Advantages and disadvantages

• Easy to be automated to make continuous observations
• Does not work well in soils with high clay content and/or EC equipment cost is very high
• Relatively expensive

Reference

Dalton, F. N. 1992. Development of time-domain reflectometry for measuring soil water content and bulk soil electrical conductivity. In (G. C. Topp et al. Eds.) Advance in measurement of soil physical properties: Bringing theory into practice. SSSA Spec. Publ. No. 30. Soil Sci. Soc. Am., Madison, WI. pp. 143-168.

Advantages and disadvantages

• It measures a sphere of about 30 cm in dia.
• Background H, bulk density, and other chemical components may influence the measuring results
• Radioactive

Reference

Greacen, E.L. 1981. Soil water assessment by the neutron method. CSIRO. Victoria, Australia.

c = g K_a

where g is instrument dependent. The instrument measures c. As mentioned earlier, θ _v is related to Ka.

Advantages and disadvantages

• Measurement can be automated
• Expensive, but not so expensive that the farmers are not interested. The time trends are very revealing and appeal to farmers in that they can see the rate water content is changing.

Consists of a porous cup (sensor), a manometer (water/mercury column, vacuum gauge, pressure transducer), and a tubing connecting the two. It measures soil matric potential.

Advantages and disadvantages

• Limitations: narrow pressure range and slow response to rapid change in matric potential
• If the tubing is relatively long, the gauge readings should be subtracted by the length of the tubing to obtain the soil matric potential.

• Two electrodes embedded in the gypsum or nylon blocks measure the electrical conductivity (resistance);
• Calibration between conductivity and matric potential/water content

Advantages and disadvantages

• The absorbent blocks can only be used under conditions where salts do not affect the calibration curve unduly.
• Blocks can be used under drier conditions than the tensiometers and are more sensitive for Y<-100 kPa.

Advantages and disadvantages

• Measurement is very reliable
• Less accurate (±0.5 - 1^oC)

Semiconductor--Resistance changes with temperature. The resistance can be measured accurately by a full bridge circuit.

Advantages and disadvantages

• High accuracy (±0.1 ^oC)
• More complex circuits

Infiltration rate measures the water intake rate at the soil surface. The common ways to measure infiltration rate use double- or single-ring infiltrometers. However, one has to keep in mind that

• Due to the fact of 3-dimensional water movement from the infiltrometers, ring size, ring insertion depth, and soil type all can affect the infiltration measurement. The measured infiltration rates by the ring infiltrometers are not the soil water intake capacity at the surface, since (1) infiltration during an irrigation event is 1-dimensional while infiltration in a ring infiltrometer is 3-dimensional; (2) there is greater ponding depth during infiltration rate measurement than in irrigation.

It was designed to create a near-normal furrow flow depth and velocity throughout the section of measurement. The device consists of a small, long-throated V-furrow flume that served both as an inflow sump and for flow rate measurement, a pump that lifts the flow from the downstream sump into the return reservoir, and a constant water supply reservoir equipped with a Marriotte tube, which maintains a constant head in the water return reservoir. Form the return reservoir the water flows through a hose to the flume at the upper end of the furrow by gravity. A valve at the reservoir outlet regulates the flow rate in the furrow. The system maintains a constant flow rate and water volume in the recirculating system, so that infiltration is equal to the water volume decrease in the supply reservoir with time.

Reference

Blair, A. W. and T. J. Trout, 1989. Recirculating furrow infiltrometer design guide. Tech. Rep. CRWR 223. Center for Research in Water Resources, University of Taxes, Austin.

This is the standard method to measure soil salinity. The extract is used to measure electrical conductivity. The measurement of conductivity is made in a cell containing two electrodes (EC meters) and the resistance of the solution between the two electrodes is measured in terms of mmhos/cm or dS/m.

Comments

• EC does not equal to TDS, but there is a certain relationship between EC and TDS.

Suction lysimeters are used to extract soil solution from soil in the field. A vacuum is applied to the lysimeter to extract soil solution through porous cups. The soil solution can be used to measure EC by EC meters.

TDSTestr (Pocket CE meter)

A simple instrument that can potentially be used for salinity management is the TDSTestr. The principle is the same as an EC meter, but it can be used in situ to directly measure soil EC through a calibration curve. One has to bear in mind that this instrument works only when

1. the soil to be measured is near water saturation. It can be achieved by pouring water on the surface where the measurement is going to take place. However, excessive water may leach the salt away and create erroneous readings.
2. each time the soil should be at similar condition, namely at similar water content.

TDR measures soil bulk electrical conductivity. The soil bulk electrical conductivity (σ) is related to the ratio of the excitation voltage (V_T) to the reflected voltage (V_R). The V_R is related to V_T by

where ε is the soil dielectric constant.

It may have different configurations, but the basic structure for each type is the same: a combination of electrical current, source and resistance meter, four electrodes and necessary connections to form an array. After the electrodes are inserted into soil to the depth of interest, electrical current source is induced between the two outer electrodes, and potential drop between the inner two electrodes is measured.

R = E/I

Calibration between R and EC will provide a relationship between EC and the measured R in the form of

EC₂₅ = k*f_T/R_T,

where k is probe constant.

The principle of the electromagnetic induction sensors is that a transmitter coil located in one end of the instrument induces circular eddy current loops in the soil. The magnitude of these loops is directly proportional to the conductivity of the soil in the vicinity of that loop. The current loops generate a secondary electromagnetic field of which the strength is proportional to the value of the current flowing in the loops. A fraction of the secondary induced EM field from each loop is intercepted by the receiver coil and the sum of these signals is amplified and formed into output voltage which is linearly proportional to the depth weighted soil bulk EC.

Reference

J. D. Rhoades. 1992. Instrumental field methods of salinity appraisal. In (G. C. Topp et al. Eds.) Advance in measurement of soil physical properties: Bringing theory into practice. SSSA Spec. Publ. No. 30. Soil Sci. Soc. Am., Madison, WI. pp. 231-248.

Infrared thermometers have been used to detect canopy surface temperatures for a relatively small area. The relatively new technology of remote thermal imaging technique enables people to detect a much larger area.

A pressure bomb can be used to measure the averaged xylem pressure of the plant material laced in the chamber. To make the measurement, a severed part is placed in the chamber with its freshly cut end protruding through a rubber seal. The air pressure in the chamber is gradually increased until it just causes the exudation of xylem sap at the cut end. At this point, the resulting pressure of the sap is equal to negative value of the applied air pressure. If the xylem osmotic pressure is negligible, then the xylem potential is the same as xylem pressure. If the plant is at an equilibrium with soil, then the soil water potential (matric + osmotic) is then equal to the xylem pressure, or the negative value of the applied air pressure.

Infrared Thermometer:	Pressure bomb:

Transpiration rates for whole plants or individual branches can be determined by measuring the rate of sap movement. There are two methods to measure the sap flow rates: heat-balance method and heat-pulse method.

• Heat-balance method - Heat (P) is applied to the entire circumference of the stem encircled by the heater. The mass flow of the sap is obtained from the balance of the heat flux into and out of the heated segment of the stem:

• Heat-pulse method - Determines the sap flow rate by measuring the velocity of a short pulse of heat carried by the moving sap stream. The method is suitable only for woody stems because the heater and temperature sensors must be installed by drilling holes into the sapwood.

Comments

• Sap flow technique measures transpiration only, suitable for plant water-use study
• If used properly, the accuracy can be within 10%
• Uncertainties in estimating water use can occur as a result of errors made in scaling transpiration from plant to stand.

Reference

D. M. Smith and S. J. Allen. 1996. Measurement of sap flow in plant stems. Journal of Experimental Botany. 47(No. 305): 1833-1844.

Agricultural remote sensing information can be generated from thermal scanner, multi-spectral video systems, or infrared film. From the images the digital files and vegetative indices can be generated. The vegetative index numbers represent both vegetation and plant health. The field crop can be divided into several classes according to the vegetative indices. Crop yield and thus water use can be estimated from the crop classes.