Soil Water Sensors: Problems, Advances and Potential for Irrigation Scheduling

1. Soil Water Sensors: Problems, Advances and Potential for Irrigation Scheduling Ogallala Aquifer Program Workshop 13 March 2012 Garden City, Kansas

2. Irrigation Scheduling • What is it? • A process to decide when to irrigate crops, how much to apply, and where to apply it in order to reach a management goal/objectives. • Goals (two examples) • Maximum profitability • Maximum sustainability • Objectives (examples) • Limit, avoid, or apply stress • Leach field, prepare for planting, …

3. Irrigation Scheduling • Why do it? • Approx. 80% of available fresh water world-wide is used for agricultural production. • Only 40 to 60% of this water is transpired. • Yield is directly related to transpiration. • Over-irrigation has negative effects. • Salinization • Ground water pollution • Declining yield and profit • Competition for water is increasing. • Profitability is impacted.

4. Water Content Based Methods • These include • Neutron probe (NP) • Time domain methods (TDR and TDT) • Capacitance and other frequency domain (FD) methods • Gravimetric sampling • Method • Observe soil water content • Irrigate at management allowed depletion • Can include forecasting

5. MAD Irrigation Scheduling Evett (2007)

7. Example of forecasting Full point Refill points and projected irrigation dates Soil moisture measurements Refill point for high use rate Refill point for low use rate Gear et al., 1977

8. θv Sensing Principles • All sensors measure a surrogate propertythat is then related to θv through a calibration. • The major surrogate properties are: • Capacitance – variable resonant frequency • Phase delay – constant frequency • Transmission time • Quasi travel time, e.g. Trime, CS616 • Time domain reflectometry (TDR) and transmission (TDT), with waveform interpretation • Thermal neutron count – neutron probe Evett et al. (2008)

9. Measurement Principles • Electromagnetic (EM) sensors respond to εa: • Apparent relative permittivity (εa) is sensitive to water content; εwater ≈ 80, εsolids ≈ 5, εair = 1, εbw = 8-40 • Interferences: • Temperature affects σdc, ε’ and ε” • σdc, ε’ and ε” affect ω, particularly for capacitance methods • ω affects measurement volume and εa sensed

10. Electromagnetic Sensor Technologies • Time Domain • Conventional TDR • Frequency Domain • Capacitance sensors • Wave Guides • Antennas • Mixed Technologies • Quasi TDR, reflectometers… Electrode Electrode Access tube

11. EM Field Geometry Electrode Electrode • Field in uniform medium → uniform geometry: • Field in medium with more or less conductive (wetter or drier) peds → geometry changed: Electrode Electrode Evett et al. (2009)

12. 2003, Winter wheat, Bushland, TX Evett et al. (2009)

13. 2003, Winter wheat, Bushland, TX 0.31 m3 m-3 NMM Grav. Trime 75 cm3 0.20 m3 m-3 Mean relative difference in storage Diviner 250 cm3 EnviroSCAN 500 cm3 PR1/6 Relative rank Evett et al. (2009)

14. EM Sensor Calibration Problems C D Mazahrih et al. (2008)

15. Sources of Imprecision/Inaccuracy • EMF is radiated as from an antenna • Volume decreases as frequency increases • Field follows conductive paths – radiates preferentially into more conductive (wetter) peds – biased to higher θv • Affected by structured soils with appreciable σa • ε’, ε” (bound water relaxation) and σdc/ω effects on εa • Clay content and type, salinity, temperature

16. Bulk EC in an 80-acre field Light to dark colors indicate salinity ranges of 7.0–13.3, 13.3–18.4, 18.4–26.4, and 26.4–35.6 dS m−1, respectively. Johnson, C.K., K.M. Eskridge, and D.L. Corwin. 2005. Apparent soil electrical conductivity: applications for designing and evaluating field-scale experiments. Computers Electronics Agric. 46:181–202.

17. EnviroSCAN Probe Design Small Measurement Volume Red lines denote axial and radial volume sensed Paltineanu and Starr, 1997 Evett et al., 2002

18. How important is small-scale variability?

19. Small-scale variability is very important in many soils

20. Automated sensing Field capacity, 0.29 m3 m-3 Refill point, 0.20 m3 m-3

21. Contrasting EM methods • Frequency domain (capacitance) • Lower frequency → larger σa/ω • Obey Gauss’ law → C = gεaε0 • Sensitive to EM field volume and geometry • Time domain (TDR and TDT) • Higher frequency → small σa/ω • Obey Maxwell’s equations → no g term • Insensitive to geometric effects

22. Results motivated OAP Project • Developed a waveguide-on-access-tube (WOAT) TDR sensor system • Sense profile water content and σa • 20-cm WOAT segments • 2 waveguides on opposite sides • Did theoretical & lab work (Casanova) • Modeled EM fields for design optimization • Characterized designs in laboratory • Did field testing – more to do (collaborators)

23. EM Field Modeling

24. Laboratory work in liquids & variably saturated soilsShown are waveformsat saturationEffects of:(a) ID(b) ϕ(c) Length

25. Calibration

26. Bulk EC Measurement • Probe constant, Kp, measured in KCl soln. is a linear function of theoretical prediction. • Bulk EC measurements are very good.

27. WOAT segment prototyping

28. Next steps • Design sensor string head • Wireless data transfer (Bluetooth, Zigbee…) • Battery life of 6 months, solar power option • Whip antenna to above crop • Internal data storage • Design installation & removal tools • Develop mixed dielectric model to ease calibration • Field test six WOAT strings (wet & dry sites)

29. Commercial prototype tests • In customer irrigated environments • Valmont, Monsanto, etc. • In collaborator fields • Texas, Kansas, Colorado, Idaho • Objectives • Test against neutron probe & gravimetric • Test usefulness in irrigation scheduling & control

30. Publications • Soil water sensing for water balance, ET and WUE • http://www.cprl.ars.usda.gov/wmru/pdfs/Evett%20et%20al%20(2012)%20Soil%20water%20sensing%20for%20water%20balance%20ET%20and%20WUE.pdf • Design and testing of access-tube TDR soil water sensor • http://www.cprl.ars.usda.gov/wmru/pdfs/Casanova%20et%20al%20(2011)%20Design%20and%20Testing%20of%20Access-tube%20TDR%20Soil%20Water%20Sensor_ASABE.pdf

31. The team • Bushland • Steve Evett, Robert Schwartz, Joaquin Casanova, Susan O’Shaughnessy and Paul Colaizzi • Cooperators • Robert Lascano (Lubbock), Jim Bordovsky (Halfway), Alan Schlegel (Tribune), Freddie Lamm (Colby)