Soil Moisture Monitoring Network Using Multi-Sensor Capacitance Probes In A Thick Vadose Zone Under Riparian Vegetation
Solis, Jose A 1 ; Rajaram, Harihar 2
1 University of Colorado, Boulder
2 University of Colorado, Boulder
Soil moisture profile measurements are an integral component of water balance estimation networks, especially in thick vadose zones and fine-grained soil environments. At field sites with silty and stiff clayey soils, installation of sensors at multiple depths poses a challenge. One approach that has gained popularity is a multi-sensor capacitance (EniroSCAN) probe system manufactured by Sentek technologies. This requires only a single borehole where a 56.5 mm PVC access tube is installed. A very attractive feature of this system is that the sensors can be positioned at different depths without having to disturb the soil since the sensors are located on a cartridge unit that rides within the access tube and can be moved in 10 cm increments.
We report the design and installation of a distributed soil moisture sensor network in a riparian zone within the Whitewater Basin in Central Kansas. The soil-moisture monitoring network used in this study is compromised of 6 profilers (each covering a depth of 2 m) with 4-5 sensors per profile, located at different depths. The network is connected to a centralized data logger, from which it can be accessed remotely through a telemetry system in real-time. The network obtains distributed soil moisture measurements every 15 minutes. All profilers exhibit a rapid response to precipitation events at depths < 1m. During the leaf-out stage, sensors at depths < 1.5 m exhibit diurnal fluctuations in response to plant water uptake. The temporal trends and response to rainfall/plant uptake is significantly different at the different profile locations, illustrating the significant heterogeneity at the site. The moisture profiles clearly demonstrate the gradual replenishment of soil moisture both by precipitation and capillary rise during fall and winter, followed by depletion (with diurnal variations superimposed) during the leaf-out stage of riparian vegetation.
We describe the calibration efforts needed to convert the dielectric permittivity measurements obtained by the capacitance sensors to moisture content values. A two point calibration (measurement in nonsaline water, Fw; and in air Fa) was found to be adequate for accurate estimation of soil moisture changes. However, more detailed calibration functions were necessary for accurate estimates of absolute water content. Characterization of unsaturated soil properties was done using the HYPROP system manufactured by UMS (Umewlt Measurement Systeme). This system determines both soil water retention and unsaturated hydraulic conductivity curves. Significant heterogeneity in unsaturated soil properties is indicated from measurements on soils at different depths and locations. The soil moisture measurements and soil properties are being used for water budget estimation at the site across a range of time scales.