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Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals. William Scheftic (Graduate Student, Atmospheric Sciences, University of Arizona) Kenneth L. Cummins and E. Philip Krider (Atmospheric Sciences, University of Arizona)

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Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals

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Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals

William Scheftic (Graduate Student, Atmospheric Sciences, University of Arizona)

Kenneth L. Cummins and E. Philip Krider (Atmospheric Sciences, University of Arizona)

David Goodrich, Susan Moran, and Russell Scott (USDA Southwest Watershed Research Center)


Discussion Outline

  • Finite soil surface conductivity has a quantifiable effect on surface-wave radio propagation in the 100 kHz to 1 MHz range

    • Brief theoretical explanation

  • Lightning Based method

    • Methodology

    • Preliminary results

    • Some Limitations

  • Radio transmission method

    • Overview of summer 2007 field campaign


Effect of finite conductivity on surface-wave signal propagation

(propagation model of Norton, 1937):

Conductivity = 10 mS/m; Propagation distance = 100 km

Signal Attenuation

Phase [rad]

Freq [Hz]


Variation with Conductivity

  • Soil – Low pass filter on propagating fields

  • Smaller conductivity or larger distance: lower cutoff-frequency


%W=0.34

Electrical Conductivity (mS/m)

σ=2.

Percent soil saturation

Electrical conductivity varies with soil moisture content


Lightning Background

Cloud-to-ground lightning electromagnetic fields: (b) First, and (c) Subsequent strokes


Rise-time determined by the highest frequencies


Propagation Animation - 75->450 km distanceConductivity = 5 mS/m

75 km


Propagation Animation - 75->450 km distance

75 km


Propagation Animation - 75->450 km distance

150 km


Propagation Animation - 75->450 km distance

225 km


Propagation Animation - 75->450 km distance

300 km


Propagation Animation - 75->450 km distance

375 km


Propagation Animation - 75->450 km distance

450 km


Rudimentary LTG Method

  • Select desired path for analysis

    • Starting at a polygon that defines the lightning-observation region

    • Ending at a sensor location (e.g., Lordsburg; Williams)

  • Evaluate risetime of lightning waveform measured by the selected sensor.

  • Convert risetime to apparent electrical conductivity

  • Convert apparent conductivity to soil moisture

  • Used North American Regional Reanalysis (NARR) as validation

Window Rock

Williams

Polygon

Yuma

Tucson

200 km

Lordsburg

Sensor

Path


Average Risetime over U.S.


Distribution of Risetime And Soil Moisture


Soil Moisture vs. Conductivity(loose criteria)


Soil Moisture vs. Conductivity(weighted count > 35)


Limitations

  • Other factors affect conductivity changes

    • Soil temperature, soil salinity, conductivity gradient over a several-meter depth

  • No perfect set of wide-area data for validation

    • NARR can miss precipitation events as occurred for at least one of the 2005 lightning events.

  • The size of the lightning region being analyzed determines how similar lightning to sensor paths really are.

  • Must have lightning!!


dab

Sb

da

Sa

Tx

Sc

Latitude

dc

Longitude

An alternate signal source is man-made narrow-band radio signals(LORAN, NDB, AM radio stations)


E

μSec

φ

The effect can be seen as changes in magnitude and phase for narrow-band radio signals


RadioTX Field Campaign ‘07

  • June->October Field Campaign in San Pedro Basin

    • Three broadband sensors

    • Remote control from PAS

    • Measure mag/phase vs. fq.

    • Derive conductivity

  • Correlate with WG and San Pedro in-situ measurements, and NARR

Note: 18 AM transmitter within 100km of Tombstone


420 kHz NDB


790 kHz AM station


Questions?


Supporting Material


Wide-area Validation:North American Regional Reanalysis

  • Specifications

    • 32 km resolution, every 3 hours

    • Available from Jan 1979 through Feb 2007

    • Adequate representation of hydrologic balance

      • Uses NOAH LSM ver. 2.6

      • 4 soil depth layers

      • Uses hourly rain gauge data and PRISM technique to assimilate precipitation


Soil Moisture: NARR Vs. ARS In-Situ


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