Soil moisture monitoring data and products in nebraska
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Soil Moisture Monitoring, Data, and Products in Nebraska. Eric Hunt, Brian Fuchs, Mark Svoboda National Drought Mitigation Center 3 March 2009. Nebraska AWDN stations. History of Nebraska Soil Moisture. Sensors installed at 14 sites beginning in 1998

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Soil moisture monitoring data and products in nebraska l.jpg
Soil Moisture Monitoring, Data, and Products in Nebraska

Eric Hunt, Brian Fuchs, Mark Svoboda

National Drought Mitigation Center

3 March 2009



History of nebraska soil moisture l.jpg
History ofNebraska Soil Moisture

  • Sensors installed at 14 sites beginning in 1998

    • Original soil moisture sites used Vitel (Stevens HydraProbe) impedance probes

  • Sensors were installed at an additional 37 sites in 2002-2004

    • New sites used another impedance probe, called the Theta

    • Theta eventually replaced the Vitel at all of the original sites by the end of 2005

  • Sensors installed using the drip loop method under natural grass cover and away from artificial sources of water (i.e., irrigation)


Sensor type and installation l.jpg

Transmission

rods

Waterproof Enclosure

Sensor type and installation

  • Waterproof enclosure that contains the measurement circuitry

  • Sensing head with rods that act as a transmission line

    • Measure the dielectric constant of the soil

    • Linear relationship between dielectric constant and volumetric water content (Topp et al., 1980)


Theta vs vitel l.jpg
Theta vs. Vitel

  • Both are impedance probes

  • However...

    • Vitel probes had 1 calibration for all soils

    • Theta probes have different calibrations for sandy and loam soils

  • Vitel probes had issues with noise in the data at various sites

  • Theta probes have been reliable with little noise



Soil moisture monitoring and data l.jpg
Soil Moisture Monitoring and Data

  • Sensors directly measure VWC at four depths

    • 10, 25, 50, and 100 cm

  • Data is collected hourly

    • Daily average is generally used

  • Daily VWC output to HPRCC Soil Moisture site

    • http://hprcc.unl.edu/awdn/soilm/





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Development of a soil moisture index

  • Continuous soil moisture measurements allowed us to develop an index to detect drought stress (Hunt et al., in press)

  • Index is based on three known quantities:

    • field capacity

    • wilting point

    • current VWC

  • Field Capacity and Wilting Point determined through field data


Development of smi l.jpg
Development of SMI

  • Assumes the point of stress/no-stress occurs at 50% available water (Baier, 1969)

  • FAW = (SM - WP) / (FC - WP)

  • FAW ranges from 1 at θFC to 0 at θWP

  • Decided to scale SMI from 5.0 to -5.0 as FAW changed from 1 to 0

    • SMI of 0.0 separates stress vs. non-stress


Soil moisture index smi l.jpg
Soil Moisture Index (SMI)

  • SMI= −5 + 10(θ − θWP)/(θFC − θWP)

  • Have generally taken the SMI at 10, 25, and 50 cm and taken a simple average of the three

    • Lower confidence in θFC and θWP at 100 cm

    • ~95% of grass roots fall within the first 50 cm below the surface (Wedin, 2004)


Why develop the smi l.jpg
Why develop the SMI?

  • Other drought indices are scaled similarly to the SMI (i.e, the SPI and the Palmer)

  • Psychologically satisfying to have positive values representing moisture and negative values representing dry conditions

  • Effective at indicating effectiveness of a rainfall event

  • Fits the goal of the HPRCC for developing a user-friendly drought index based on soil moisture observations

    • Effective at indicating flash drought


Conclusions l.jpg
Conclusions

  • Soil moisture observations greatly improve ability to detect the onset of drought or the end of agricultural drought

    • Ability to derive an index such as the SMI

    • Verification of the effectiveness of a rainfall event

  • Theta probes are highly recommended for all soil types

    • HPRCC has had good luck with them

  • An expansion of a soil moisture network would be very beneficial for drought monitoring efforts



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