Calibration of a water content reflectometer and soil water dynamics for an agroforestry practice

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  • Calibration of a water content reflectometer and soil waterdynamics for an agroforestry practice

    Ranjith P. Udawatta Stephen H. Anderson

    Peter P. Motavalli Harold E. Garrett

    Received: 9 March 2010 / Accepted: 19 November 2010 / Published online: 4 December 2010

    Springer Science+Business Media B.V. 2010

    Abstract Water content reflectometers allow tem-

    poral and continuous assessment of spatial differ-

    ences in soil water dynamics. We hypothesized that

    volumetric soil water content estimated by the water

    content reflectometers (CS616 Campbell Sci. Inc.,

    Logan, UT) is influenced by clay content and

    temperature and therefore site- and or soil-specific

    equations are required for accurate estimations of soil

    water. Objectives of the study were to develop

    calibration equations and to evaluate soil water

    dynamics for an agroforestry system using the

    improved calibration equation. Putnam silt loam

    (fine, smectitic, mesic Vertic Albaqualfs) and Menfro

    silt loam (fine-silty, mixed, superactive, mesic Typic

    Hapludalfs) soils were selected with 2354% clay.

    Soils were packed in cylinders and sensors were

    monitored at 5, 10, 15, 20, 25, 30, 35, and 40C.Calibration equations for volumetric water content

    (hv) as a function of sensor measured period,temperature, and clay content were developed. Coef-

    ficient of determination (r2) and root mean square

    error (RMSE) were used to compare goodness of fit.

    RMSE varied between 0.028 and 0.040 m3 m-3 for

    soil specific and soil-combined linear and quadratic

    equations with period. Coefficients of determination

    ranged between 0.89 and 0.96 for these calibrations.

    RMSE decreased and r2 increased as temperature was

    included. The effect of temperature varied with water

    content, with the strongest effect at high water

    contents. Clay content did not contribute significantly

    to improve predictability. Water content estimated by

    the linear calibration equation with period and

    temperature showed differences in hv influenced byvegetation and soil depth, and closely followed

    precipitation events and water use by vegetation.

    The field study showed significant differences

    between the two treatments. Also the importance of

    temperature correction is emphasized during periods

    with large diurnal fluctuations and site specific

    calibration equations. Results of the study showed

    that water content reflectometers can be used to

    estimate hv with less than 4% error and may needsite specific calibration and a temperature correction

    to research more precise estimates.

    Keywords Cornsoybean CS616 CS615 Sensitivity analyses Soil water sensors


    CEC Cation exchange capacity

    EC Electrical conductivity

    K Dielectric constant

    R. P. Udawatta (&) H. E. GarrettCenter for Agroforestry, University of Missouri,

    203 Anheuser-Busch Natural Resources Building,

    Columbia, MO 65211, USA


    R. P. Udawatta S. H. Anderson P. P. MotavalliDepartment of Soil, Environmental and Atmospheric

    Sciences, University of Missouri, 302 Anheuser-Busch

    Natural Resources Building, Columbia, MO 65211, USA


    Agroforest Syst (2011) 82:6175

    DOI 10.1007/s10457-010-9362-3

  • hv Volumetric soil water content m3 m-3

    RMSE Root mean square error

    C PeriodTDR Time domain reflectometry

    WCR Water content reflectometer


    Accurate and continuous estimation of soil water

    content is important in many plantsoilwater and

    hydrologic studies. Gravimetric, nuclear, electromag-

    netic, and tensiometer methods can be used to

    estimate soil water content (Zazueta and Xin 1994).

    Capacitance sensors (Dean et al. 1987; Kelleners

    et al. 2004a), impedance sensors (Hilhorst et al. 1993;

    Seyfried and Murdock 2004), and transmission line

    oscillators (Campbell and Anderson 1998) are elec-

    tromagnetic approaches to measure soil water content

    which are often preferred over neutron probe meth-

    odology. Relatively inexpensive CS616 water content

    reflectometer (WCR) sensors (Campbell Sci. Inc.,

    Logan, UT), a type of a transmission line oscillator,

    uses a technique similar to time domain reflectom-

    eters (TDR) but does not require a separate pulse and

    sampling unit (Kelleners et al. 2005). WCR are

    increasingly being used in field and laboratory

    experiments to research water balance, plant water

    use, irrigation, precision farming, and movement of

    chemicals and ions (Seyfried and Murdock 2001,

    2004; Seobi et al. 2005; Anderson et al. 2009). Some

    possible reasons for the preference for these units are

    ease of installation, fewer regulatory and safety

    concerns, and cost effectiveness. Data can be col-

    lected continuously and either stored on-site or

    transmitted to a remote computer via a telephone or

    radio line (Seyfield and Murdock 2001). Therefore,

    they are easier to use in an in-field monitoring


    In WCR sensors, two wave guides 30 cm long and

    0.32 cm diameter with a 3.2 cm spacing are attached

    to a probe head with embedded circuitry; thus

    allowing an increase in the distance between the

    sensor and a data logger (Seyfried and Murdock

    2001; Chandler et al. 2004). Inside the probe head,

    voltage pulses are generated and the reflected pulse

    triggers the next pulse. The output is proportional to

    the number of reflections per second. Reflections are

    divided by a scaling factor which can be read by a

    data logger as period. Sensors can be vertically

    installed to estimate integrated soil profile water

    content or horizontally to measure water content by

    soil depth.

    The wide disparity between dielectric permittivity

    (k) of air (1), soil (2.43.5), and water (80) is used to

    measure water content; thus it is an indirect mea-

    surement of soil water content. The WCR technique

    measures equilibrium oscillation frequency or period

    of an applied voltage, which is directly related to

    k. The travel time varies with the k of the medium in

    which the wave guide is inserted (Fellner-Feldegg

    1969). With an empirical calibration equation, the

    measured wave period in microseconds is then related

    to volumetric soil water content (hv; Chandler et al.2004). Dielectric permittivity also varies with tem-

    perature. For example, dielectric constants are 87.9,

    78.4, and 55.6 for water at 0, 25, and 100C,respectively, and 1.0059 for 100C air.

    Manufacturer provided calibration estimates water

    content in sand reasonably well (Seyfried and Mur-

    dock 2001). In contrast, studies have shown that

    factory calibration overestimates soil water content in

    many soils (Seyfried and Murdock 2001; Quinones

    et al. 2003; Stangl et al. 2009). The WCR sensors use

    1545 MHz frequency range to estimate hv (Seyfriedand Murdock 2001) where TDR probes use up to

    about 1 GHz (Or and Wraith 1999). The frequency

    range used in the WCR sensors is affected by

    variations in clay content, clay type, and soil

    electrical-conductivity (Campbell 1990; Seyfried

    and Murdock 2004). However, the effect of clay

    content can be corrected by using simple linear or

    quadratic functions (Chandler et al. 2004). Further-

    more, due to this low frequency range, WCR

    estimates are often affected by temperature and

    requires soil specific calibrations (Seyfried and

    Murdock 2001; Chandler et al. 2004).

    Clay content, especially soils containing smectitic

    clays found in subsurface horizons of poorly-drained

    claypan soils of Major Land Resource Area 113

    (USDA-NRCS 1998) or in other regions, may affect

    WCR readings. Smectitic clays have relatively high

    surface charge which may attenuate the signal from

    WCR sensors and affect their ability to estimate

    profile hv. In addition, soils with high smectitic claycan undergo as much as 30% volume change due to

    wetting and drying. Therefore, soils characterized by

    62 Agroforest Syst (2011) 82:6175


  • clay-rich subsurface horizons affect water movement,

    retention, and hv. These soils often retain water forextended periods of time and their shrinkage cracks

    will seal during wet periods, or channel flow through

    these cracks and infiltrate soil water during dry

    periods. Therefore, soil profiles with relatively high

    water infiltration in surface horizons and relatively

    low infiltration in subsurface horizons may need

    individual calibration equations to better understand

    water dynamics. In support of this, Serrarens et al.

    (2000) observed that the measurement error doubled

    when a single calibration equation was used in a TDR

    calibration study with six soil depths.

    A good and accurate understanding of plantwater

    use, hydrologic relationships, and soil water dynam-

    ics are especially important in agroforestry alley

    cropping practices where grass, trees, and crops may

    share the same area. Roots of this mixed vegetation

    may occupy the same soil volume but with different

    densities and distribution patterns. Trees and grass in

    agroforestry alley cropping practices use soil water

    from different soil depths over a longer period as

    compared to crop plants since annual crops have

    relatively shallow root systems and shorter growing

    seasons than


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