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The increased use of soil moisture retrieval from satellites has heightened the need for improved accuracy of point measurements that are used to validate remotely sensed soil moisture products. A wide range of devices can be installed for operational monitoring of soil moisture; however, many of these devices have not been tested in situ in soils with a very high reactive clay content. The objective of this study was to evaluate the accuracy in field performance of five soil moisture sensors: the enviroSCAN probe, the Diviner 2000 (both from Sentek Technologies), the Hydra Probe soil sensor (Stevens Water Monitoring Systems), the Thetaprobe ML2x (Delta-T Devices), and the ECH2O EC-5 (decagon devices) in soils that had about 71% clay content. The instruments' default calibrations were tested against observed soil moisture from core samples using the thermogravimetric method. New calibration equations were developed for each device, which were evaluated using an independent data set. The ThetaProbe had the lowest root mean square error (RMSE) of 0.025 m³ m-3 and mean bias error (MBE) of 0.002 m³ m-3 in the precalibration analysis. Although the Hydra Probe showed the highest precalibration errors, the instrument made the greatest improvement in post-calibration analysis, with an RMSE of 0.129 m³ m-3 using the default equation reduced to 0.014 m³ m-3 using in situ calibration, and the 0.110 m³ m-3 MBE was reduced to 0 after applying in situ calibration. [ABSTRACT FROM AUTHOR]

Two standard tests for soil stability in water involve assessment of the readily dispersible clay (RDC) content of predried soil samples. It is shown that this predrying limits the applicability of the tests to very dry soils. Ways to avoid this limitation are developed. Undisturbed soil samples were collected from the 10-to 15-cm depth of arabic topsoils at Villamblain and Faux Perche and from the 0- to 5-cm depth from plowed and direct-drilled plots at Boigneville. These soils in north-central France were collected from the field when moist (near field capacity). A range of water contents and corresponding pore water potentials (or stictions h) was produced by slow drying in air. For moist soils at field capacity (h = 100 hPa) the amount of RDC was large. Readily dispersible clay decreased progressively with increasing antecedent suction, h. For soil samples dried to beyond h > 10 MPa, the RDC was reduced to small values. With such intensely dried soils, rewetting for up to 2 wk did not reverse the effect on RDC. At h = 4500 hPa, re-equilibration to h = 100 hPa reversed the effect of this moderate drying on RDC. Two standard conditions (moist and dry) are proposed for measurement of clay dispersion from soil into water. The moist condition is relevant to soil at the end of along, wet period (e.g., at the end of the winter in temperate climates), whereas the dry condition is relevant to dry surface crusts and dry topsoils. We conjecture that the soil "resets" during long, wet periods to values of stability and RDC at equilibrium with field capacity. Experimental procedures used to determine the content of RDC in moist and dry soils are described. [ABSTRACT FROM AUTHOR]