Measuring soil water

Field methods of measuring soil water are primarily designed to measure water in the range of -10 to -1500 kPa of pressure. However, different instruments have different ranges as shown in Table 5.1. Tensiometers, porous blocks, and thermocouple psychrometers are usually installed in the field and measurements are taken on a regular basis. Neutron probe and time domain reflectometry (TDR) equipment are usually carried to the field each time a... 

TABLE 5.1. Common Instruments for Measuring Soil Water Content in the Field... 

Klein, K. and Santamarina, J.C. (1997) Methods for broad-band dielectric permittivity measurements (soil-water mixtures, 5 Hz to 1.3 GHz), ASTM Geotechnical Testing Journal 20, 168-178... 

Holm, P. E., Christensen, T. H., Lorenz, S. E., Hamon, R. E., Domingues, H. C., Sequeira, E. M., and McGrath, S. P. (1998). Measured soil water concentrations of cadmium and zinc in plant pots and estimated leaching outflows from contaminated soils. Water Air Soil Pollut. 102, 105—115. 

It is also a measure of the moisture stress in plants or soil, measured in megapascals. A more negative value indicates greater moisture stress. Soils with no moisture stress have a water potential of 0 to -1 mPa. Two methods of measuring soil water potential are the heat-dissipation method or the electrical-resistance method. 

This is a method for determination of soil moisture status in situ without disturbing the system In the neutron prohe method number of hydrogen neuclei present per rmit volume of soil is measured. Since hydrogen neuclei have a marked property for scattering and slowing neutrons, the same is exploited in the neutron method for measuring soil water content. 

Fig. 2.7. Neutron moisture meter for measuring soil water content.

Fig. 2.8. A schematic representation of a typical calibration curve for measuring soil water content by neutron probe method.

Nielsen, D. R., Reichardt, K., and Wierenga, P. J. (1983). Characterization of field-measured soil-water properties. In Isotope and Radiation Techniques in Soil Physics and Irrigation Studies 1983. pp. 55-78. Report lAEA-SM-267/40, International Atomic Energy Agency, Vienna. 

The soil texture types mainly are sand and loamy sand according to American soil texture classification, the ranges of sand, silt and clay contents are 74.08% 99.6%, 0.40% 23.52%, 0 2.4% (Table 1), sand content is far higher than silt and clay, and clay is the lowest, therefore, soil samples are assumed coarse textured. Silt and Clay have a strong variation with the variation coefficients are larger than 75%, while BD and Sand have a weak variation. Bulk density ranges from 1.35 to 1.76 g/cm, possibly for reason that soil sand content is very high. The 120 measured soil water... 

Turbidity n/a Tt3 Turbidity, a measure of water cloudiness, is used to indicate water quality and filtration effectiveness (e.g., whether disease-causing organisms are present). Higher turbidity is associated with higher levels of microorganisms such as viruses, parasites and some bacteria. These organisms can cause symptoms such as nausea, cramps, diarrhea, and associated headaches. Soil runoff... 

The term aggressive is often used to imply some approximately quantitative estimate of the likelihood of corrosion and depends on measuring factors such as soil water (resistivity), pH, redox potential, salt concentrations and bacterial populations in order to establish criteria for the prediction of corrosion rates . Similar measurements for predicting corrosion... 

BCF factors in fish ranging from 1.08 to 1.85, indicating that bioconcentration of methyl parathion is not an important fate process (Crossland and Bennett 1984). In another study, methyl parathion was added to the water of a carp-rearing pond and the concentration of methyl parathion was measured in water, soil, macrophytes, and carp over a 35-day period. Results showed that methyl parathion accumulated in macrophytes for 1 day and in carp for 3 days following exposure, and then dissipated. The concentrations of methyl parathion decreased in macrophytes by 94% by day 35 and by 98% in carp tissue by day 28 (Sabharwal and Belsare 1986). These data indicate the potential for biomagnification in the food chain is likely to be low because methyl parathion appears to be metabolized in aquatic organisms. 

In many cases, where one is concerned with the effects of specific environmental factors it is appropriate to replace the general term stress by the appropriate quantitative measure (e.g. soil water content or water potential) together with an appropriate measure of the plant response (e.g. growth rate). 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Fig. 11. Bulk soil water content and root ABA content in consecutive 10 cm soil layers on days 3, 9,12 and 18 after withholding water (A) from maize plants growing in 1 m deep soil columns. Well-watered plants (A) received water daily throughout the experimental period. Points are means of five measurements. Modified from Zhang Davies (1989).

Zhang, J. Davies, W.J. (1989). Abscisic acid produced in dehydrating roots may enable the plant to measure the water status of the soil. Plant, Cell and Environment, 12, 73-81. 

In marked contrast to hydrophobic compounds, more polar ones tend to be less adsorbed and to reach relatively high concentrations in soil water. Phenoxyalkanoic acids such as 2,4-D and MCPA are good examples (Figure 4.3). Their half-lives in soil are measured in weeks rather than years, and they are more mobile than OC insecticides in soils. When first applied they are lost only slowly. After a lag period of a... 

Elicks BB, Baldocchi DD, Meyers TP, Hosker Jr RP, Matt DR. 1987. A preliminary multiple resistance routine for deriving deposition velocities from measured quantities. Water Air Soil Pollut 36 311-330. 

Figure 14 Comparison of actual volumetric water contents (measured by time domain reflectom-etry) and calculated soil-water flux values (Penman equation) at four soil depths...

More sophisticated methods that actually measure volumetric water content can also be used, such as time domain reflectometry (TDR). In Figure 14, an example of TDR results is presented. Both the calculated and measured (i.e., TDR) volumetric water contents provide a similar picture of the profile water status by depth with time. Proper soil characterization data, such as those shown in Table 6, are necessary for these calculations and improve understanding of the test system. The determination of water-holding capacity (WHC) at 0.03 MPa field capacity (FC) and 1.5 MPa... 

Gravity force can be measured by means of the mass of the water. The direction of the force is, obviously, downwards toward the Earth s center. The gravitational potential of soil water at each position is determined by the elevation of the position relative to some reference level. If we only consider the elevation potential and the related velocity energy, then a water body at a higher elevation will flow to a lower elevation, decreasing the elevation potential but increasing its velocity. 

Installation of the pumping well is more time consuming than digging a trench. There is a lag period between the start of pumping, the formation of the depression cone, and containment of the plume. This limits its use as a rapid containment measure. The water table depression must be kept constant otherwise, if the water table is allowed to fluctuate, gasoline droplets may adhere to soil particles and get trapped below the water table, especially when the depth of the cone of depression gets lower. 

Performance data Percolation is being measured with a lysimeter connected to flow monitoring systems, soil moisture is being measured with water content reflectometers, and soil matric potential and soil temperature are being monitored with heat dissipation units. From November 1999 to July 2002, the capillary barrier cover system had a cumulative percolation of 0.5 mm. Total precipitation was 837 mm over the 32-month period. Additional field data were collected through 2005. 

Consequently, if the free ions in the soil water are leached out, the double layers swell tremendously, pinching off flow paths and resulting in very low hydraulic conductivity. Data have shown hydraulic conductivity to be as much as two to three orders of magnitude lower when measured with distilled water than with other kinds of water. For this reason, distilled water should not be used in testing the hydraulic conductivity of a clay liner. 

Dissolved organic matter can be thought of, for simplicity, as soluble organic matter present in soil solution. However, although some workers have measured dissolved organic matter in soil leachates or extracted soil solutions, many use the extract from a 1 2 w/v soil/water extraction... 

Toxaphene is persistent in soils, water, and other environmental compartments, with residence times measured in years. 

---