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# Measurements for water: Soil water

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### Measurements for water: Soil water

1. 1. Measurements for waterA.M.J. CoendersWaterbalans: Soil Water
2. 2. Water in the soilDelftUniversity ofTechnologyChallenge the future
3. 3. Unsaturated zone/ Vadose zone 2
4. 4. Why is soil moisture important? 3
5. 5. Methods to measure soil moisture• Direct • Gravimetric method• Indirect by di-electric permittivity • Capacitance probe • Time Domain Reflectrometer (TDR) • Frequency Domain Reflectometry (FDR)• Indirect by thermal conductivity • Heat Pulse Sensors 4
6. 6. Gravimetric method M sample − M dry Vwθ= = M sat − M dry Vt+ Cheap and straightforward- Destructive and not continue 5
7. 7. Dielectric permittivity• Permittivity (ε) is a measure of the ability of a material to be polarized by an electric field• κ =ε/ε0 (ε0 vacuum = 8 x 10-12 F/m)• Dielectric permittivity is function of water content • κ vacuum = 1 (per definition) • κ soil = 2-5 • κ water = 80 6
8. 8. Capacitor probe• Soil as a capacitor, C= κ A/S• Measures charge time, t Capacitor=κ A/S 7
9. 9. Time Domain Reflectometry (TDR)• Measures travel time of high frequency electromagnetic wave 2  tc  κ =   2L • with L the transmission line, c= speed of light• t short => ε low => θ low- expensive+ highly accurate, less sensitive for salinity and temperature 8
10. 10. Frequency Domain Reflectometry (FDR)• Measures frequency difference transmitted and reflected pulse+ cheaper than TDR- needs per soil type calibration 9
11. 11. Heat Pulse Sensorsρ c p = 1.92X m + 2.51X o + 4.18θ v Q  −r 2 T (r , t ) = exp   4πκ t  4κ t  qQ= ρcp qρcp = exp π r 2 ∆TM 10
12. 12. Methods to measure soil moisture• Direct • Gravimetric method• Indirect by di-electric permittivity • Capacitance probe • Time Domain Reflectrometer (TDR) • Frequency Domain Reflectometry (FDR)• Indirect by thermal conductivity • Heat Pulse Sensors 11