temperature and (B) measured water content using multisensor capacitance probe (MCAP) and (C) time domain reflectometry (TDR) sensors for sealed nearly saturated soil columns. Increased soil temperature resulted in a corresponding increase of water content measured by both the MCAP and TDR probes

View larger version (24K): [in a new window]   Fig. 5 The multisensor capacitance probe (MCAP)-measured soil water content with depth before and after resaturation with saline water