Determining Water Retention in Seasonally Frozen Soils Using Hydra Impedance Sensors
2012
Kelleners, T. J. | Norto, J. B.
The soil freezing characteristic, defined as the relationship between freezing soil temperature and unfrozen water content, can be used to determine the soil water retention curve in situ. The objective of this study was to investigate whether freezing characteristics measured with Hydra impedance sensors (Stevens Water Monitoring Systems, Portland, OR) result in accurate depthwise soil water retention curves. Hydra sensors measuring the complex permittivity and soil temperature were installed at five depths at five sites in southeastern Wyoming and monitored for approximately 2 yr. A dielectric mixing model was calibrated using unfrozen soil data to predict the liquid water content and ice content in frozen soils. The total water potential in the frozen soils was calculated from the Hydra sensor soil temperature measurements using the Clapeyron equation. Soil water pressure heads were calculated from the total soil water head by subtracting the osmotic head. Comparison of the resulting Hydra sensor water retention data with depthwise laboratory retention data using a dew point potentiometer and a pressure plate extractor showed mixed results (coefficient of determination 0 ≤ R² ≤ 0.94). The best results were obtained for the shallowest sensors at the five sites (0.74 ≤ R² ≤ 0.93) because of the more significant and more prolonged soil freezing at these depths, resulting in relatively wide ranges for the calculated soil water pressure heads. Fitted curves for the Hydra sensor water retention data yielded unreliable parameters because of insufficient information on the wet end of the water retention curves.
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