Measurements for

water

A.M.J. Coenders

Waterbalans: Soil Water

Challenge the future

DelftUniversity ofTechnology

Water in the soil

2

Unsaturated zone/ Vadose zone

3

Why is soil moisture important?

4

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

5

Gravimetric method

+ Cheap and straightforward

- Destructive and not continue

sample dry w

sat dry t

M M V

M M Vθ

−= =

−

6

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

7

Capacitor probe

• Soil as a capacitor, C= κ A/S

• Measures charge time, t Capacitor=κ A/S

8

Time Domain Reflectometry (TDR)

• Measures travel time of high

frequency electromagnetic wave

• with L the transmission line, c=

speed of light

• t short => ε low => θ low

- expensive

+ highly accurate, less sensitive for

salinity and temperature

2

2

tc

Lκ =

9

Frequency Domain Reflectometry (FDR)

• Measures frequency difference

transmitted and reflected pulse

+ cheaper than TDR

- needs per soil type calibration

10

p m o v

2

2

c = 1.92X + 2.51X + 4.18

( , ) exp4 4

exp

p

pM

Q rT r t

t t

qQ

c

qc

r T

ρ θ

πκ κ

ρ

ρπ

−=

=

=∆

Heat Pulse Sensors

11

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