Soil Moisture Monitoring, Data, and Products in Nebraska

1. Soil Moisture Monitoring, Data, and Products in Nebraska Eric Hunt, Brian Fuchs, Mark Svoboda National Drought Mitigation Center 3 March 2009

2. Nebraska AWDN stations

3. 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)

4. 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)

5. 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

6. Example of noise with the Vitel:

7. 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/

8. Soil Moisture Products

9. More Soil Moisture Products

10. More Soil Moisture Products

11. 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

12. 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

13. 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)

14. 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

23. 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

24. Questions? • Contact: ehunt2@unl.edu