A capacitance probe has been employed to measure the spatial and temporal change of soil moisture in a weathered soil pro®le with heterogeneous texture characteristics. Measurements were made to a depth of 2 m at intervals of 2 cm. The ®eld work was carried out during a wetting-up period in a subtropical monsoonal environment in the Middle Hills of Nepal. Calibrations of the frequency readings against gravimetric and volumetric soil moisture are compared. It was found that the frequency reading correlates much better with the volumetric soil moisture. Although the soil is spatially heterogeneous in its particle size, porosity and bulk density, it was found that a single regression equation can be used to represent the relationship between the frequency reading of the capacitance probe and volumetric soil moisture. The results demonstrate that the capacitance probe enables the investigation of soil moisture variation temporally and spatially at high resolution and can measure a wide moisture range. # THE CAPACITANCE PROBEThe soil moisture capacitance probe, despite being based on the dielectric constant like TDR (Time Domain Re¯ectometry) and its obvious advantages in some aspects, has been documented less well than TDR probe or the neutron probe for soil moisture measurement.The development of the capacitance probe was described by Dean et al. (1987) and was tested by Bell et al. (1987). As with the TDR technique, the capacitance probe makes use of the large dierence between the relative dielectric constant of water (80) and that of soil (2±4) and air (1). Hence the overall dielectric constant of moist soil is very sensitive to the proportion of water present and measurement of the dielectric constant can be used to determine water content accurately. Evett et al. (1995) did a comparison of one brand of capacitance probe and two brands of neutron probe. They concluded that the capacitance probe compared poorly with the neutron probes for routine soil water content measurement because the R 2 value for the correlation between volumetric moisture content and capacitance probe readings was only 68±71%. A similar comparison (Tomer and Anderson,1995) showed a dierent picture. Tomer and Anderson found that in ®ne sand the capacitance probe gave greater soil water content estimates than the neutron probe. Robinson et al. (1994) also addressed the important eect of iron-rich minerals, such as magnetite, haematite and goethite, on the accuracy of the reading. They found that the uncertainty caused can be up to 60% of volumetric moisture with the presence of 15% magnetite. Robinson and Dean (1993) discovered a very close correlation between soil moisture and the reading of a portable surface capacitance insertion probe. The linear equations, derived from their experiment for two soils at 5 and 10 cm, have R 2 of 96 . 6% and 94 . 4%, respectively.
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